Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session A1: Focus Session: Novel Instrumentation and measurements for Biomedical Research
Sponsoring Units: GIMSChair: Larry Nagahara, National Cancer Institute
Room: 203
Monday, February 27, 2012 8:00AM - 8:12AM |
A1.00001: Instrumentation of Molecular Imaging on Site-Specific Targeting Fluorescent Peptide for Early Detection of Breast Cancer Ping Yu, Lixin Ma In this work we developed two biomedical imaging techniques for early detection of breast cancer. Both image modalities provide molecular imaging capability to probe site-specific targeting dyes. The first technique, heterodyne CCD fluorescence mediated tomography, is a non-invasive biomedical imaging that uses fluorescent photons from the targeted dye on the tumor cells inside human breast tissue. The technique detects a large volume of tissue (20 cm) with a moderate resolution (1 mm) and provides the high sensitivity. The second technique, dual-band spectral-domain optical coherence tomography, is a high-resolution tissue imaging modality. It uses a low coherence interferometer to detect coherent photons hidden in the incoherent background. Due to the coherence detection, a high resolution (20 microns) is possible. We have finished prototype imaging systems for the development of both image modalities and performed imaging experiments on tumor tissues. The spectroscopic/tomographic images show contrasts of dense tumor tissues and tumor necrotic regions. In order to correlate the findings from our results, a diffusion-weighted magnetic resonance imaging (MRI) of the tumors was performed using a small animal 7-Telsa MRI and demonstrated excellent agreement. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A1.00002: Digital Image Speckle Correlation for the Quantification of the Cosmetic Treatment with Botulinum Toxin Type A (BTX-A) Divya Bhatnagar, Nicole Conkling, Miriam Rafailovich, Alexander Dagum The skin on the face is directly attached to the underlying muscles. Here, we successfully introduce a non-invasive, non-contact technique, Digital Image Speckle Correlation (DISC), to measure the precise magnitude and duration of facial muscle paralysis inflicted by BTX-A. Subjective evaluation by clinicians and patients fail to objectively quantify the direct effect and duration of BTX-A on the facial musculature. By using DISC, we can (a) Directly measure deformation field of the facial skin and determine the locus of facial muscular tension(b)Quantify and monitor muscular paralysis and subsequent re-innervation following injection; (c)~Continuously correlate the appearance of wrinkles and muscular tension. Two sequential photographs of slight facial motion (frowning, raising eyebrows) are taken. DISC processes the images to produce a vector map of muscular displacement from which spatially resolved information is obtained regarding facial tension. DISC can track the ability of different muscle groups to contract and can be used to predict the site of injection, quantify muscle paralysis and the rate of recovery following BOTOX injection. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A1.00003: Ultra-low field SQUID electron paramagnetic resonance for biomedical applications P. Bhupathi, B.H. Eom, K.I. Penanen, P.K. Day, I. Hahn We have constructed a SQUID-magnetometer system operating at 4K, for electron paramagnetic resonance (EPR) detection from room temperature samples in magnetic fields of the order of Earth$^\prime$s field. The magnetometer consists of a home-built gradiometer pick-up coil inductively coupled to the input of a commercially available 2-stage dc SQUID amplifier. Operation at low EPR excitation frequency of a few MHz has advantages of negligible sample heating and finite penetration depth effects in biological systems. The same system can be adapted to detect NMR signals at several kHz range. We describe our detection scheme and discuss the prospects for \textit{in vivo} biomedical EPR imaging. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A1.00004: Single cell microfluidics for systems oncology Invited Speaker: Rong Fan The singular term ``cancer'' is never one kind of disease, but deceivingly encompasses a large number of heterogeneous disease states, which makes it impossible to completely treat cancer using a generic approach. Rather systems approaches are urgently required to assess cancer heterogeneity, stratify patients and enable the most effective, individualized treatment. The heterogeneity of tumors at the single cell level is reflected by the hierarchical complexity of the tumor microenvironment. To identify all the cellular components, including both tumor and infiltrating immune cells, and to delineate the associated cell-to-cell signaling network that dictates tumor initiation, progression and metastasis, we developed a single cell microfluidics chip that can analyze a panel of proteins that are potentially associated inter-cellular signaling network in tumor microenvironment from hundreds of single cells in parallel. This platform integrates two advanced technologies -- microfluidic single cell handling and ultra-high density protein array. This device was first tested for highly multiplexed profiling of secreted proteins including tumor-immune signaling molecules from monocytic leukemia cells. We observed profound cellular heterogeneity with all functional phenotypes quantitatively identified. Correlation analysis further indicated the existence of an intercellular cytokine network in which TNF$\alpha $-induced secondary signaling cascades further increased functional cellular diversity. It was also exploited to evaluate polyfunctionality of tumor antigen-specific T cells from melanoma patients being treated with adoptive T cell transfer immunotherapy. This platform could be further extended to analyze both solid tumor cells (e.g. human lung carcinoma cells) and infiltrating immune cells (e.g. macrophages) so as to enable systems analysis of the complex tumor microenvironment from small amounts of clinical specimens, e.g. skinny needle biopsies. Thus, it could potentially become a clinical tool for patient stratification based upon the inter-cellular signaling network and designing new anti-cancer therapy by targeting microenvironmental components. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A1.00005: High resolution phase-sensitive optical coherence microscopy tracking of magnetic microbeads for cellular mechanics Vasilica Crecea, Benedikt Graf, Qingshan Wei, Hyon-min Song, Alexander Wei, Stephen Boppart We present a real-time multi-modal near-infrared imaging technology that tracks externally induced axial motion of magnetic microbeads in mouse macrophages, human breast epithelial cells, and human breast primary ductal carcinoma cells. The imaging technique includes nanometer scale phase-sensitive magnetomotive integrated optical coherence (MM-OCM) and multiphoton (MPM) microscopy. MPM is used to visualize multifunctional microbeads based on their fluorescence, while MM-OCM detects sinusoidal displacements of the magnetic microbeads induced by a magnetic field. This methodology demonstrates imaging of cell dynamics and has the potential for measuring cell biomechanical properties, which are important in assessing cell health. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A1.00006: Reagent-free ultrasensitive spectroscopic probes for long term diabetes monitoring N.C. Dingari, I. Barman, J.W. Kang, G. Horowitz, Ramachandra Rao Dasari Long-term glycemic control is essential in developing therapeutics for diabetics. Glycated hemoglobin (HbA1c) and glycated albumin have been increasingly accepted as a functional metric of glycemic control over the past two to three months and three weeks, respectively. In this talk, we present the first demonstration of non-enhanced Raman spectroscopy as a novel analytical method for quantitative detection of HbA1c and glycated albumin. Using the drop coating deposition Raman technique, we observe that the non-enzymatic glycosylation of these proteins results in subtle, but consistent, changes in vibrational features, which with the help of multivariate classification techniques can be used to discriminate the glycated proteins from their unglycated variants with 100\%. Additionally, the developed multivariate calibration models show a high degree of prediction accuracy even at substantially lower concentrations than those typically encountered in clinical practice. The excellent accuracy and reproducibility achieved in this proof-of-concept study opens substantive avenues for basic investigations of glycated proteins as well as in high-throughput glycemic marker sensing in multi-component mixtures and potentially even in serum and whole blood samples. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A1.00007: The measurand problem in infrared breath alcohol testing Ted Vosk Measurements are made to determine the value of a quantity known as a measurand. The measurand is not always the quantity subject to measurement, however. Often, a distinct quantity will be measured and related to the measurand through a measurement function. When the identities of the measurand and the quantity actually measured are not well defined or distinguished, it can lead to the misinterpretation of results. This is referred to as the measurand problem. The measurand problem can present significant difficulties when the law and not science determines the measurand. This arises when the law requires that a particular quantity be measured. Legal definitions are seldom as rigorous or complete as those utilized in science. Thus, legally defined measurands often fall prey to the measurand problem. An example is the measurement of breath alcohol concentration by infrared spectroscopy. All 50 states authorize such tests but the measurand differs by jurisdiction. This leads to misinterpretation of results in both the forensic and legal communities due to the measurand problem with the consequence that the innocent are convicted and guilty set free. Correct interpretation of breath test results requires that the measurand be properly understood and accounted for. I set forth the varying measurands defined by law, the impact these differing measurands have on the interpretation of breath test results and how the measurand problem can be avoided in the measurement of breath alcohol concentration. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A1.00008: Nanoscale neuroelectronic interface based on open-ended nanocoax arrays Jeffrey R. Naughton, Binod Rizal, Michael J. Burns, Jee Yeom, Shannon Heyse, Michelle Archibald, Stephen Shepard, Gregory McMahon, Thomas C. Chiles, Michael J. Naughton We describe the development of a nanoscale neuroelectronic array with submicron pixelation for recording and stimulation with high spatial resolution. The device is composed of an array of nanoscale coaxial electrodes, either network- or individually-configured. As a neuroelectronic interface, it will employ noninvasive real-time capacitive coupling to the plasma membrane with potential for extracellular recording of intra- and interneural synaptic activity, with one target being precision measurement of electrical signals associated with induced and spontaneous synapse firing in pre- and post-synaptic somata. Subarrays or even individual pixels can also be actuated for precisely-localized stimulation. We report initial results from measurements using the rat adrenal pheochromocytoma PC12 cell line, which terminally differentiates in response to nerve growth factor, as well as SH-SY5Y neuroblastoma cells in response to retinoic acid, characterizing the basic performance of the fabricated device. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A1.00009: Phosphorus-31 MRI of bones using quadratic echo line-narrowing Merideth Frey, Sean Barrett, Karl Insogna, Joshua VanHouten There is a great need to probe the internal composition of bone on the sub-0.1 mm length scale, both to study normal features and to look for signs of disease. Despite the obvious importance of the mineral fraction to the biomechanical properties of skeletal tissue, few non-destructive techniques are available to evaluate changes in its chemical structure and functional microarchitecture on the interior of bones. MRI would be an excellent candidate, but bone is a particularly challenging tissue to study given the relatively low water density and wider linewidths of its solid components. Recent fundamental research in quantum computing gave rise to a new NMR pulse sequence - the quadratic echo - that can be used to narrow the broad NMR spectrum of solids. This offers a new route to do high spatial resolution, 3D $^{31}$P MRI of bone that complements conventional MRI and x-ray based techniques to study bone physiology and structure. We have used our pulse sequence to do 3D $^{31}$P MRI of \textit{ex vivo} bones with a spatial resolution of (sub-450 $\mu$m)$^3$, limited only by the specifications of a conventional 4 Tesla liquid-state MRI system. We will describe our plans to push this technique towards the factor of 1000 increase in spatial resolution imposed by fundamental limits. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A1.00010: Phosphorus-31 MRI of cell membranes using quadratic echo line-narrowing Sean Barrett, Merideth Frey, Joseph Madri, Michael Michaud Soft biological tissues have phosphorus concentrated in the membranes, metabolites, RNA and DNA of cells. This leads to a complicated, multi-peak $^{31}$P nuclear magnetic resonance spectrum (including a broad membrane peak and narrow metabolite peaks), which precludes high-resolution $^{31}$P MRI of soft tissues. This long-standing barrier has been overcome by a novel pulse sequence - the quadratic echo - recently discovered in fundamental quantum computation research. Applying time-dependent gradients in synch with a repeating pulse block enables a new route to high spatial resolution, three-dimensional $^{31}$P MRI of the soft solid components of cells and tissues. This is a functionally different kind of MR image, since conventional $^1$H MRI probes the intracellular and extracellular free water, whereas our $^{31}$P MRI signal is dominated by the cell membrane contribution, which in turn depends on the density of mitochondria. The unique aspects of the signal should provide new insights into cellular and tissue function that compliment the information revealed by $^1$H MRI. So far, various \textit{ex vivo} soft tissue samples have been imaged with (sub-mm)$^{3}$ voxels. We will describe plans to enhance the spatial resolution in future work, to open a new window into cells. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A1.00011: Accelerating the magnetic resonance imaging of hard and soft solids Zachary Sethna, Merideth Frey, Sean Barrett Recent fundamental research in quantum computing gave rise to a new NMR pulse sequence - the quadratic echo - that can be used to narrow the broad MR spectrum of solids by orders of magnitude. This advance enables high spatial resolution, 3D MRI of hard and soft solids (e.g., the $^{31}$P MRI of bone and soft tissues, which has recently been demonstrated in our group). While this is great progress, the next challenge to address is the slow imaging time in solids. The $T_1$ time of solids can be orders of magnitude larger than that of a liquid (e.g., for some of our samples the $T_1$ time can be $>$100 s). Since imaging sequences wait a repetition time (of order $T_1$) between scans, a three-dimensional image (requiring many scans) can take $>$24 hours. Here we discuss approaches under study that aim to decrease the total imaging time of our MRI of solids technique. One method is to implement a variant on the driven equilibrium approach. Another method is to take less data, namely, sparse MRI (i.e., under-sampling of \textbf{k}-space). This leads to artifacts in standard FFT image construction; more advanced alternatives, such as a difference map algorithm, may be used to produce an image closer to the ideal, which is a promising approach to reduce total imaging time. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A1.00012: A Comparison of Raman Spectral Features of Frozen and Deparaffinized Tissues in Neuroblastoma and Ganglioneuroma Suneetha Devpura, Jagdish S. Thakur, Janet M. Poulik, Raja Rabah, Vaman M. Naik, Ratna Naik We have investigated the cellular regions in neuroblastoma and ganglioneuroma using Raman spectroscopy and compared their spectral characteristics with those of normal adrenal gland. Thin sections from both frozen and deparaffinized tissues, obtained from the same tissue specimen, were studied in conjunction with the pathological examination of the tissues. We found a significant difference in the spectral features of frozen sections of normal adrenal gland, neuroblastoma, and ganglioneuroma when compared to deparaffinized tissues. The quantitative analysis of the Raman data using chemometric methods of principal component analysis and discriminant function analysis obtained from the frozen tissues show a sensitivity and specificity of 100{\%} each. The biochemical identification based on the spectral differences shows that the normal adrenal gland tissues have higher levels of carotenoids, lipids, and cholesterol compared to the neuroblastoma and ganglioneuroma frozen tissues. However, deparaffinized tissues show complete removal of these biochemicals in adrenal tissues. This study demonstrates that Raman spectroscopy combined with chemometric methods can successfully distinguish neuroblastoma and ganglioneuroma at cellular level. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A1.00013: Expanding Cancer Detection Using Molecular Imprinting for a Novel Point-of-Care Diagnostic Device Yingjie Yu, Miriam Rafailovich, Yantian Wang, Alina Ranjbaran, Tom Wang, David Nam We propose the use of a potentiometric biosensor that incorporates the efficient and specific molecular imprinting (MI) method with a self-assembled monolayer (SAM). We first tested the biosensor using carcinoembryonic antigen, CEA, a biomarker associated with pancreatic cancer. No change in detection efficiency was observed when detection was performed in the presence of 100{\%} serum albumin, indicating that the sensor is able to discriminate for the template analyte even in concentrated solution of similar substances. Computer simulations of the protein structure were performed in order to estimate the changes in morphology and determine the sensitivity of the biosensor to conformational changes in the proteins. We found that even small changes in PH can generate rotation of the surface functional groups, without significant change in the morphology. Yet, the results show that only when the detection and imprinting conditions are similar, robust signals occurs. Hence we concluded that both morphology and surface chemistry play a role in the recognition. [Preview Abstract] |
Session A2: Invited Session: Teaching Quantum Information Science at Liberal Arts College
Sponsoring Units: GQI FEdChair: Ian Durham, Saint Anselm College
Room: 204AB
Monday, February 27, 2012 8:00AM - 8:36AM |
A2.00001: Doing quantum theory at a liberal arts college Invited Speaker: Benjamin Schumacher An academic career at a liberal arts college poses some challenges for a researcher in quantum information and quantum foundations, most notably the lack of local collaborators and graduate students. Are there also complementary advantages and opportunities? [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A2.00002: Quantum Information in Non-physics Departments at Liberal Arts Colleges Invited Speaker: Michael Westmoreland Quantum information and quantum computing have changed our thinking about the basic concepts of quantum physics. These fields have also introduced exciting new applications of quantum mechanics such as quantum cryptography and non-interactive measurement. It is standard to teach such topics only to advanced physics majors who have completed coursework in quantum mechanics. Recent encounters with teaching quantum cryptography to non-majors and a bout of textbook-writing suggest strategies for teaching this interesting material to those without the standard quantum mechanics background. This talk will share some of those strategies. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A2.00003: Theoretical Research Fostered by an Undergraduate Environment Invited Speaker: William Wootters An undergraduate setting entails certain challenges for a faculty member doing research in theoretical physics, but it also offers particular opportunities. While it is possible to succeed in any area of theoretical research at an undergraduate institution, the experience of teaching at the undergraduate level and doing research with undergraduate students naturally leads one to think about problems that, though possibly very interesting, do not require a high level of technical expertise. Such problems tend to be ``close to the trunk of the tree of physics,'' as a colleague has suggested, meaning that they may be closer to the central ideas of physics than many highly specialized problems. In this talk I give examples of research projects in quantum information and quantum foundations that have been stimulated in part by undergraduate-level teaching and carried out with undergraduate collaborators. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A2.00004: Is Theoretical Physics Locus-dependent? Quantum Information at Liberal Arts Colleges Invited Speaker: Herbert Bernstein |
Monday, February 27, 2012 10:24AM - 11:00AM |
A2.00005: Fundamental Physics with Table-Top Optics Invited Speaker: Enrique Galvez Experimental physics research at an undergraduate institution poses the challenge of requiring the production of publishable results while also being a suitable setting for mentoring undergraduates in research. It is tempting for young faculty to copy/adapt ``big-machine'' university research, but that is a tough enterprise that is hard to sustain. An option that suits the undergraduate setting better is a small, table-top, apparatus devoted to projects with promising potential. In optics I found such a setting, and a successful transition from big-machine to table-top setups. Optics is a fertile ground for student projects that does not involve heavy maintenance, can achieve substantial depth, and enables student independence. It also allows the creation of projects that match the abilities of the not-so-high achievers. At the conference I will present examples of recent projects involving the study of classical light beams that carry optical vortices and non-classical beams that encode qubits. This work has been funded by Research Corporation, NSF and Air Force. [Preview Abstract] |
Session A3: Invited Session: Superconducting Fluctuations in Cuprates
Sponsoring Units: DCMPChair: N. Peter Armitage, John Hopkins University
Room: 205AB
Monday, February 27, 2012 8:00AM - 8:36AM |
A3.00001: Decrease of pairing strength with underdoping in cuprate superconductors Invited Speaker: Johan Chang The transition temperature $T_{c}$ of cuprate superconductors decreases at low hole doping $p$, but it is still unclear whether the pairing strength decreases or increases. Different interpretations of the pseudogap lead to opposite conclusions. Different estimates of the upper critical field $H_{c2}$ are in sharp contradiction. In this talk, we resolve the latter contradiction by showing that superconducting fluctuations in the underdoped cuprate Eu-LSCO, measured via the Nernst effect, obey the theory of Gaussian fluctuations, as in conventional superconductors [1, 2]. The extracted critical field $H_{c2}$ is small, and it dips at $p$ = 0.11, showing that pairing strength is weak where stripe order is strong. In the archetypal cuprate superconductor YBCO, $H_{c2}$ extracted from other measurements~[3] has the same doping dependence, also with a minimum at $p$ = 0.11, again where stripe order is present [4, 5]. We conclude that competing states such as stripe order weaken the pairing strength and this, rather than phase fluctuations, is the predominant cause for the low $T_{c}$ of underdoped cuprates. Work done in collaboration with N. Doiron-Leyraud, E. Hassinger, J.-Ph. Reid, O. Cyr-Choini\`{e}re,~F. Lalibert\'{e}, R. Daou, S. Pyon, T. Takayama, H. Takagi,~~and Louis Taillefer. \\[4pt] [1] M. N. Serbyn \textit{et al}., Phys. Rev. Lett. \textbf{102}, 067001 (2009). \\[0pt] [2] K. Michaeli and A. M. Finkel'stein, Europhys. Lett. \textbf{86}, 27007 (2009).\\[0pt] [3] Y. Ando and K. Segawa, Phys. Rev. Lett. \textbf{88}, 167005 (2002).\\[0pt] [4] F. ~Lalibert\'{e}~\textit{et al.}, Nature Comm. \textbf{2}, 432 (2011).\\[0pt] [5] T. Wu \textit{et al}., Nature \textbf{477}, 191 (2011). [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A3.00002: Pseudogap phase and superconducting fluctuation regime of the cuprate superconductors Invited Speaker: Neven Bari\v{s}i\'{c} The pseudogap phenomenon in the cuprates is one of the most investigated topics in the field of correlated materials. A related question is the extent to which superconducting fluctuation persist between the pseudogap temperature (\textit{T*}) and superconducting transition temperature (\textit{T$_c$}). We have addressed this question by combining several experimental probes: planar dc-resistivity [1], microwave conductivity [2,3], and torque mangetometry [4]. dc-resistivity measurements in the simple tetragonal model compound HgBa$_2$CuO$_{4+\delta}$ [5], which features the highest \textit{T$_c$} (97 K) among all single-layer cuprates, reveal four characteristic temperatures: \textit{T*}, coincident with the onset of novel \textbf{q}=0 magnetic order revealed by neutron diffraction [6]; a second, lower pseudogap temperature \textit{T**} associated with a further rearrangement of the sates at the Fermi level; \textit{T'}, which marks the onset of superconducting fluctuations; and finally \textit{T$_c$}. Notably, \textit{T'} lies only 10-20 K above \textit{T$_c$} and closely tracks the superconducting dome with doping. The superconducting fluctuation regime is further investigated by microwave conductivity and torque magnetometry, and these results confirm the latter conclusion. The results for HgBa$_2$CuO$_{4+\delta}$ are complemented by a comprehensive investigation of other cuprates (La$_{2-x}$Sr$_x$CuO$_4$, YBa$_2$Cu$_3$O$_{6+\delta}$, Bi$_2$Sr$_{2-z}$La$_z$CuO$_{6+\delta}$), which leads to new insights into the phase diagram of cuprate superconductors.\\[4pt] [1] N. Bari\v{s}i\'{c} \textit{et al.}, \textit{preprint}.\\[0pt] [2] M.S. Grbi\'{c} \textit{et al.}, \textit{Phys. Rev. B} \textbf{80}, 094511 (2009).\\[0pt] [3] M.S. Grbi\'{c} \textit{et al.}, \textit{Phys. Rev. B} \textbf{83}, 144508 (2011).\\[0pt] [4] G. Yu \textit{et al.}, \textit{preprint}.\\[0pt] [5] N. Bari\v{s}i\'{c} \textit{et al.}, \textit{Phys. Rev. B} \textbf{78}, 054518 (2008).\\[0pt] [6] Y. Li \textit{et al.}, \textit{Nature} \textbf{455}, 372 (2008). [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A3.00003: Fluctuations of Superconductivity in La$_{2-x}$Sr$_x$CuO$_4$: A Terahertz Conductivity Study Invited Speaker: Lucas Bilbro In the underdoped pseudogap regime of the high-temperature superconductors, one expects that due to low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties. However, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap regime, and their significance for understanding high-temperature superconductivity. We use THz time-domain spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity above the critical temperature (T$_C$) in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) thin films over a doping range that spans almost the entire superconducting dome. Signatures of the fluctuations persist in the conductivity in a narrow temperature range above T$_C$. Our measurements show that superconducting correlations do not make an appreciable contribution to the charge transport anomalies of the pseudogap in LSCO at tempera tures well above T$_C$.\footnote{L.S. Bilbro \textit{et al.}, Nature Physics 7, 298 (2011).} I will compare our results for an underdoped (x=0.095) sample with measurements of diamagnetism in a similarly doped crystal of La$_{1.905}$Sr$_{0.095}$CuO$_4$. I will show, through a vortex-plasma model, that if the fluctuation diamagnetism originates solely in vortices, then these vortices must exhibit an anomalously large vortex diffusion constant, more than two orders of magnitude larger than the Bardeen-Stephen estimate.\footnote{L.S. Bilbro \textit{et al.}, Phys. Rev. B 84, 100511 (2011).} This points to either the extremely unusual properties of vortices in the underdoped d-wave Cuprates or a contribution to the diamagnetic response that is not superconducting in origin. Finally, I will introduce preliminary results of THz conductivity measurements of critically underdoped LSCO films, where superconductivity is fully suppressed. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A3.00004: Distinct Ranges of Superconducting Fluctuations and Pseudogap in YBa$_2$Cu$_3$O$_{6+x}$ Invited Speaker: Florence Rullier-Albenque The contribution of superconducting fluctuations (SCF) to the ab-plane conductivity has been determined accurately in a series of YBa$_2$Cu$_3$O$_{6+x}$ single crystals using high magnetic fields to restore the normal state behavior [1]. This allows us to determine within the same set of transport measurements both the field $H^{\prime}_c(T)$ and the temperature $T^{\prime}_c$ above which the SCFs are fully suppressed, and the pseudogap temperature $T^{\star}$. A careful investigation near optimal doping shows that $T^{\star}$ becomes smaller than $T^{\prime}_c$, which unambiguously evidences that the pseudogap cannot be assigned to preformed pairs [2]. In the nearly optimally doped samples, the SCF contribution to conductivity can be accounted for by Gaussian Aslamazov-Larkin fluctuations in the Ginzburg-Landau approach [3]. A phase fluctuation contribution might be invoked in the most underdoped sample in a $T$ range which increases when controlled disorder is introduced by electron irradiation. The analysis of the fluctuation magnetoconductance allows us to determine the critical fields $H_{c2}(0)$ which are found to be very similar to $H^{\prime}_c(0)$ and to increase with hole doping. These two depairing fields which are directly connected to the magnitude of the superconducting gap therefore follow the evolution of $T_c$, which is at odds with the sharp decrease of the pseudogap with increasing hole doping. \\[4pt] [1] F. Rullier-Albenque, H. Alloul, Cyril Proust, P. Lejay, A. Forget, and D. Colson, Phys. Rev. Lett. \textbf{99}, 027003 (2007). \\[0pt] [2] H. Alloul, F. Rullier-Albenque, B. Vignolle, D. Colson, A. Forget, Europhys. Lett. \textbf{91}, 37005 (2010). \\[0pt] [2] F. Rullier-Albenque, H. Alloul, G. Rikken, Phys. Rev. B \textbf{84}, 014522 (2011). [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A3.00005: Fluctuoscopy of Superconductors Invited Speaker: Andrey Varlamov The study of superconducting fluctuations (SF) is a subject of fundamental and practical importance. Since the moment of discovery SF became a noticeable part of research in the field of superconductivity (SC) and a variety of fluctuation effects have been detected. The interest to SF in SC was regenerated by the discovery of HTS, where, due to extremely short coherence length and low effective dimensionality of the electron system, SF manifest themselves in a wide range of temperatures. The characteristic feature of SF is their strong dependence on temperature and magnetic field. This allows to separate SFs from other contributions and to use them as a tool for characterization of SC systems (``fluctuoscopy'') for example to extract the values of $T_c$, $H_{c2}(T)$ and phase-breaking time from experimental data. We present the complete results for fluctuation magneto-conductivity (FMC) and Nernst signal (FNS) of impure 2D superconductor in the whole phase diagram above the transition line $H_{c2}(T)$, including the domain of quantum fluctuations. Along some line $H_0(T)$, in agreement with experimental findings, FMC becomes zero and beyond it remains small and negative. The corresponding surface in coordinates $(T,H)$ becomes in particular non-trivial at low temperatures and close to $H_{c2}(0)$, where it is trough-shaped. The observation of large FNS in HTS and conventional SC above $T_c(H)$, has attracted much attention recently. The idea to attribute it to the entropy transport by analogy to vortices was proposed. On the other hand this giant effect, close to $T_c(0)$, was explained in terms of SF. Our general results allow to successfully fit the available experimental data in a wide range of magnetic fields and temperatures, to extract the value of the ``ghost'' field and other parameters of SC. We offer also a qualitative consideration, which gives a natural explanation for the giant value of FNS attributing it to a strong dependence of the fluctuation Cooper pair (FCP) chemical potential on temperature. Close to zero temperature, when the magnetic field approaches $H_{c2}(0)$, a peculiar dynamic state consisting of clusters of coherently rotating FCP forms. We estimate the characteristic size and lifetime of such clusters and present the scenario of fluctuation nucleation of Abrikosov's lattice. [Preview Abstract] |
Session A4: Focus Session: Hybrid Systems and Quantum Information Science in Atomic, Molecular, and Optical Physics
Sponsoring Units: DAMOPChair: Mikhail Lukin, Harvard University
Room: 205C
Monday, February 27, 2012 8:00AM - 8:12AM |
A4.00001: A Light-Matter Interface with NV Centers Brendan Shields, Nathalie de Leon, Birgit Hausmann, Yiwen Chu, Michael Burek, Patrick Maletinsky, Qimin Quan, Alexander Zibrov, Hongkun Park, Marko Loncar, Mikhail Lukin NV centers in diamond offer much promise as solid state qubits for scalable quantum communication and information processing. However, for bulk diamond systems the low collection efficiency and large phonon sideband emission represent substantial limitations for applications in quantum information science. In this talk we will describe the realization of nanoscale photonic cavities containing NV centers with desired optical properties. The experimental realization of spontaneous emission control and strong coupling regime of cavity QED will be discussed. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A4.00002: Probing the motion of a mechanical resonator via coherent coupling to a single spin qubit Shimon Kolkowitz, Quirin Unterreithmeier, Ania Bleszynski Jayich, Steven Bennett, Peter Rabl, J.G.E. Harris, Mikhail Lukin Mechanical systems can be influenced by a wide variety of extremely small forces, ranging from gravitational to optical, electrical, and magnetic. When mechanical resonators are scaled down to nanometer-scale dimensions, these forces can be harnessed to enable coupling to individual quantum systems. In this talk we will present results showing that the coherent evolution of a single electronic spin associated with a Nitrogen Vacancy (NV) center in diamond can be coupled to the motion of a magnetized mechanical resonator. Specifically we use coherent manipulation of the spin to sense the driven and Brownian motion of the resonator under ambient conditions at a picometer length scale. We will discuss potential applications of this technique including the decetion of the zero-point fluctuations of a mechanical resonator, the realization of strong spin-phonon coupling at a single quantum level, and the implementation of quantum spin transducers. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A4.00003: A nanoscale quantum interface for single atoms Tobias Tiecke, Jeff Thompson, Johannes Feist, Chun Yu, Alexey Akimov, Darrick Chang, Alexander Zibrov, Vladan Vuletic, Hongkun Park, Mikhail Lukin Neutral atoms are ideal quantum systems: they have long ground-state coherence times and strong optical cycling transitions that enable state detection and preparation. Building quantum networks of atoms interacting through photons is challenging, however, as many schemes for atom-photon interaction are inefficient or hard to scale. We propose a scheme to trap neutral atoms near silver nanowires, which are tightly confining waveguides for surface plasmons. The nanowire tip is used to generate a near-field optical trapping potential, and to enhance and efficiently collect spontaneous emission from the atom. We present experimental results on using the atom to sense the optical field at submicron distances from the wire and our current efforts towards loading the nanotrap. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A4.00004: Estimating and characterizing electromechanical coupling of superfluid helium to a microwave resonator Yang Ge, Bing Li, Andreas Fragner, Rob Schoelkopf, David Schuster Electrons on helium is a unique two-dimensional electron gas system formed at the interface of a quantum liquid (superfluid helium) and vacuum. The motional state of single-electron quantum dots defined on such systems has been proposed as a good candidate for hybrid quantum computing and as a gateway to measuring the electron spin [1,2]. Incoherent fluctuations of the thickness or density of the helium film are potential sources of motional dephasing, hence require further experimental characterization. In addition, if these ripplons or phonons could be coherently coupled to an electromagnetic cavity one could realize a quantum electro-mechanical system. Here, I will present estimates and preliminary experimental characterization of the electromechanical couplings as well as progress with electrons on helium. \\[4pt] [1] S. Lyon, Phys. Rev. A. 74, 5 (2006) \\[0pt] [2] D.I. Schuster, et al. Phys. Rev. Lett. 105, 040503 (2010) [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A4.00005: Tunable High Q Superconducting Microwave Resonator for Hybrid System with $^{87}$Rb atoms Zaeill Kim, K.D. Voigt, Jongmin Lee, J.E. Hoffman, J.A. Grover, S. Ravets, V. Zaretskey, B.S. Palmer, M. Hafezi, J.M. Taylor, J.R. Anderson, A.J. Dragt, C.J. Lobb, L.A. Orozco, S.L. Rolston, F.C. Wellstood We have developed a frequency tuning system for a ``lumped-element'' thin-film superconducting Al microwave resonator [1] on sapphire intended for coupling to hyperfine ground states of cold trapped $^{87}$Rb atoms, which are separated by about $f_{Rb}=6.83$ GHz. At \emph{T}=12 mK and on resonance at 6.81 GHz, the loaded quality factor was 120,000. By moving a carefully machined Al pin towards the inductor of the resonator using a piezo stage, we were able to tune the resonance frequency over a range of 35 MHz and within a few kHz of $f_{Rb}$. While measuring the power dependent response of the resonator at each tuned frequency, we observed anomalous decreases in the quality factor at several frequencies. These drops were more pronounced at lower power. We discuss our results, which suggest these resonances are attributable to discrete two-level systems.\\[4pt] [1] Z. Kim \emph{et al}., AIP ADVANCES \textbf{1}, 042107 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A4.00006: Coupling quantum microwave circuits to quantum optics via cavity electro-optic modulators Mankei Tsang Experimental circuit quantum electrodynamics has made great strides in recent years, but it remains an open question how the quantum information stored in the microwave circuits can be transferred for long distances. Just as in classical information, the most promising solution is to convert the microwave fields to optical frequencies, where ultra-low-loss photonic devices such as optical fibers can be used. Here I propose the use of cavity electro-optic modulators for coherent coupling between microwave and optical fields. The electro-optic effect is the change in optical refractive index in certain materials, such as lithium niobate, under an applied voltage. Leveraging the fact that cavity electro-optics has the same physics as cavity optomechanics, a cavity electro-optic modulator can realize various joint quantum operations on the microwave and optical fields, including coherent frequency conversion, laser cooling of microwave resonance, hybrid entanglement, hybrid parametric amplification and oscillation, and optical quantum-nondemolition measurements of microwave quadrature and energy. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A4.00007: Hybrid Quantum Systems with Circuit Quantum Electrodynamics Invited Speaker: David Schuster Quantum Information Processing presents daunting challenges, with competing requirements of fast manipulation, long storage, and long distance transport of fragile quantum states. In aggregate, many of the challenges quantum computation have been met with nanosecond manipulations (quantum circuits), coherence times measured in seconds (atomic ions/nuclear spins), and entanglement transported over kilometers (linear optics), yet thus far no system has achieved all of the necessary components simultaneously. One promising direction is to leverage the best aspects of each system in a hybrid system, much as is done in a conventional computer, where transistors provide fast processing, magnetic memory provides massive long term storage, and information is transmitted via microwaves or fiber optics. A review of the constituent quantum systems, and the types of couplings between them will be presented. The coupling of a superconducting cavity/qubit system to electrons floating on helium will be discussed as an example of how to construct a hybrid system. Recent results on trapping and detection of electrons on helium using a superconducting cavity will be presented. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A4.00008: Nonlinear optics quantum computing with circuit QED Prabin Adhikari, Mohammad Hafezi, Jacob Taylor One approach to quantum information processing is to use photons as quantum bits and rely on linear optical elements for most operations. However, some optical non-linearity is necessary to enable universal quantum computing. We consider a circuit-QED approach to linear optics quantum computing in the microwave regime, including a deterministic two-photon phase gate. Our model is a hybrid quantum system comprising an LC resonator coupled to a flux or phase superconducting qubit, which will be used to implement a non-linear two photon phase shift operation. Using this model, we show how fast, low-noise two-qubit gates between photons are possible, and discuss limitations of these ideas based on current technology. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A4.00009: Towards Hybrid Quantum Information Processing with Electrons on Helium Andreas Fragner, David Schuster, Mark Dykman, Stephen Lyon, Luigi Frunzio, Robert Schoelkopf Electrons above the surface of superfluid helium form a two-dimensional electron gas in which single-electron quantum dots can be defined using electrostatic gates submerged under the helium film. The quantized motion and spin state of such a trapped electron on helium can be coupled to a high finesse superconducting cavity in a hybrid circuit QED architecture [1]. The cavity is used for nondestructive readout and as a quantum bus mediating interactions between distant electrons or an electron and a superconducting qubit. Coupling between motional states and individual photons in the cavity is estimated at a Rabi frequency of $g/2\pi\sim 20$ MHz with coherence times exceeding 20 $\mu$s for charge and 1 s for spin [1, 2]. Here I will discuss recent experiments in which we successfully trap and detect a two-dimensional electron gas on helium in a dc-biased superconducting cavity. Experimental progress towards the single-electron regime will also be presented. \newline [1] D.I. Schuster, A.Fragner, M.I. Dykman, S. Lyon and R.J. Schoelkopf, Phys. Rev. Lett. 105, 040503 (2010) \newline [2] S. A. Lyon, Phys. Rev. A 74, 052338 (2006) [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A4.00010: An ab-initio microscopic theory of anomalous heating in planar ion traps H.R. Sadeghpour, A. Safavi-Naini, P. Rabl, P.F. Weck Anomalous heating of trapped ions limits the scalability of the planar trap architecture for quantum computation. Measurements of the electric field noise present in ion traps indicate that the noise-induced heating scales as the inverse fourth power of the distance from the trap electrodes to the ion and its spectral density scales with the inverse of frequency [1]. These measurements also suggest that some thermally activated random process is at work. In this work, we present an ab-initio theory of this noise due to oscillating dipoles on the trap electrode surface [2]. The dipoles are formed when atoms are adsorbed on the trap surface, whose interaction with the surface is described with density functional theory (DFT). We present calculations for the spectral noise density and its distance, frequency and temperature dependencies. We consider both independent and correlated dipoles.\\[4pt] [1] Q. A. Turchette et. al., Phys. Rev. A. 61, 63418 (2000).\\[0pt] [2] A. Safavi-Naini, P. Rabl, P. F. Weck, H. R. Sadeghpour, Phys. Rev. A. 84, 023412 (2011). [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A4.00011: Quantum logic for molecular quantum information processing J. Mur-Petit, J. Perez-Rios, J. Campos-Martinez, M.I. Hernandez, S. Willitsch, J.J. Garcia-Ripoll Very recently, molecular ions have been trapped and cooled to the mK regime in well defined internal states [1] opening a new window for precision spectroscopy of molecular species and quantum information with cold molecular ions. A first requirement for both applications is the ability to control and measure the state of molecular ions. I will present our proposal [2] of a fast, non-destructive and temperature independent spectroscopy method suitable to study electronic, vibrational, rotational and Zeeman transitions in complex ions that implements quantum logic schemes~[3] between an atomic ion and the molecular ion of interest, using optical forces on the atom, and optical forces or magnetic field gradients on the molecule. This method sets a starting point for a hybrid quantum computation scheme with molecular and atomic ions, covering the measurement and entangling steps. Finally I will discuss the remarkable decoherence properties of two Zeeman states of the $^{16}$O$_2^+$ molecular ion that make it a promising system for QIPC purposes~[4].\\[4pt] [1] X. Tong {\em et al.}, Phys. Rev. Lett. {\bf 105}, 143001 (2010).\\[0pt] [2] J. Mur-Petit {\em et al.}, arXiv:1106.3320\\[0pt] [3] P. O. Schmidt {\em et al.}, Science {\bf 309}, 749 (2005).\\[0pt] [4] J. Mur-Petit {\em et al.}, in preparation. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A4.00012: Possibility of ``magic'' trapping of three-level system for Rydberg blockade implementation Muir J. Morrison, Andrei Derevianko The Rydberg blockade mechanism has shown noteworthy promise for scalable quantum computation with neutral atoms. Both qubit states and gate-mediating Rydberg state belong to the same optically-trapped atom. The trapping fields, while being essential, induce detrimental decoherence. Here we theoretically demonstrate that this Stark-induced decoherence may be completely removed using powerful concepts of ``magic'' optical traps. We analyze ``magic'' trapping of a prototype three-level system: a Rydberg state along with two qubit states, which are hyperfine states attached to a $J=1/2$ ground state. Our numerical results show that the group IIIB metals such as Al are suitable candidates. Such trapping may or may not be possible for the alkalis, as ``magic" conditions depend sensitively on the the trap-Rydberg interaction. Calculations of these effects are ongoing, and the results will be presented. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A4.00013: Cooling in the single-photon regime of optomechanics Andreas Nunnenkamp, Kjetil Borkje, Steven Girvin Optomechanics experiments are rapidly approaching the regime where the radiation pressure of a single photon displaces the mechanical oscillator by more than its zero-point uncertainty. We show that in this limit the power spectrum has multiple sidebands and that the cavity response has several resonances in the resolved-sideband limit [Phys.~Rev.~Lett.~\textbf{107}, 063602 (2011)]. We then discuss how red-sideband cooling is modified in this nonlinear regime. Using Fermi's Golden rule we calculate the transition rates induced by the optical drive. In the resolved-sideband limit we find multiple cooling resonances for strong single-photon coupling. They lead to non-thermal steady states and are accompanied by multiple mechanical sidebands in the optical output spectrum. Our study provides the tools to detect and take advantage of this novel regime of optomechanics. [Preview Abstract] |
Session A5: Surfaces: Structure, Transitions and Morphological Evolution
Sponsoring Units: DCMPChair: Sanjay Khare, University of Toledo
Room: 206A
Monday, February 27, 2012 8:00AM - 8:12AM |
A5.00001: The Relaxation of Vicinal (001) with ZigZag [110] Steps Micah Hawkins, Ajmi BH Hamouda, Diego Luis Gonz\'alez-Cabrera, Theodore L. Einstein This talk presents a kinetic Monte Carlo study of the relaxation dynamics of $\left[110\right]$ steps on a vicinal $\left(001\right)$ simple cubic surface. This system is interesting because $\left[110\right]$ steps have different elementary excitation energetics and favor step diffusion more than close-packed $\left[100\right]$ steps. In this talk we show how this leads to relaxation dynamics showing greater fluctuations on a shorter time scale for $\left[110\right]$ steps as well as 2-bond breaking processes being rate determining in contrast to 3-bond breaking processes for $\left[100\right]$ steps. The existence of a steady state is shown via the convergence of terrace width distributions at times much longer than the relaxation time. In this time regime excellent fits to the modified generalized Wigner distribution (as well as to the Berry-Robnik model when steps can overlap) were obtained. Also, step-position correlation function data show diffusion-limited increase for small distances along the step as well as greater average step displacement for zigzag steps compared to straight steps for somewhat longer distances along the step. Work supported by NSF-MRSEC Grant DMR 05-20471 as well as a DOE-CMCSN Grant. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A5.00002: Embedded Atom Method Potential for Ni-Cu Alloys and Its Applications for Ni, Cu growth on Cu(111) Berk Onat, Sondan Durukanoglu We developed a semi-empirical, many-body type model potential to investigate static and dynamic properties of Ni-Cu alloys. The formalism is based on the embedded atom method with improved optimization techniques. The Ni-Cu alloy potential was determined by fitting to data on lattice parameters, cohesive energies for L1$_{0}$, L1$_{1}$, L1$_{2}$, and L1$_{3}$ phases, together with vacancy formation energies, bulk modulus and elastic properties for L1$_{2}$, L1$_{3}$ phases. Our preliminary calculations for energy barriers for the diffusing Ni and Cu atoms on Cu(111) based on the nudged elastic band method are found to be consistent with the available experimental and other theoretical results. Our ultimate goal is to describe the varying characteristics in growing islands of pure Cu, Ni atoms and mixed Ni-Cu combinations on Cu(111) [S. Pons et al., Surf. Sci., \textbf{511}, 449, (2002)]. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A5.00003: Computational diffusion model of reconstructed regions in Ag/Si epitaxial growth Joseph Driscoll, Kelly Roos, D. Wall, M. Horn-von Hoegen, F.-J. Meyer zu Heringdorf The thermal decay of Ag islands, grown epitaxially in a Stranski-Krastanov mode on Si(001) and Si(111) surfaces, has been studied experimentally with photoemission electron microscopy (PEEM). In a range of elevated temperatures the islands decay mainly by dissociation of Ag atoms from island edges, rather than by direct desorption into the gas phase. On the surrounding surface, the Ag atoms are subject to thermally-activated diffusion and desorption. The Ag surface concentration decreases with distance from the island edges. Where the local concentration is above a critical value, coverage-dependent reconstructed overlayers form surrounding the islands. The spread of the overlayers, relative to the position of the decaying island, depends on competition between diffusion and desorption. Previous quasi-static models [1] have shown that the observed reconstructed regions are related to the atomistic parameters describing surface diffusion, and have been applied to extract diffusion coefficients from the experimental data. Here we present results from a dynamic diffusion model that captures many of the qualitative and quantitative time- and temperature-dependent phenomena observed in the experiments. \\[4pt] [1] K.R. Roos et. al. PRL 100, 016103 (2008); D. Wall et. al. NJP 12 (2010) 103019 [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A5.00004: Plasma-assisted molecular beam epitaxy growth of ZnSnN$_{2}$ Nathaniel Feldberg, James Aldous, Yuan Yao, Imtiaz Tanveer, Benjamin Keen, Wojciech Linhart, Tim Veal, Young-Wook Song, Roger Reeves, Steve Durbin The Zn-IV-nitrides are a promising series of ``earth abundant element'' semiconductors with a predicted band gap range of 0.6 eV to 5.4 eV, which, like the (Al,Ga,In)N family, spans the entire visible solar spectrum. Considering this alternative family has a number of advantages, including the avoidance of indium, the price of which has varied almost an order of magnitude over the past decade, and surface electron accumulation which is present in the In-rich alloys. Not all members of this family have yet been synthesized, in particular ZnSnN$_{2}$, the most important member for PV with its predicted band gap of approximately 2 eV. We have successfully grown a series of these films using plasma-assisted molecular beam epitaxy using elemental Zn and Sn sources. In this report, we discuss the relationship between process parameters and microstructure, as well as stoichiometry as determined by Rutherford backscattering spectrometry. Additionally, we provide preliminary estimates for its bandgap energy based on photoluminescence and optical absorption. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A5.00005: Transition between two patterns on an Au-deposited Si(111) surface F.K. Men, A.L. Chin, C.P. Chang Two distinct patterns have been observed by depositing sub-monolayer Au onto a Si(111)-(7$\times $7) surface with a small miscut angle. Upon depositing Au at 600\r{ }C, we find that a stripe of (5$\times $2) reconstruction forms at the upper step edge in every terrace. For 700\r{ }C deposition, one entire terrace out of several terraces transforms into the (5$\times $2) reconstruction while the other terraces are totally unaffected by the Au deposition. The relative population between the (5$\times $2) and the (7$\times $7) terrace is governed by the amount of deposited Au. After annealing at a temperature above 700\r{ }C the striped (5$\times $2) pattern transforms into the (5$\times $2)-terrace pattern. The similarity between this ?coarsening? of (5$\times $2) reconstruction and the Ostwald ripening of clusters is striking and will be discussed. One application using the (5$\times $2) pattern as a template to grow nanostructure in designated regions will be demonstrated. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A5.00006: Unusual Island Formations of Iridium on Ge(111) Studied by STM Marshall van Zijll, Cory Mullet, Bret Stenger, Emilie Huffman, Dylan Lovinger, William Mann, Shirley Chiang We have used scanning tunneling microscopy (STM) to characterize the growth of iridium onto Ge(111). Iridium was deposited onto the Ge(111) c(2x8) surface at different coverages less than 1ML, and the samples were annealed to temperatures between 550K and 750K. A new form of growth was observed, consisting of pathways connecting larger iridium islands. As the annealing temperature increased, the iridium growth first formed unusual shapes with finger-like protrusions. Next, these shapes broke apart into smaller islands, which ultimately formed into larger islands at higher temperatures. High resolution images have been obtained, which allow insight into the atomic arrangements. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A5.00007: Thickness-dependent Adatom-Adatom Binding Energy on Pb(111): The Effect of Quantum Size Effect on Critical Nucleus Miao Liu, Feng Liu We perform first-principles calculations to investigate adatom-adatom binding energies on Pb(111) as a function of film thickness. An odd-even thickness-dependent oscillation is found in the adatom-adatom binding energies, in analogy to the similar behavior in adatom surface binding energies and diffusion barriers found previously. We will discuss these results in relation to the thickness-dependent island nucleation density and compactness, as observed in recent experiments. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A5.00008: De-wetting of thin films: Analytic theory of cluster coarsening dynamics Adi Constantinescu, Leonardo Golubovic, Artem Levandovsky Long range de-wetting forces acting across thin films, such as the fundamental van der Waals interactions, may drive the formation of large clusters (tall multi-layer islands) and pits, observed in thin films of soft materials (polymers), as well as in thin films of liquid and solid metals. These long range de-wetting interactions introduce a distinct long lasting early-time scaling behavior characterized by a slow growth of the cluster height/lateral size aspect ratio (i.e., a time-dependent Young angle), and by effective coarsening exponents that depend on cluster size. In this study, we develop an analytic theory capable to calculate these effective size-dependent coarsening exponents characterizing the cluster growth in the early-time cross-over regime. Such a pronounced cross-over behavior has been indeed seen in experiments; however its physical origin has remained elusive to this date. Our results attribute these observed phenomena to ubiquitous long range de-wetting interactions acting across thin films. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A5.00009: Surface morphological stabilization of crystalline solids under the simultaneous action of electric, thermal, and mechanical fields Dwaipayan Dasgupta, Georgios Sfyris, Rauf Gungor, Dimitrios Maroudas We report a detailed analysis of the morphological stability of planar surfaces of electrically and thermally conducting stressed crystalline elastic solids under the simultaneous action of an electric field, an imposed temperature gradient, and uniaxial tension. Our analysis is based on linear stability theory and self-consistent dynamical numerical simulation according to a fully nonlinear model that accounts for curvature- and stress-driven surface diffusion, surface electromigration and thermomigration, as well as surface diffusional anisotropy. Our self-consistent dynamical simulations combine a front tracking method for monitoring surface morphological evolution with Galerkin boundary-integral computations of the displacement and temperature fields and the electrostatic potential. We determine the surface morphological stability domain boundaries and the critical values of the applied electric-field strength and temperature gradient required to stabilize the planar surface morphology. We explore the synergistic or competing effects on the surface morphological response of the simultaneously applied thermal and electric fields, aiming at optimization of the electric-field strength and temperature gradient requirements for planar surface stabilization. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A5.00010: Electromigration-driven morphological evolution of monolayer-thick epitaxial islands on substrates Dimitrios Maroudas, Dwaipayan Dasgupta, Georgios Sfyris Electromigration-driven dynamics, with and without the simultaneous action of elastic strain, can lead to pattern formation of surface morphological features that may have significant impact on nanofabrication. An important example is epitaxial islands on substrates; for heteroepitaxial islands, misfit strain is induced due to lattice mismatch with the substrate. We develop a fully nonlinear model for the driven morphological evolution of monolayer-thick coherently strained islands on crystalline elastic substrates with diffusional mass transport limited to the island circumference. We carry out self-consistent dynamical simulations of such island dynamics, combining front tracking methods with solutions to the corresponding electrostatic and elastostatic boundary-value problems. We develop a universal scaling theory that explains the simulation results for the dependence of the island migration speed on the island size, electric field, and epitaxial system parameters for isolated, morphologically stable islands. We investigate systematically thermal, elastic, and size effects on the migration and morphological evolution of heteroepitaxial islands. We also find and characterize a variety of stable asymptotic states in the driven dynamical response of such heteroepitaxial islands. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A5.00011: Electromigration-driven surface morphological stabilization of coherently strained thin films on elastically deformable substrates Georgios Sfyris, Rauf Gungor, Dimitrios Maroudas We study the surface morphological stability of a coherently strained thin film grown epitaxially on a substrate and subjected to an external electric field; both infinitely thick and finite-thickness elastic substrates are examined. Due to its lattice mismatch with the substrate material, the film may undergo a Stranski-Krastanow (SK) instability, resulting in formation of islands on the film surface. To examine the morphological stability of the epitaxial film's planar surface state, we conduct a linear stability analysis based on a three-dimensional model of driven surface morphological evolution. We also consider the use of thin compliant substrates, which partly accommodate elastically the lattice-mismatch strain. We find that, regardless of the substrate type, the simultaneous action of a properly applied and sufficiently strong electric field is necessary to stabilize the planar morphology; in such cases, surface electromigration can inhibit SK-type instabilities and control the onset of island formation on the film surface. Our analysis shows that the critical electric-field strength required to stabilize the planar morphology of a thin film on a compliant substrate can be reduced by up to two orders of magnitude compared to that for a conventional thick substrate. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A5.00012: Quantum Monte Carlo Study of Surface Energy Cheng-Rong Hsing, Ching-Ming Wei The accuracy of Density Functional Theory (DFT) is based on the exchange-correlation approximation used and needs to be checked by highly accurate quantum many-body approaches. We have performed calculations of the surface energies using the state-of-the-art diffusion quantum Monte Carlo (QMC) method to examine the accuracy of LDA and GGA (PBE) functionals in the study of surface energy. The systems studied include NaCl(100), MgO(100), CaO(100), TiO$_{2}$(110), Si(100)-(2x2), C(100)-(2x2), and Ge(100)-(2x2) surfaces. Our results indicate that (i) the surface energy by DMC is always larger than the surface energy by LDA; and (ii) the surface energy by LDA is always larger than the surface energy by GGA. For the surface energies of NaCl(100) and MgO(100), the DMC results reproduce the experimental measured values accurately. To conclude, when compared the surface energies obtained by DFT and DMC, the results predicted by DFT using either LDA or GGA functional are underestimated. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A5.00013: Reconstructions of the GaN(10$\bar{1}$$\bar{1}$) surfaces: Density functional theory calculations Jung-Min Hyun, Yong-Sung Kim, Hanchul Kim GaN has been extensively studied for its potential applicability in optoelectronics as well as in spintronics. The functional performance in such applications depends on the surface characteristics of thin films. Thin films of GaN are typically grown along the polar [0001] direction, but their light-emission efficiency is reduced due to the electron-hole separation. A strategy to remedy such an undesired effect is to grow films along nonpolar or semipolar directions. In this presentation, we will address the reconstructions of the Ga-terminated semipolar (10$\bar{1}$$\bar{1}$) surface. We performed the density functional theory calculations using the generalized gradient approximation, the projector augmented wave potentials, and the repeated slabs. From the calculated energetics of various reconstructions, we found that there exist a few structural motifs of GaN(10$\bar{1}$$\bar{1}$). They are short Ga chains and Ga vacancies. For instance, a 4 $\times$ 2 reconstruction with a Ga tetramer and surface Ga vacancies is stable in the N-rich condition, which is significantly different from the previous results [Akiyama \emph{et al}, Jpn. J. Appl. Phys. 48, 100201 (2009)]. Our results would provide a comprehensive understanding on the Ga-terminated semipolar surfaces. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A5.00014: Cross-sectional scanning tunneling microscopy at GaN (10-10) Holger Eisele, Lena Ivanova, Svetlana Borisova, Mario Daehne, Philipp Ebert Group-III nitrides are the materials of choice for optoelectronic devices in the green to ultraviolet wavelength range. Recently, the question arose, whether the growth of GaN based devices could be improved by switching from polar surfaces to the non-polar ones. For non-polar GaN surfaces only little is known about the exact energetic positions of surface states, and thus their possible influence on the Fermi level. Furthermore, GaN still suffers from high dislocation densities, far above that of zincblende type III-V semiconductor crystals, which are detrimental for optoelectronic applications of GaN. Therefore we investigated the GaN(10-10) cleavage surface by cross-sectional scanning tunneling microscopy nd spectroscopy. We were able to identify the energetic positions of the intrinsic surface states and the Fermi level. We found that both, the filled N-derived and empty Ga-derived dangling bond states are outside the fundamental band gap, the latter one being 0.1--0.2~eV above the conduction band minimum. The observed band gap is 3.4$\pm$0.2~eV, in agreement with the nominal value from the bulk. The observed Fermi level pinning of about 1.0~eV below the conduction band edge is attributed to a high defect density at the surface, but not to intrinsic surface states. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A5.00015: Geometrical and Electronic structures of Planar and Buckled Silicene on Ag(111) Chun-Liang Lin, Ryuichi Arafune, Noriyuki Tsukahara, Kazuaki Kawahara, Noriaki Takagi, Maki Kawai Silicene grown on the Ag(111) surface was investigated by scanning tunneling microscopy / spectroscopy (STM / STS). Two atomic arrangements of honeycomb lattice, planar and buckled, were found. The planar silicene is consisted of Si atoms in the same height while the buckled one shows slightly twisted structure in the vertical direction. Compared to the planar silicene, the lateral distance between two neighboring atoms in the buckled silicene is reduced with about 7{\%}. Assuming that there is no change in the length of Si-Si bond, the angle between the Si-Si bond and the axis normal to the surface is 110 degrees, which is close to the bond angle of 109 degrees in the sp$^{3}$ hybridization. This might suggest that the Si-Si bonding in the buckled silicene is formed mainly by sp$^{3}$ hybridization rather than sp$^{2}$ However, our STS observations certainly showed a Dirac cone feature at the Fermi level for both types of silicene. Thus, we conclude that the electronic configuration of buckled silicene partially remains in sp$^{2}$ hybridization and the charge carriers still behave as the massless Dirac fermions. [Preview Abstract] |
Session A6: Focus Session: van der Waals Bonding in Advanced Materials - Physisorption and Self Assembly on Metals
Sponsoring Units: DMPChair: Talat Rahman, University of Central Florida
Room: 206B
Monday, February 27, 2012 8:00AM - 8:36AM |
A6.00001: Application of vdW-DF Methods to Hydrogen Adsorptions and an Organic Ferroelectric Invited Speaker: Kyuho Lee In recent years, several schemes have been proposed to include van der Waals (vdW) interactions into the framework of density functional theory (DFT). These offer the potential to extend the scope and usefulness of DFT, allowing applications to an entire new class of sparse materials. For finite systems, we validated our non-empirical van der Waals density functionals (vdW-DFs) with respect to very accurate quantum chemical calculations of the potential energy curves (PECs) for small molecular duplexes [1]. For extended systems, however, typical tests focus on comparisons with a few accessible observations, such as binding energy and bond length [1]. In this talk, we present a third approach in which full PECs from accurate experiments are used for the assessment of vdW methods for extended systems [2]. We calculate the PECs of the H$_2$ molecule and light atoms on the (100), (110), and (111) surfaces of Cu. The gas-surface-scattering experiments provide the rich data bank that covers results for the whole shape of the physisorption potentials. We also present an application of vdW-DF2 to an organic ferroelectric, phenazine-chloranilic acid [3]. In spite of extensive experimental efforts to characterize this rare organic material, the origin of the long-range order is unclear and even the ground state structure is not completely determined yet. We study its structure, energetics, ferroelectric properties, structural instability, and the proton-transfer process in it in comparison with PBE results and experiments wherever possible. \\[4pt]% [1] K.~Lee, E.~Murray, L.~Kong, B.~I.~Lundqvist, and D.~C.~Langreth, \textit{Phys. Rev. B} \textbf{82}, 081101 (2010). \\[1pt]% [2] K.~Lee, A.~K.~Kelkkanen, K.~Berland, S.~Andersson, D.~C.~Langreth, E.~Schr\"oder, B.~I.~Lundqvist, and P.~Hyldgaard, \textit{Phys. Rev. B} \textbf{84}, 193408 (2011). \\[1pt]% [3] K.~Lee, B.~Kolb, T.~Thonhauser, D.~C.~Langreth, and D.~Vanderbilt, \textit{in preparation}. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A6.00002: Structure and Energetics of Benzene Adsorbed on Transition-Metal Surfaces: Density-Functional Theory with Screened van der Waals Interactions Wei Liu, Victor G. Ruiz-L\'{o}pez, Guo-xu Zhang, Xinguo Ren, Matthias Scheffler, Alexandre Tkatchenko The adsorption of benzene on metal surfaces is an important benchmark system for more complex hybrid inorganic/organic interfaces. Here, the recently developed DFT+vdW$\rm^{surf}$ method (density-functional theory including screened van der Waals (vdW) interactions) [1] is used to study the structure and energetics of benzene on transition-metal surfaces (Cu, Ag, Au, Pd, Pt, Rh, and Ir). Benzene adsorbs in a planar configuration at coinage metal surfaces, with almost zero distortion and a flat potential-energy surface. In contrast, benzene is strongly bound to the (111) surface of Pd, Pt, Rh, and Ir, and located at the bridge-30$^\circ$ site. The vdW interactions significantly enhance the binding energy by more than 0.75 eV for all metals. The screening of the vdW energy plays a critical role in coinage metals, shortening the equilibrium distance by 0.2 {\AA}, and lowering the binding energy by 0.25 eV. The validity of our results is confirmed by comparison with calculations using the random-phase approximation including renormalized single excitations (EX+cRPA+rSE scheme [2]), and the experimental data from temperature-programmed desorption and calorimetry measurements. [1] V. G. Ruiz-L\'{o}pez et al., submitted. [2] X. Ren et al., Phys. Rev. Lett. 106, 153003 (2011). [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A6.00003: Physisorption of three amine terminated molecules (TMBDA, BDA, TFBDA) on the Au(111) Surface: The Role of van der Waals Interaction Maral Aminpour, Duy Le, Talat S. Rahman Recently, the electronic properties and alignment of tetramethyl-1,4-benzenediamine (TMBDA), 1,4-benzenediamine (BDA) and tetrafluro-1,4-benzenediamine (TFBDA) molecules were studied experimentally. Discrepancies were found for both the binding energy and the molecule tilt angle with respect to the surface, when results were compared with density functional theory calculations [1]. We have included the effect of vdW interactions both between the molecules and the Au(111) surface and find binding energies which are in very good agreement with experiments. We also find that at low coverages each of these molecules would adsorb almost parallel to the surface. N-Au bond lengths and charge redistribution on adsorption of the molecules are also analyzed. Our calculations are based on DFT using vdW-DF exchange correlation functionals. For BDA (since we are aware of experimental data), we show that for higher coverage, inclusion of intermolecular van der Waals interaction leads to tilting of the molecules with respect to the surface and formation of line structures. Our results demonstrate the central role played by intermolecular interaction in pattern formation on this surface.\\[4pt] [1] M. Dell'Angela et al, Nano Lett. 2010, 10, 2470; M. Kamenetska et al, J. Phys. Chem. C, 2011, 115, 12625 [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A6.00004: Effect of physisorbed molecules and an external external fields on the metallic Shockley surface state of Cu(111): A density functional theory study Kristian Berland, T.L. Einstein, Per Hyldgaard To manipulate the Cu(111) partially-filled Shockley surface state, we study its response to an external field\footnote{KB, TLE, PH; arXiv 1109:6706} $E$ and physisorbed PAHs and quinone molecules. We use density-functional theory calculations with periodic-boundary conditions. The van der Waals density functional version vdW-DF2 accounts for the molecular adsorption. The issue that the Kohn-Sham wave functions couple to both sides of the Cu slab is handled with a decoupling scheme based on a rotation in Hilbert space. A convergence study reveals that to obtain a proper Shockley surface state, 6 Cu layers is sufficient, while 15 is optimal. We use 6 layers for the response to the molecules and 15 to external field. We find that the surface state displays isotropic dispersion (up to order $k^6$), free-electron like until the Fermi wave vector but with a significant quartic component beyond. The shift in band minimum and effective mass depend linearly on $E$, with a smaller fractional change in the latter. Charge transfer occurs beyond the outermost copper atoms, and most of the screening is due to bulk electrons. We find that the molecular physisorption increases the band minimum, with the effect the of a quinone being much stronger than the corresponding PAH. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A6.00005: Computational Study of Supramolecular Self-Assembly Using CH/$\pi$ Bonds C. Han, Q. Li, S. Horton, M. Fuentes-Cabrera, B. Sumpter, W. Lu, J. Bernholc, P. Maksymovych, M. Pan Self assembly is an important research area in supramolecular engineering. We show that CH/$\pi$ bonds can be exploited as a vehicle to assemble clusters of well-defined sizes on metal surfaces. Specifically, we theoretically explain the observations of largely uniform distribution of phenylacetylene magic clusters, each consisting of six molecules, on Au(111) surfaces. Using density functional theory with a van der Waals functional, we discuss the reasons for the preference of the hexamer structure, the key effect of CH/$\pi$ bonding on the self-assembly, and the critical role of the metal surface. Our calculated STM images and electronic properties are in good agreement with experiment. The cooperative, multi-center CH/$\pi$ interactions offer an attractive tunability via chemical functionalization, and thus may provide a new avenue towards rationally designing a desired supramolecular shape and size. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A6.00006: Noble Gases on Metal Surfaces: New Insights on Adsorption Site Preference De-Li Chen, Wissam Al-Saidi, Karl Johnson Experiments have previously found that noble gases (Kr, Xe) adsorb on low-coordination atop sites on several different metal surfaces, rather than on high-coordination hollow sites. This unexpected preference for low-coordination sites has been previously ascribed to reduced Pauli repulsion mostly due to exchange energy at the atop site, based on density functional theory calculations within the local density approximation (LDA). In contrast, our calculations using non-local van der Waals (vdW-DF) density functional show that site preference is due to a delicate balance between the electrostatics which favor the hollow site and kinetic energy which favors the atop site; exchange-correlation energies has a very little role. Moreover, we find, using LDA, GGA, and vdW-DF functionals, that the hollow site is a saddle point of index 1 or 2 on the 2-dimensional potential energy surface, while the atop site is the only true minimum. Therefore, the reason that hollow site occupation is not observed is that it is a transition state and so has a very short life-time. Our results show that the inclusion of non-local vdW interactions is crucial for obtaining results in quantitative agreement with experiments for adsorption energies, equilibrium distances, and vibrational energies. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A6.00007: Adsorption of polyvinylpyrrolidone on Ag surfaces: Insight into the workings of a structure-directing agent Wissam Al-Saidi, Haijun Feng, Kristen Fichthorn We use density-functional theory to resolve the role of polyvinylpyrrolidone (PVP) as a structure-directing agent in the shape-selective synthesis of Ag nanostructures. We identify several different binding states for PVP segments on Ag(100) and Ag(111) and find an energetic preference for Ag(100), which arises from a surface-sensitive balance between direct binding and van der Waals attraction. At the chain level, correlated segment binding leads to a strong preference for PVP bind to Ag(100). Our study underscores differences between small-molecule and polymeric structure-directing agents. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A6.00008: Adsorption of halogenated molecule on stepped metallic surfaces: the role of van der Waals interactions Rengin Pekoz, Karen Johnston, Davide Donadio The deposition of halogenated aromatic molecules on metallic surfaces is the first step for the bottom-up fabrication of atomically precise graphene nanoribbons [1]. Interest in the binding properties in this type of systems and in general in organic/inorganic interfaces has stimulated the inclusion of van der Waals (vdw) interactions within density functional theory (DFT). Using a fully non-local Van der Waals density functional (vdW-DF) [2,3] we have studied the adsorption of dichlorobenzene on stepped Au(332) and Pt(332) surfaces and on flat (111) surfaces. For several different adsorption sites, the energies, and equilibrium geometries have been computed, and electron density analysis has been performed using both conventional generalized gradient approximation (GGA) and vdW-DF. The two approaches yield qualitatively different results, highlighting the importance of non-local dispersions in this class of systems. The non-trivial role of steps edges on adsorption energies and geometries is also elucidated. [1] J. Cai \textit{et al}., Nature 466, 470 (2010). [2] M. Dion \textit{et al}., Phys. Rev. Lett. 92, 246401 (2004). [3] A. Gulans\textit{ et al}. Phys. Rev. B 79, 201105 (2009). [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A6.00009: The Nature of Binding in the Phenalenyl Dimer and its Derivatives Brian Kolb, Timo Thonhauser, Miklos Kertesz The biradical phenalenyl \(\pi\)-dimer and its derivatives have attracted interest recently because of their potentially useful electrical, optical, and magnetic properties. These properties can be tuned by adjusting the binding characteristics between the monomers within the dimer. Typically, this is done by substituting electron withdrawing or donating groups onto the \(\alpha\) or \(\beta\)-site carbons. An understanding of this binding lies at the heart of useful application of these materials. In this work, the binding characteristics of phenalenyl dimers were investigated using density functional theory. In particular, the vdW-DF functional was used to explore the role of van der Waals interactions in the binding within this system. A comparison of the binding curves with those of the closed shell derivatives wherein the central carbons have been replaced by either nitrogen or boron sheds light into the nature of the interactions between the monomers. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A6.00010: Binding nature of adenine and C60 on graphene: a van der Waals density functional analysis Per Hyldgaard, Kristian Berland, Valentino Cooper, Anders Bergvall, Sergey Kubatkin, Tomas L\"ofwander, Elsebeth Schr\"oder Based on van der Waals density functional theory (using vdW-DF1 and vdW-DF2), we study and analyze the adsorption of adenine\footnote{K. Berland et al; J. Phys.: Condens. Matter 23 135001 (2001)} and C60\footnote{A. Bergvall et al; Phys. Rev. B 84, 155451 (2011)}on graphene. Understanding molecular binding on graphene helps development of functionals because the infinite graphene shifts the balance between short-range and long-range contributions to binding as compared dimers or molecular crystals. The potential of graphene as contacts in single-molecule electronics also motivates the study of the interaction between aromatic molecules and graphene; the binding separation affects the magnitude of hopping parameters. We present results on binding energy and separation, vibrational states, overlayers, and charge transfer. We find that the hexagonal ring in C60 binds closer to the graphene sheet than what a flat molecule such as adenine does. The role played by the difference in geometry between the flat (adenine) and spherical (C60) shape is discussed. [Preview Abstract] |
Session A7: Focus Session: Computational Design of Materials - Structure Prediction
Sponsoring Units: DMP DCOMPChair: Richard Hennig, Cornell University
Room: 207
Monday, February 27, 2012 8:00AM - 8:12AM |
A7.00001: Crystal structure prediction: a novel approach based on minima hopping Maximilian Amsler, Stefan Goedecker With increasing computational resources the prediction of crystal structures from first principle calculations has become feasible, but still remains a demanding task. A reliable method to perform an efficient, systematic search for the ground state structure based solely on the system's composition is essential. Motivated by the promising results of the minima hopping method obtained on isolated systems, we have generalized the algorithm for crystal structure prediction. Optimized moves in the configurational space spanned by both atomic coordinates and simulation cell variables are performed to escape from local enthalpy minima, and revisiting known minima is avoided, thus allowing a fast exploration of the enthalpy surface. The predictive power of the novel method has been shown in several applications, of which the following will be presented. Superconducting phases in hydrogen rich materials were investigated, leading to the discovery of novel ground state structures. For the longstanding question of the crystal structure of cold compressed graphite a new candidate phase could be identified to perfectly match experimental results. And at last, new low energy structures for materials with possible applications in hydrogen storage are presented. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A7.00002: Adaptive Genetic Algorithm method for Crystal Structure Prediction and Materials Discovery Cai-Zhuang Wang, Min Ji, Shunqing Wu, Koichiro Umemoto, Renata Wentzcovitch, Kai-Ming Ho We developed a fast and efficient method for crystal structure prediction and materials discovery. The method is based on the cut-and-paste genetic algorithm (GA) scheme introduced by Deaven and Ho [1]. In the evaluation of energies of target structures, first-principles calculations are accurate but time-consuming. Our method performs GA searches uses auxiliary model potentials to screen the energy of candidate structures, selecting only a few for more extensive first principles evaluation. Parameters of the auxiliary potentials are adaptively adjusted to reproduce the first-principles results during the course of the GA search. Our method combines the speed of empirical potential searches with the accuracy of first principles calculations. We will present results on applications to various systems including metallic alloys and ultrahigh pressure SiO$_{2}$, H$_{2}$O and Mg-Si-O systems. \\[4pt] [1] D. M. Deaven, K. M. Ho, Phys. Rev. Lett. 75, 288 (1995). [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A7.00003: Periodic structure optimization via local heat pulse-quench cycles employing the GULP code Arnulf M\"{o}bius, J. Christian Sch\"{o}n We present an optimization algorithm for problems with many continuous degrees of freedom and a huge number of local minima. It is based on the thermal cycling approach, originally developed for combinatorial optimization tasks [1]. The main idea is to cyclically disturb a few randomly chosen degrees of freedom of the current best local minimum and to quench this state by a highly efficient local search code. As the optimization proceeds, the amplitude of the disturbance slowly decreases. This approach is applied to a lattice structure prediction problem. We use the general utility lattice program (GULP) by J.D. Gale and co-workers [2] for local search. As test, the hypothetical periodic Mg$_{10}$Al$_4$Ge$_2$Si$_8$O$_{36}$ compound is studied, where both the cell parameters and the atom positions are free to vary. The results demonstrate that the proposed procedure is robust and far more efficient than the previous approaches to this problem by means of multi-start local search, simulated annealing, and evolutionary algorithms in Ref. 3. \\[4pt] [1] A. M\"{o}bius et al., Phys. Rev. Lett. 79 (1997) 4297. \\[0pt] [2] J.D. Gale and A.L Rohl, Mol. Simul. 29 (2003) 291. \\[0pt] [3] A.R. Oganov et al., in ``Modern Methods of Crystal Structure Prediction,'' ed. A.R. Organov, (Wiley, 2011), p. 223. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A7.00004: Mapping the Materials Genome through Combinatorial Informatics Invited Speaker: Krishna Rajan The recently announced White House Materials Genome Initiative provides an exciting challenge to the materials science community. To meet that challenge one needs to address a critical question, namely \textit{what} is the materials genome? Some guide on how to the answer this question can be gained by recognizing that a ``gene'' is a carrier of information. In the biological sciences, discovering how to manipulate these genes has generated exciting discoveries in fundamental molecular biology as well as significant advances in biotechnology. Scaling that up to molecular, cellular length scales and beyond, has spawned from genomics, fields such as proteomics, metabolomics and essentially systems biology. The ``omics'' approach requires that one needs to discover and track these ``carriers of information'' and then correlate that information to predict behavior. A similar challenge lies in materials science, where there is a diverse array of modalities of materials ``discovery'' ranging from new materials chemistries and molecular arrangements with novel properties, to the development and design of new micro- and mesoscale structures. Hence to meaningfully adapt the spirit of ``genomics'' style research in materials science, we need to first identify and map the ``genes'' across different materials science applications On the experimental side, combinatorial experiments have opened a new approach to generate data in a high throughput manner, but without a clear way to link that to models, the full value of that data is not realized. Hence along with experimental and computational materials science, we need to add a ``third leg'' to our toolkit to make the ``Materials Genome'' a reality, the science of Materials Informatics. In this presentation we provide an overview of how information science coupled to materials science can in fact achieve the goal of mapping the ``Materials Genome''. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A7.00005: Diamondlike carbo-boride C5B compound? Testing crystal structures and stability with Global Space Group Optimization ( GSGO) Arkadiy Mikhaylushkin, Xiuwen Zhang, Alex Zunger Diamond-like C5B has been alleged to be a new exciting material discovered at high pressures and temperatures. Using density-functional based evolutionary Global Space Group Optimization (GSGO) we established the likely structures of BC3 and BC5 phases and the known B4C compound. Examining the ground state line between solid Carbon and solid Boron we find only B4C is a ground state structure. C5B and C3B are high energy structure at high pressure. The reaction between BC3 and solid carbon producing C5B is found to have a positive reaction enthalpy that depends only weakly on pressure. In contrast to the previous reports, we argue that at 0 T the BC5 is less stable than BC3 in access of carbon. However, a small positive enthalpy of the reaction BC5 $\rightarrow$ BC3 +2C cannot rule out formation of the BC5 from the BC3 precursor material as thermally stabilized due to vibrational entropy or/and disorder effects. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A7.00006: Hands-free thermodynamic alloy modeling of $\sim$700 binary alloys using a Bayesian approach: Part I Gus Hart, Lance J. Nelson, Stefano Curtarolo, C. Shane Reese Bayesian approaches have become useful in recent years as increasing computing power has made them practical. Bayes rule itself is nothing more than a simple statement of conditional probability but can be used to make strong inferences. We discuss the general idea behind Bayes rule and how to use it to build physical models. Using a database of about 150,000 first principle calculations, we are building models for $\sim$700 binary alloys. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A7.00007: Hands-free thermodynamic alloy modeling of $\sim$700 binary alloys using a Bayesian approach: Part II Lance J. Nelson, Stefano Curtarolo, C. Shane Reese, Gus L.W. Hart In recent years, Bayesian statistics has become more popular as a scientific tool. This is mainly due to advances in computing power, which make the Bayesian formalism tractable. Baye's rule, which is a simple statement of conditional probability, is the foundation for Bayesian statistics. When used in conjunction with sampling algorithms, such as Metropolis-Hastings and Gibbs, Baye's rule provides a powerful framework for characterizing important parameters and for making inference. We demonstrate how the Bayesian framework can be applied to alloy modeling, providing the solution to several typical problem areas in this field. Together with our large database of first-principles data this Bayesian framework helps us quickly and accurately characterize hundreds of binary alloys. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A7.00008: Sorting stable from unstable hypothetical compounds and determining the electronic structure of interesting candidates : The case of Half-Heusler Filled Tetrahedral ABX structures Xiuwen Zhang, Liping Yu, Andriy Zakutayev, David Ginley, Alex Zunger Electronic structure theory has recently been used to propose hypothetical compounds, seeking new useful functional materials. In some cases, such hypothetical compounds in presumed crystal structures may be significantly higher in energy than (i) their lowest-energy structures or, than (ii) a combination of their constituents. We use the first-principles thermodynamics to sort (i) the lowest-energy structure and (ii) the thermodynamic stability with respect to disproportionation of the 488 unreported ABX octet compounds. We find that as many as 235 of the 488 are unstable with respect to decomposition, whereas other 235 of the unreported compounds are predicted to be thermodynamically stable. (18 materials are too close to call). The electronic structures of these predicted stable compounds are evaluated based on GW approximation for electron's self-energy. To support the theoretical predictions of new materials, thin film samples of AgYGe were synthesized using combinatorial Pulsed Laser Deposition. AgYSi-type ground state crystal structure and Y-rich composition stability range of AgYGe agree well with theory. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A7.00009: Band-structure predictions for A2BX4 discovery compounds Stephan Lany, V. Stevanovic, A. Zunger The inverse design of materials requires to predict the existence and the properties of previously unknown materials. We have performed a computational search for thermodynamically stable materials within the family of A2BX4 compounds (A, B = main group and 3d cations; X = O, S, Se, Te) resulting in the theoretical discovery of about 100 previously unreported compounds. The challenge for the prediction of band-structures and optical spectra is to obtain accurate results for a wide range of materials within a single computational scheme, so that unknown materials can be predicted with confidence. Whereas the main group chalcogenides are rather accurately predicted by many-body GW calculations, large deviations from experiment are observed for many 3d oxides. In particular, we find that the 3d orbitals consistently occur at too high energies, independent on whether they are occupied (e.g., Cu2O) or unoccupied (e.g., TiO2). While the exact nature of these issues are under investigation, we pursue here a pragmatic approach, using attractive on-site potentials with a single parameter for each 3d element, which leads to good agreement with experiment for binary and ternary 3d oxides. We use this approach to predict the band-structures of the discovery compounds. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A7.00010: New Crystal Structures Identified for PtO and PtO$_2$ using Density Functional Theory Calculations Juarez L.F. Da Silva, Ricardo K. Nomiyama, Mauricio J. Piotrowski Platinum plays an important role in catalysis and electrochemistry, and it has been known that the direct interaction of oxygen with Pt surfaces can lead to the formation of platinum oxides (PtO$_x$), which can affects the reactivity. To contribute to the atomistic understanding of the atomic structure of PtO$_x$, we report a density functional theory study of the atomic structure of bulk PtO$_x$ ($1 \leq x \leq 2$). From our calculations, we identified a lowest energy structure (GeS-type, space group $Pnma$) for PtO, which is 0.181~eV lower in energy than the structure suggested by Moore and Pauling (PtS-type). Furthermore, two atomic structures were identified for PtO$_2$, which are almost degenerate in energy with the lowest energy structure reported so far for PtO$_2$ (CaCl$_2$-type). Based on our results and analysis, we suggest that Pt and O atoms tends to form octahedron motifs in PtO$_x$ even at lower O composition by the formation of Pt$-$Pt bonds. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A7.00011: Simulation of structural and electronic properties of amorphous tungsten oxycarbides Kaliappan Muthukumar, Harald O. Jeschke, Roser Valenti Electron beam induced deposition with tungsten hexacarbonyl W(CO)$_6$ as precursors leads to granular deposits with varying compositions of tungsten, carbon and oxygen. Depending on the deposition conditions, the deposits are insulating or metallic. We employ an evolutionary algorithm to predict the crystal structures starting from a series of chemical compositions that were determined experimentally. We show that this method leads to better structures than structural relaxation based on guessed initial structures. We approximate the expected amorphous structures by reasonably large unit cells that can accommodate local structural environments that resemble the true amorphous structure. Our predicted structures show an insulator to metal transition close to the experimental composition at which this transition is actually observed. Our predicted structures also allow comparison to experimental electron diffraction patterns. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A7.00012: Predicting Two-Dimensional Boron-Carbon Compounds by the Global Optimization Method Xinyu Luo, Jihui Yang, Xingao Gong, Hongjun Xiang, Hanyu Liu, Yanchao Wang, Yanming Ma We adopt a global optimization method to predict two-dimensional (2D) nanostructures through the particle-swarm optimization (PSO) algorithm. By performing PSO simulations, we predict new stable structures of 2D boron-carbon (B-C) compounds for a wide range of boron concentrations. Our calculations show that: (1) All 2D B-C compounds are metallic except for BC3 which is a magic case where the isolation of carbon six-membered ring by boron atoms results in a semi-conducting behavior. (2) For C-rich B-C compounds, the most stable 2D structures can be viewed as boron doped graphene structures, where boron atoms typically form 1D zigzag chains except for BC3 in which boron atoms are uniformly distributed. (3) The most stable 2D structure of BC has alternative carbon and boron ribbons with strong in-between B-C bonds, which possesses a high thermal stability above 2000 K. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A7.00013: Quantum confined Schottky barriers: Tuning the Schottky-Mott and Bardeen Limits James Chelikowsky, Tzu-Liang Chan, Kai-Ming Ho, Cai-Zhuang Wang, Shengbai Zhang Even though metal-semiconductor junctions are an essential component in electronic devices, an atomistic understanding of the electronic structure of such junctions has remained elusive. Hundreds, if not thousands, of atoms may be required to model such interfaces, owing to lattice mismatches at the interface. The absence of a detailed understanding of the interface structure has inhibited the application of electronic structure calculations to examine the evolution of Fermi level pinning. Here we capitalize on recent computational breakthroughs and apply them to a prototypical system: a Pb(111) film on a Si(111) substrate. We consider up to 1,500 atoms in our first principle calculations based on real space pseudopotentials, and explicitly model the atomistic details of the metal-semiconductor interface. We propose a pinning mechanism for the Fermi level that depends critically on the role of quantum confinement in the overlaying metallic film. By changing the thickness of the overlying film, the pinning mechanism can be tuned from the abrupt interface description of Schottky and Mott to the metal induced gap state description of Bardeen. [Preview Abstract] |
Session A8: Focus Session: Frustrated Magnetism - Triangular Lattice
Sponsoring Units: DMP GMAGChair: Jason Gardner, NIST
Room: 208
Monday, February 27, 2012 8:00AM - 8:12AM |
A8.00001: Spin Correlations in the quasi-triangular magnet, Cu$_{2}$(OH)$_{3}$NO$_{3}$ Jason S. Gardner, Georg Ehlers, Fletcher Werner, S.A. Solin We have investigated the structural and magnetic properties of the spin S = $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ antiferromagnetic quasi-triangular lattice materials: Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$ (0 $<$ x $<$ 0.65) using a.c. susceptibility, heat capacity [1,2] and neutron scattering. After a brief introduction to the bulk properties of this family of materials, we will discuss recent inelastic neutron scattering results from the pure and doped materials. The temperature dependence of the quasielastic scattering reveals an abundance of slow spin dynamics at elevated temperatures. This scattering collapses as the system is cooled through its ordering temperature (11 K) and a \textbf{Q}-independent mode is observed at finite energy. These results will be compared to those seen in other triangular systems with a Kagome motif. \\[4pt] [1] J. Wu, J.S. Wildeboer, F. Werner, A. Seidel, Z. Nussinov, and S.A. Solin, Europhysics Letters,~\textbf{93,}~67001 (2011).\\[0pt] [2] J. Wu, A. K. Gangopadhyay, P. Kanjanaboos and S. A. Solin, J. Phys.: Condens. Matter \textbf{22,} 334211 -- 334222 (2010). [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A8.00002: Optical spectroscopy of the Triangular Lattice Antiferromagnets CuCrO$_2$ and $\alpha$-CaCr$_2$O$_4$ Michael Schmidt, Zhe Wang, F. Mayr, S. Toth, B. Lake, A.T.M.N. Islam, V. Tsurkan, A. Loidl, J. Deisenhofer We will compare and discuss our results obtained by optical spectroscopy on CuCrO$_2$ and $\alpha$-CaCr$_2$O$_4$. While CuCrO$_2$ is famous for its multiferroicity [1], in $\alpha$-CaCr$_2$O$_4$ a polarization can only be observed under the application of electric or magnetic field, despite having a closely related structure [2]. At near infrared and visible light frequencies we observe Cr$^{3+}$ crystal field absorptions and below T$_N$ excitons and exciton-magnon-transitions appear. The width of these exciton-magnon transitions is analyzed with respect to the existence of Z$_2$ vortices as proposed by Kojima et al. [3]. \\[4pt] [1] S. Seki et al., Phys. Rev. Lett. 101, 067240 (2008)\\[0pt] [2] K. Singh et al., Phys. Rev. B 84, 064129 (2011)\\[0pt] [3] N. Kojima et al., J. Phys. Soc. Jpn. 62, 4137 (1993) [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A8.00003: Quantum Spin Fluctuations for an Incommensurate Spiral Randy Fishman Quantum spin fluctuations are investigated for the incommensurate spiral state of a geometrically-frustrated triangular-lattice antiferromagnet. With increasing anisotropy, the average spin amplitude becomes larger but the spiral becomes more distorted. Quantum fluctuations enhance both the wavevector of the distorted spiral and the critical anisotropy above which it undergoes a first-order transition into a collinear state. An experimental technique is proposed to isolate the effects of quantum fluctuations from the classical distortion of the spiral. Results of this work are used to estimate the change in spin amplitude and ellipticity in the multiferroic state of CuFeO$_{2}$. Research sponsored by the Division of Materials Sciences and Engineering, U.S. Department of Energy under contract with UT-Battelle, LLC. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A8.00004: Phase Diagram of the Spatially Anisotropic Heisenberg Model on a Triangular Lattice Sedigh Ghamari, Catherine Kallin, Sung-Sik Lee, Erik Sorensen The spatially anisotropic spin-1/2 Heisenberg model on a triangular lattice is examined using a renormalization group (RG) analysis in the limit of weak interchain coupling ($J'/J \ll 1$ or large anisotropy). In agreement with a previous similar study, a collinear antiferromagnetic (CAF) state is found for large anisotropies. Retaining a marginal term that was not previously included, elucidates the strong competition between spiral and CAF order and suggests a direct transition at a fairly large anisotropy to incommensurate spiral order with a significantly renormalized wave vector that smoothly connects to the commensurate spiral state at the isotropic point. The significant renormalization of the ordering vector of this incommensurate spiral phase is argued to explain why many numerical studies found spin liquid behavior close to the isotropic point, (i.e. $J'/J \sim 0.85$). The agreement between the predictions of our RG analysis for the correlation function of the staggered magnetization on next-nearest-neighbor chains with DMRG results for small anisotropies further supports this picture. Finally, the effect on the phase diagram of Dzyaloshinskii-Moriya interactions, relevant to Cs$_2$CuCl$_4$, will be discussed. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A8.00005: Variational Monte Carlo study of quantum spin=1 liquid phases in the extended triangular-lattice Heisenberg model Samuel Bieri, Maksym Serbyn, Todadri Senthil, Patrick Lee Recent experiments in the compound Ba3NiSb2O9 [PRL 107, 197204] indicated that quantum-spin liquid phases in a spin S=1 anti-ferromagnet may exist. Motivated by these experiments, we construct quantum spin=1 liquid states with three flavors of fermionic spinons. We use variational Monte Carlo calculations to investigate the phase diagram of a triangular-lattice quantum Heisenberg model with single-ion anisotropy, bi-quadratic, and ring-exchange terms. We compare the energies of the spin-liquid states with conventional magnetically ordered states. We find that in some parameter ranges, an exotic gapless U(1) spin liquid is stabilized. In other parameter ranges, a BCS pairing instability with unconventional symmetry gaps out some of the spinons. We discuss our findings in relation with present and future experiments. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A8.00006: Magnetic Structure and Spin Waves in Co$_{2}$(OD)$_{3}$Cl Sachith Dissanayake, S. Ji, C. Chan, T.K. Ng, J. Lee, Y. Qiu, K.C. Rule, B. Lake, M. Green, X.G. Zheng, S.-H. Lee We have examined the magnetic structure of Co$_{2}$(OD)$_{3}$Cl with magnetic Co$^{2+}$ (3$d^{7}$; s= 3/2) ions using neutron powder diffraction data. Magnetic structure that yields the best refinement factor is an ``umbrella''-type antiferromagnetic structure with ab-components of magnetic moments in the kagome plane forming a q=0 120$^{\circ}$ structure and the moments are canted out of the plane by $\sim $ 40$^{\circ}$. The magnetic moments in the triangular plane are aligned ferromagnetically along the c-axis. We have performed linear spin wave calculations considering the nearest neighbor interactions J within the kagome plane and J$_{F}$ between the kagome and the triangular plane. The effects of J$_{F}$/J and the canting angle to the spin wave dispersion was studied thoroughly, to find out an effective spin hamiltonian that explains our inelastic neutron scattering data with two prominent excitation modes centered at 3 meV and 19 meV. Single ion-type anisotropy was also included in the spin hamiltonian to study its effect to the spin wave excitation. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A8.00007: Partially disordered state and spin-lattice coupling in an $S$=3/2 triangular lattice antiferromagnet Ag$_2$CrO$_2$ M. Matsuda, H. Yoshida, M. Isobe, C. de la Cruz, R.S. Fishman Ag$_2$CrO$_2$ consists of triangular lattice planes of CrO$_2$, which are well separated by the metallic Ag$_2$ layers. [1] This compound is an $S$=3/2 frustrated triangular lattice antiferromagnet without orbital degree of freedom. We performed neutron diffraction experiments on a powder sample of Ag$_2$CrO$_2$ on a neutron powder diffractometer HB-2A and a triple-axis neutron spectrometer HB-1, installed at HFIR in Oak Ridge National Laboratory. With decreasing temperature, a short-range 4-sublatice spin state develops. However, a long-range partially disordered state with 5 sublattices abruptly appears at $T\rm_N$=24 K, accompanied by a structural distortion, and persists at least down to 2 K. The spin-lattice coupling stabilizes the anomalous state, which is expected to appear only in limited ranges of further-neighbor interactions and temperature. It was found that the spin-lattice coupling is a common feature in triangular lattice antiferromagnets with multiple-sublattice spin states, since the triangular lattice is elastic. \\[4pt] [1] H. Yoshida {\it et al.}, to appear in J. Phys. Soc. Jpn. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A8.00008: Quantum Critical Magnetization Behaviors of the Kagome- and Triangular-Lattice Antiferromagnets Toru Sakai, Hiroki Nakano Magnetization process of the S=1/2 isotropic Heisenberg antiferromagnets on the kagome and triangular lattices are studied. Data from numerical-diagonalization method up to 39-spin systems, are reexamined from the viewpoint of the derivative of the magnetization with respect to the magnetic field. We find that the behavior of the derivative around the 1/3 height of the magnetization saturation is quite different from the cases of typical magnetization plateaux for the kagome-lattice antiferromagnet. This new phenomenon is called the ``magnetization ramp'' [1]. We also compare it with the 1/3 magnetization plateau of the triangular antiferromagnet. The critical exponent analysis indicates a clear difference between the magnetization plateau and ramp [2]. In addition using the numerical diagonalization up to 42-spin systems we suggest that the kagome-lattice antiferromagnet has a gapless singlet-triplet excitation in the thermodynamic limit [3].\\[4pt] [1] H. Nakano and T. Sakai: J. Phys. Soc. Jpn. 79 (2010) 053707.\\[0pt] [2] T. Sakai and H. Nakano: Phys. Rev. B 83 (2011) 100405(R).\\[0pt] [3] H. Nakano and T. Sakai: J. Phys. Soc. Jpn. 80 (2011) 053704. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A8.00009: Phase Diagram of Spin-1/2 Triangular Antiferromagnet in a Magnetic Field Ru Chen, Hyejin Ju, Hongchen Jiang, Oleg Starykh, Leon Balents We investigate the spin-1/2 quantum Heisenberg antiferromagnet on both two-dimensional triangular lattice and N-leg triangular ladder. The model describes isotropic Heisenberg chains (exchange constant $J$) coupled antiferromagnetically through interchain diagonal bonds (exchange constant $J$'). We study different regions using various controlled theoretical methods. Primarily we focus on the region slightly below saturation field. We show that the cone-coplanar state transition is absent, while commensurate-incommensurate transition emerges right below the saturation field for two-dimensional triangular lattice. We also determine the ground states in the limit $J'\ll J$, using one-dimensional bosonization, renormalization group methods and current algebra. Finally, we compare our theoretical result with DMRG result for N-leg ladder. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A8.00010: Spin Correlations and Excitations in the Quasi-2D Triangular Bilayer Spin Glass LuCoGaO4 K. Fritsch, G.E. Granroth, A.T. Savici, H.M.L. Noad, H.A. Dabkowska, B.D. Gaulin LuCoGaO4 is a layered magnetic-bilayer material wherein Co2+ magnetic moments and nonmagnetic Ga3+ ions are randomly distributed on planar triangular bilayers. This makes it an ideal case to study the interplay between geometric frustration, site disorder and low dimensionality and its influence on the magnetic ground of the system. This novel material has been grown for the first time in single crystal form at McMaster University. We have performed magnetization measurements, revealing a previously identified spin glass transition near Tf$\sim $19K, and a Curie Weiss temperature of Tcw$\sim $-96K, consistent with antiferromagnetic interactions[1]. We discuss time-of-flight neutron scattering measurements using SEQUOIA at SNS which elucidate the evolution of the static and dynamic spin correlations in LuCoGaO4 over a range of temperatures from T$<<$ Tf to T$>$Tcw. We observe quasielastic scattering at (1/3,1/3,L) positions in reciprocal space and rods of scattering along the c*-direction, consistent with short range antiferromagnetic correlations within decoupled bilayers, and which comfirm the 2-dimensional character of this system. Inelastic scattering measurements show a gapped $\sim $ 12 meV spin excitation which softens and broadens in energy, filling in the gap on a temperature scale of $\sim $ Tcw/2. [1] Cava et al., J. Solid State Chem. 140, 337 (1998). [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A8.00011: Emergence and lifting of frustration for dipolar molecules Sebastian Huber, Ehud Altman, Eugene Demler Ising spins on a triangular lattice are like a ``harmonic oscillator'' of geometric frustration. We address the general question of how this frustration is lifted in a system of dipolar molecules confined to 2+epsilon dimensions. When the confinement to two dimensions is strong, the dipoles arrange in a triangular lattice. Upon reduction of the confinement, the dipoles undergo a transition out of the two dimensional plane, leading to an Ising degree of freedom. Spin--lattice coupling gives rise to an effective model in terms of quantum dimers. We discuss the resulting phases and the implications to current experiments with hetero-nuclear molecules. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A8.00012: Spin dynamics in Kagom\'{e}-staircase multiferroic Ni$_3$V$_2$O$_8$ Andrey Podlesnyak, G. Ehlers, M. Frontzek, R.S. Fishman, O. Zaharko, M. Kenzelmann, S. Barilo The coupling of magnetic and ferroelectric order has recently been drawing a lot of interest in condensed matter science given the fundamental interest and potential applications. Ni$_3$V$_2$O$_8$ (NVO) is a $S=1$ magnet with Ni$^{2+}$ ions arranged in a weakly coupled buckled Kagom\'{e}-staircase planes. Its complex magnetic phase diagram exhibits four different zero field incommensurate and commensurate magnetic phases below 10~K, with only one developing ferroelectric order. We present here a detailed study of low temperature magnetic dynamics in this geometrically frustrated spin system. Using single crystal inelastic neutron scattering technique we map the magnetic excitation spectra across all the magnetic phase transitions. We found that the spin-waves, well formed in the base temperature nonferroelectric phase at $T < 3$~K, are considerably damped when the system enters the low-temperature incommensurate phase with ferroelectric order ($3.9 < T < 6.3$~K). Finally, we discuss models that describe the coupling between magnetic and ferroelectric properties in the incommensurate magnets. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A8.00013: Temperature-dependent Raman studies of the magnetodielectric vanadates: Ni$_{3}$V$_{2}$O$_{8}$, Co$_{3}$V$_{2}$O$_{8}$, and K$_{2}$V$_{3}$O$_{8}$ M. Kim, Y. Gim, S.L. Cooper, R.J. Cava The kagome staircase compounds, Ni$_{3}$V$_{2}$O$_{8}$ and Co$_{3}$V$_{2}$O$_{8}$, are known to exhibit dielectric anomalies at the magnetic transitions; and inhomogenously mixed-valent K$_{2}$V$_{3}$O$_{8}$ has been shown to have a strong dependence of the dielectric constant on applied magnetic field. However, while strong spin-lattice coupling is generally associated with the complex phases and phenomena in these materials, there has been little microscopic information about this coupling. In this talk, we present temperature-dependent Raman scattering studies of Ni$_{3}$V$_{2}$O$_{8}$, Co$_{3}$V$_{2}$O$_{8}$, and K$_{2}$V$_{3}$O$_{8}$ that allow us to investigate the microscopic connection between the strong spin-lattice coupling and the magnetodielectric effects in these materials. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A8.00014: Competing Interactions in the $S$=3/2 Kagome Staircase Co$_{3}$V$_{2}$O$_{8}$: Evolution of the Commensurate and Incommensurate Phases in a Magnetic Field Joel Helton, Ying Chen, Jeffrey Lynn, Georgii Bychkov, Sergei Barilo, Nyrissa Rogado, Robert Cava Single crystal neutron diffraction studies have been performed on the $S$~=~3/2 kagome staircase compound Co$_{3}$V$_{2}$O$_{8}$ with a magnetic field applied along the magnetization easy-axis ($\vec{H}$~$||$~$\vec{a}$). Previous zero-field measurements reported incommensurate, transversely polarized spin density wave (SDW) phases [with a temperature dependent propagation vector of $\vec{k}$~=~(0~$\delta$~0)] interspersed with multiple commensurate lock-in transitions at temperatures above the ferromagnetic ground state. For small applied fields along $\vec{a}$, $\mu_{0}H$~$\approx$~0.05~T, the commensurate lock-in phases are destabilized in favor of the incommensurate SDW, while slightly larger applied fields restore the commensurate lock-in phase with $\delta$~=~1/2 and yield a new commensurate phase with $\delta$~=~2/5. For measurements in an applied field, higher-order scattering is observed that corresponds to the second-harmonic. [Preview Abstract] |
Session A9: Focus Session: Magnetic Oxide Thin Films And Heterostructures - Cobaltite And Ferrite Thin Films
Sponsoring Units: GMAG DMPChair: Chris Palmstrom, University of California, Santa Barbara
Room: 209
Monday, February 27, 2012 8:00AM - 8:12AM |
A9.00001: Ferromagnetic insulating state in tensile-strained LaCoO$_3$ thin films Han Hsu, Peter Blaha, Renata Wentzcovitch With local density approximation + Hubbard $U$ (LDA+$U$) calculations, we show that the ferromagnetic (FM) insulating state observed in tensile-strained LaCoO$_3$ epitaxial thin films is most likely a mixture of low-spin (LS) and high-spin (HS) Co, namely, a HS/LS mixture state. Compared with other FM states, including the intermediate-spin (IS) state (\textit{metallic} within LDA+$U$), which consists of IS Co only, and the insulating IS/LS mixture state, the HS/LS state is the most favorable one. The FM order in HS/LS state is stabilized via the superexchange interactions between adjacent LS and HS Co. We also show that Co spin state can be identified by measuring the electric field gradient (EFG) at Co nucleus via nuclear magnetic resonance (NMR) spectroscopy. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A9.00002: Aberration-corrected STEM-EELS studies of epitaxial La0.5Sr0.5CoO3 thin films Maria Varela, Jaume Gazquez, Neven Biskup, Stephen Pennycook, Maria Torija, Manish Sharma, Shameek Bose, Chris Leighton Cobaltite thin films provide a unique opportunity to study magneto-electronic phase separation, which can be strong in this reduced dimensionality environment. Here we present an investigation of epitaxial La0.5Sr0.5CoO3 thin films on SrTiO3 and LaAlO3 substrates by scanning transmission electron microscopy and electron energy loss spectroscopy. The different degrees of strain and also different orientations of the substrates (such as (001) vs. (110)) induce major changes of the crystal structure and the depth profile of the chemical composition, observed both in the La/Sr and O sub-lattices. These effects can lead to lower effective doping level at the interface, favoring interfacial magneto-electronic phase separation. Research Council Starting Investigator Award (JS, NB) and the U.S. Dept. of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Div. (MV, SJP). Work at UMN supported by NSF and DOE (scattering). [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A9.00003: Atomic-Scale Imaging and Control of Interface Magnetic States in Vacancy Ordered Cobaltite Thin Films Albina Borisevich, Young-Min Kim, Michael Biegalski, Jun He, Hans Christen, Sokrates Pantelides, Stephen Pennycook Magnetic properties of complex oxide thin films are strongly affected by strain, chemical composition, and octahedral tilt of the substrate. Here, we study lanthanum/strontium cobaltite (La$_{0.5}$Sr$_{0.5}$CoO$_{3-x,}$ LSCO) thin films via quantitative aberration-corrected scanning transmission electron microscopy and Electron Energy Loss Spectroscopy (EELS) to explore the coupling between magnetic properties, ionic behavior, and oxygen octahedral tilts. LSCO films were grown by PLD in identical conditions on two different substrates, LSAT (cubic) and NGO (orthorhombic). These substrates have nearly identical lattice parameters, but different octahedral tilts. The film on NGO appears to be La$_{0.5}$Sr$_{0.5}$CoO$_{2.5}$, while the film on LSAT is less oxygen deficient. Comparison of measured lattice parameters with the first-principles calculations allows us to determine oxygen content in the film. In La$_{0.5}$Sr$_{0.5}$CoO$_{2.5}$/NGO films, EELS reveals different valence states of Co at the interface depending on termination, resulting in different magnetic states. Therefore changes in octahedral tilts can induce changes in oxygen stoichiometry and interface magnetic states of the vacancy ordered structures. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A9.00004: Stress and magnetism in LaCoO$_{3}$ films Invited Speaker: Alex Demkov Cobaltates exhibit a wide variety of exciting electronic properties resulting from strong electron correlations; these include superconductivity, giant magnetoresistance, metal-insulator transition, and strong thermoelectric effects. This makes them an excellent platform to study correlated electron physics, as well as being useful for various applications in electronics and sensors. In the ground state in the bulk, the prototypical complex cobalt oxide LaCoO$_{3}$ is in a spin-compensated low-spin state (t$_{2g}^{6})$, which results in the ground state being nonmagnetic. In a recent experiment, Fuchs \textit{et al.} (\textit{Phys. Rev. B} \textbf{75}, 144402 (2007)) have demonstrated that a ferromagnetic ground state could be stabilized by epitaxial tensile strain resulting in a Curie temperature ($T_{C})$ of $\sim $90 K when LaCoO$_{3}$ (LCO) is grown on SrTiO$_{3}$ (STO) using pulsed laser deposition. In this talk I will discuss our recent successful attempt to integrate a LCO/STO heterostructure with Si (001) using molecular beam epitaxy. We have grown strained, epitaxial LaCoO$_{3}$ on (100)-oriented silicon using a single crystal STO buffer (Appl.Phys. Lett. \textbf{98}, 053104 (2011)). SQUID magnetization measurements confirm that the ground state of the strained LaCoO$_{3}$ is ferromagnetic with a $T_{C}$ of 85 K. Our first-principles calculations of strained LaCoO$_{3}$ using the LSDA+$U$ method show that beyond biaxial tensile strain of 2.5{\%} local magnetic moments, originating from the high spin state of Co$^{3+}$, emerge in a low spin Co$^{3+}$ matrix. Ferromagnetism found in tensile-strained LaCoO$_{3}$ is tightly coupled to the material's orbital and structural response to applied strain. Theoretical calculations show how LaCoO$_{3}$ accommodates tensile strain \textit{via} spin state disproportionation, resulting in an unusual sublattice structure. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A9.00005: Ordering of defects induced by epitaxy in LaCoO$_{3}$ films Virat Mehta, Neven Biskup, Franklin Wong, Elke Arenholz, Maria Varela, Yuri Suzuki In the bulk, LaCoO3 (LCO) undergoes a spin state transition from a diamagnet to a paramagnet with increasing temperature. Recent studies of epitaxial LCO thin films have resulted in the stabilization of a higher spin state and ferromagnetic ordering at low temperatures. Here, we explore the effects of epitaxy on the electronic structure of LCO films with X-ray absorption spectroscopy (XAS) and scanning transmission electron microscopy (STEM). We find differences in XAS spectra in coherently strained thinner films compared to the thicker partially relaxed films which may be due to differences in Co valence and bonding. STEM and electron energy loss spectroscopy of thinner LCO films reveal ordered defect planes that appear to be associated with a change in the O and Co bonding environments. In films on LaAlO3 strained in compression periodic planes occur parallel to the substrate-film interface, while films on SrTiO3 strained in tension have perpendicular defect planes. Correlation with magnetic data suggests that defect rich regions may exhibit greater ferromagnetism. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A9.00006: Spin state disproportionation and ferromagnetism in strained LaCoO3: Ab-initio study Seo Hosung, Alexander Demkov Strain engineering of artificial oxide heterostructures opens up routes for the creation of novel electronic phases that do not exist in the bulk. To fully exploit the functionalities of the oxide, understanding its electronic and structural response to epitaxial strain is crucial. One example is the recent demonstration of biaxial tensile strain stabilizing an insulating ferromagnetic ground state in normally non-magnetic LaCoO3. However, theoretical understanding is incomplete. In this talk, using the LSDA+U method we discuss the origin of strain induced transition to insulating ferromagnetic ground state in LaCoO3. We show that beyond biaxial tensile strain of 2.5{\%} local magnetic moments, originating from high spin state of Co3+, emerge in low spin Co3+ matrix. We further show that these local moments are ferromagnetically coupled via superexchange interaction. In contrast, we find that compressive strain by itself is not able to stabilize a magnetic state, that agrees with recent experiment. Ferromagnetism found in tensile-strained LaCoO3 is tightly coupled to the material's orbital and structural response to applied strain. We discuss how LaCoO3 accommodates tensile strain via spin state disproportionation, resulting in an unusual sublattice structure. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A9.00007: Percolation-Type Ferromagnetic Order in Epitaxial Strained LaCoO$_3$ Thin Films George Sterbinsky, Philip Ryan, Jong-Woo Kim, Evguenia Karapetrova, J. Ma, Jing Shi, Joseph Woicik Support for percolation-type ferromagnetic order in LaCoO$_3$ thin films is provided by x-ray diffraction and Co $K$-edge x-ray absorption fine structure (XAFS) spectroscopy. X-ray diffraction shows considerable changes in structure with respect to bulk LaCoO$_3$, and extended XAFS (EXAFS) demonstrates a large Jahn-Teller like distortion of the oxygen octahedra in highly strained films. Structural distortions of the oxygen octahedra are strongly coupled to the hybridization between orbitals of Co and O atoms, as shown by x-ray absorption near edge spectroscopy (XANES). Our results indicate that increased Co-O hybridization, and therefore increased magnetic exchange energy, does not cause ferromagnetism to occur in LaCoO$_3$ thin films. Instead, we suggest that the strain-induced distortions of the oxygen octahedra increase the population of $e_g$ electrons and concurrently depopulate $t_{2g}$ electrons beyond a stabilization threshold for ferromagnetic order. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A9.00008: Imaging Local Magnetic Domain Rearrangement in Strained LaCoO$_3$ Thin Films Using Magnetic Force Microscopy Morgann Berg, Neliza Leon, Agham Posadas, Alfred Lee, Jeehoon Kim, Alex de Lozanne, Alex Demkov Previous studies we have conducted on thin films of lanthanum cobaltate (LCO) under tensile strain have revealed a tendency toward local magnetic domain rearrangement into streak-like configurations near the ferromagnetic to paramagnetic phase transition. Moreover, the persistence of these streak-like characteristics to lower temperatures after field-cooling appears to be linked to the strength of the applied magnetic field in which these films are field-cooled. This tendency has not yet been verified for thin films of LCO under compressive strain which could indicate whether this magnetic domain rearrangement is intrinsic to thin film samples of LCO or is merely an effect of tensile strain. Using magnetic force microscopy, we investigate the microscale magnetic properties of a thin film of LCO under compressive strain, prepared by molecular beam epitaxy and deposited on a lanthanum aluminate substrate. We observe these properties across a wide temperature range and compare our results to global magnetic characteristics of this film as measured by a SQUID magnetometer. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A9.00009: Origin of ferromagnetic ordering in LaCoO$_{3}$ epitaxial thin films Woo Seok Choi, J.-H. Kwon, Hyungjeen Jeen, Vladimir Hinkov, M. Kim, Ho Nyung Lee LaCoO$_{3}$ (LCO) film has received attention due to its unexpected ferromagnetic (FM) ordering, which is distinctly different from the bulk counterpart. Although the exact origin has not been understood, previous studies have suggested that the epitaxial strain should play an important role. In this work, we show that the FM ordering could be related to a locally-ordered microstructure. We used PLD to deposit epitaxial LCO thin films on various substrates in order to impose different degree of strain. XRD and XAS studies showed that the films are of high quality, without any secondary phases or changes in the Co valence. In addition, all the films were coherently-strained. From the STEM investigation, however, we noticed that some of the films had an unexpected stripe-like superstructure along the $<$100$>$ direction. While the microstructure resembles that of oxygen-vacancy or charge ordering, typically found in doped transition metal oxides, we could rule out such possibilities and interpret it as a nanoscale twin boundary. The strain induced structural change seems to originate the FM ordering. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A9.00010: Magnetic properties of LuFeO$_3$ thin films Xiaoshan Xu, Wenbin Wang, Zheng Gai, Xuemei Cheng, Leyi Zhu, David Keavney, Paul Snijder, Thomaz Ward, Jian Shen In order to extract their intrinsic magnetic properties, we have grown LuFeO$_3$ thin films epitaxially on Al$_2$O$_3$ (0001), a substrate with minimum magnetic impurities, using pulsed laser deposition. The magnetization measurements reveal strong anisotropy between in-plane and out-of-plane, not only in terms of coercivity and remanence, but also obvious in the zero field cool and field cool splitting. Further experiment using X-ray magnetic linear dichroism suggest magnetic ordering higher than 290 K and a spin reorientation at lower temperature. Over all, the films appear weak-ferromagnetic. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A9.00011: Fabrication and properties of LuFeO3 thin film Wenbin Wang, Xiaoshan Xu, Zheng Gai, Nina Balke, Miaofang Chi, Thomas Z. Ward, Paul C. Snijders, Jian Shen We have succeeded in growing the hexagonal LuFeO3 single crystalline thin films on Al2O3(0001) substrates using Pulsed Laser Deposition (PLD). The structures, epitaxial relation between film and substrate, ferroelectric and magnetic properties of the samples were characterized by RHEED, LEED, XRD, AFM, TEM, PFM and SQUID magnetometry. The structure of our hexagonal LuFeO3 films is consistent with that of YMnO3, and the samples exhibit a piezoelectric effect at room temperature. RHEED data are consistent with a structural change from a polar P63cm (185) to non-polar P63/mmc (194) at 1050 K. SQUID measurements reveal strong magnetic order in the thin film. All the data suggests a coexistence of ferroelectricity and magnetic order in hexagonal LuFeO3 films. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A9.00012: Enhancement of the Magnetic Moment in Ultrathin Fe-doped CoFe$_{2}$O$_{4}$ Jarrett Moyer, Carlos Vaz, Divine Kumah, Dario Arena, Victor Henrich The magnetic properties of magnetic oxides can be drastically altered through a reduction in film thickness. It has previously been demonstrated that the magnetic moments of CoFe$_{2}$O$_{4}$ and NiFe$_{2}$O$_{4}$ are enhanced for ultrathin films; however, the physical mechanisms for this enhancement are still unknown. To determine the physical cause of this increased magnetic moment and to examine the effect of Fe doping, thin films of Co$_{1-x}$Fe$_{2+x}$O$_{4}$ (0 $\le x\le $ 0.8) are grown epitaxially on MgO (001) substrates by MBE at thicknesses ranging from 3 -- 20 nm. SQUID magnetometry measures the bulk magnetic properties of the samples and confirms that there is an increase in the magnetic moment for all stoichiometries as the film thickness is reduced. XAS, XMLD and XMCD measurements examine the cation-specific magnetic moments and spin directions to explain the physical mechanisms that lead to an enhanced magnetic moment in ultrathin Fe-doped CoFe$_{2}$O$_{4}$ films. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A9.00013: Electronic states and magnetic structure at the Co$_{3}$O$_{4}$ (110) surface: a first principles study Jia Chen, Annabella Selloni Tricobalt tetraoxide (Co$_{3}$O$_{4})$ is an important catalyst and Co$_{3}$O$_{4}$(110) is a frequently exposed surface in Co$_{3}$O$_{4}$ nanomaterials. We employed Density-functional theory with on-site Coulomb repulsion U term to study the atomic structures, energetics, magnetic and electronic properties of the two possible terminations, A and B, of this surface. These calculations predict A as the stable termination in a wide range of oxygen chemical potentials, consistent with recent experimental observations. The Co$^{3+}$ ions do not have a magnetic moment in the bulk, but become magnetic at the surface, which leads to surface magnetic orderings different from the one in the bulk. Surface electronic states are present in the lower half of the bulk band gap and cause partial metallization of both surface terminations. These states are responsible for the charge compensation mechanism stabilizing both polar terminations. Furthermore, our calculations predict that the critical thickness for polarity compensation is 4 layers. [Preview Abstract] |
Session A10: Invited Session: Computational Assessment of a Sustainable Energy Future: The Earth-abundant Materials Approach
Sponsoring Units: DCOMPChair: Vincent Meunier, RPI
Room: 210A
Monday, February 27, 2012 8:00AM - 8:36AM |
A10.00001: Computational Design of Solar Energy Harvesting Materials Made of Earth-Abundant Elements Invited Speaker: Yiyang Sun Very large-scale deployment of photovoltaic (PV) technology based on both the first and second generation solar cells posts serious questions on the materials supply as they rely on either high-purity and high-quality silicon crystals or rare elements such as indium and tellurium. ``Ancient'' PV materials made of earth-abundant elements, such as oxides and sulfides of copper and iron, have attracted resurgent interests. There is also intensive research devoted to the search for ``modern'' earth-abundant PV materials, with a recent promising example being Cu$_{2}$ZnSnSe$_{4}$. Computational approaches play a key role in this endeavor by guiding the screening and optimization of the materials toward high device performance. In this paper, I will focus on two aspects of computational design of earth-abundant PV materials. First, I will discuss the methods for accurately predicting band gaps of semiconductor materials. The emphasis will be on the performance of hybrid functional method on different classes of materials. Based on these understandings, I will discuss how to tune the band gap of a material to match the solar spectrum. For example, one could reduce of the band gap of anatase to 1.5 eV by the chemical codoping approach. Second, I will discuss the methods for accurate computation of defect properties, which is important as the defectiveness is intrinsic to the low-cost synthesized materials. I will introduce a method for calculation of defect formation energies by minimizing the error due to the ``band-gap problem'' of the density functional theory. I will also discuss approaches to mitigating the effects of defects, e.g., by passivation. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A10.00002: Computational approaches to finding earth-abundant thermoelectric materials Invited Speaker: Vidvuds Ozolins Good thermoelectric materials should possess a combination of seemingly incompatible properties, such as high electronic mobility and low lattice thermal conductivity. Therefore, search for crystalline materials with glass-like thermal conductivity has been an active field of research. Several cubic I-V-VI$_2$ semiconductors, the paradigm for which is AgSbTe$_2$, have been shown to exhibit minimal values of lattice thermal conductivity at ambient temperatures when the phonon mean free path equals the interatomic distance. These modes are due to the existence of highly polarizable lone $s^2$ electron pairs on the group V cations. Electrostatic repulsion between the lone $s^2$ pairs and the valence charge on group VI anions tends to favor locally distorted bonding configurations and may lead to unstable phonons. We present the results of first-principles density functional theory (DFT) calculations of phonon dispersion and electron-phonon interactions in cubic I-V-VI$_2$ semiconductors, where the group I elements are Cu, Ag, Au or alkali metals, the group V elements are P, As, or Bi, and the group VI elements are S, Se, or Te. Compounds that have only marginally stable phonons have extremely large Gr\"uneisen parameters that result in a thermal conductivity limited by Umklapp processes to values at the amorphous limit above 200 K. Following the {\it ab initio\/} calculation, we synthesized AgSbTe$_2$, AgSbSe$_2$, AgBiTe$_2$, NaSbTe$_2$, NaSbSe$_2$, and NaBiTe$_2$ and report their thermal conductivity and specific heat: in all cases, the experiments confirm the theory. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A10.00003: Solar energy into fuels - the importance of interface catalysis Invited Speaker: Frank Abild-Pedersen Finding sustainable energy solutions for the future will rely heavily on the energy influx from the sun. One convenient way of storing solar energy is by transforming that energy into a chemical form - like a fuel. The efficiency of such a transformation will require catalysts that are optimized for specific reactions, and we will need to find new catalysts for a number of processes, if we are to successfully synthesize fuels from sunlight. A fundamental insight into the way the catalysts work at the molecular level is an essential ingredient if one wants to speed up the discovery process. In this presentation I will discuss some of the challenges in catalyst discovery. In particular, I will focus on the conversion of syngas to methanol, an important sub-reaction in the biomass to fuels process. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A10.00004: Key electronic processes in organic solar cells: a theoretical perspective Invited Speaker: Veaceslav Coropceanu In this contribution, we discuss state-of-the-art quantum-chemical approaches used to derive the microscopic parameters and model the key electronic processes in organic solar cells. We illustrate the application of recently developed computational methods by computing the electronic couplings and the rates of exciton dissociation and charge recombination in several model donor-acceptor complexes. The contributions of both intra-molecular and inter-molecular vibrations to the electron-vibrational interaction will be discussed in detail. The impact on the charge-transport characteristics of the interplay between electron-vibration coupling and electronic coupling is investigated in the framework of band, disorder, and semi-classical models. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A10.00005: Harvesting the energy from the sun: insights from ab initio materials modeling Invited Speaker: Giulia Galli Can quantum and atomistic simulations make valuable contributions to the search of materials for sustainable energy sources? We will address this question by discussing some specific examples where quantum simulations have been used to predict optical properties of nanostructured semiconductors and of complex oxides in contact with water. We will also discuss issues related to the comparisons with increasingly complex experiment. [Preview Abstract] |
Session A11: Graphene: Transport in Devices and Structures
Sponsoring Units: DCMPChair: Xu Du, Stony Brook University
Room: 210B
Monday, February 27, 2012 8:00AM - 8:12AM |
A11.00001: Transport in Graphene: Ballistic, Anomalous, or Diffusive? Mario Borunda, Holger Hennig, Eric Heller The discovery of graphene, consisting of a layer of carbon atoms arranged in a honeycomb lattice, represents a conceptually new class of materials that are only one atom layer thick. Recent experiments achieved extremely high mobilities in suspended graphene leading to ballistic transport. In this talk, we study implications of diffusive and ballistic transport in graphene devices and ask what could be possible signatures of ``anomalous'' (or superdiffusive) transport. Experimental setups to differentiate between these stochastic processes are discussed. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A11.00002: Transport in high-mobility mesoscale graphitic devices Andrei Garcia, Francois Amet, James Williams, David Goldhaber-Gordon Recent advances in graphene fabrication have allowed for high-mobility structures to be created. We report on the fabrication and measurement of mesoscale devices of graphene on boron-nitride. We present the details by which graphene is transferred on to boron-nitride substrates, where we observed enhanced mobility over similar devices fabricated on silicon dioxide substrates. Transport measurements at low temperature are described with focus given to reconfigurable, mesoscale devices in graphene. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A11.00003: Gate defined quantum confinement in suspended bilayer graphene Monica Allen, Jens Martin, Amir Yacoby Devices that confine electrons in graphene have sparked substantial interest due to applications ranging from spin-based quantum computation to valley filters. However, the absence of an intrinsic bandgap in graphene has limited such devices to on-chip nanopatterned structures to date. Here we present high quality quantum dots in suspended bilayer graphene with tunnel barriers defined by external electric fields that break layer inversion symmetry, thereby eliminating both edge and substrate disorder. We demonstrate clean electron confinement in two regimes: at zero magnetic field B using the single particle energy gap induced by a perpendicular electric field and at B$>$0 using the quantum Hall ferromagnet v=0 gap. Our devices exhibit clean quantum transport behavior at magnetic fields ranging from zero to seven Tesla, including a highly resistive v=0 quantum Hall state and over forty consecutive Coulomb blockade oscillations with symmetric source-drain coupling. The data indicate that the dots are defined by local top gating and are not disorder formed. Geometric control over oscillation periodicity is confirmed by electrostatic simulations based on lithographic gate geometry. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A11.00004: Quantum Transport in Double-gated Trilayer Graphene \textit{pnp} Junctions Yongjin Lee, Jairo Velasco Jr., Lei Jing, Wenzhong Bao, David Tran, Marc Bockrath, Chun Ning (Jeanie) Lau Using trilayer graphene \textit{pnp} junctions with suspended top gates, we perform transport measurements. At a magnetic field B=0, by an applied perpendicular electric field, the conductance is increased that it is suggestive of a semi-metallic band overlap. At B=8T we observe quantum Hall conductance with fractional values, which can be explained equilibration of edge state between differentially-doped regions, and the presence of an insulating state at filling factor $\nu $=0. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A11.00005: Superconducting proximity effect in graphene: Injecting Cooper pairs in quantum Hall edge states Katsuyoshi Komatsu, Chuan Li, Sandrine Autier-Laurent, Alik Kasumov, Helene Bouchiat, Sophie Gueron A superconductor-graphene(SG) hybrid system, such as an SGS junction or an SG interface, provides an ideal platform to investigate the relativistic nature of Dirac fermions combined with superconductivity. Instead of the retro-reflection of carriers in an ordinary superconductor-normal metal interface, an SG interface is theoretically predicted to show the specular reflection of quasiparticle carriers. We show that a supercurrent flows through a SGS junction with Nb electrodes even through a very long graphene distance of 1.2$\mu $m, more than 3 times the length previously reported. This supercurrent disappears in the vicinity of the Dirac point, indicating a strong sensitivity of the transmission of Andreev pairs to the formation of charge puddles with size greater than the superconducting coherence length. We also present data on similar size graphene samples with superconducting electrodes with a high critical field (more than 7Tesla) for which the properties of the normal state are dominated by quantum Hall physics. Whereas the behavior of the supercurrent is similar to the Nb/Graphene/Nb system in zero field, new features are observed in the high field quantum Hall regime. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A11.00006: Gate-Tunable Superconducting-Insulating Transition in Tin-Decorated Graphene Vincent Bouchiat, Zheng Han, Adrien Allain We report the measurement of electrostatically tuned superconducting-insulating transition in macroscopic, CVD-Grown samples of graphene which decorated with tin nanoparticles. The self assembled network of Tin islands generates superconducting correlations locally in the Graphene by means of proximity effect. Correlations eventually leads to percolation of a supercurrent This system exhibits features related to granular superconductivity, a giant magnetoresistance peak, as well as an intermediate metallic behavior. We emphasize outstanding dynamics of the transition, which exhibit a change in resistance of more than 7 orders of magnitude within 40V of gate voltage, thus realizing a real electrostatically driven superconducting-insulating transition. The intense positive magnetoresistance observed for fields below the critical field of Tin nanoparticles is a signature of the localization of Cooper pairs. This hybrid superconductor provides a model system to better understand the physics of inhomogeneous superconductivity, as crossing the transition by adjusting the carrier density is conceptually simpler than using a magnetic field. It also allows to cross the transition continuously and under constant disorder. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A11.00007: Transport through Andreev Bound States in a Graphene-base Quantum Dot Yanjing Li, Nadya Mason We perform tunneling spectroscopy on a graphene-quantum dot (QD)-superconductor junction, a system in which sharp, gate-tunable Andreev bound states (ABS) in the spectra have been observed [1]. Here we extend previous results, particularly regarding the origins of the QD. In particular, we discuss how a discontinuous layer of AlO$_{x}$ between the superconductor and the graphene plays a role in the formation of the QD. We also discuss additional spectroscopic features that may be due to multiple QDs and energy levels. Finally, we show that a robust superconducting tunneling junction can be created in a lead-graphene structure, without the explicit deposition of a tunneling barrier. \\[4pt] [1] Dirks, T., Nature Physics 7, 386--390 (2011) [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A11.00008: Direct Measurement of the Fermi Energy in Graphene Using a Double Layer Structure Seyoung Kim, Insun Jo, David Dillen, Domingo Ferrer, Babak Fallahazad, Zhen Yao, Sanjay Banerjee, Emanuel Tutuc The Fermi energy is a fundamental property of an electron system. Here we introduce a direct measurement technique of the relative Fermi energy as a function of carrier density, using transport measurement in a double layer structure where one of the layers is graphene. The principle of this method is that the Fermi energy in the target material is equal to the applied inter-layer bias required to bring the graphene layer to charge neutrality point. Using a double layer graphene structure, we illustrate the technique by measuring the Fermi energy in one of the graphene layers. By mapping the top graphene layer zero density line as a function of bottom and inter-layer bias, we measure the Fermi energy as a function of carrier density at zero and in high magnetic fields. We extract the Fermi velocity, Landau level spacing and Landau level broadening. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A11.00009: Magnetic field dependence of the carrier's effective mass and the g-factor in graphene Andrii Iurov, Godfrey Gumbs, Oleksiy Roslyak, Danhong Huang It has been established that the intrinsic Zeeman energy is one half of the cyclotron energy for ``\emph{bare}'' electrons in graphene. Consequently, there could be Landau-level mixing between the energy bands. We investigate how the band mixing is affected by the Coulomb interaction. Pairing of the electrons and holes in the presence of a circularly polarized light is also considered for several filling factors. We calculate the quasiparticle effective mass and effective $g$-factor for dressed electrons and holes in monolayer graphene. As an intermediate step of these calculations, we obtain the dielectric function for the case of electron dressed states and investigate how the magetoplasmons modes are affected by the electron-photon interaction. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A11.00010: Magnetic-field tunable electron-phonon coupling in graphite Li-Chun Tung, W. Yu, P. Cadden-Zimansky, M. Kindermann, D. Smirnov, Z. Jiang Electron-phonon coupling (EPC) plays a pivotal role in condensed matter physics, governing intriguing phenomena such as superconductivity, ballistic transport, and excited-state dynamics. In graphitic systems, EPC is found to be strongly related to the quasiparticle excitations, electronic states and optical properties. Thus, the EPC may be manipulated via external parameters, such as electric field or magnetic field, and provide alternative access to adjust the characteristics of carbon-based devices. Here, we explore the EPC in graphite via magnetophonon resonance using cyclotron resonance (CR) spectroscopy. A marked avoided-level-crossing splitting of the CR and a Fano resonance-like behavior are observed, when the CR energies intersect the specific phonon energies via tuning the magnitude of the magnetic field. We attribute these results to the resonance between the CR excitations and the large momentum zone-edge, and the long-wavelength zone-center phonons, respectively. This work suggests that rich interacting physics exists in graphitic materials, which may have profound implications in future optoelectronics. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A11.00011: High frequency high magnetic field response of graphene monolayers Ivana Petkovic, Francis Williams, Fabien Portier, Patrice Roche, Keyan Bennaceur, Christian Glattli We study the electronic magnetotransport in graphene at rf frequencies (5-50GHz). Our aim is to investigate the dynamics of charge carriers in the quantum Hall regime. The graphene sample is placed in a break made in a coplanar waveguide and the transmitted power is measured. In order to isolate the response of the sample from the direct transmission between the input and output waveguides, the graphene electron density distribution is modulated with a side gate and the resulting modulation in the transmitted power detected via a standard lock-in technique. The fixed frequency graphene response as a function of magnetic field reveals two different components. One is symmetric in B and dominates under large side gate voltage, and the other shows reproducible fluctuations revealed only at low gate voltage modulation amplitude. The first part is thought to be related to the bulk conductivity and the fluctuations to the carrier dynamics close to the edge. The amplitude of the fluctuations depends on the trajectory of the carriers, since the parity with respect to magnetic field reversal is not conserved. We thus demonstrate the chiral nature of the transport. We assume that the fluctuations of impedance originate in the scattering from localized states close to the sample edge. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A11.00012: Impurity-limited carrier transport in graphene nanoribbons C.W. Smith, Masahiro Ishigami We have measured the transport property of graphene nanoribbons as a function of impurity density in ultra high vacuum. Specifically, the impact of Coulomb and van der Waals impurities on the transport and source-drain gap of nanoribbons is investigated. Our results have direct consequences on fundamental science using graphene constrictions and graphene-based devices. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A11.00013: Electrical Transport Study of Suspended Graphene Nanoribbons Ming-Wei Lin, Kulwinder Dhindsa, Lezhang Liu, Qing Lan, Mark Ming-Cheng Cheng, Luis Agapito, Nicholas Kioussis, Zhixian Zhou Suspended graphene nanoribbon field effect transistors from unzipped multiwall carbon nanotubes have been fabricated. Electrical transport measurements show that current-annealing effectively removes the adsorbed impurities on the suspended graphene nanoribbons. Further increasing the annealing current creates a narrow constriction in the ribbons with non-negligible disorder, leading to the formation of a large band-gap and subsequent high on/off ratio. On the other hand, uniform suspended graphene nanoribbons with ultra-low-disorder reveal a high mobility exceeding 3000 cm2 V-1 s-1 and an intrinsic band gap. The width and length dependence of the electrical transport properties of ultra-low-disorder graphene nanoribbons with nearly atomically smooth edges will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A11.00014: Quantum Transport in Graphene Nanoribbon Networks Andr\'es Rafael Botello-M\'endez, Eduardo Cruz-Silva, Jos\'e Manuel Romo-Herrera, Florentino L\'opez-Ur\'Ias, Mauricio Terrones, Bobby G. Sumpter, Humberto Terrones, Jean-Christophe Charlier, Vincent Meunier Focusing on systems that can be realized experimentally, both in-plane conductance of inter-connected graphene nanoribbons and tunneling conductance in out-of-plane nanoribbon intersections are investigated. The quantum transport properties of such networks are computed using first-principles calculations based on the density functional theory formalism. The electronic transport through in-plane nanoribbon cross-points is found to be significantly affected by scattering at the intersections with the exception of all zigzag nanoribbon terminals arranged at a 60 degree angle. This result demonstrates the possibility of designing graphene nanoribbon networks capable of guiding electron along desired and predetermined paths. In addition, the electron transport properties of out-of-plane nanoribbons cross-points with realistic size are described within a simple tight-binding approach. The stacking angle is predicted to play a key role on the electronic transmission through nanoribbon networks. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A11.00015: Current Quantization in Corrugated Graphene Nanoribbons Upali Aparajita, Oleksiy Roslyak, Godfrey Gumbs, Danhong Huang Corrugated graphene provides such a phenomenon as curvature induced $p-n$ junction band gap opening and decoherence. We report yet another effect of current quantization in graphene nanoribbons via energy minigaps induced by the corrugation. Effects of edge roughness and long range charged scatterers on the quantization are investigated. Comparison is drawn with acoustically induced minigaps in carbon nanotubes [Talyanskii et al.,PRL 87,276082(2001)]. [Preview Abstract] |
Session A12: Graphene: Electronic Structure and Interactions - Adsorbates, Doping and Defects
Sponsoring Units: DCMPChair: Jian-Hao Chen, University of California, Berkeley
Room: 210C
Monday, February 27, 2012 8:00AM - 8:12AM |
A12.00001: Transport study of electrochemically decorated and intercalated graphene Dmitri K. Efetov, Kin Fai Mak, Yinsheng Guo, Tony F. Heinz, Louis Brus, Philip Kim Due to the surface-only properties of graphene, the decoration and/or intercalation of single, bi- and multi-layer graphene with foreign atoms can severely modify its electronic interactions, similar to those observed in its 3D counterpart the graphite intercalation compounds. Supported by a highly increased density of state due to a strong charge transfer above 10$^{14}$ cm$^{-2 }$into the graphene $\pi $-bands, certain adatoms are expected to induce strong electronic interactions to the graphenes own Dirac fermions, where theoretical predictions reach from the Kondo-effect and magnetism to as far as superconductivity in graphene. In this study we will present evidence of specific adsorption and intercalation of diverse atomic species by electrochemical means. We will present a detailed transport study, including resistivity-, Hall- and magneto-resistivity measurements of single-, bi- and multi-layer graphene devices which were subjected to electrochemical doping by a variety of electrolytes and ionic species such as Li$^{+}$, ClO$_{4}^{-}$, Cs$^{+}$, Ca$^{2+}$, etc. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A12.00002: Electronic detection of phase transition of adsorbed water on graphene Shudong Xiao, Michael Fuhrer Graphene is sensitive to overlayers on its surface through charge transfer and also through changes in dielectric constant, which can alter the scattering by static disorder in graphene. We performed transport measurement on graphene devices \textit{in situ} in ultra-high vacuum at low temperature. Water vapor is introduced to the chamber and is adsorbed on graphene at low temperature. After deposition of a few monolayers of water, the resistivity of graphene at fixed gate voltages is measured as the temperature is raised from 40K to room temperature. Sharp features in the temperature-dependent resistivity mark reproducible, irreversible (upon re-cooling) changes in the sample which we interpret as phase transitions in the adsorbed water overlayer, likely corresponding to dewetting and desorption. This work has been supported by the University of Maryland NSF-MRSEC under Grant No. DMR 05-20471 with supplemental funding from NRI. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A12.00003: \textit{GW} study of the effect of various defects on the band gap of fluorographene Young-Moo Byun, Jorge Sofo, Vincent Crespi Recently synthesized fluorographene, fully fluorinated graphene in a chair configuration, is a wide band-gap ($E_{g})$ semiconductor with an experimental optical band gap of $\sim$3 eV. However, first-principles calculations have shown that pristine fluorographene should have E$_{g}$ of 5.4 to 7.5 eV. To explain this discrepancy, we have studied the effect of F vacancies, a Stone-Wales (SW) defect, C single vacancies and C double vacancies on E$_{g}$ of fluorographene using density functional theory and the \textit{GW} approximation. F vacancies and a SW defect are not likely to affect $E_{g}$ of fluorographene, whereas a C single vacancy with a doubly fluorinated C atom, a C double vacancy, and a C double vacancy with two doubly fluorinated C atoms lead to a \textit{GW} band gap of $\sim$4 eV, which is consistent with the optically measured E$_{g}$, and they are energetically more favorable than other C vacancies at a wide range of chemical potential of F ($\mu_{F})$. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A12.00004: Controlled electrochemical functionalization of epitaxial graphene Calvin Chan, Thomas Beechem, Taisuke Ohta, David Wheeler, Keith Stevenson Chemical functionalization is a promising means of modifying graphene for applications ranging from nanoelectronics to transparent electrodes. Various schemes have been demonstrated, but control over functionalization density with well-specified molecules is still a challenge. We report on the controlled electrochemical functionalization of epitaxial graphene with trifluoromethylphenylene (CF$_3$Ph), where the functionalization density was controlled by the electron injection rate. CF$_3$Ph peaks were observed in x-ray photoemission spectroscopy, along with binding energy shifts consistent with bonding between CF$_3$Ph and graphene. A maximum functionalization density of one molecule per six graphene carbons was inferred from the peak intensities. Spectroscopic Raman mapping revealed increasing graphene D:G peak intensity ratios that scaled with increasing functionalization-induced localized defects. While a minimal shift in the $\pi$ orbital structure and the emergence of CF$_3$Ph related features were observed in ultraviolet photoemission spectroscopy, a work function increase by $0.5$ eV in CF$_3$Ph-graphene suggests a shift of the electron distribution towards the CF$_3$ moieties on the surface. This work has positive implications for transparent electrode applications. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A12.00005: Adsorption of NH$_{2}$ on Graphene in the Presence of Primary Defects Chad Junkermeier, Stefan Badescu, Thomas Reinecke The primary amine, NH$_{2}$, is of interest as a linker between graphene and organic molecules in novel biotechnologies using graphene platforms. We are using \textit{ab initio} electronic structure calculations to study NH$_{2}$ adsorption on graphene. We find that the adsorption energy on pristine graphene is on the order of 0.778 eV, a relatively weak bond. We are interested in situations in which the bonding of NH$_{2}$ is stronger and are studying systems in which NH$_{2}$ adsorbs near defects. We find the adsorption energy of a NH$_{2}$ molecule near a second NH$_{2}$ molecule is as high as 1.037 eV and that the adsorption near a substitutional N atom is 1.063 eV. We find that there is a RKKY-like interaction between the adsorbate molecules in the case of two NH$_{2}$. We will also give results for NH$_{2}$ adsorption near other defects. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A12.00006: Influence of subsurface hydrogen on the properties of single layer graphene grown on Ru(0001) Bogdan Diaconescu, Frank Hagelberg, Maxwell Grady, Karsten Pohl Graphene has aroused tremendous interest due to its remarkable electronic and mechanical properties. The lack of a band-gap, however, causes a serious challenge for implementing graphene as a material for electrical switches and therefore creative ways of inducing this band-gap are needed. We will present a STM/LEED/DFT study of the single layer graphene on Ru(0001) system in the presence of hydrogen. Structural studies show arrays of Moire superlattices with sizes ranging from 0.9 to 3.0 nm in the presence of hydrogen on the compact surface of ruthenium. First principle calculations help explain the appearance of these arrays of graphene reconstructions driven by the H presence at the Ru(0001) interface, and furthermore, predict the appearance of a bandgap with values correlated with the Moire superstructure sizes in the presence of hydrogen. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A12.00007: Graphene symmetry-breaking with molecular adsorbates: modeling and experiment M.A. Groce, M.K. Hawkins, Y.L. Wang, W.G. Cullen, T.L. Einstein Graphene's structure and electronic properties provide a framework for understanding molecule-substrate interactions and developing techniques for band gap engineering. Controlled deposition of molecular adsorbates can create superlattices which break the degeneracy of graphene's two-atom unit cell, opening a band gap. We simulate scanning tunneling microscopy and spectroscopy measurements for a variety of organic molecule/graphene systems, including pyridine, trimesic acid, and isonicotinic acid, based on density functional theory calculations using VASP. We also compare our simulations to ultra-high vacuum STM and STS results. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A12.00008: Overcoming Doping Difficulty in Graphene via Substrate Su-Huai Wei, Bing Huang, Hongjun Xiang Controlling the type and density of charge carriers by doping is the key step for developing graphene electronics. However, direct doping of graphene is rather challenge. Using first-principles method we find that doping could be strongly enhanced in epitaxial graphene grown on silicon carbide substrate. Compared to free-standing graphene, the formation energies of the dopants can decrease by as much as 8 eV. The type and density of the charge carriers of epitaxial graphene layer can be effectively manipulated by suitable dopants and surface passivation. More importantly, contrasting to the direct doping of graphene, the charge carriers in epitaxial graphene layer are weakly scattered by dopants due to the spatial separation between dopants and conducting channel. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A12.00009: Calculations of double-sided coverage of transition metals on graphene Timothy Hecht, Bret Hess We study the properties of transition metal atoms adsorbed in high coverage on graphene using first principles density functional theory. While there have been many studies on single-sided coverage of adatoms on graphene, we focus on coverage of both sides of graphene. We have observed systems with significantly stronger binding with double-sided coverage than systems with only single-sided coverage. We discuss the effect of double-sided coverage on the electronic structure of these systems. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A12.00010: Excited Carriers Relaxation and Hydrogen Dissociation on Hydrogenated Graphene: A Theory Junhyeok Bang, Y.Y. Sun, D. West, S.B. Zhang, S. Meng, Z.G. Wang, F. Gao Using \textit{ab initio} molecular dynamics coupled with time-dependent density functional theory (TDDFT), we show that the energy transfer of photo-excited carriers into atomic kinetic energy on hydrogenated graphene depends sensitively on the surface H coverage. Here, the energy transfer rate plays a crucial role in the determination of the H dissociation dynamics from graphene. In the low density ``isolated'' hydrogen atom limit, the energy transfer is significantly suppressed 80 fs after the excitation. Thus, it is difficult to dissociate hydrogen due to the faster energy dissipation from H into carbon backbone, despite that initially the H kinetic energy had increased to around 1.5 eV and the C-H bondlength had starched to 2.4 {\AA}. In sharp contrast, at the high-density graphane limit, an efficient energy transfer channel is established when the C-H bondlength exceeds 1.4 {\AA}. A fraction of the H readily dissociates within 15 fs. This is because ionized H forms a charged layer that expels, and as such accelerates the H ions with higher initial thermal velocities flying away. Our study thus reveals the importance of performing TDDFT calculations for excited carrier dynamics as from the widely adopted ground-state or constrained DFT dynamics one would expect the C-H bonds in graphane to be significantly stronger, due to full surface passivation, than that of isolated H. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A12.00011: Theory of hydrogen induced giant spin-orbit coupling in graphene Martin Gmitra, Denis Kochan, Jaroslav Fabian Adatoms seem for now the most perspective way of increasing and controlling spin-orbit coupling in graphene. Hydrogen in articular is a role representative, as it gives both lattice deformation and covalent bonding, both contributing towards sigma-pi hybridization needed for the increase of the spin-orbit coupling around K. To establish the relevant physics of the H induced spin-orbit coupling in graphene, we have performed systematic calculations of hydrogenated graphene. We found that the magnitude of the coupling is of the order of meVs, exactly what is needed to explain the experimental data on spin relaxation. Doing both first-principal calculations and tight-binding modeling we calculate the spin-orbit splittings and introduce an effective hopping model that can be used in realistic investigations. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A12.00012: Understanding electron-phonon interactions in doped graphene: the case of Li-intercalated graphite Andrew Walters, Mark Dean, Christopher Howard, Mark Ellerby, Jonathan Payne, Michael Krisch, Alexei Bosak, Gianni Profeta, Matteo Calandra, Francesco Mauri The recent explosion of research on doped graphene systems together with the discovery of superconductivity in CaC$_6$ has reignited the interest in graphite intercalation compounds (GICs). While it is generally agreed that the superconductivity observed in GICs is BCS-like, there is still much controversy over which electrons and which phonons are primarily involved in the electron-phonon (e-ph) coupling leading to superconductivity. Moreover, thanks to the close similarity between the electronic structure of GICs and doped graphene, the study of e-ph interactions in GICs provides a unique approach to help elucidate the complex e-ph interactions in doped graphitic systems. We present inelastic x-ray scattering measurements of the high energy ($\sim$ 200 meV) graphitic phonons in LiC$_6$ across the Brillouin zone. The LiC$_6$ phonons are much softer than in pure graphite, as the electron doping destabilizes the C-C bonds. We observe large phonon broadening for all phonons at the graphite Brillouin zone center, suggestive of unusual e-ph interaction phenomena. We discuss our results in the light of the e-ph coupling reported from angle-resolved photoemission spectroscopy and in relation to strong non-adiabatic effects observed using Raman scattering. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A12.00013: p-n codoping induced improvement of adsorption, magnetism, and electronic structure in 3d transition metal adatoms on graphene Shifei Qi, Zhenyu Zhang, Xiaohong Xu Generating ferromagnetism and preserving its unique properties in graphene are crucial to the development of graphene-based spintronics. Using first-principles calculations, we investigate the effects of p-n codoping method on absorption, magnetic properties, and of electronic structures 3d transition metal adatoms (TMs, i.e., Fe, Co, and Ni) on graphene. It is found that p-n codoping can strengthen the adsorption of TMs on graphene, and enhance the magnetic moments of Fe and Co adatoms on graphene. It can also cause Ni to transition from nonmagnetic to magnetic states. Furthermore, magnetic coupling between two p-n pairs is also explored. Electronic structure analysis indicates that p-type dopant turns graphene into an electron-deficient system, and compensates for the shift in Fermi level caused by adsorption of TMs. Therefore, p-n codoping can bring about increases in the magnetic moment and adsorption of TM-adsorbed graphene systems while preserving the unique properties of graphene. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A12.00014: Directed Assembly of Molecules on Graphene/Ru(0001) L.Z. Zhang, H.G. Zhang, J.T. Sun, Y. Pan, Q. Liu, J.H. Mao, H.T. Zhou, T. Low, H.M. Guo, S.X. Du, H.-J. Gao Recently, the graphene monolayers have been seen to adopt a superstructure - moir\'e pattern - on Ru(0001). By using low temperature scanning tunneling spectroscopy, we identified the laterally localized electronic states on this system. The individual states are separated by 3 nm and comprise regions of about 90 carbon atoms. This constitutes a highly regular quantum dot-array with molecular precision. It is evidenced by quantum well resonances with energies that relate to the corrugation of the graphene layer. By using scanning tunneling microscopy/spectroscopy, we demonstrate the selective adsorption and formation of ordered molecular arrays of FePc and pentacene molecules on the graphene/Ru(0001) templates. With in-depth investigations of the molecular adsorption and assembly processes we reveal the existence lateral electric dipoles in the epitaxial graphene monolayers and the capability of the dipoles in directing and driving the molecular adsorption and assembly. When increasing the molecular coverage, we observed the formation of regular Kagome lattices that duplicate the lattice of the moir\'e pattern of monolayer graphene. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A12.00015: Study on the Adsorption of Small Gas Molecules on graphene by the Density Functional Theory Calculations Yiming Mi, Xinxin Zhao, Shuichi Iwata The absorption of different small gas molecules on graphene is investigated based on the pseudopotential method within the density functional theory formalism. The preferred adsorption site (among the top, bridge, and hollow positions) and orientations of these molecules on the graphene surface are analyzed and the related adsorption energies are calculated. The charge transfer between the absorpted molecules and the graphene is discussed as well. [Preview Abstract] |
Session A13: Focus Session: Magnetic Nanostructures-Patterned Nanostructures and Nanowires
Sponsoring Units: DMP GMAGChair: Andrew Baruth, University of Minnesota
Room: 211
Monday, February 27, 2012 8:00AM - 8:12AM |
A13.00001: High Frequency Excitation of Nanometer-Scale, Strongly Coupled FM / NM / FM Disks Javier Pulecio, Peter Warnicke, Shawn Pollard, Dario Arena, Yimei Zhu There is great interest in the manipulation of magnetic domains in nanostructures from both a fundamental and applications perspective. In particular, the use of resonant frequency excitations permits a power reduction of the driving forces necessary to induce detectable motion in magnetic vortex structures. Here we present an experimental and numerical study of patterned tri-layered disk stacks which are composed of 25nm Permalloy$\vert $1nm Copper$\vert $15nm Permalloy, excited at resonance, ranging from 250-500nm in radii. In-situ Lorentz microscopy was used to acquire time averaged real space images of the vortices' gyrotropic motion and micromagnetic simulations were implemented to further understand the coupled dynamics between the ferromagnetic layers across the thin non-magnetic spacer layer. We discuss the effects of interlayer coupling on the vortex trajectories and resonant frequencies for the individual ferromagnetic layers. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A13.00002: Controlling the formation process of vortex states in magnetic nanodots with asymmetric geometry Mi-Young Im, Peter Fischer, Andreas Vogel, Guido Meier Magnetic vortex structures in nanoscale elements are currently highly attractive, since they offer exciting topological spin configurations to study on a fundamental level nanoscale spin behavior and they show great potential for applications in data storage and memory technologies. One of the primary questions is to tailor the nucleation process of vortex structures, which would open the opportunity for the control of magnetic/spin phenomena in magnetic vortices such as the resonant motion of a magnetic vortex core, etc. We have investigated the stochastical character of the formation process of vortex states in permalloy (Ni$_{80}$Fe$_{20})$ nanodots with different geometries by direct imaging of vortex structures with high resolution magnetic transmission soft X-ray microscopy (MTXM). We observe that the formation process of vortex state sensitively depends on the geometry of nanodots. Based on our experimental result, we will discuss the possibility to control the generation process of magnetic vortex states by directed modifications of the geometry of nanodots. \\[4pt] References: \\[0pt] T. Shinjo, et al., Science \textbf{289}, 930 (2000). \\[0pt] M.-W. Yoo, et al., Phys. Rev. B \textbf{82}, 174437 (2010). \\[0pt] P. Fischer, et al., Phys Rev B \textbf{83} 212402 (2011) \\[0pt] H. Jung, et al., NPG Scientific Reports 1 59 (2011) \\[0pt] M.-Y. Im, et al., Phys Rev Lett 102 147204 (2009) [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A13.00003: Quantum depinning of the magnetic vortex core in micron-size permalloy disks Ricardo Zarzuela, Sa\"ul V\'elez, Joan Manel Hernandez, Javier Tejada, Valentyn Novosad The vortex state being characterised by an in-plane closed flux domain structure and an out-of-plane magnetization at its centre (known as the vortex core) is one of the magnetic equilibria of thin soft ferromagnetic micron-size dots. The vortex core is a mesoscopic object and so it is a suitable candidate to observe quantum tunneling of its magnetic moment between classically stable magnetic configurations. For the first time, we report experimental evidence of quantum dynamics of the vortex core of micron-size Permalloy (Fe$_{19}$Ni$_{81})$ disks induced by the application of an in-plane magnetic field. It is attributed to the quantum tunneling of the vortex core through pinning barriers, which are associated to structural defects in the dots, towards its equilibrium position. The crossover temperature from the thermal to the quantum regime is obtained within the framework given by the Caldeira-Leggett theory. Comparison between experiments and theory points to tunneling of the vortex core by steps of the order of 0.3 nm and gives estimates to the parameters characterising the pinning barriers. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A13.00004: Energy barriers for vortex nucleation and annihilation in sub-100 nm magnetic dots Andrew T. King, Igor V. Roshchin Understanding energy barriers involved in nucleating and annihilating magnetic vortices in nanodots is important for magnetic memories and nano-oscillators. We used a ``rigid-vortex approximation'' and micromagnetic approach to calculate the total magnetic energy of a nanodot for various magnetic configurations. This was done for 20 nm-thick iron nanodots with different diameters (30, 40, 65, and 80 nm) as a function of applied magnetic field. By analyzing the energy landscape for different magnetic configurations, we calculated the energy barrier for switching from the vortex to the single-domain state (vortex annihilation) and the converse (vortex nucleation). The applied fields required to overcome these two barriers are compared to those obtained from the simulations directly and to the experimental values.\footnote{R. K. Dumas, \textit{et. al.}, Appl. Phys. Lett. \textbf{91}, 202501 (2007).} The role of the thermal fluctuations in the temperature dependence of these critical fields will be discussed by comparison of the energy barriers with the thermal energy, kT. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A13.00005: A Novel Non-lift-off Block Copolymer Nanolithography Technique for Etch-damage Susceptible Magnetic Materials A. Baruth, A. Shankar, K. Walster, M.D. Rodwogin, M.J. Erickson, M.A. Hillmyer, C. Leighton Nanolithographic techniques based on self-assembled block copolymer templates offer exceptional potential for fabrication of large-area nanostructure arrays from a wide variety of functional materials. Despite significant progress with control of the template ordering and development of pattern transfer schemes, significant issues exist with common techniques such as lift-off and etching. Here, we demonstrate successful execution of a nanolithographic process based on climate-controlled solvent annealing of easily degradable cylinder-forming poly(styrene-$b$-lactide) block copolymer films that avoids both lift-off, and some of the most challenging aspects of etching. In particular, our overfill/planarize/etch-back scheme leads to retention of robust ferromagnetism even in 24 nm diameter dots of a material (Ni$_{80}$Fe$_{20})$ that is both magnetically soft and susceptible to etch damage. The result is a large-area array of 24 $\pm $ 1.6 nm diameter magnetic nanodots with exceptional hexagonally-close-packed long range order that retain their crystallinity and $\sim $ 70 {\%} of the bulk magnetization. Extensive diffraction, microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation and fidelity. Funded by NSF MRSEC. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A13.00006: Quenching of the initial ac Susceptibility in Single Domain Ni Nanobars Zheng Gai, Xiaoguang Zhang, Ivan I. Kravchenko, Scott T. Retterer, J. Wendelken The ac susceptibility measurement probes the dynamic properties of a magnetic material, which is believed to consist of magnetization rotation and domain wall motion contributions. Here we report the observation of a complete quenching of the initial ac susceptibility for a single domain Ni nanobar array, when the ac field is aligned with the long axis of the bars. The vanishing of the susceptibility in one direction is a unique nanoscale phenomena, allowing an unambiguous determination of the magnetic state of the nanostructure and a clean separation of different contributions to its dynamic properties. For example, an unambiguous determination of the temperature dependent surface anisotropy energy is obtained when the field is applied perpendicular to the long axis. This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A13.00007: Microwave assisted magnetization reversal in cylindrical antidot arrays with in-plane and perpendicular anisotropy Mehmet Yumak, Kerim Ture, Gulen Aktas, Victor Vega, Victor Prida, Carlos Garcia Porous anodic alumina is a particularly attractive self-ordered system used as template to fabricate nanostructures. The anodic film contains a self-ordered hexagonal array of parallel pores with tunable pore size and interpore distance, and whose pore locations can be templated. Deposition of magnetic films onto porous alumina leads to the formation of porous magnetic films, whose properties differ significantly from those of unpatterned films. The study of antidot arrays has both technological and fundamental importance. Although porous alumina films are typically synthesized in a planar geometry, in this work we deposited NiFe and Ti/CoCrPt magnetic films with in-plane and out-of-plane anisotropy onto cylindrical-geometry porous anodic alumina substrates to achieve cylindrical antidot arrays. The effect of both, the magnitude of the AC current and the circular magnetic field on the magnetization reversal has been studied for in-plane and perpendicular anisotropies. The level of reduction in the switching field was found to be dependent on the power, the frequency of the microwave pulses and the circular applied magnetic field. Such a reduction is associate with the competition between pumping and damping processes. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A13.00008: DC and High-frequency Magnetic Properties of Patterned Ferromagnetic Nanostructures Huajie Ke, Yitzi Calm, Mark Tuominen Magnetic mesoscopic and nanostructures have promising applications such as high-density data storage, magnetic field sensors, and microwave devices. Patterned magnetic structures are especially interesting because their constitutive material, sizes and geometry are easily adjustable in fabrication. This work aims to study dc and radio frequency magnetic properties of Co and permalloy patterned structures and the effect of magnetic coupling. We use electron-beam lithography and complementary techniques to ferromagnetic nanostructures with various separations to control the strength of magnetic interaction. SQUID and complimentary MFM characterization are performed to observe the dc magnetic properties. AC susceptibility is used to investigate the low frequency response. Microstrip transmission lines are then incorporated to measure the scattering parameters between 300kHz and 6GHz. The equivalent RLCG circuit elements can be extracted to obtain the effective magnetic permeability for different ferromagnetic structures. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A13.00009: Precise control of vortex chirality and polarity in ``Pac-Man''-like magnetic nanodots by in-plane magnetic field Vladimir Cambel, Jaroslav T\'{o}bik, Goran Karapetrov Here we explore size-dependent magnetic states of sub-100 nm Permalloy nanomagnets of specific geometry. The geometry is suitable for independent setting and readout of vortex polarity and chirality by applying \emph{in-plane magnetic fields} only. Micromagnetic calculations show that in ``Pac-Man''-like magnetic nanodots the relaxation channels to specific chirality and polarity states from uniform magnetization state are deterministic and are not influenced by the presence of moderate out-of-plane fields. The particular geometry opens straight channel for magnetization relaxation towards stable closure-domain vortex state with specific chirality and polarity. We explore a wide geometrical phase space in search for stable and predictable remanent vortex configurations. We find that in these nanomagnets the write process is simple and the signal is easily readable. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A13.00010: Direct imaging of complex domain walls and chirality sensors in magnetic nanostripes S.R. Bowden, J. Unguris, Y. Jang, M. Mascaro, C.A. Ross Domain walls (DWs) in patterned ferromagnetic nanostripes are increasingly being considered for non-volatile and radiation hard data storage and logic applications. We use scanning electron microscopy with polarization analysis (SEMPA) to image the formation of complex domain walls in nanostripes and local patterned structures used for sensing DW chirality. The DWs studied have a transverse orientation, where the in-plane spin direction of a 180\r{ }DW is perpendicular to the nanostripe axis. We demonstrate a technique where two 180\r{ }DWs of alternating chirality may interact to form a stable DW with 360\r{ } rotation as opposed to DW annihilation. Higher order DWs with n$\pi $ rotation are demonstrated, where n is an integer number of interacting 180\r{ }DWs. The detection of moving 180\r{ }DWs via external fields is studied by placing patterned magnetic triangular elements above and below the nanostripe in-plane. As the DW propagates across the wire, the stray field interacts and switches the magnetization of the triangles. The chirality of the DW may be sensed by designing the triangles to respond to the inherent asymmetry of the DW's stray field. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A13.00011: A high-spin atomic 1-D system with a spin-induced CDW instability Nader Zaki, Chris Marianetti, Peter Johnson, Richard Osgood We report on low-temperature STM measurements of a Co atomic-wire system that has been realized by the technique of self-assembly on a vicinal Cu(111) substrate [1,2]. We show that for this bimetallic case, the Co-wire system undergoes a CDW instability leading to a 1-D high-spin system. This type of instability does not appear to have been previously reported for a bi-metallic system, particularly for a chain of Co atoms. Using ab initio theoretical calculations, it is deduced that the CDW instability is spin-induced by way of symmetry breaking in the spin population. This result presents a fundamental electronic-structure mechanism for CDW instability that is distinct from previously reported metal-semiconducting systems [3], in that spin clearly plays an essential role in lowering the energy of the system. Furthermore, the calculations indicate that the high-spin correlated state of the constituent Co atoms is a necessary consequence of this CDW instability. Finally, the ferromagnetic nature of this realized system raises questions with regard to substrate spin mediation, such as the possible role of Kondo and RKKY interaction. [1] N. Zaki et al, Phys. Rev. B 80, 155419 (2009) [2] N. Zaki et al, Phys. Rev. B 83, 205420 (2011) [3] P. C. Snijders and H. H. Weitering, Rev. Mod. Phys. 82, 307 (2010) [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A13.00012: Spin resonance in Luttinger liquid with spin-orbit interaction Oleg Tretiakov, K.S. Tikhonov, V.L. Pokrovsky The spin-orbit interaction leads to a narrow spin resonance at low temperatures, even in the absence of an external magnetic field [1]. We study the effect of electron-electron correlations on the resonance. These correlations are strong in quantum wires and cannot be neglected. We show that the electron correlations change the shape and width of the resonance and produce an additional weak resonance at the plasmon frequency. \\[4pt] [1] Ar. Abanov, V. L. Pokrovsky, W. M. Saslow, and P. Zhou, arXiv:1008.1225. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A13.00013: Telegraph Noise in LSMO Nanowires Nicholas Bronn, Xiaoqian Chen, Jason Hoffman, Anand Bhattacharya, Peter Abbamonte, Nadya Mason Hole-doped manganites with the perovskite structure exhibit a variety of superlative properties because of close competition among ferromagnetic metallic, paramagnetic insulating, as well as various charge, spin, and orbitally ordered phases. We have recently observed random telegraph noise (RTN) in low-temperature conductance measurements of epitaxially-grown La$_{2/3}$Sr$_{1/3}$MnO$_3$ nanowires patterned by electron-beam lithography and ion milling to widths of $\sim$ 80nm. The RTN is apparent at temperatures less than 30K. It is thought that the RTN is the result of domain fluctuations, which are more clearly observable in such narrow wires. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A13.00014: Direct Imaging of Non-Adiabatic Spin Torque Effects on Vortex Core Orbits Shawn Pollard, Lei Huang, Kristen Buchanan, Dario Arena, Yimei Zhu Recently high frequency, current induced vortex motion has received a great deal of interest from a spintronic perspective, as it suggests a possible low power, high speed writing process. However, understanding the processes that govern this motion, specifically the relative contributions of adiabatic and non-adiabatic spin torque effects, has been difficult due to experimental constraints. We developed a novel TEM sample stage in which we apply high frequency currents in-situ to excite resonant motion in Permalloy disc structures (2000x2000x50nm) with high spatial resolution ($<$5nm for dynamic measurements). We have imaged the time-averaged vortex trajectory through resonance. We find that the orbital amplitudes are drastically different for clockwise and counterclockwise chiralities, indicating the presence of both Oersted fields and non-adiabatic spin torque effects, and that the orbital size scales linearly with current density varied between (7.1-10.0)x10$^{10}$ A/m$^{2}$. These results allow us to extract a value for the non-adiabatic spin torque with unprecedented precision. Additionally, we report on off-resonance effects, such as tilting and variations in the ellipticity of the orbit as it is swept through resonance, with first of their kind experimental observations. [Preview Abstract] |
Session A14: Focus Session: Spins in Semiconductors - Magnetic Semiconductors I
Sponsoring Units: GMAG DMP FIAPChair: Paul Crowell, University of Minnesota
Room: 212
Monday, February 27, 2012 8:00AM - 8:12AM |
A14.00001: Oxygen Annealing Studies of SnO$_{2}$:Co Thin Films Deposited by RF Sputtering Gratiela Stoian, P.A. Stampe, R.J. Kennedy, Y. Xin, S. von Molnar We report on post-deposition oxygen annealing studies of SnO$_{2}$:Co thin films to examine the origin of the room temperature ferromagnetism (RTFM) observed in such materials. Materials are deposited on r-cut sapphire substrates by RF sputtering from a doped target with 5 at.{\%} Co nominal concentration. Magnetization measurements reveal that as-grown samples in Ar atmosphere are non-magnetic at RT. However, by annealing them in low O$_{2}$ pressure (10$^{-4 }$- 2x10$^{-4}$ Torr), the saturation moment increases to $\sim $0.78 $\mu _{B}$/Co at RT, somewhat lower than the expected value for Co$^{2+}$ ions. This verifies that the Co ions are incorporated in the matrix. X-ray diffraction data show a decrease in crystallinity for the most magnetic samples annealed in O$_{2 }$at 2x10$^{-4}$ Torr. To confirm this, further structural and temperature-dependent magnetic measurements for various annealing protocols are underway to determine the nature of magnetism in SnO$_{2}$:Co sputtered thin films. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A14.00002: Embedded Ferromagnetic GdN Nano-Islands in GaN by Molecular Beam Epitaxy T.F. Kent, J. Yang, L. Yang, M.J. Mills, R.C. Myers Nano-islands of GdN are embedded into a GaN matrix by plasma-assisted molecular beam epitaxy. X-ray diffractometry shows that the cubic rocksalt islands are (111) oriented to the c-axis of the hexagonal wurtzite GaN matrix. Cross-sectional scanning transmission electron microscopy allows for the study of island formation, which occurs after 1.2 monolayers of GdN coverage, forming discrete particles, which the GaN matrix grows epitaxially around. Magnetometry reveals two ferromagnetic phases, one due to the GdN particles with Curie temperature of 70K and an anomalous phase with ferromagnetism persistent to room temperature. This room temperature ferromagnetic phase is strongly anisotropic, with out of plane magnetization nearly 300{\%} larger than in-plane at fields less than 1T. Optical characterization reveals that GdN, when strained to GaN, is a semiconductor with direct and indirect bandgaps at 1.2eV and 0.75eV, respectively. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A14.00003: Synthesis and Characterization of Co-doped ZnO Dilute Magnetic Semiconducting Nanorods N. Das, S. Khanra, S. Bhamidipati, K. Manivannan, P. Kahol, K. Ghosh Transition-metal doped ZnO dilute magnetic semiconducting nanomaterials are considered as ideal systems for carrying out research in the field of spintronics as they can successfully combine magnetism and electronics in a single substance. ZnO is a wurtzite-type wide-bandgap semiconductor of the II-VI semiconductor group with band gap energy of 3.37 eV. Hydrothermal synthesis of undoped ZnO and Co-doped ZnO nanorods is carried out using aqueous solutions of Zn(NO$_{3})_{2}$.6H$_{2}$O, Co(C$_{2}$H$_{3}$OO)$_{2}$.4 H$_{2}$O, and using NH$_{4}$OH as hydrolytic catalyst. Nanomaterials of different sizes and shapes were synthesized by varying the process parameters such as molarity (0.15M, 0.3M, 0.5M) and pH (8-11) of the precursors, growth temperature (130$^{\circ}$C), and annealing time during the hydrothermal Process. Structural, morphological, optical and magnetic properties are studied using various techniques such as XRD, SEM, UV-vis spectroscopy, and SQUID magnetometer. XRD and SEM studies reveal nanorods with hexagonal wurtzite structure with length in the range of 200 to 500 nm, and cross section in the range of 30 to 60 nm. Detailed structural, optical, and magnetic properties will be discussed in this presentation. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A14.00004: Defect-Induced High-$T_{C}$ Ferromagnetism and Spinodal Nanodecomposition in MgO Masayoshi Seike, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida Based on a first-principles study of the magnetic properties of MgO with Mg vacancies [1,2], we calculated the electronic structures and exchange coupling constants for Monte Carlo Simulation (MCS) of the Curie temperature (\textit{Tc}). We also performed MCS of the spinodal nanodecomposition based on the calculated chemical pair interactions between the vacancies. In this study, it was found that hole-doping by Mg vacancies leads to a ferromagnetic ground state, invoking long-range magnetic interaction, and that \textit{Tc} can reach room temperature at sufficient vacancy concentrations of 15 at.{\%} under a homogeneously distributed condition. However, it was also found that the chemical pair interactions between vacancies are significantly attractive and that the system can form super-paramagnetic clusters of vacancies with strong ferromagnetic coupling in the clusters. These results suggest that, by the spinodal nanodecomposition, the \textit{Tc} or blocking temperature ($T_{B})$ can be enhanced and reach room temperature at smaller vacancy concentrations compared with those estimated for room-temperature ferromagnetism under the homogeneous distribution condition.\\[4pt] [1] M. Seike, et al. Jpn. J. Appl. Phys. 50, 090204 (2011).\\[0pt] [2] K. Sato, et al. Rev. Mod. Phys. 82, 1633 (2010). [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A14.00005: Gd in GaN: the role of magnetic vacancy clusters Alexander Thiess, Peter H. Dederichs, Stefan Bl\"ugel, Walter R.L. Lambrecht Five years after the experimental discovery of colossal magnetic moments and ferromagnetic ordering above room temperature in gadolinium doped gallium nitride the identification of its microscopic origin is still not accomplished. Here, we are proposing a new model explaining the origin: the clustering of magnetic gallium vacancies. First, we show that such clustered gallium vacancies indeed can preferentially occur by utilizing a simplified growth model, for which we provide the input by large-scale first-principles Green function calculations. The same calculations reveal that the dangling bond nitrogen states around gallium vacancies become significantly spin-polarized. Moreover, we are able to extract a rich set of information on the magnetic exchange interactions between those spin-polarized atoms. These exchange interactions are the basis for our study on the thermal behavior of magnetic vacancy clusters by means of Monte-Carlo simulations. We present the resulting magnetic properties of our simulations and highlight important similarities to the experiment that all point at gallium vacancy clusters as the origin of the experimentally observed magnetic properties in GaN:Gd. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A14.00006: Photoluminescence studies of various magnetic phases in (Zn,Mn)Te/ZnSe QDs Joseph Murphy, Lars Schweidenback, Biplob Barman, Rafal Oszwaldowski, Alexander Cartwright, Bruce McCombe, Athos Petrou, Igor Zutic, Ian Sellers, Wen Chung Chou, Wu-Ching Fan We have carried out a magneto-PL study of a single layer of (Zn,Mn)Te/ZnSe quantum dots. The CW PL from a previously studied 5 layer sample exhibits high circular polarization but very small Zeeman splitting. Time-resolved PL exhibited a temperature independent temporal red shift, associated with the magnetic polaron formation. These data have been interpreted as due to the presence of a antiferromagnetic phase for the Mn ions. In the present study, the CW magneto-PL spectra exhibit high circular polarization, but significantly larger Zeeman splittings. Furthermore the Zeeman splitting depends strongly on temperature and vanishes at 50 K. These results indicate the presence of a different magnetic phase for the Mn ions in the single layer QD sample. Time-resolved PL experiments suggest the formation of magnetic polarons in this sample. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A14.00007: Controlling Curie Temperature of (GaMn)As Through Location of the Fermi Level Within the Impurity Band Invited Speaker: Margaret Dobrowolska This talk will address the unresolved issue of ferromagnetism in the ferromagnetic semiconductor GaMnAs, a material whose understanding is of central importance to semiconductor-based spintronics. The above unresolved issue concerns the question of whether the ferromagnetic order in GaMnAs is mediated by valence band holes or by holes residing in the impurity band. The results to be presented are based on the investigation of a wide range of GaMnAs samples using a comprehensive set of experiments that include magnetization, electrical transport and magneto-optics, along with studies of microscopic composition by channeling Rutherford back-scattering and particle-induced x-ray emission. These experiments show unambiguously that the holes underlying ferromagnetic order in GaMnAs reside in the impurity band; and that it is not only the concentration of Mn and of holes, but also the specific location of the Fermi level in the impurity band that establishes the Curie temperature of this material. Specifically, we show that having the Fermi level near the middle of the impurity band, where the states are most extended, is more important for raising Tc than increases in the effective Mn or hole concentrations. Since the location of the Fermi level can be controlled by a variety of means both during and after growth, this new understanding automatically provides practical guidelines for increasing the critical temperature. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A14.00008: Interstitial Cu Codoping Method for High Curie Temperature (Ga,Mn)As Hitoshi Fujii, Kazunori Sato, Hiroshi Katayama-Yoshida, Lars Bergqvist, Peter H. Dederichs Based on first principles calculations, we propose a solubility control method of magnetic impurities in dilute magnetic semiconductors (DMSs). We show that donor atoms, such as Cu, introduced at the interstitial sites in GaAs enhance the solubility of Mn. As a result, Mn can be doped to more than 20 percent in GaAs in the thermal equilibrium condition [1]. Due to the carrier-induced nature of the ferromagnetism in DMSs, the ferromagnetism is suppressed because of the compensation of hole from Mn acceptors by the codoped interstitial Li or Cu. In order to recover the ferromagnetism, we propose low temperature annealing after crystal growth to remove only the interstitials. Our NEB(Nudged Elastic Band method) calculation results show that the effective migration barrier of Cu in GaMnAs is about 0.2eV. This value is small compared with the migration barrier of Li in GaMnAs (about 0.5eV). Even if Li, it is possible to diffuse Li in (Ga$_{0.7}$,Mn$_{0.3}$)As at 0.12 micron in 24 hours [2]. In case of Cu, therefore, we can expect further annealing distance than Li case because of the low migration barrier.\\[4pt][1] H. Fujii, et al.:Appl. Phys. Express 4 (2011) 043003.\\[0pt] [2] L. Bergqvist, K. Sato: Phys. Rev. B 83, 165201 (2011) [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A14.00009: Tight binding study of single ion magnetic anisotropy of $\rm{Mn^{2+}}$ in Ga(Mn)As Hemachander Subramanian, Jong Han Bulk uni-axial magnetic anisotropy of Ga(Mn)As observed in experiments has not been well understood as much as cubic magnetic anisotropy in the same material. We propose that the uni-axial anisotropy arises due to the coupling of local lattice distortions around $\rm{Mn^{2+}}$ impurity ion to its spin state through spin-orbit coupling of holes bound to the impurity ion. We model the coupling using nearest-neighbor tight-binding and many-body perturbation theory. The model includes intra-atomic Coulomb interaction inside $\rm{Mn^{2+}}$ ion, spin-orbit interaction of holes at the $\Gamma$ point, $p-d$ hopping interaction between $\rm{Mn^{2+}}$ ion $d$ orbitals and As ion $p$ orbitals, and strain due to local lattice distortions. We observe breaking of tetrahedral symmetry around the $\rm{Mn^{2+}}$ ion when the system is paramagnetic. We explore the effect of this broken symmetry in stabilizing certain magnetization directions through spin-orbit coupling in the ferromagnetic regime. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A14.00010: Infrared to visible magneto-optical effects in (Ga,Mn)As N. Tesarova, C.T. Ellis, G. Acbas, A. Mukherjee, K. Vyborny, J. Cerne, J. Subrt, T. Ostatnicky, P. Nemec, V. Novak, J. Sinova (Ga,Mn)As is perceived as a model material for future low-power spintronics devices due to its carrier mediated ferromagnetic properties. Despite the extensive theoretical and experimental studies, the energy band structure and the mechanism of ferromagnetic order (of Mn spins) still remains a matter of controversy [Ohya, Nature Physics 2011; Jungwirth, PRL 2010]. In our study, we employ magneto-optical Faraday and Kerr measurements to probe the character of the states near the Fermi energy, which is expected to be responsible for Mn-ordering. We also study the spectral dependence of magnetic linear dichroism that is mainly sensitive to the states mediating the Mn-Mn interaction [Kimel, PRL 2005]. The measurements are performed from the infrared to visible range (100~-- 2600~meV) on high quality samples with different Mn concentration (1.5 -- 14~{\%}) with Curie temperatures up to 190~K. The results are compared with the predictions of $k.p$ theory with antiferromagnetic $p-d$ exchange. We acknowledge the financial support provided by NSF-DMR1006078 and Faculty of Mathematics and Physics, Charles University in Prague. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A14.00011: Spin-Transfer-Torque Driven Domain Wall Motion in (Ga,Mn)(As,P) E.K. Vehstedt, L.P. Zarbo, K. Vyborny, E. De Ranieri, H.G. Katzgraber, J. Wunderlich, T. Jungwirth, J. Sinova Precise control of domain wall (DW) motion in magnetic materials is a prerequisite for the realization of novel non-volatile and down-scalable logic/memory devices which promise to overcome the limitations of current technologies. While magnetic fields are the obvious choice for DW manipulation, in spin-orbit (SO) coupled materials, electric fields provide an additional means of control via current-induced spin torque. We extend the existing theoretical framework used to describe magnetization dynamics in uniform ferromagnets (FM) to dilute FM semiconductors. Analogous to the study of homogeneous systems, we compute the current-induced internal fields (CIF) corresponding to the spin torques and perform a quantitative analysis of the effect of CIFs on DW motion by solving the phenomenological Landau-Lifshitz-Gilbert equations. Microscopic calculations based on an accurate description of the SO coupling effects are used to estimate the observed anisotropies. Our theoretical efforts are complemented by experimental studies in the SO coupled FM (Ga,Mn)(As,P). [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A14.00012: Magnetic properties of Ga1-xMnxAs/Ge heterojunctions Jonathan Leiner, Xinyu Liu, Jacek Furdyna, Margaret Dobrowolska It has been shown that the incorporation of Mn in substitutional and interstitial positions of GaAs is linked to the Fermi level of GaMnAs during the growth [1]. Additionally, experiments with samples that are doped with either Be or Si reveal the role of the Fermi level in determining Tc [2]. To further investigate this effect, we deposited Ge in varying thicknesses on top of GaMnAs layers. Germanium is very nearly lattice matched to GaMnAs, and the valence band offset of the two materials ($\sim$0.54 eV) places the top of the valence band as well as the Mn acceptor level of GaMnAs significantly below the top of the valence band of Ge. Thus, when Ge is grown on GaMnAs, the incorporation of Mn has already been fixed during its growth, but the holes are drained off into Ge. SQUID measurements on these samples show that the Tc of the GaMnAs drops very rapidly when layers of Ge are deposited over it, the decrease in Tc scaling roughly with the thickness of the Ge layers. This behavior is consistent with the expected ``draining away'' of holes from the GaMnAs layer into the Ge. Results of our efforts to fine-tune the amount of holes removed from GaMnAs by Ge will be presented.\\[4pt] [1] Wojtowicz et al., Physica E 25, 171 (2004).\\[0pt] [2] Cho et al., JAP 103, 07D132 (2008); APL 93, 262505 (2008). [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A14.00013: Spin-dependent Transport in GaAs/MnAs Core/shell Nanowires Jing Liang, Jian Wang, A. Paul, B.J. Cooley, D.W. Rench, N.S. Dellas, S.E. Mohney, R. Engel-Herbert, N. Samarth Hybrid GaAs/MnAs core/shell nanowires synthesized by molecular beam epitaxy [APL {\bf 97}, 072505 (2010)] are of potential interest for proof-of-concept room temperature nanospintronics applications. Magnetic order in these nanostructures is directly influenced by a unique competition between the magnetocrystalline and shape anisotropies in MnAs. We report four probe measurements of the temperature dependence of the resistivity and the anisotropic magnetoresistance (AMR) in single nanowires over a temperature range 1 K - 300 K and in magnetic fields ranging up to 80 kOe, applied both parallel and perpendicular to the nanowire axis. We used the measured AMR in conjunction with micromagnetic simulations to reveal the detailed magnetization reversal process in the MnAs nanoshell. The micromagnetic simulations also provide insights into interesting structures for spin engineering at the nanoscale. Supported by NSF-MRSEC and ONR. [Preview Abstract] |
Session A15: Focus Session: Spins in Metals - Thermal Effects on Magnons and Spin Currents
Sponsoring Units: DMP FIAP GMAGChair: Stewart Barnes, University of Miami
Room: 213
Monday, February 27, 2012 8:00AM - 8:12AM |
A15.00001: Theory of phonon-driven spin Seebeck effect Hiroto Adachi, Jun-ichiro Ohe, Saburo Takahashi, Sadamichi Maekawa Spin Seebeck effect refers to a thermal spin injection occurring over millimeter scales from a ferromagnet into an attached nonmagnetic metal [Uchida et al., Nature 455, 778 (2008)]. We discuss the importance of the phonon-drag process in the spin Seebeck effect. Our theory of phonon-drag spin Seebeck effect [Adachi et al., Appl. Phys. Lett. 97, 252506 (2010)] explains simultaneously the local nature of the spin Seebeck effect [Jaworski et al., Nature Materials 9, 898 (2010); Uchida et al., Nature Materials 10, 737 (2011)] and the signal enhancement at low temperatures [Jaworski et al., Phys. Rev. Lett. 106, 186601 (2011)]. We also discuss the difference between our approach and that developed in Xiao et al., Phys. Rev. B 81, 214418 (2010). [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A15.00002: Spin Seebeck Effect Measurements on Ferromagnetic Thin Films Using Micromachined Thermal Isolation Platforms Azure D. Avery, Dain Bassett, Matt R. Pufall, Barry L. Zink The newly discovered thermoelectric effect, called the spin Seebeck effect (SSE), refers to a spin imbalance generated by a thermal gradient. This spin imbalance, capable of driving a pure spin current into a contact, is detected by measuring the conversion of the spin current into a transverse voltage ($V_{T}$) via the inverse spin Hall effect. A robust theoretical treatment of the SSE has so far eluded the community at large, and more experimental data are needed to understand the underlying physics. In this talk we present $V_{T}$ measurements associated with the SSE and AMR measurements for Ni, Fe, and Ni-Fe alloy thin films, along with Au as a control, made using our micromachined thermal isolation platforms. The sensitive thermal transport measurements we make using these platforms offer several advantages including concurrent measurements at hot and cold ends of the sample, equal heating of the sample ends to isolate traditional thermoelectric effects, and a large reversible thermal gradient. Additionally, the substrate thickness results in a virtually 2-D thermal platform that dramatically reduces the likelihood of thermal gradients perpendicular to the sample. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A15.00003: Search For Spin Seebeck effect in in situ grown thin films Priyanga Jayathilaka, Dustin Belyea, Hillary Kirby, Casey W. Miller The Spin Seebeck Effect (SSE) is a phenomenon in which the application of a temperature gradient cross a ferromagnet causes a measurable electric potential difference transverse to the gradient when a normal metal is grown onto the ferromagnet. The spin current diffusing into the normal metal is transduced to a voltage via the Inverse Spin Hall Effect. Measuring the SSE accurately is challenging due to presence of other effects, possibly including regular Seebeck effect and anomalous thermo-magnetic phenomena. Here we report on our efforts to measure the SSE in thin films of NiFe and Co, using Au and Ta as the electrodes. All samples were grown on Si/SiOx substrates by magnetron sputtering through contact masks. The mask exchange was done in situ in a chamber where the base pressure was 2.0x10-7 Torr in order to limit contamination of the interfaces. The samples were measured using a rig with a reversible temperature gradient of 15K/cm and the resultant voltage was measured at the hot and cold ends of the sample using nanovoltmeters. The voltage signal we observe is strongly correlated with the magnetic hysteresis loops measured by Magneto-Optical Kerr Effect magnetometer. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A15.00004: Intrinsic spin-dependent thermal transport Invited Speaker: C.L. Chien Spin caloritronic effect, such as spin Seebeck effect, has attracted a great deal of attention recently. In most cases such studies have been made on patterned ferromagnetic thin films on substrates. The mechanism of spin Seebeck effect has evolved from intrinsic difference in the spin chemical potentials to magnon-phonon interaction through the substrate. We use patterned ferromagnetic thin film to demonstrate the profound effect of a substrate on the spin-dependent thermal transport. With different sample patterns and on varying the direction of temperature gradient, both longitudinal and transverse thermal voltages exhibit asymmetric instead of symmetric spin dependence. This unexpected behavior is due to an out-of-plane temperature gradient imposed by the thermal conduction through the substrate and the mixture of the anomalous Nernst effects. Only with substrate-free samples have we determined the intrinsic spin-dependent thermal transport with characteristics and field sensitivity similar to those of anisotropic magnetoresistance effect. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A15.00005: Thermoelectric coating based on the spin Seebeck effect Akihiro Kirihara, Ken-ichi Uchida, Yosuke Kajiwara, Masahiko Ishida, Yasunobu Nakamura, Takashi Manako, Shigeru Kohmoto, Eiji Saitoh, Shinichi Yorozu Thermoelectric (TE) technologies have been drawing great interest, since they can directly generate electricity from thermal energy that is available in various places. However, their complicated module structure, which is based on a number of thermocouples, still makes it difficult to fabricate large-area TE devices at low cost. In this work, we show a novel concept based on the spin Seebeck effect (SSE) called TE coating, which is characterized by a simple film structure, convenient scaling capability, and easy fabrication. We fabricated a TE-coating film with a bismuth-substituted yttrium iron garnet (Bi:YIG) by a highly productive spin-coating-based process on a nonmagnetic substrate, and demonstrated the SSE-induced TE conversion. The TE-coating layer amounts to only 0.01 {\%} of the total sample thickness, suggesting that such an ultrathin magnetic film can work as a useful thermal-energy collector. This new concept may enable us to implement low-cost and large-area TE functions on various objects, opening opportunities for innovative energy harvesting applications. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A15.00006: Linear response theory for magnon transport in ferromagnetic insulators Shuichi Murakami, Ryo Matsumoto We study transverse response of magnons in ferromagnetic insulators within linear response theory. In analogy with the corresponding theory for electrons [1], magnon transverse response is described, including the Hall effect, Nernst effect, and thermal Hall effect. As is also the case for electrons [1], the response functions for magnons consist of the Kubo-formula term, and the term corresponding to the orbital angular momentum. We can rewrite the response functions in terms of the Berry curvature in momentum space [2]. We apply this theory to the (quantum-mechanical) magnons and to the classical magnetostatic waves. For the magnetostatic waves, the eigenmodes are given by a generalized eigenvalue problem, giving rise to the special form of the Berry curvature [2]. We explain various properties of this Berry curvature for the generalized eigenvalue problem, and discuss its implications for the physical properties of magnetostatic modes. [1] L. Smrcka and P. Streda, J. Phys. C, 10, 2153 (1977); H. Oji, P. Streda, Phys. Rev. B 31, 7291 (1985); [2] R. Matsumoto and S. Murakami, Phys. Rev. Lett. 106, 197202 (2011); Phys. Rev. B 84, 184406 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A15.00007: Thermally-assisted magnetization reversal in nanomagnets with spin-transfer torque: a GPU approach Daniele Pinna, Andrew Kent, Aditi Mitra, Daniel Stein Spin transfer magnetization reversal has a direct impact on magnetic information storage technologies. The probability that a nanomagnet switches under an applied magnetic field is expected to follow a simple thermally activated LLG model\footnote{M. L. N\'eel, Ann. Geophys. 5, 99 (1949); W. F. Brown, Phys. Rev. B 130, 1677 (1963).}. However, a direct current applied to a nanomagnet produces a spin-transfer torque that drives the magnetization out of equilibrium\footnote{J. C. Slonczewski, JMMM. 159, L1 (1996)}. Such dynamics have been studied in limits where both the low and high current regimes allow analytical treatment\footnote{J. Sun, Phys. Rev. B 62 1 (2000)}. Nonetheless, the inability to study numerically the long time behavior has tampered with theoretical verification and comparison to current experimental data\footnote{D. Bedau et al. Appl. Phys. Lett. 97, 262502 (2010)}. In this talk, we present results obtained by employing modern GPU computational techniques to massively parallelize the Langevin equations of the model. We test the numerics by considering a simplified uniaxial case. The full current spectrum is reviewed, verified and compared to the present literature. We then proceed to break the symmetries in the problem and explore the general macrospin model. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A15.00008: Can heat flow induced spin currents move a magnetic domain wall? Aakash Pushp, Timothy Phung, Luc Thomas, Konstantinos Alexandrou, Xin Jiang, See-hun Yang, Brian Hughes, Charles Rettner, Stuart Parkin It has been established in the past few years that heat flow within a ferromagnet can induce a spin current and an associated voltage. This Spin Seebeck effect, initially reported in ferromagnetic metals, has also been observed in magnetic semiconductors as well as magnetic insulators. An open question has been whether heat flow induced spin currents can also move magnetic domain walls in 'racetrack' magnetic nanowires. In order to answer this question, we investigate the interaction of a magnetic domain wall with spin currents induced by sharp temperature gradients in magnetic nanowire spin valves. We use optical as well as electrical techniques to create sharp temperature gradients on the order of 1-10 K/nm on nanosecond timescales. We will describe our experimental setup and present data that show the various roles that temperature plays on the saturation magnetization of the material as well as on the induced spin currents that influence magnetic domain wall motion. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A15.00009: Observation of spin-wave cooling effect in magnets Toshu An, Ken-ichi Uchida, Kazuya Harii, Yosuke Kajiwara, Kazuya Yamagichi, M.B. Jungfleisch, A.V. Chumak, V.I. Vasyuchka, Burkard Hillebrands, Eiji Saitoh We focused on utilizing a surface spin wave (Damon-Eshbach mode); traveling on top and bottom surfaces in a non reciprocal manner, as a good carrier of heat. As a sample, Yttrium iron garnet (YIG) was chosen because the spin waves excited in the YIG is known to have a long coherence length propagating distances even a few millimeters. By exciting the surface spin wave of only one side, heat transportation was successfully observed by measuring sample temperature with an infrared thermocamera. More interestingly, the temperature where the spin wave is initially excited shows cooling effect to drop its temperature just after the excitation of the surface spin wave. Here we call this effect as microwave cooling effect which is introducing a new cooling principle. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A15.00010: Thermodynamics of magnetic systems from first principles Markus Eisenbach, Gregory Brown, Aurelian Rusanu, Don M. Nicholson Density functional calculations have proven to be a useful tool in the study of ground state properties of many materials. The investigation of finite temperature magnetism on the other hand has to rely usually on the usage of empirical models that allow the large number of evaluations of the system's Hamiltonian that are required to obtain the phase space sampling needed to obtain the free energy, specific heat, magnetization, susceptibility, and other quantities as function of temperature. We have demonstrated a solution to this problem that harnesses the computational power of today's large massively parallel computers by combining a classical Monte-Carlo calculations with our first principles multiple scattering electronic structure code (LSMS) for constrained magnetic states. Here we will present recent advances in our method that improve the convergence as well as applications to 3d element based ferromagnets. This research was performed at Oak Ridge National Lab and sponsored in parts by the Center for Nanophase Material Sciences, Scientific User Facilities Division, the Center for Defect Physics, an Energy Frontier Research Center funded by the US DOE Office of Basic Energy Sciences and the Division of Materials Science and Engineering, Office of Basic Energy Science of [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A15.00011: Effect of Temperature and Spin Torque on the Stoner-Wohlfarth Astroid of a Nanomagnet Yu-Jin Chen, Jordan Katine, Juergen Langer, Mark Lewis, Graham Rowlands, Jian Zhu, Pedram Khalili Amiri, Kang Wang, Ilya Krivorotov We report measurements of the Stoner-Wohlfarth switching astroid curves of the free layer nanomagnet in CoFeB/MgO/CoFeB/Ru/CoFe/PtMn elliptical nanoscale magnetic tunnel junctions made as a function of applied voltage and temperature. Measurements of the astroid area as a function of temperature allow us to determine the magnetic anisotropy energy barrier of the free layer and thereby quantify its thermal stability - an important performance parameter of spin torque nonvolatile magnetic memory. Measurements of the astroid as a function of voltage (V) applied to the junction at the bath temperature of 4 K reveal significant voltage-induced deformations of the astroid curve. We observe a decrease of the hard-axis length of the astroid, which arises from ohmic heating of the junction. Comparison of the hard-axis astroid length measured at T = 4 K and |V| > 0 to the hard-axis astroid length measured at T > 4 K and V = 0 allows us to quantify ohmic heating of nanoscale tunnel junctions by the applied voltage. The applied voltage reduces the easy-axis length of the astroid as well, but the reduction is asymmetric for positive and negative easy-axis directions. This easy-axis asymmetry reverses upon the applied voltage sign reversal and thus it can be attributed to spin transfer torque. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A15.00012: Magnetization dynamics at elevated temperatures Lei Xu, Shufeng Zhang The conventional Landau-Lifshitz (LL) equation is the basis for simulation of magnetic structure and dynamics as long as the temperature is not too close to Curie temperature. In order to model the magnetization dynamics at elevated temperatures, one needs to extend the LL equation by including a finite longitudinal relaxation. Here within the self-consistent mean-field treatment of ferromagnetism, we propose an effective equation which is capable of addressing magnetization dynamics for a wide range of temperatures. At low temperatures, the equation reduces to the Landau-Lifshitz equation, namely, the transverse relaxation governs the dynamics. At high temperatures, it reduces to paramagnetic Block equation. Near the Curie temperature, the longitudinal relaxations play a more important role on the magnetization reversal. We present numerical calculations to simulate a heat-assisted-magnetic-recording process when the temperature is heated and cooled through the Curie temperature. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A15.00013: Temperature dependence of magnetic losses in GMR and TMR devices Kevin Haughey, Ryan Stearrett, Arif Ozbay, Edmund Nowak Thermally induced magnetization fluctuations in the free and reference magnetic layers of giant and tunneling magnetoresistance (GMR and TMR) spin valves (SV) devices typically give rise to a low-frequency 1/f power spectral density that has been related to local dissipative processes [1]. Understanding the origin of these magnetic losses is essential for increasing the magnetic field sensitivity of GMR and TMR sensors. The low-frequency magnetic losses can be parameterized through a loss angle, $\varepsilon $(T, H). $\varepsilon $(T) for the reference layer in our GMR SV is non-monotonic: first decreasing with increasing T, then exhibiting a minimum near 50K and what may be the onset of a plateau or peak near 300K. The peak and minimum shift to lower temperatures when the applied field is oriented perpendicular to the exchange pinning direction. Data for TMR devices show similar trends. The measurements will be described in the context of a model involving thermally activated kinetics and a field-dependent distribution of activation energies for the nanoscale magnetic fluctuators. \\[4pt] [1] Z. Diao et al., PRB \textbf{84}, 094412 (2011) [Preview Abstract] |
Session A16: Liquid and Solid Helium
Sponsoring Units: DCMPChair: Souleymane Diallo, Oak Ridge National Laboratory
Room: 251
Monday, February 27, 2012 8:00AM - 8:12AM |
A16.00001: Solid helium in rigid torsional oscillators D.Y. Kim, M.H.W. Chan In torsional oscillator experiments on solid helium, separating out the effect of shear modulus stiffening is important to measure true non-classical rotational inertia (NCRI). An increase in the shear modulus of solid helium stiffens the oscillator and causes the resonant period to drop thus mimicking NCRI. This effect can be multiplied in a torsional oscillator that is not completely rigid. We have carried out measurements in torsional oscillators specially designed to minimize the shear modulus effect. These oscillators are different from conventional ones in two aspects. Annulus sample spaces are located in the outermost part from rotational axes and all other parts consist of rigid metal. In this design, the resonant period change by loading solid helium is maximized and the effect of shear modulus change is minimized. We found a NCRI fraction on the order of $3 \times 10^{-5}$ in these torsional oscillators. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A16.00002: Thermal Conductivity of Solid $^{4}$He in Vycor Zhigang Cheng, Samhita Banavar, Stefan T. Omelchenko, Moses H.W. Chan In addition to non-classcial rotational inertia (NCRI) observed in torsional oscillator (TO), we have carried out thermal conductivity measurement of solid $^{4}$He embedded in porous Vycor glass to search for possible anomaly related to the onset of NCRI. Because of the high thermal conductivity of bulk solid $^{4}$He, it is difficult to resolve any small `extra' thermal conductivity signal due to the onset of supersolidity. However, when it is confined in porous Vycor, the thermal conductivity of solid $^{4}$He is significantly reduced. This makes it easier to single out possible anomaly. Preliminary measurements show a rounded peak in the thermal conductivity near 0.1 K. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A16.00003: NMR studies of very dilute concentrations of 3He in solid 4He Sung Su Kim, C. Huan, L. Yin, J. Xia, D. Candela, N. S. Sullivan We compare the results of recent measurements of the nuclear spin-lattice relaxation time ($T_1$) and nuclear spin-spin relaxation time ($T_2$) for very dilute concentrations of $^3$He ($16 \leq x_3 \leq 2000$ ppm) in solid $^4$He with results from previous studies in the temperature range where the relaxation is attributed to the quantum tunneling of $^3$He atoms in the $^4$He lattice. The comparison shows that the results cannot be explained in terms of a unique correlation time and the effects of $^4$He lattice are important. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A16.00004: Observation of superfluid components in solid $^{4}$He Hans Lauter, Eckhard Krotscheck, Efim Kats, Alexander Puchkov, Valeria Lauter, Vesa Apaja, Ivan Kalinin, Marek Koza Neutron scattering demonstrated that localized superfluid components exist at high pressure within solid helium in aerogel [1]. Two sharp phonon-roton spectra are clearly distinguishable from modes in bulk superfluid helium. These roton excitations exhibit different roton gap parameters than the roton observed in the bulk fluid at freezing pressure. One of the roton modes disappears after annealing. Comparison with theoretical calculations suggests that the model that reproduces the observed data best is that of superfluid double layers within the solid and at the helium-substrate interface. The elastic scattering evidenced in addition to the hcp phase also the bcc-phase. both consisting of a small crystallites as a consequence of the confinement. The structural aspect of coexisting hcp and bcc phases in the aerogel matrix seems to be important for the creation of the localized superfluid components.\\[4pt] [1] H. Lauter, E. Krotscheck, E. Kats, A. V. Puchkov, V. V. Lauter, V. Apaja, I. Kalinin, M. Koza, PRL submitted [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A16.00005: Evidence for a BCC to HCP Phase Transition for Solid Helium in Porous Vycor Glass at 100 bar and 800 mK Laurence Lurio, Sambhunath Bera, Jonathan Maloney, Norbert Mulders, Zhigang Cheng, Moses Chan, Clement Burns, Zhan Zhang In 2004, Kim and Chan\footnote{Kim, E. and Chan, M. H. W., Nature 427, 225 (2004).} found evidence that solid helium grown in porous vycor glass exhibited changes in rotational inertia at millikelvin temperatures similar to that observed for superfluids. The nature of this putative supersolid phase is a question of current debate. Subsequent experiments have shown that supersolid behavior depends strongly on crystal quality. For the case of solid helium in vycor, little is known about the crystal quality, or even the crystal phase. In the present work we have performed transmission x-ray diffraction experiments on solid helium in porous vycor glass, over a range of pressures from 1 bar to 162 bar. At pressures up through 96 bar a single peak is observed in the diffraction pattern. At 114 bar and above this peak is observed to split into three peaks. We tentatively identify the low pressure phase as BCC and the high pressure phase as coexistence between BCC and HCP. We interpret the absence of higher order peaks as due to a combination of zero-point motion and defects. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A16.00006: Simultaneous observation of 10 MHz ultra sound in solid $^4$He contained in torsional oscillator Bettina Hein, John Goodkind, Harry Kojima Kim and Chan[1] were motivated in part by an anomalous ultra sound propagation observed by Ho et al.[2] in solid $^4$He near 200 mK to carry out torsional oscillator(TO) experiments which led to the discovery of supersolid phenomena. We constructed a 270 Hz TO oscillator which incorporated quartz transducers for simultaneously observing 10 MHz longitudinal ultra sound and torsional oscillation of solid $^4$He samples. We are searching for correlation between behaviors in ultra sound propagation and TO response. The length and density of dislocations extracted from ultra sound and the frequency shifts of TO measured in some half a dozen solid $^4$He samples have not shown clear correlation.\\[4pt] [1] E. Kim and M. Chan, Nature \textbf{427}, 225(2004).\\[0pt] [2] P. Ho, I. Bindloss and J. Goodkind, JLTP \textbf{109}, 409(1997). [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A16.00007: More on the shear modulus of solid helium Norbert Mulders In experiments on solid $^{4}$He one finds a striking similarity between the response of torsional oscillators and the shear modulus. It has been suggested that in fact the observed increase in the torsional oscillator frequency at low temperature is just a reflection of the stiffening of the solid helium, and thus that of the oscillator as a whole. In some cases this may indeed be the case, but a recent experiment by Keiya Shirahama and Eunseong Kim's groups in which a measurement of the shear modulus was incorporated in a torsional oscillator cell seemed to indicate that the response of the solid to shear is decoupled from that of the oscillator. However, one may object that a) the shear was not applied at a frequency different from that of the oscillator, and that b) the direction of the applied shear was orthogonal to the stress imposed by the motion of the oscillator. In a series of experiments using two stacks of piezoelectric transducers that can be excited, and are able to detect, in orthogonal shear directions, we show that shearing the solid at one frequency affects the shear modulus at different frequencies, and similarly, that shearing in one direction affects the modulus as measured at the orthogonal direction. We conclude that quite generally shear and torsional oscillator responses are decoupled. And while they may both reflect the same underlying physics, it is unlikely that one causes the other. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A16.00008: Simultaneous Measurement of Non-Classical Rotational Inertia and Shear Modulus of Solid $^{4}$He Wonsuk Choi, Jaeho Shin, Hyoung Chan Kim, Keiya Shirahama, Eunseong Kim A failure to rotate or oscillate is the essential nature of low temperature superfluid helium, and more technically known as non-classical rotational inertia (NCRI). It is counter-intuitive, but NCRI is also found in solid helium-4 below $\sim$200 mK [1,2]. Recently, shear modulus showed unusual increase with striking resemblance to those of NCRI [3]. Extended measurements show the NCRI occurs only in a stiffened Bose solid, but it is not understood how they are related. Here we report the first simultaneous measurement of shear modulus and NCRI in solid helium to elucidate the fundamental connection between them. Both emerge at remarkably similar temperatures, whereas no quantitative agreement between the increase of the shear modulus and the magnitude of NCRI is found. The increase of shear modulus seems to be the necessary condition for the onset of NCRI.\\[4pt] [1] E. Kim and M. H. W. Chan \textit{Nature} \textbf{427}, 225-227 (2004)\\[0pt] [2] E. Kim and M. H. W. Chan \textit{Science} \textbf{305}, 1942 (2004)\\[0pt] [3] J. Day and J. Beamish \textit{Nature} \textbf{450}, 853-856 (2007) [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A16.00009: Interplay of Aerogel Anisotropy and Superfluid $^3$He Textures Jia Li, J. Pollanen, C.A. Collett, W.J. Gannon, W.P. Halperin The effect of aerogel anisotropy on the $^3$He superfluid order parameter and the relative stability of $A$ and $B$-phases has been investigated. We have performed pulsed NMR on $^3$He in high porosity aerogel samples that have different types of anisotropy, characterized with an optical, cross-polarization technique. One aerogel sample has $14.3\%$ growth-induced axial stretching. Its superfluid phase diagram is occupied by the $A$-phase. Linewidth analysis gives the distribution of the orbital angular momentum, $\vec{l}$. The orientation of $\vec{l}$ is consistent with an easy plane distribution that is perpendicular to the strain axis. A second aerogel sample is axially compressed mechanically by $22.5\%$. The major part of the zero magnetic field phase diagram is occupied by the $B$-phase. Additionally, our results show that aerogel anisotropy introduced by compressing and stretching have different orienting effects on the $^3$He superfluid order parameters. This work was supported by the National Science Foundation, DMR-1103625. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A16.00010: Experiments with $^{3}$He in 10{\%} uniaxially compressed aerogel: the superfluid phase diagram Nikolay Zhelev, Robert Bennett, Eric Smith, Johannes Pollanen, William Halperin, Jeevak Parpia Entraining $^{3}$He in aerogel provides a way to introduce disorder in the otherwise ideal quantum fluid. Motivated by the recent prediction that uniaxially compressed aerogel can stabilize the anisotropic A phase over the isotropic B phase, we use a torsional oscillator technique to measure the superfluid phase diagram of $^{3}$He entrained in 10{\%} axially compressed, 98{\%} porous aerogel. We observe that a broad region of the temperature-pressure phase diagram is occupied by the metastable A phase. The reappearance of the A phase on warming from the B phase, before superfluidity is extinguished at T$_{c}$, is in contrast to its absence in uncompressed aerogel. We also find that the anticipated alignment of the angular momentum vector by compression is not observed. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A16.00011: Application of MEMS Devices to the Study of Liquid $^{3}$He Miguel Gonzalez, Pan Zheng, Erik Garcell, Ho Bun Chan, Yoonseok Lee We report measurements on the mechanical properties of a micro-electro-mechanical (MEMS) resonator submerged in liquid $^{3}$He at millikelvin temperatures and at pressures 3, 21 and 29 bar. The device consists of a pair of parallel plates with a well-defined gap of 0.75 $\mu $m. The sub-micron gap size and geometry of the device gives access to physics in the high Knudsen regime and allows the investigation of surface scattering effects in thin films of quantum fluids. Details of design, fabrication, and operation will be presented along with a study of the damping characteristics of the submerged resonator through a wide range of temperatures spanning from classical fluid to degenerate Fermi liquid. The device shows potential for the use in low temperature experiments and to investigate novel phenomena in quantum fluids at the micro/nano scale such as superfluid $^{3}$He films. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A16.00012: Partial depinning of dislocation in $^4$He from $^3$He impurities induced by thermal roughening Darya Aleinikava, Anatoly Kuklov The mechanism of the roughening induced partial depinning of gliding dislocations from $^3$He impurities is proposed [1] as an alternative to the standard ``boiling off'' scenario [2]. We give a strong argument that $^3$He remains bound to dislocations even at large temperatures due to very long equilibration times. This conjecture is based on two observations: First, the experimental data [2] for shear modulus temperature dependence obtained at very different $^3$He concentrations (1 ppb and 300 ppb) can be, practically, collapsed on each other by a simple rescaling of temperature; Second, both moduli can be fit by the Monte-Carlo simulations data within the assumption that the impurities remain confined to the spatial region occupied by dislocation. Conversely, impurities evaporation violates strongly the data collapse and is absolutely inconsistent with the simulations. We propose that such long equilibration time is due to the very narrow band of $^3$He impuritons [3]. \\[4pt] [1] D.Aleinikava, A.B.Kuklov. arXiv:1110.5884v1;\\[0pt] [2] J. Day and J. Beamish, Nature {\bf 450}, 853 (2007); J. Day,O. Syshchenko, and J.Beamish,Phys. Rev. B {\bf 79}, 214524 (2009);\\[0pt] [3] A.F. Andreev, Sov. Phys. Usp. {\bf 19}, 137 (1976) [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A16.00013: Microscopic Dynamics of Liquid Helium Confined in 1D Nanopores Timothy Prisk, Paul Sokol, Narayan Das, Souleymane Diallo, Nobuo Wada, Shinji Inagaki Recently, Toda et al. performed torsional oscillator and heat capacity measurements on liquid He$^4$ confined within FSM-16. This porous silica glass has 1D pores with a very narrow diameter of $d = $ 2.8 nm. This system is an example of a 1D quantum fluid in the sense that the thermal wavelength $\lambda$ is longer than the pore diameter $d$. Because neutron time-of-flight (ToF) spectroscopy probes elementary excitations, it can be used to study the microscopic dynamics underlying the thermodynamic properties of this 1D quantum liquid. Using the Cold Neutron Chopper Spectrometer (CNCS) at the Spallation Neutron Source (SNS), we performed the first direct measurements of the elementary excitation spectrum of liquid He$^4$ confined in FSM-16. Measurements were performed at full pore at temperatures T = 33, 80, 800, and 1500 mK with an elastic energy resolution of approximately 80 $\mu$eV. We will discuss the temperature $T$ dependence of the static structure factor $S(Q)$, the energies $\hbar\omega$ and line widths $\Gamma$ of the phonon-roton spectrum, and the evidence for 2D layer modes in this system. This research was supported by NIST and employed facilities sponsored by the Scientific User Facilities Division of the US Department of Energy. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A16.00014: Path-Integral Monte Carlo Simulations of the Ideal Strength of HCP Helium 4 Maurice de Koning, Edgar Josu\'e Landinez Borda Using path-integral Monte Carlo simulations we assess the ideal strength of solid He-4 in its HCP phase. This fundamental material parameter is defined as the stress necessary to produce irreversible deformation in a defect-free crystal. For this purpose we impose slowly increasing homogeneous deformations to defect-free He-4 crystals and measure the corresponding internal stress state. In this manner, we determine the ideal shear strength in the basal plane as a function of the shear orientation, as well as the tensile and compressive strength perpendicular to this plane. Our results establish upper bounds to the strength of real HCP He-4 crystals. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A16.00015: Fitting of m*/m with Divergence Curve for He$_{3}$ Fluid Monolayer using Hole-driven Mott Transition Hyun-Tak Kim The electron-electron interaction for strongly correlated systems plays an important role in formation of an energy gap in solid. The breakdown of the energy gap is called the Mott metal-insulator transition (MIT) which is different from the Peierls MIT induced by breakdown of electron-phonon interaction generated by change of a periodic lattice. It has been known that the correlated systems are inhomogeneous. In particular, He$_{3}$ fluid monolayer [1] and La$_{1-x}$Sr$_{x}$TiO$_{3}$ [2] are representative strongly correlated systems. Their doping dependence of the effective mass of carrier in metal, m*/m, indicating the magnitude of correlation (Coulomb interaction) between electrons has a divergence behavior. However, the fitting remains unfitted to be explained by a Mott-transition theory with divergence. In the case of He$_{3}$ regarded as the Fermi system with one positive charge (2 electrons + 3 protons), the interaction between He$_{3}$ atoms is regarded as the correlation in strongly correlated system. In this presentation, we introduce a Hole-driven MIT with a divergence near the Mott transition [3] and fit the m*/m curve in He$_{3}$ [1] and La$_{1-x}$Sr$_{x}$TiO$_{3}$ systems with the Hole-driven MIT with m*/m=1/(1-$\rho ^{4})$ where $\rho $ is band filling. Moreover, it is shown that the physical meaning of the effective mass with the divergence is percolation in which m*/m increases with increasing doping concentration, and that the magnitude of m*/m is constant.\\[4pt] [1] Phys. Rev. Lett. 90, 115301 (2003).\\[0pt] [2] Phys. Rev. Lett. 70, 2126 (1993).\\[0pt] [3] Physica C 341-348, 259 (2000); Physica C 460-462, 1076 (2007). [Preview Abstract] |
Session A17: Focus Session: Thermoelectrics - Nanostructured and Oxide TE
Sponsoring Units: DMP GERA FIAPChair: Li Shi, University of Texas at Austin
Room: 252A
Monday, February 27, 2012 8:00AM - 8:12AM |
A17.00001: Thermoelectric properties of filling-controlled zinc-antimonides with layer structure T. Suzuki, M.S. Bahramy, R. Arita, Y. Taguchi, Y. Tokura Thermoelectric properties have been investigated for polycrystalline samples of layer-structured $R_{1-x}A_x$ZnSbO ($R$=La, Ce; $A$=Ca, Sr) as two-dimensional analogues of a conventional thermoelectric semiconductor ZnSb[1]. By substituting $A^{2+}$ for $R^{3+}$ in the charge-reservoir layers, carrier concentration can be successfully controlled without lowering the carrier mobility. The hole doped materials showed low thermal conductivity and moderately high thermopower, whose temperature- and doping-dependence were well explained by theoretical calculation. The values of dimensionless figure of merit $ZT$ were found to increase without showing any sign of saturation up to 390 K, and even higher values can be expected along the conducting ZnSb layers for a single crystal. These results indicate the potential of the hole-doped $R$ZnSbO as a good thermoelectric material. This work was in part supported by FIRST program on \lq\lq Quantum Science on Strong Correlation\rq\rq \ from JSPS. \\[4pt] [1] T. Suzuki, M. S. Bahramy, R. Arita, Y. Taguchi, and Y. Tokura, Phys. Rev. B 83, 035204 (2011) [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A17.00002: Enhancing thermoelectric properties of FeSb$_{2}$ by altering stoichiometry and nanostructure Mani Pokharel, Huaizhau Zhao, Kevin Lukas, Zhifeng Ren, Cyril Opeil FeSb$_{2}$ is a strongly correlated semiconductor that has been shown to have an extraordinarily large Seebeck coefficient in single crystal samples. Bentien \textit{et al.} report a Seebeck Coefficient of -45000 microV/K at 10K. The peak value of the dimensionless figure of merit (ZT$_{max})$ for single crystal samples is calculated to be approximately 0.005 at 10 K and is constrained by its relatively high thermal conductivity. In our previous studies, we find that nanocomposites (NC) tend to decrease thermal conductivity substantially by introducing phonon mismatches between crystal grains. Given that the Seebeck coefficient on the FeSb$_{2}$ system is quite sensitive to carrier concentration, we focus on the effects of stoichiometric changes that heighten thermoelectric properties of FeSb$_{x}$ where x=2.04, 2.00, 1.96, 1.92. By tuning the stoichiometry and using the nanocomposite method, the peak value of ZT$_{max}$ was found to be 0.0123 at 43K. Carrier concentration and Hall-mobility measurements will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A17.00003: Local Structure of Thermoelectric Clathrates Scott Medling, Frank Bridges, Toshiro Takabatake We present local structure results from extended X-ray absorption fine structure (EXAFS) analysis of two new kinds of clathrates: type I clathrates with a light 'rattler' atom and type VIII clathrates. The rattler atom is extremely important for the thermoelectric properties as it is what strongly scatters phonons and lowers the thermal conductivity without affecting the electrical conductivity. In most thermoelectric clathrates, the rattler atom is a heavy atom, typically Ba or Eu, and such type I clathrates have been discussed extensively in the literature; a smaller rattler atom such as K results in more free space inside the cage. Type VIII clathrates have the same chemical formula as type I clathrates, but have a different, more distorted, cage structure resulting in differing electronic properties. Recent studies indicate a thermoelectric figure of merit at least as high as 1.2 at 400 K is attainable. For both kinds of clathrates, we collect and analyze temperature-dependent data over the range 10-300 K and compare the results with our models. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A17.00004: Nanostructured Thermoelectrics and the New Paradigm Invited Speaker: Mercouri Kanatzidis A comprehensive and stable energy strategy would require proportionate attention to all three legs of the ``energy stool''; supply (sources), demand (efficiency) and storage/transport (delivery). Thermoelectric materials, that convert waste thermal energy into useful electrical energy, have an important role to play in any and all these three legs. The efficacy and efficiency of thermoelectrics is reflected in the figure of merit ZT, which is directly proportional to the power factor (comprising electrical conductivity and Seebeck coefficient) and inversely proportional to thermal conductivity (comprising carrier and lattice contributions). The recent emergence of nanostructured thermoelectrics has ushered in a new era for bulk thermoelectrics, which show considerable promise to enhance the ``contra-indicating'' parameters of high electrical conductivity and low thermal conductivity. This is achieved by introducing nanostructures in bulk thermoelectric host materials to significantly reduce lattice thermal conductivity via effective scattering of heat carrying phonon through hierarchical architecture of nanostructured thermoelectrics. The presentation will cover recent developments, current research in our EFRC and future prospects for high performance bulk materials. Systems based on lead chalcogenides (e.g., PbTe, PbSe, PbS) present key science challenges with promising properties and are given particular emphasis. We have achieved excellent control of synthesis and crystal growth of such materials resulting in record enhancements in the figure of merit. These enhancements derive from very large reductions in lattice thermal conductivity possible with nanostructuring. We have experimentally realized concurrent synergistic effect of phonon blocking and charge transmission via the endotaxial placement of nanocrystals in thermoelectric material host. In particular, we have shown that the enhanced performance is due to nanostructuring of thermoelectric host matrix, with a compelling influence of hierarchy of length-scales associated with these systems. The presentation will outline possible future strategies for enhancing the thermoelectric figure of merit of bulk thermoelectric materials. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A17.00005: Assessing the thermoelectric properties of sintered compounds via high-throughput ab initio calculations Shidong Wang, Zhao Wang, Wahyu Setyawan, Natalio Mingo, Stefano Curtarolo In order to identify promising thermoelectric materials, we study several thousand compounds from the ICSD database. In particular, we consider nano-grained sintered power thermoelectric compounds with the high-throughput {\it ab initio} \textsf{AFLOW} framework (http://aflowlib.org and http://materials.duke.edu/aflow.html). By regression analysis, we find that the power factor is positively correlated to electronic band gap, carrier effective mass, and the number of atoms per unit cell. This work illustrates the important role that experimental and theoretical databases can play in the development of novel materials. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A17.00006: Enhancement of the figure-of-merit in strongly correlated multilayers Veljko Zlatic, Jim Freericks A theory of charge and heat transport in inhomogeneous multilayers (ML) with correlated electrons is presented. We consider the device which consists of several strongly correlated metallic planes (channel) sandwiched between two semi-infinite Mott insulators (barriers). A driving field is applied parallel to the ML planes and the gate voltage perpendicular to the ML planes. The device is described by the Falicov-Kimball model and transport coefficients are calculated by the linear response theory using the DMFT. The device is tuned by i) increasing the correlation strength so as to enhance the slope of the local density of states in the channel-planes at the chemical potential and ii) changing the gate voltage so as to shift the position of the chemical potential with respect to the band-edge. Both effects have a large impact on the thermoelectric properties and are used to optimize the figure-of-merit, ZT, of the device. The effect of the number of planes is discussed as well. The results for the electrical conductivity, the Seebeck coefficient, the power factor, the Lorenz number and ZT are presented. Optimal tuning gives ZT $\gg$ 1. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A17.00007: Thermoelectric and Structural Properties of the Chemically Doped Ca$_{3}$Co$_{4}$O$_{9}$ System Jianming Bai, Tao Wu, Trevor A. Tyson, Haiyan Chen The Cu and Y doped thermoelectric oxide system [Ca$_{2}$CoO$_{3}$][CoO$_{2}$]$_{1.61}$, also referred to as Ca$_{3}$Co$_{4}$O$_{9}$, was prepared by solid state reaction followed by annealing under oxygen. The temperature dependent thermoelectric properties, including resistivity ($\rho )$, Seebeck coefficient (S) and thermal conductivity ($\kappa )$, were measured on Cu doped [Ca$_{2}$Co$_{1-x}$Cu$_{x}$O$_{3}$][CoO$_{2}$]$_{1.61}$ and Y doped [Ca$_{2-x}$Y$_{x}$CoO$_{3}$][CoO$_{2}$]$_{1.61}$. In order to understand the origin of the changes in ZT with doping, local (XAS) and long range (XRD) structural measurements as a function of doping were conducted. Identification of the locations of the doping sites and the impact on ZT will be discussed. This work is supported by DOE Grant DE-FG02-07ER46402. The Physical Properties Measurements System was acquired under NSF MRI Grant DMR-0923032 (ARRA award). [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A17.00008: The effects of increased Co-ion spin states on the Seebeck coefficient in thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$ Robert Klie, Tadas Paulauskas, Qiao Qiao, Alejandro Rebola, Serdar Ogut, Dipanjan Mazumdar, Arun Gupta, Stanislaw Kolesnik, Juan-Carlos Idrobo Thermoelectric oxides have attracted increasing attention due to their high thermal power and temperature stability. In particular, Ca$_{3}$Co$_{4}$O$_{9 }$(CCO), a misfit layered structure consisting of single layer hole-doped CoO$_{2}$ sandwiched between insulating Ca$_{2}$CoO$_{3}$ rocksalt layers, exhibits a high Seebeck coefficient at 1,000 K. It was previously suggested that the Seebeck-coefficient can be further improved by stabilizing an increased Co-ion spin state in the CoO$_{2}$ layers. Here we report a significant increase in the room-temperature in-plane Seebeck coefficient of 40 nm thick CCO films grown by pulsed laser deposition on SrTiO$_{3}$ substrates. We combine aberration-corrected Z-contrast imaging, atomic-column resolved electron energy-loss spectroscopy, and density-functional calculations to show that the increase is caused by CoO$_{2}$ stacking faults with Co$^{4+}$-ions in a higher spin state compared to that of bulk CCO. The higher Seebeck coefficient makes the CCO system suitable for many high-temperature waste-heat-recovery applications. The role of dopants, such as Bi and Ti will also be explored. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A17.00009: Anisotropic thermopower and magnetothermopower in a misfit-layered calcium cobaltite Huaihong Guo, Teng Yang, Zhidong Zhang An unusual anisotropy of thermopower and magnetothermopower has been observed in the powerful thermoelectric Ca$_3$Co$_4$O$_{9+\delta}$ single crystal. The in-plane thermopower is about twice as big as the out-of-plane thermopower. Combining {\em ab initio} band structure calculation with semi-classical model analysis, we understand this anisotropy with band structure effects and especially with anisotropic Fermi surface. We find that a strong anisotropy in the topology of Fermi surface leads to the anisotropy of (magneto)thermopower. This study may also shed light on anisotropic properties of other layered cobalt oxides. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A17.00010: The Origin of Enhanced High Temperature Electron Transport in Thermoelectric Ca$_{3}$Co$_{4}$O$_{9}$ Tao Wu, Trevor A. Tyson, Haiyan Chen, Jianming Bai, Cherno Jaye Temperature dependent measurements of resistivity, crystal structure and heat capacity reveal significant hysteresis occurring near 400 K in Ca$_{3}$Co$_{4}$O$_{9}$. The largest changes in structure occur in the CoO$_{2}$ layer associated with electron transport: manifested mainly by $b_{2}$ axis changes. Application of magnetic fields up to 8 T reduces the area of the resistivity hysteresis loop with saturation at $\sim $4 T. Reduced resistivity associated with this first order phase transition from metallic to semiconducting behavior enhances the thermoelectric properties at high temperatures and points to the metal-insulator transition as a mechanism for improved ZT in high temperature thermoelectric oxide. This work is supported by DOE Grant DE-FG02-07ER46402. The Physical Properties Measurements System was acquired under NSF MRI Grant DMR-0923032 (ARRA award). [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A17.00011: Electrical and Thermal Transport Properties of Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{9-\delta }$ Single Crystals and Thin Films Zhenyu Diao, H.N. Lee, M. Chisholm, Rongying Jin Layered Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{9-\delta }$ possesses rich physical properties, promising for thermoelectric applications. We have successfully synthesized Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{9-\delta }$ in both single crystal and epitaxial thin film forms by applying various oxygen pressures. We found that their electrical and thermal transport properties are sensitive to the oxygen content, suggesting that the oxidation state of Co plays an important role in thermoelectric properties. Comparison of power factor between single crystals and thin films will be presented. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A17.00012: Transport Properties of La- doped SrTiO$_{3}$ Ceramics Prepared Using Spark Plasma Sintering Arash Mehdizadeh Dehkordi, Sriparna Bhattacharya, Terry M. Tritt, Husam N. Alshareef In this work, thermoelectric transport properties of La-doped SrTiO$_{3}$ ceramics prepared using conventional solid state reaction and spark plasma sintering have been investigated. Room temperature power factor of single crystal strontium titanate (SrTiO$_{3})$, comparable to that of Bi$_{2}$Te$_{3}$, has brought new attention to this perovskite-type transition metal-oxide as a potential n-type thermoelectric for high temperature applications. Electronic properties of this model complex oxide, SrTiO$_{3}$ (ABO$_{3})$, can be tuned in a wide range through different doping mechanisms. In addition to A site (La-doped) or B site (Nb-doped) substitutional doping, introducing oxygen vacancies plays an important role in electrical and thermal properties of these structures. Having multiple doping mechanisms makes the transport properties of these perovskites more dependent on preparation parameters. The effect of these synthesis parameters and consolidation conditions on the transport properties of these materials has been studied. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A17.00013: Transition-metal-based perovskite oxides for enhanced thermopower Stanislaw Kolesnik, Bogdan Dabrowski, Krzysztof Wojciechowski, Konrad Swierczek Due to the enhancement of thermopower by spin and orbital degrees of freedom, transition-metal-based perovskite oxides are good candidates for stable and nontoxic materials with a large thermoelectric figure of merit ZT. We have investigated the most promising Mn-, Co- and Ti-based perovskite oxides. Electron doping of SrMnO$_{3}$ materials on either Mn or Sr sites induces a rapid decrease in both electrical resistivity and thermopower with the doping level due to the introduction of itinerant charge carriers. The thermopower of electron-doped SrTiO$_{3}$ materials satisfy the basic Heikes description, however, no additional enhancement is observed. The hole-doped $R$CoO$_{3}$ perovskites exhibit limited solubility of alkaline earth's for small rare earth ion sizes. The dependence of thermopower on charge doping and temperature appears to follow the extended Heikes formulation only at low doping and below 300 K, which indicates that Co$^{3+}$ and Co$^{4+}$ exist in several spin states beyond that range. Among all investigated compounds the largest ZT$\sim $0.3 values were observed for 3-8{\%} Nb-substituted SrTiO$_{3}$ materials at about 700 K. Supported by the U.S. DOE-BES DE-AC02-06CH11357. [Preview Abstract] |
Session A18: Dynamics in II-VI Nanocrystals
Sponsoring Units: DCMPChair: Joseph Tischler, Naval Research Laboratory
Room: 252B
Monday, February 27, 2012 8:00AM - 8:12AM |
A18.00001: Blinking and spectral diffusion of CdSe/ZnS nanoparticles Axel Lorke, Daniel Braam, Andreas M\"{o}lleken, Matthias Offer, G\"{u}nther Prinz, Martin Geller Even though the tunable optical properties of colloidal nanoparticles have been studied extensively, their luminescent behaviour is still not fully understood. The random emission intermittency and the power-law of on- and off-times as well as shifts in the emission wavelength still lack a comprehensive understanding [1]. We investigate the excitonic structure of CdSe/ZnS core/shell nanoparticles using a micro-photoluminescence (PL) setup with confocal as well as imaging optics. The nanoparticles are dispersed in toluene with 1\% PMMA and deposited by spin-coating on different substrates (bare Si/SiO$_{2}$ as well as Si/SiO$_{2}$ covered with different rough metallic layers). Depending on the substrate, we observe emission intermittency or nearly blinking-free emission with spectral jumps of 25 meV in the emission energy. Both can be assigned to excitonic transitions affected by additional charge inside or outside the nanoparticle [2]. Furthermore, we observe a phonon replica of 25 meV and smaller ($<$10 meV) energetic shifts of the emission lines, which are likely caused random charge variations in the environment of the nanoparticle. \\[4pt] [1] P. Frantsuzov et al., Nature 4, 519 (2008). \\[0pt] [2] A. Efros, Nature Mat. 7, 612 (2008) [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A18.00002: Photoluminescence blinking and carrier dynamics in giant nanocrystals with different electron confinement Sid Sampat, Yijun Guo, Javier Vela, Anton Malko Quantum dots have shown great promise as high quantum yield photon sources for applications in bioimaging, LEDs, lasers, etc. However, their photoluminescence (PL) intermittency (blinking) often complicates practical implementations. Recently, a new breed of giant nanocrystal quantum dots (gNQDs) with a large number of shell monolayers (ML) has been developed that show strongly suppressed blinking\footnote{Y. Chen \textit{et al.}, \textit{JACS}\textbf{ 130}, 5026 (2008)} and existence of multiexcitons.\footnote{Y.S. Park \textit{et al.,} \textit{Phys. Rev. Lett}. \textbf{106}, 187401 (2011)}$^,$\footnote{A. V. Malko \textit{et al.}, \textit{Nano Lett}., accepted (2011)} So far, their PL emission has been limited to around 630nm. In this work, we broadened this approach and extended gNQD emission to shorter wavelength in the visible spectrum. We investigated photostable CdSe/CdS gNQDs with small (480nm emission) core and compared them to large (625nm emission) core non-blinking gNQDs with similar shell thickness (14-17 ML). The small core dots show increased blinking behavior and shorter PL decay times in comparison to large core dots. The observed difference in blinking behavior is suggestive of different carrier confinement regimes leading to enhanced electron trapping at the dot's surface as well as modifications to non-radiative Auger recombination rates. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A18.00003: Lifetime Blinking in Non Blinking Quantum Dots Victor Klimov, Yagnaseni Ghosh, Andrea Steinbrueck, Jennifer Hollingsworth, Han Htoon, Christophe Galland Photoluminescence (PL) blinking is a common property of nanoscale light emitters. Nanocrystal quantum dots have often been used as model systems in studies of this intriguing phenomenon. Here, we use recently developed thick-shell CdSe/CdS NQDs to demonstrate a new regime of blinking where discrete fluctuations in the PL lifetime (``lifetime blinking'') occur without appreciable changes in the PL intensity. Single-dot measurements under controlled electrochemical charge injection [1] yield the PL lifetimes of neutral and charged excitons. We show that the observed ``lifetime blinking'' are due to random charging/discharging of the nanocrystal [2]. Indeed, the injection of electrons does not appreciably modify the PL quantum yield, which explains the coexistence of a nonblinking intensity with a ``blinking'' lifetime. At higher excitation power, charged excitons dominate the PL emission. We build a quantitative model showing that nanocrystal charging is caused by Auger-assisted ejection of a hole, producing negatively charged species. Importantly, Auger recombination that involves excitation of an electron is suppressed while hole-based processes remain efficient.\\[4pt] [1] Galland \textit{et al.}, Nature \textbf{479}, 203-207 (2011)\\[0pt] [2] Galland \textit{et al}., Submitted (2011) [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A18.00004: Two types of luminescence blinking revealed by spectroelectrochemistry of single quantum dots Christophe Galland, Yagnaseni Ghosh, Andrea Steinbrueck, Milan Sykora, Jennifer Hollingsworth, Victor Klimov, Han Htoon The phenomenon of fluorescence intermittency (blinking between ON/OFF states) has been observed for both naturally occurring fluorophores and artificial nanostructures. This study aims to resolve the long-standing controversy surrounding the origin of photoluminescence blinking in core/shell nanocrystal quantum dots. Researchers usually evoke the Auger, or A-type, mechanism in which photo-ionization of the dot leads to the OFF state, but recent observations have raised doubts about this explanation. Here we report time-resolved photoluminescence studies of individual nanocrystal quantum dots performed while electrochemically controlling the degree of their charging [1]. We find that a second mechanism (called B-type) is the dominant cause for blinking. During B-type blinking, a photo-excited, ``hot'' electron is trapped in a surface state before being released to the core; the luminescence is quenched without any Auger process. By controlling the applied potential and the shell thickness, we can control the frequency and type of blinking, or suppress it completely. \\[4pt] [1] Galland \textit{et al.}, Nature \textbf{479}, 203-207 (2011). [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A18.00005: Ultrafast Cathodoluminesence Studies of Colloidal Nanocrystals Lazaro Padilha, Wan Ki Bae, Victor Klimov, Jeffrey Pietryga, Richard Schaller Despite possessing numerous desirable properties, including excellent photostability, high stopping-power, and fast, efficient fluorescence, semiconductor nanocrystal quantum dots are only mediocre gamma-ray scintillator materials. Efforts to improve performance are forestalled by a lack of understanding of how those materials respond to high energy radiation, which in turn comes from a lack of appropriate ultrafast tools for examining the relaxation of gamma-excited quantum dots. To this end, we have developed a 20keV as a surrogate for spontaneous gamma irradiation. We apply this technique to study the time-resolved response of thin films of CdSe/ZnS core/shell quantum dots. Energetic excitation produces a variety of excited states that can be separately resolved by consideration of their established energies and relaxation dynamics. We analyze the relative branching ratios of single excitonic, multi-excitonic and charged excitonic states to derive surprising conclusions regarding the physics of highly-excited quantum dots, as well as the probable source of poor gamma-scinitillating performance. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A18.00006: Dynamic fluctuations in ultrasmall nanocrystals induce white light emission Timothy J. Pennycook, James R. McBride, Sandra J. Rosenthal, Stephen J. Pennycook, Sokrates T. Pantelides Nanocrystals typically emit monochromatically at their size-dependent energy gaps. Recently, it was found that by pushing the size of a nanocrystal to its lower limits, absorption occurs at increasingly larger energies, but the expected blue to ultraviolet emission does not occur. Instead, ultrasmall nanocrystals begin to emit a broader spectrum. For instance, ultrasmall CdSe nanocrystals emit white light [1]. We have investigated small to ultrasmall CdSe nanocrystals using a combination of state-of-the-art scanning transmission electron microscopy and finite-temperature quantum molecular dynamics simulations. Our findings indicate that following excitation, partial thermalization sets the ultrasmall nanocrystals into a fluxional state, with a continuously varying energy gap, which results in white light emission when averaged over time. Furthermore, although the larger monochromatic emitting nanocrystals we have observed possess stable crystal cores, their surfaces are fluxional. Dynamic fluxionality should be taken into consideration when optimizing nanocrystals for applications. This work is supported by DOE grant DE-FG02-09ER46554 and Basic Energy Sciences Materials Sciences and Engineering Division.\\[4pt] [1] Bowers II, M.J. et al. J. Am. Chem. Soc 127, 15378 (2005). [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A18.00007: Dynamics of charge separation and multicarrier recombinaiton in CdS/CdTe heterodimer nanocrystals Seiji Taguchi, Masaki Saruyama, Toshiharu Teranishi, Yoshihiko Kanemitsu Heterostructured semiconductor nanocrystals provide new ways to manipulate electronic wave functions and carrier recombination pathways. Recently, we developed a novel anion exchange reaction and synthesized anisotropically phase-segregated CdS/CdTe heterodimers with staggered band-edge alignment [1]. Here we report the ultrafast carrier dynamics in the heterodimers measured by transient absorption spectroscopy. While pump laser energy was tuned to create excitons only in the CdTe phase, we observed the bleaching of the CdS excitonic transition. The bleaching of CdS exciton states should be induced by electron injection from the CdTe phase to the CdS phase. Very rapid electron transfer time was evaluated to be about 400 fs. Moreover we found that temporal evolutions of CdS excitonic bleaching are almost independent of excitation intensity over a wide range, implying the suppression of Auger recombination. Our results indicate that charge separation efficiency of the heterodimers is enhanced due to their centrosymmetry-broken structures, and the designed nanocrystals are useful for next generation solar cells. [1] Saruyama et al., J. Am. Chem. Soc. 133, 17598 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A18.00008: Transient Photoluminescence in Ligand-Exchanged Quantum Dot Assemblies Michael E. Turk, Patrick M. Vora, Aaron T. Fafarman, Benjamin T. Diroll, Christopher B. Murray, Cherie R. Kagan, James M. Kikkawa Improving electronic contact between nanocrystals (NCs) within self-assembled NC solids is a long standing goal, and recent work on ligand exchange using ammonium thiocyanate has demonstrated marked improvements in the charge mobility of NC films over longer, more insulating ligands [1]. Here, we use transient photoluminescence (PL) to study changes in radiative lifetime caused by ligand exchange. Our work begins by examining differences in lifetime and radiative efficiency between liquid dispersions and solid films of CdSe semiconductor NCs, and moves on to study additional changes in both types of samples correlated with ligand substitution. We use a combination of time-correlated single photon counting and non-linear optical Kerr gating to study PL on nanosecond and sub-picosecond time frames, respectively. We discuss the relationship between nanosecond and picosecond dynamics, and examine temperature dependence from 300 to 5 Kelvin. Initial data indicate a decrease in PL lifetime with ligand exchange, which we discuss in the context of increased transport mobilities and decreased interparticle separations, as well as changes in the steady-state optical spectra of these systems. [1] A.T. Fafarman, et al, J. Am. Chem. Soc., 133, 15753 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A18.00009: Optically Excited Exciton Transfer of Spherical Quantum Dots via Optical Near-Field Interactions Togay Amirahmadov, Pedro Ludwig Hernandez Martinez, Hilmi Volkan Demir We study a system composed of a mixture of different-sized spherical quantum dots (QDs) involving optical near-field (ONF) interactions to induce effective optical excitation transfer. Here energy transfer was explained by resonant energy transfer via the optical near-field interaction between the first excited state of small QDs and the second excited state of large QDs. The energy transfer in a film of different-sized QDs made of CdTe and CdSe were experimentally demonstrated. An analysis between the optical near field transfer rate [1] and F\"{o}rster type transfer rate was made. The proper understanding of the exciton transfer between these QDs is important for the design and implementation of near-field photonic devices employing them. [1] M. Ohtsu, et al., Principles of Nanophotonics, CRC Press (2008). [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A18.00010: Efficient Exciton Transfer from an Epitaxial Quantum Well to an Energy Gradient Structure Composed of Layer-by-Layer Assembled Colloidal Quantum Dots Sedat Nizamoglu, Pedro Ludwig Hernandez Martinez, Evren Mutlugun, Hilmi Volkan Demir In this work, we study exciton migration from a violet-emitting epitaxial quantum well (QW) to an energy gradient structure that consists of layer-by-layer assembled, green- and red-emitting quantum dot (QD) bilayer. In the experimental study, the energy gradient of these green and red QDs provides an increase of 64.2{\%} in the exciton transfer efficiency with respect to the bilayer of only red-emitting QDs. These results suggest that the energy difference between the QD layers significantly boosts the QW-QD exciton transfer rate compared to the mono-dispersed case. To support this experimental observation, we propose a theoretical model based on optical near field and density matrix to investigate the effects of energy difference between the QD layers. The strong exciton transfer from the epitaxial QWs to the colloidal QDs is essential to the energy efficiency of hybrid optoelectronic devices [1-3].\\[4pt] [1] A. Ruland, et al., Adv. Mater. 23, 4573--4577 (2011).\\[0pt] [2] M. Naruse, et al., Phys. Rev. 82, 125417 (2010).\\[0pt] [3] S. Nizamoglu, et al., Appl. Phys. Lett. 98, 163108 (2011). [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A18.00011: Anomalous photo-induced spectral changes in CdSe/ZnS quantum dots Georgiy Shcherbatyuk, Richard Inman, Patrick Talbot, Sayantani Ghosh We study photo-induced static and dynamic spectral changes in self-assembled CdSe/ZnS quantum dot (QD) thin films with varying QD concentrations under ambient conditions. Using spatially resolved scanning photoluminescence microscopy in conjunction with spectrally resolved time-correlated photon counting, we measure the variations in spectral intensity, emission wavelength, and recombination lifetimes as functions of photo-exposure time. We find that at low concentrations photo-darkening and photo-oxidation rates slow down with increasing QD density, but in the high concentration limit these rates are strongly enhanced. Our measurements lead us to conclude that the interplay of photo-induced surface trap discharging with preferential photo-oxidation of smaller QDs is further modulated by resonant energy transfer driven by strong inter-dot interactions in highly concentrated samples.~ Finally, we extend our studies to thin films with two different QD diameters to vary the ratio of donors to acceptors and modify the energy transfer efficiency [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A18.00012: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 10:24AM - 10:36AM |
A18.00013: Waveguide Integrated Colloidal Quantum Dot Emitters Mingming Jiang, Dongfang Li, Rashid Zia High-yield II-VI colloidal quantum dots (QDs) have received considerable attention as quantum emitters for photonic device applications. Given their scalable synthesis and self-assembly, these QDs could serve as the basis for large arrays of single photon sources. Here, we present preliminary results on the integration of CdSe-based QDs into dielectric waveguides. Specifically, we will show how the local optical environment can be used to direct and enhance QD emission into specific spatial and spectral modes. First, we will present fabrication techniques to safely embed QDs into thin film dielectric waveguides. Then, we will present experimental back-focal-plane (BFP) measurements that demonstrate how thin-film interference effects can be used to the control the QD angular emission patterns. Experimental BFP data will be discussed and analyzed in terms of theoretical predictions based on the local density of optical states (LDOS), thus demonstrating how interference effects can be used to effectively direct emission from randomly oriented QDs. If time permits, we will also show how the extension of these techniques to individual QD emitters in ridge waveguides. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A18.00014: Comparison of carrier multiplication yields in the PbX salts Jayson Stewart, Lazaro Padilha, Jeffrey Pietryga, Aaron Midgett, Joseph Luther, Matthew Beard, Arthur Nozik, Victor Klimov In this talk I will present recent results of a collaborative effort investigating the quantum efficiency of carrier multiplication (CM) of the lead salt nanocrystals: PbS, PbSe and PbTe. These materials are promising candidates for exploring generation-III photovoltaic concepts that rely on carrier multiplication, the process in which a single photon generates more than one electron-hole pair. Despite the many apparent similarities of these materials in their bulk form, we find that these compounds exhibit strikingly different CM yields in their nanocrystalline form. We suggest that the difference in CM yields in these nanomaterials is the consequence of different competing relaxation rates, such as phonon emission. Indeed, we estimate the rate of energy dissipation due to phonon emission mediated by a polar Frohlich-type interaction for these three materials and find excellent qualitative agreement with the data. This approach could prove useful for predicting future materials to investigate for increasingly high CM yields. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A18.00015: ABSTRACT WITHDRAWN |
Session A19: Invited Session: New Correlated Electron Physics Using Scanning Tunneling Microscopy and Other Probes
Sponsoring Units: DCMPChair: Abhay Pasupathy, Columbia University
Room: 253AB
Monday, February 27, 2012 8:00AM - 8:36AM |
A19.00001: Local Kondo screening and spatial coherence in YbRh$_2$Si$_2$ Invited Speaker: Steffen Wirth Heavy fermion metals are often characterized by a variety of relevant energy scales and competing interactions which may result in such fascinating phenomena as quantum criticality and unconventional superconductivity. Therefore, these materials have advanced to suitable model systems by means of which electronic interactions can be studied in detail. This will be discussed for the interplay of localized and itinerant electronic states in Kondo lattice systems in which heavy charge carriers are generated. We investigate the generic Kondo lattice system YbRh$_2$Si$_2$, one of the heaviest heavy fermion metals, by utilizing atomically resolved Scanning Tunneling Spectroscopy (STS) [1]. An analysis of the topography allows for a determination of the terminating surface as well as a comparison to results from chemical analysis. Importantly, the crystal field excitations are unambiguously reflected by our STS measurements, clearly relating STS to bulk properties. The hybridization of conduction and 4$f$ electrons results in a gap-like feature in the tunneling conductance. In addition, a strongly temperature dependent peak in the tunneling conductance is attributed to a resonance resulting from the Kondo lattice. The experimental data are discussed in relation to results obtained within the non-crossing approximation (NCA) and renormalized band structure calculation. In a brief outlook we discuss further investigations by STS, e.g.\ with respect to the quantum critical phenomena observed in YbRh$_2$Si$_2$ [2], to substitutions-induced changes of the relevant energy scales [3], or on heavy fermion superconductors. \\[4pt] [1] S. Ernst {\it et al.}, Nature {\bf 474} (2011) 362.\\[0pt] [2] S. Friedemann {\it et al.}, Proc. Natl. Acad. Sci. USA {\bf 107} (2010) 14547.\\[0pt] [3] S. Friedemann {\it et al.}, Nature Phys. {\bf 5} (2009) 465. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A19.00002: Correlated Spin Phenomena in Molecular Systems Invited Speaker: Wilson Ho While a great deal of work has been carried out on molecular magnets, the spatial distribution of the spin wave function and the many body interactions between the delocalized molecular spin and its surrounding electrons can now be obtained with atomic scale resolution with the scanning tunneling microscope (STM). The combination of surface science, self-assembly, and STM enables correlated spin phenomena, such as the Kondo state, to be probed in a wide range of well characterized systems from single molecules to a two-dimensional lattice of interacting spins. Nonlocality, Kondo gap, and the Kondo lattice in correlated electron physics are revealed by the atomic-scale spatial resolution and high energy resolution spectroscopy and imaging with the STM from oxygen to porphyrins and phthalocyanine molecules adsorbed on metal and oxide surfaces. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A19.00003: Long-range Kondo signature of a single magnetic impurity Invited Speaker: Martin Wenderoth Scanning tunneling spectroscopy (STS) has provided an approach to study the Kondo effect - one of the oldest many particle phenomena known in condensed matter physics - in real space. In spite of the high spatial resolution of scanning tunneling spectroscopy, experiments performed on single magnetic atoms on metal surfaces, have shown that the fingerprint of the Kondo effect is only visible if the tip is placed directly above the impurity (e.g. [1]). In the present work we follow a novel route and investigate single isolated Co and Fe impurities not on top but buried below a Cu(100) surface. It has been shown recently [2] that the anisotropy of the copper Fermi surface leads to a strongly directional propagation of quasi particles called electron focusing which gives access to individual bulk impurities in a metal. We have studied the energy-dependent scattering characteristics for single isolated atoms of Ag, Co and Fe buried under a Cu(100) surface using low temperature scanning tunnelling spectroscopy (STS). For the case of a non-magnetic Ag impurity a Friedel oscillation in the local density of states is observed. For both magnetic impurity atoms we observe, in contrast to previous works, a long range Kondo signature which is periodic with the distance to the impurity [3]. The comparison of Co and Fe atoms demonstrates that both impurity species show similar behavior on completely different energy scales, which is determined by the Kondo temperature. We investigate the scattering amplitude as well as the phase. A theoretical interpretation based on a combined approach of band structure and many-body numerical renormalization group calculations is able to describe the rich spatially and spectroscopically resolved experimental data. \\[4pt] [1] M. Ternes et al., Journal of Physics: Condensed Matter 053001 (2009) \\[0pt] [2] A. Weismann et al., Science 323, 1190 (2009) \\[0pt] [3] H. Prueser et al., Nature Physics 7,203 (2011) [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A19.00004: Defects in Heavy-Fermion Materials: Unveiling Strong Correlations in Real Space Invited Speaker: Dirk Morr Heavy-fermion materials exhibit a plethora of puzzling phenomena which are believed to arise from the competition between Kondo screening and antiferromagnetic ordering. The microscopic origin of these phenomena, such as the non-Fermi-liquid properties observed in the quantum critical region, is still a topic of debate. Recent groundbreaking scanning tunneling spectroscopy (STS) experiments [1-4] have shed new light on this debate by providing important insight into the electronic and magnetic structure of heavy fermion materials. In this talk, I review some recent progress made in our theoretical understanding of the differential conductance, dI/dV and the resulting quasi-particle interference (QPI) patterns [5-7], observed in these experiments. In particular, I will demonstrate that defects in heavy-fermion materials provide an unprecedented opportunity to disentangle electronic correlations arising from Kondo screening, and antiferromagnetic correlations between the magnetic moments by inducing perturbations in the electronic and magnetic structure that exhibit characteristically different spatial patterns. The spatial extent of these perturbations grows with the strength of the magnetic interactions, and thus directly reflects the degree of correlations [5]. In addition, I show that non-magnetic defects (Kondo holes) in heavy fermion materials can give rise to the formation of an impurity bound. Our prediction of spatial hybridization oscillations and the formation of an impurity bound state were recently confirmed by STS experiments [4]. Moreover, I will demonstrate that QPI spectroscopy, utilizing spatial oscillations in the LDOS induced by defects, does not only provide important insight into the electronic structure of heavy fermion materials, but also in the entanglement of electronic and magnetic states [6,7]. Finally, the strongly correlated nature of heavy-fermion materials leads to a highly non-linear quantum interference between defects, and the creation of order from disorder. These results provide unique insight into the spatial complexity of heavy fermion materials. \\[4pt] [1] A.R. Schmidt et al., Nature 465, 570 (2010). \\[0pt] [2] P. Aynajian et al., PNAS 107, 10383 (2010). \\[0pt] [3] S. Ernst et al., Nature 474, 362 (2011). \\[0pt] [4] M. Hamidian et al., PNAS 108, 18233 (2011). \\[0pt] [5] J. Figgins and D.K. Morr, Phys. Rev. Lett. 107, 066401 (2011). \\[0pt] [6] J. Figgins and D.K. Morr, Phys. Rev. Lett. 104, 187202 (2010). \\[0pt] [7] T. Yuan, J. Figgins, and D.K. Morr, arXiv:1101.2636. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A19.00005: Tunable Kondo Effect in SrTiO$_3$ Invited Speaker: James Williams Correlated, low-dimensional systems offer the possibility of tuning complex physical phenomena. Using strong electric fields applied with an electrolytic gate, we continuously tune through three regimes of transport in SrTiO$_3$: an insulator, a metal and a Kondo metal. Two of these regimes -- the metallic and Kondo metal state -- are investigated as a function of temperature and magnetic field, where clear signatures of each regime are evident and discussed. This diverse system not only displays behavior distinct from conventional two-dimensional electron gases, but also show similarities to the LaAlO$_3$/SrTiO$_3$ interface, elucidating on some of the phenomena observed in this heterostructure of debated origin. [Preview Abstract] |
Session A20: Invited Session: Advanced Quantum Materials for Future Information Technology
Sponsoring Units: FIAPChair: Cherry Murray, Harvard University
Room: 253C
Monday, February 27, 2012 8:00AM - 8:36AM |
A20.00001: Graphene-based Electronics and Optoelectronics Invited Speaker: Phaedon Avouris Graphene has rather unique electrical and properties and there is currently strong interest in taking advantage of these properties for technological applications. In my talk I will review some of the key properties of free graphene, how these properties are affected by environmental interactions and under technologically relevant conditions, and how they can be utilized in electronics and optoelectronics. In electronics, I will focus on high frequency ($\ge $ 300 GHz) graphene transistors and simple IC circuits, as well as related device physics problems, such as the role of electrical contacts, scattering effects, graphene topology, device size scaling, energy dissipation, etc. I will then review the key optical properties of graphene and their use in optoelectronics. Specifically, I will focus on the far-infrared and THz range of the spectrum, on ways of controlling graphene's absorption in this spectral range and provide examples of corresponding applications. I will also discuss photocurrent generation in graphene and its use in ultrafast graphene photodetectors. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A20.00002: Visualizing Helical Metals on Topological Insulators Invited Speaker: Ali Yazdani During the last few years, it has become apparent that there can be a distinct type of insulator, which can occur because of the topology of electronic wavefunctions in materials comprised of heavier elements. Strong interaction between the spin and the orbital angular momentum of electrons in these compounds alters the sequence in energy of their electronic states. The key consequence of this topological characteristic (and the way to distinguish a topological insulator from an ordinary one) is the presence of metallic electrons with helical spin texture at their surfaces. I will describe experiments that directly visualize these novel quantum states of matter and demonstrate their unusual properties through spectroscopic mapping with the scanning tunneling microscope (STM). These experiments show that the spin texture of these states protects them against backscattering and localization. These states appear to penetrate through barriers that stop other electronic states. I will also describe more ongoing efforts focused on unraveling the physics of topological surface states and their potential for device-like applications. \\[4pt] References: \\[0pt] P. Roushan et al, Nature \textbf{460 }1106 (2009). \\[0pt] J. Seo et al, Nature \textbf{466 }343 (2010). \\[0pt] H. Beidenkopf et al, to appear Nature Physics (2011). [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A20.00003: Quantum information processing with defect spins in diamond and silicon carbide Invited Speaker: William Koehl Many proposals for quantum information technologies require quantum systems that can be easily manipulated by an outside observer, while remaining largely unaffected by destructive interactions with the surrounding environment. One system that matches this description is a defect in the crystal lattice of diamond known as the nitrogen-vacancy (NV) center. Electrons trapped at this defect form an atomic scale spin state that can be used as an individually addressable, solid state quantum bit (qubit) even at room temperature. The exceptional quantum properties of the diamond NV have motivated recent efforts to search for similar defects in other semiconductors, as these would expand the technological opportunities available to defect-based quantum systems [1]. We discuss these efforts, which make use of techniques from both computational materials science and experimental quantum physics, focusing on explorations of the 4H polytype of silicon carbide (4H-SiC). In particular, we present recent experimental results that identify several defect spin states in 4H-SiC that function as analogs to the diamond NV. Using optical and microwave techniques similar to those used with diamond NV qubits, the spins of these defects can be optically addressed and coherently controlled in the time domain at temperatures ranging from 20 -- 300 K. Additionally, these defects are optically active near telecom wavelengths, inhabit a host material for which there already exist industrial scale crystal growth and advanced microfabrication techniques, and possess desirable spin coherence properties comparable to those of the diamond NV. This makes them promising candidates for various photonic, spintronic, and quantum information applications that merge quantum degrees of freedom with classical electronic and optical technologies [2]. \\ \\ {[1]} J. R. Weber*, W. F. Koehl*, J. B. Varley*, A. Janotti, B. B. Buckley, C. G. Van de Walle, and D. D. Awschalom, \emph{Proc.~Natl~Acad.~Sci.~USA} \textbf{107}, 8513 (2010).\\ {[2]} W. F. Koehl, B. B. Buckley, F. J. Heremans, G. Calusine, and D. D. Awschalom, \emph{Nature} \textbf{479}, 84 (2011); A. Dzurak, \emph{Nature} \textbf{479}, 47 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A20.00004: Quantum Transport in Dirac Materials Invited Speaker: Pablo Jarillo-Herrero Over the past few years, the physics of low dimensional electronic systems has been revolutionized by the discovery of materials with very unusual electronic structures. Among these, graphene and topological insulators have taken center stage due to their relativistic-like electron dynamics and their potential applications in nanotechnology. In this talk I will briefly review the properties of graphene and topological insulators and discuss some of our recent quantum electronic transport experiments in these systems. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A20.00005: GHz - THz plasmonic circuits using low dimensional electronic systems Invited Speaker: Donhee Ham Nature offers a broad variety of plasma systems consisting of electrons unbound from atoms, e.g.; astrophysical plasmas in intergalactic, interstellar, and stellar media; the Earth's ionosphere; and solid-state plasma, the free electrons in metals and semiconductors, only to name a few. A key feature of many plasma systems is collective motions of electrons; as the electron density profile is perturbed from equilibrium, Coulomb restoring forces (and sometimes quantum pressure in dense plasma) arise to power these collective motions, usually in the form of bulk electron density oscillations or electron density waves. Solid-state plasmas are particularly interesting, as the fabrication technologies available for solid-state materials allow us to alter the boundaries and interfaces of the plasma media in various ways to engineer the collective motion. A notable example is the surface plasmons, which have been a source of many breakthroughs in photonics. I will talk about a set of our recent developments where the plasmons are brought down to the electronics-regime (GHz$\sim $THz) and manipulated to produce a range of functionalities, while offering unique advantages to electronics over their purely electromagnetic counterparts. (Co-workers) William Andress (Harvard), Hosang Yoon (Harvard), Kitty Yeung (Harvard), Ling Qin (Harvard), Ken West (Princeton), and Loren Pfeiffer (Princeton). [Preview Abstract] |
Session A21: Superconductivity: Josephson, Tunneling and Proximity Effects
Sponsoring Units: DCMPChair: John Wei, University of Toronto
Room: 254A
Monday, February 27, 2012 8:00AM - 8:12AM |
A21.00001: Equal-spin Andreev reflection from quasiparticle interference effects in high-T$_{C}$/half metallic ferromagnet junctions C. Visani, R. Bernard, J. Briatico, M. Bibes, A. Barth\'el\'emy, J.E. Villegas, J. Tornos, Z. Sefrioui, J. Santamaria We report evidence for long-range superconducting correlations in the half-metallic ferromagnet (F) La$_{0.7}$Ca$_{0.3}$MnO$_{3}$, obtained from conductance measurements along the c-axis in SFS and SF junctions (with S the high-T$_{C}$ superconductor YBa$_{2}$Cu$_{3}$O$_{7})$. Well below the superconducting T$_{C}$, we observed oscillations in the differential conductance as a function of the voltage bias, in which we identified two sets of resonances: i) quasiparticle interferences in the S (top) layer (Tomasch resonances) and ii) in the half-metallic F layer (McMillan-Rowell resonances). Both of them imply Andreev-like reflections at the SF interface and the coherent propagation of the resulting phase-conjugated quasiparticles through the entire S and F layers thickness (up to 30 nm for the F layer). Because conventional Andreev reflection and the coherent long-range propagation of the outcoming opposite-spin electron/hole pairs are heavily suppressed in strongly polarized ferromagnets, the observation of the above effect implies the occurrence of equal-spin Andreev reflection at the studied SF interfaces. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A21.00002: Proximity Effect in Mesoscopic Superconductor-Normal-Superconductor Arrays Serena Eley, Sarang Gopalakrishnan, Paul Goldbart, Nadya Mason Systems of superconducting islands on normal metal films provide a tunable medium with which to study the superconducting proximity effect, phase transitions, and vortex dynamics. Such systems are predicted to exhibit 2D zero-temperature metallic states. Although there has been experimental evidence of such states, they cannot be explained by conventional transport theory. Here, we report transport measurements on triangular arrays of mesoscopic, proximity-coupled Nb islands placed on normal metal Au films. The arrays undergo a two-step transition to a superconducting state; we characterize the superconducting transitions in these systems as a function of island thickness and spacing. The temperature of the first step of the transition linearly decreases with increasing island spacing, and the spacing-dependence of the second step deviates from conventional theories. Moreover, the trends of both steps suggest that the system is approaching zero-temperature metallic states. Through a phenomenological model, we resolve these transitions as a consequence of intra- and inter-island coupling between superconducting phases of individual Nb grains. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A21.00003: Coherent Two Photon Production in Superconductor-Semiconductor Heterostructures Paul Baireuther, Ilya Vekhter, J\"org Schmalian Connecting a thin (direct band gap) p-n junction to a superconductor allows Cooper pairs to tunnel into the junction. This leads to an enhancement of the luminescence at the junction via Cooper pair based radiative recombination[1,2], an effect that has recently been observed experimentally[3]. Due to the proximity-induced Cooper pairs in the junction, anomalous photon production related to coherent two photon processes becomes allowed. Using a simple model for direct band gap luminescence we study a superconductor-p-n-superconductor heterostructure where the two photon state depends on the relative phase between the two superconductors. We investigate to what extend the production rate of entangled photons is controlled by the phase difference between the attached superconductors. \newline \newline [1] E. Hanamura, Phys. Stat. Sol. (b) 234, 166 (2002). \newline [2] Y. Asano, I. Suemune, H. Takayanagi, and E. Hanamura, Phys. Rev. Lett. 103, 187001 (2009). \newline [3] I. Suemune, T. Akazaki, K. Tanaka, M. Jo, K. Uesugi, M. Endo1, H. Kumano, E. Hanamura, H. Takayanagi, M. Yamanishi and H. Kan, Jpn. Journ. of Appl. Phys. 45, 9264 (2006). [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A21.00004: Field-Dependent Effects in Superconductors Coupled to Semiconducting Heterostructures Stephanie Law, Wade DeGottardi, Smitha Vishveshwara, Nadya Mason, James Eckstein We report transport measurements between superconducting leads separated by a small InAs gap. The NbTi superconducting layer is grown in-situ on top of the InAs to allow good contact. The samples are then fabricated into Hall bars with narrow gaps between the superconducting leads. Differential resistance and IV characteristics are measured in two and four terminal setups at 300mK both on and off quantum Hall plateaus. Multiple Andreev reflection peaks are observed (up to n=4 in some cases) and their field dependence measured. As the field increases, the MAR peaks shift to lower voltages and follow a general scaling law which holds across devices with different gap lengths. One device shows a supercurrent at all fields and in this device, MAR peaks corresponding both to the superconducting gap as well as a proximity-induced normal gap are seen. Explanations for the field-dependence of the MAR peaks will be discussed. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A21.00005: Transport Properties Characterized by Magnetic Domain-Wall Motion in Josephson Junction Michiyasu Mori, Sin-ichi Hikino, Wataru Koshibae, Sadamichi Maekawa Nano-scale magnetic materials for spintronics devices are extensively studied due to many advantages. Non-volatile memory using a magnetic domain wall (DW) is one example of such devices, and the oscillatory DW is examined toward several applications. Once such devices are realized in a microscopic circuit, one needs to measure the DW motion more precisely. In this talk, we discuss the transport properties characterized by a magnetic domain wall (DW) motion in a ferromagnetic Josephson junction, which is composed of a ferromagnetic wire with DW and two superconducting electrodes. Our previous theory [1] is developed to include the DW motion into the Josephson current. By supposing a simplified DW structure, we find that the current-voltage curve exhibits stepwise structures, when DW oscillates in the ferromagnetic wire. The mechanism behind this result is common to the electric field generated by the vortex motion in the superconductor. \\[4pt] [1] S. Hikino, M. Mori, S. Takahashi, and S. Maekawa, J. Phys. Soc. Jpn, {\bf 77}, 053707 (2008). [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A21.00006: Dramatic suppression of the Josephson effect in floating superconductors Maria Jose Martinez-Perez, Fabio Taddei, Francesco Giazotto A number of devices exploiting the Josephson effect consist of floating superconducting islands. A well-known example are SISIS junctions (where S and I stand for superconductor and insulating barrier, respectively) present in many applications such as SQUIDs. The Josephson effect relies on the macroscopic phase describing the wave function of the electrons in the superconducting electrodes. This phase is fixed by the electrochemical potential of the superconductor that is settled experimentally. An intriguing situation arises then when dealing with floating superconducting islands that have no well-defined reference potential. Here we present an experimental characterization of Al/AlOx/Al/AlOx/Al junctions. We focus, on the one hand, on genuine SISIS junctions in which the central aluminium island remains floating and, on the other, on virtually grounded samples in which the central aluminium island is referred to a fixed potential through a metallic contact. We observe that, under identical circumstances, floating junctions exhibit a dramatic suppression of their Josephson current compared to their grounded counterparts. Possible reasons such as the overheating of the central island or quasiparticle fluctuations are addressed. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A21.00007: Escape and retrapping phenomena in HTS and LTS Josephson Junctions L. Longobardi, D. Stornaiuolo, D. Massarotti, L. Galletti, F. Carillo, G. Papari, A. Kawakami, G.P. Pepe, A. Barone, G. Rotoli, F. Tafuri We investigate escape and retrapping dynamics in Josephson junctions characterized by different levels of dissipation. Measurements are carried out both on high (HTS) and low (LTS) critical temperature superconductor Josephson systems, characterized by different types of barriers, i.e. grain boundary and standard insulating layers. Based on the damping level we observe various regimes ranging from macroscopic quantum tunneling, thermal activation and phase diffusion processes. Experimental data are compared with a numerical model allowing a precise determination of the damping parameter. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A21.00008: Dominant second harmonic in the Josephson current--phase relation: Manifestation of the long-range spin-triplet proximity effect in SFF'S junctions Zoran Radovic, Luka Trifunovic, Milos Knezevic We present theoretical study of the Josephson effect and pairing correlations in planar SFF'S junctions that consist of conventional superconductors connected through two metallic monodomain ferromagnets. Both singlet and triplet pair amplitudes, the Josephson current-phase relations, and density of states for arbitrary orientation of magnetizations are calculated from the self-consistent solutions of Eilenberger equations in the clean limit and for moderate disorder in ferromagnets. We find that in highly asymmetric SFF'S junctions the long-range spin-triplet proximity effect manifests itself as a dominant second harmonic in the Josephson current-phase relation [1] and gives distinctive tunneling conductance spectra [2]. Unambiguous detection of the long-range spin-triplet proximity effect by tunneling spectroscopy and experimental realization of the Josephson junctions ground state degeneracy (like at 0-pi transtions) is accessible for small interface roughness and moderate disorder in ferromagnets at low temperatures. \\[4pt] [1] L. Trifunovic, Z. Popovic and Z. Radovic, Phys. Rev. B 84, 064511 (2011).\\[0pt] [2] M. Knezevic, L. Trifunovic, and Z. Radovic (to be published). [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A21.00009: Andreev bound states in proximitized InAs nanowires Willy Chang, Vladimir Manucharyan, Merritt Moore, Ferdinand Kuemmeth, Charles Marcus, Thomas Sand Jespersen, Jesper Nygard We present measurements of individual Andreev bound states (ABS) in semiconducting InAs nanowires contacted by a superconductor. The two ends of a U-shaped Al lead are deposited on the nanowire to form low-resistance contacts, between which a normal lead is then deposited to form a high-resistance tunnel contact to the nanowire. A tunneling current through the nanowire is formed by grounding the Al leads and by applying a voltage bias to the tunnel contact. Measurements of the differential conductance of the device as a function of voltage bias, magnetic field, and backgate show resonances that are associated with the density of states in the proximitized nanowire. This sub-gap structure depends periodically on magnetic flux, with a period of \textit{$\phi $}$_{0}=h$/2$e$. At voltages and magnetic fields exceeding the gap and the critical field of the Al leads, this structure disappears completely. Further control is achieved via a global backgate, with the tunneling current completely switched off at sufficiently low gate voltages. We interpret this structure as ABS arising from the normal electronic properties of the nanowires. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A21.00010: A Superconductor in the Presence of an Exchange Field and Spin-orbit Interaction Marius Eich, Johannes C. Leutenantsmeyer, Peng Wei, Markus Muenzenberg, Jagadeesh S. Moodera A superconductor (SC) in contact with a magnetic semiconductor experiences a large internal exchange field (IEF) that acts on the spins [1,2]. The quasi particle density of states (\textit{q-DOS}) in the SC splits by the Zeeman energy, 2$\mu $H$_{EX}$ where $\mu $ is the electron magnetic moment and H$_{EX}$ is the IEF [1,2]. Here we combine IEF and spin-orbit (S-O) interaction to investigate the properties of Al by superconductive (SIS) tunneling spectroscopy [1]. Thin film sandwich junction 4EuS/4.5Al/($d$-Au)/Al$_{2}$O$_{3}$/8Al (film thickness in nm and $d$ from 0 to 0.06nm) was studied: ferromagnetic EuS layer provides the IEF whereas the Au layer creates the S-O scattering in Al. Tunnel conductance measured at 0.4K showed a large Zeeman splitting of \textit{q-DOS} in Al film due to the IEF, even at H$_{appl}$ = 0. In zero H the large asymmetry in the conductance peaks and reduction in the Zeeman splitting were observed due to S-O scattering when compared to control junctions (no Au). The sharp features in the SIS tunnel conductance allowed these studies even at the lowest level of S-O scattering.\\[4pt] [1] R. Meservey and P. M. Tedrow, Phys. Rep. 238, 173 (1994).\\[0pt] [2] J. S. Moodera, T. S. Santos and T. Nagahama, J. Phys.: Condens. Matter 19, 165202 (2007) [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A21.00011: Quantum tunneling of Normal-Superconducting interfaces in a type-I superconductor Saul Velez, Antoni Garcia-Santiago, Ricardo Zarzuela, Joan Manel Hernandez, Javier Tejada, Eugene Chudnovsky The magnetic irreversibility in the intermediate state in various disks of pure type-I superconducting lead (Pb) with structural defects of diverse nature is presented. The results are discussed in terms of the contributions of the shape-dependent geometrical barrier (which is the origin of the so-called topological hysteresis) and the energy barriers associated to stress defects that act as pinning centers on normal-superconductor interfaces (NSI). The effect of the defects is to enhance the capability of the system to trap magnetic flux along the descending branch of the hysteresis cycle driving the system in a set of metastable states that would originate the occurrence of time-dependent phenomena. Magnetic relaxation studies reveal that the dynamics of the intermediate state of a type-I superconductor with defects is ruled by nonthermal processes for low enough temperatures. It is attributed to quantum tunneling of NSI mediated by the formation/flattening of bumps at the defects of the sample. The average value of the tunneling barriers is estimated and the temperature of crossover from the thermal to the quantum regime is obtained from the Caldeira-Leggett theory. Comparison between theory and experiment points to tunneling of interface segments of a size comparable to the coherence length, by steps of the order of 1 nm. Finally, the effect of an applied magnetic field on the quantum dynamics of the system is also explored. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A21.00012: Josephson superjunction: an SIS junction with a correlated insulator David Stroud, Chris Porter, Kwangmoo Kim We consider the properties of an SIS Josephson junction, in which the insulating region is itself a superconductor, but below its superconducting-insulating transition. The junction could, for example, be made of a superconducting film, in which the central region is much thinner than the two regions on either side. To treat quantum fluctuations in both the insulating and the superconducting region, we model the insulator as itself an SIS Josephson array below its S/I transition and the superconductor as an array above that transition. We calculate the coupling energy of the junction as a function of insulator thickness using two methods. The first is a mean-field approach, generalized to allow for spatial variation of the order parameter. The second approach is to use quantum Monte Carlo simulation, which we use to calculate the phase stiffness as a function of insulator thickness. In both cases, the properties of the junction can be tuned by varying the thickness of the insulating region, and the properties of both superconducting and insulating materials. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A21.00013: Neural dynamics in superconducting networks Kenneth Segall, Dan Schult, Patrick Crotty, Max Miller We discuss the use of Josephson junction networks as analog models for simulating neuron behaviors. A single unit called a ``Josephson Junction neuron'' composed of two Josephson junctions [1] displays behavior that shows characteristics of single neurons such as action potentials, thresholds and refractory periods. Synapses can be modeled as passive filters and can be used to connect neurons together. The sign of the bias current to the Josephson neuron can be used to determine if the neuron is excitatory or inhibitory. Due to the intrinsic speed of Josephson junctions and their scaling properties as analog models, a large network of Josephson neurons measured over typical lab times contains dynamics which would essentially be impossible to calculate on a computer We discuss the operating principle of the Josephson neuron, coupling Josephson neurons together to make large networks, and the Kuramoto-like synchronization of a system of disordered junctions.\\[4pt] [1] ``Josephson junction simulation of neurons,'' P. Crotty, D. Schult and K. Segall, Physical Review E 82, 011914 (2010). [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A21.00014: Half-metallic $d$-wave Josephson junctions Henrik Enoksen, Jacob Linder, Asle Sudb{\O} We examine the dc Josephson effect in a ballistic superconductor/half-metal/superconductor junction by means of the Bogoliubov--de Gennes equations. We study the role of spin-active interfaces and compare how different superconductor symmetries affect the Josephson effect. We analyze critical current as a function of junction width, spin-flip strength and direction, and temperature. We show that the temperature-dependence of the supercurrent in the $d_{xy}$-symmetry case differs qualitatively from the $s$- and $d_{x^2-y^2}$-symmetries. Finally, we have found a general analytical expression for the Andreev Bound State-energies which shows how we can either induce $0-\pi$-transitions, or continuously change the ground state phase of the junction by controlling the magnetic misalignment at the interfaces. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A21.00015: A Microfabricated Phonon Spectrometer to Spectrally Resolve Phonon Transport Obafemi Otelaja, Jared Hertzberg, Richard Robinson In this work we show how frequency-dependent phonon transport can be probed with non-equilibrium phonons. We demonstrate a method of generating and detecting non-equilibrium phonons in micron- and nanoscale structures [1]. Our scalable method of fabricating phonon generators and detectors involves formation of Al-AlxOy-Al superconducting tunnel junctions on the sidewalls of a silicon mesa. In the line-of-sight path in these mesas, we generate and detect non-equilibrium, ballistically propagating phonons with a frequency $\sim$ 100 GHz. Phonons are generated through the decay of quasiparticles injected into one superconducting film of the generator. This process excites phonons in a tunable, non-thermal spectral distribution. The phonons radiate into the mesa and are observed by the detector after passing through the mesa. By utilizing electron-beam lithography and plasma etching, we demonstrate the fabrication of high aspect ratio nanostructures along the ballistic path in order to observe surface scattering effects. This work is supported by DOE (DE-SC0001086).\\[4pt] [1] J. B. Hertzberg et al, Rev. Sci. Instrum. 82, 104905 (2011). [Preview Abstract] |
Session A22: Focus Session: Fe-based Superconductors - Gap Structure
Sponsoring Units: DMP DCOMPChair: Wai-Kwong Kwok, Argonne National Laboratory
Room: 254B
Monday, February 27, 2012 8:00AM - 8:12AM |
A22.00001: Anisotropic Energy-Gaps of Iron-based Superconductivity from Intra-band Quasiparticle Interference in LiFeAs Andreas W. Rost, Milan P. Allan, Andrew P. Mackenzie, Yang Xie, J.C. Davis, K. Kihou, C.-H. Lee, A. Iyo, H. Eisaki, T.-M. Chuang Cooper pairing in the iron-based high-$T_C$ superconductors is thought to occur due to the projection of the antiferromagnetic interactions between neighboring iron atoms onto the complex momentum-space electronic structure. A key consequence is that distinct anisotropic energy gaps $\Delta_i(k)$ with specific relative orientations should occur on the different electronic bands $i$. However, the high-precision spectroscopy required to demonstrate anisotropy of the energy gaps, and to determine the relationship between the $\Delta_i(k)$ on different bands, has not been achieved. Here we introduce intra-band Bogoliubov quasiparticle scattering interference (QPI) to iron-based superconductor studies, focusing specifically on LiFeAs. This approach provides direct spectroscopic confirmation of multiple anisotropic energy gaps on different bands. We identify the QPI signatures of the three hole-like bands assigned by photoemission studies to be $\gamma$, $\alpha_2$ and $\alpha_1$. Then, by introducing a new QPI technique, we determine the magnitude and relative orientations of the anisotropic $\Delta_i(k)$. Intra-band Bogoliubov QPI therefore yields the spectroscopic information required to identify the mechanism of superconductivity in iron-based superconductors. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A22.00002: Low temperature specific heat of the codoped iron-arsenic superconductor Ba$_{0.55}$K$_{0.45}$Fe$_{1.95}$Co$_{0.05}$As$_{2}$ K. Gofryk, J. Lashley, A. Leithe-Jasper, W. Schnelle, F. Ronning, M. Nicklas, F. Weickert, H. Rosner, J.L. Smith Despite large experimental and theoretical efforts the structure of the superconducting gap and the origin of the pairing mechanism in iron-based superconductors in still unresolved. Measurements of the low temperature specific heat and its magnetic response inside the superconducting state give important information about the symmetry of the gap. Here, we present results of our studies of codoped Ba$_{0.55}$K$_{0.45}$Fe$_{1.95}$Co$_{0.05}$As$_{2}$ with a $T_{c}$ of 32.5 K. The high quality of the material is marked by a pronounced peak at $T_{c}$ as well as by a low residual specific heat $\gamma_{0}$ = 2.4 mJ/mol K$^{2}$. We will discuss the implications of the new specific heat results on the symmetry of the order parameter in this system. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A22.00003: Gap structure of the iron-based superconductor KFe$_2$As$_2$ via thermal conductivity A. Juneau-Fecteau, R.T. Gordon, J.-Ph. Reid, N. Doiron-Leyraud, L. Taillefer, M.A. Tanatar, R. Prozorov, T. Saito, H. Fukazawa, Y. Kohori, K. Kihou, C.H. Lee, A. Iyo, H. Eisaki The thermal conductivity of the iron-based superconductor KFe$_2$As$_2$ was measured at temperatures down to 50 mK in magnetic fields up to 15 T, as a way to probe the superconducting gap structure. A large residual linear term in the $T=0$ limit is observed in zero field, showing that the gap structure contains nodes, consistent with a previous report [J.K. Dong et al., PRL 104, 087005 (2010)]. We discuss the possible interpretations for the nature of these nodes, in light of three different theoretical proposals: accidental line nodes in an extended $s$-wave state which are either horizontal [K. Suzuki et al., arXiv:1108.0657] or vertical [S. Maiti et al., arXiv:1111.0306], or symmetry-imposed vertical line nodes in a $d$-wave state [R. Thomale et al., PRL 107, 117001 (2011)]. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A22.00004: Gap structure of iron-based superconductors via directional thermal conductivity Invited Speaker: Jean-Philippe Reid Because the structure of the superconducting gap as a function of direction reflects the pairing interaction, it can shed light on the nature of the pairing mechanism. In the iron pnictides, the experimental situation in this respect remains unclear and so far suggests the lack of a universal picture. Here I present a systematic study of the superconducting gap structure through directional thermal conductivity measurements [1] on hole-doped K-Ba122 [2,3], electron-doped Co-Ba122 [4,5], self-doped LiFeAs [6] and the chalcogenide FeTeSe. We observe a general trend for the evolution of the superconducting gap with doping. At optimal doping, the gap structure is nodeless and isotropic (3D). Away from optimal doping, nodes appear on the Fermi surface at the edges of the superconducting dome, as seen for K-Ba122 and Co-Ba122. This strongly suggests that the presence of these nodes is accidental and therefore not imposed by symmetry. It would instead depend on the competition between intra- and inter-band interactions controlled by the evolving band structure and Fermi surface, and by the onset of antiferromagnetic order.\\[4pt] Work done in collaboration with M. A. Tanatar, X. G. Luo, H. Shakeripour, R. Gordon, A. Juneau-Fecteau, N. Doiron-Leyraud, S. Ren\'e de Cotret, F. Lalibert\'e, E. Hassinger, J. Chang, N. Ni, S. L. Bud'ko, P. C. Canfield, H. Kim, R. Prozorov, B. Shen, H. Luo, Z. Wang, H.-H. Wen, K. Cho, Y. J. Song, Y. S. Kwon, and Louis Taillefer.\\[4pt] [1] H. Shakeripour et {\it al}., New Journal of Physics {\bf 11}, 055065 (2009).\\[0pt] [2] X. G. Luo et {\it al}., Physical Review B {\bf 80}, 140503 (2009).\\[0pt] [3] J.-Ph. Reid et {\it al}., arXiv:1105:2232.\\[0pt] [4] M. A. Tanatar et {\it al}., Physical Review Letters {\bf 104}, 067002 (2010).\\[0pt] [5] J.-Ph. Reid et {\it al}., Physical Review B {\bf 82}, 064501 (2010).\\[0pt] [6] M. A. Tanatar et {\it al}., Physical Review B {\bf 84}, 054507 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A22.00005: Proximity fingerprint of $s_{\pm}$ superconductivity Alexei Koshelev, Valentin Stanev We suggest a straightforward and unambiguous test to identify possible opposite signs of superconducting order parameter in different bands proposed for iron-based superconductors ($s_{\pm}$-state). We consider proximity effect in a weakly coupled sandwich composed of a $s_{\pm} $-superconductor and thin layer of $s$-wave superconductor. In such system the $s$-wave order parameter is coupled differently with different $s_{\pm} $-gaps and it typically aligns with one of these gaps. This forces the other $s_{\pm}$-gap to be anti-aligned with the $s$-wave gap. In such situation the aligned band induces a peak in the $s$-wave density of states (DoS), while the anti-aligned band induces a dip. Observation of such contact-induced negative feature in the $s$-wave DoS would provide a definite proof for $s_{\pm}$-superconductivity. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A22.00006: Doping dependence of the specific heat of single crystal BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ Carlos Chaparro, Lei Fang, Helmut Claus, George Crabtree, Valentin Stanev, Wai-Kwong Kwok, Ulrich Welp, Morten Eskildsen, Andreas Rydh We present a systematic study of the specific heat transitions on a series of BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ single crystals with phosphorous doping ranging from near optimum doped x = 0.3 to strongly over doped x = 0.55. Our results reveal that BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ follows the scaling \textit{$\Delta $C/T}$_{c }\sim \quad T_{c}^{2}$ remarkably well.\footnote{S. L. Bud'ko, N. Ni, P. C. Canfield, Phys. Rev. B \textbf{79}, 220516 (2009).} The clean-limit nature of this material imposes new restraints on theories aimed at explaining the scaling. We find that the Ginzburg-Landau parameter decreases significantly with doping whereas the superconducting anisotropy is $\Gamma \sim $2.6, independent of doping. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A22.00007: Gap Symmetry in KFe$_2$As$_2$ Saurabh Maiti, Maxim Korshunov, Andrey Chubukov We revisit the issue of the gap symmetry in K$Fe_2$As$_2$, which is an Fe-pnictide superconductor with only hole pockets. Previous theoretical studies mostly argued for a $d-$wave gap in K$Fe_2$As$_2$ since transport and thermodynamic measurements point to the presence of the gap nodes. However, a $d-$wave gap is inconsistent with recent laser-based angle-resolved photoemission measurements. We propose the scenario for a nodal $s-$wave superconductivity induced by a non-magnetic intra-band and inter-band interactions between fermions near the two hole pockets at $\Gamma$ point. The superconducting gap that we find changes sign between the two hole pockets at $\Gamma$ point and has $\cos {4\theta}$ angular dependence and can have accidental nodes on one or several hole pockets. We argue that strong angle dependence is the consequence of near-degeneracy between inter-pocket and intra-pocket interaction on the hole pockets. We also provide a connection between the the relative phase of 4$\theta$ oscillations and the shapes of the Fermi surface and discuss the implications in the light of photoemission and tunneling experiments. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A22.00008: Disorder induced transition between $s_{\pm}$ and $s_{++}$ states Fe-based superconductors M.M. Korshunov, D.V. Efremov, O.V. Dolgov, A.A. Golubov, P.J. Hirschfeld The symmetry and structure of the superconducting gap in recently discovered Fe-based materials is one of the main challenges in this exciting new field (see, e.g. P.J. Hirschfeld, M.M. Korshunov, and I.I. Mazin, Rep. Progr. Phys. 2011). We have reexamined the problem of disorder in 2-band superconductors, and shown within the framework of the $T$-matrix approximation that the suppression of $T_c$ can be described by a single parameter depending on the intra- and interband impurity scattering rates. $T_c$ is shown to be more robust against nonmagnetic impurities than would be predicted in the trivial extension of Abrikosov-Gor'kov theory. We find disorder-induced transition from the $s_{\pm}$ state to a gapless and then to a fully gapped $s_{++}$ state, controlled by the sign of the average coupling constant. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A22.00009: Doping - dependent anisotropic superconducting gap in Na$_{1-\delta}$FeAs and NaFe$_{1-x}$Co$_x$As pnictides Kyuil Cho, M.A. Tanatar, N. Spyrison, H. Kim, R. Prozorov, G. Tan, J. Yan, P. Dai, C. Zhang London penetration depth, $\lambda$ (T), was measured in single crystals of self electron-doped Na$_{1-\delta}$FeAs and chemically electron-doped NaFe$_{1-x}$Co$_{x}$As superconductors. Doping level $\delta$ in self-doped ones was controlled by the deintercalation of Na$^+$ ions, stimulated by ultrasonic treatment. Use of the two doping techniques allowed us to cover the whole doping phase diagram from underdoped parent NaFeAs to heavily Co-overdoped compositions, with the optimal doping, $T_c \sim$ 25 K, achieved for $x = $ 0.025. Use of two protocols also allowed us to monitor the effect of disorder, introduced by chemical substitution in Fe sublattice. The low-temperature variation of $\lambda$ (T), measured as a function of doping, was analyzed using a power-law fit, $\Delta \lambda$ = A T$^n$. The exponent, $n$, changes from $n \sim$ 1.85 at the optimal doping to much lower values in the underdoped, $n \sim$ 1.1, and heavily overdoped, $n \sim$ 1.3, samples. This doping-evolution of $\lambda$ (T) cannot be explained by isotropic gap with scattering and suggests that while the superconducting gaps are isotropic at the optimal doping, at least one of them develops strong anisotropy at the dome edges. This scenario appears to be common for many other Fe-based superconductors. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A22.00010: Proximity effect of iron-based superconductor in conventional $s$-wave superconducting thin films Nick Groll, Thomas Proslier, Alex Koshelev, Valentin Stantev, Duck-Young Chung The proximity effect has been proposed as a mechanism to unambiguously identify the possible $s_{\pm}$-state in iron-based superconductors.\footnote{A. E. Koshelev, V. Stanev, Europhysics Letters, Vol. 96, 27014 (2011)} With a thin $s$-wave superconductor atop a $s_{\pm}$-superconductor it is suggested that the $s$-wave order parameter will couple to the $s_{\pm}$-gaps differently, inducing a correction to the $s$-wave density of states that can be probed using electron tunneling spectroscopy. In this talk, we will present recent results of the superconducting proximity effect in s-wave MoGe thin films sputtered on top of bulk superconducting Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ (T$_{c}$=35K) pnictide. Electron tunneling spectroscopy measurements were performed for several MoGe film thicknesses using a homemade point contact setup. Finally, results will also be presented for similar measurements using two conventional $s$-wave superconductors. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A22.00011: Volovik effect in the s$\pm$ state of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ from high field NMR Sangwon Oh, Andrew Mounce, William Halperin, Chenglin Zhang, Pengcheng Dai, Philip Kuhns, Arneil Reyes he spatially averaged density of states of an unconventional $d$-wave superconductor is magnetic field dependent, proportional to $H^{1/2}$, owing to the Doppler shift of quasiparticle excitations in a background of vortex supercurrents.~[1, 2] This phenomenon, called the Volovik effect, is absent in an $s$-wave state; however, it has been predicted~[3] to exist for a sign changing $s\pm$ state with a characteristic field dependence, proportional to $H$. We have observed this behavior in the $^{75}$As NMR spin-lattice relaxation rate of a single crystal of Ba$_{0.67}$K$_{0.33}$Fe$_{2}$As$_{2}$ studied over a wide range of fields up to 28 T. Our spatially resolved measurements show that Doppler contributions to the rate increase toward the vortex core, consistent with the superconducting state having unconventional $s\pm$ symmetry.\\ {[1]} G. E. Volovik, J. Phys. C. {\bf 21}, L221 (1988)\\ {[2]} G. E. Volovik, JETP Lett. {\bf 58}, L221 (1988)\\ {[3]} Y. Bang, Phys. Rev. Lett. {\bf 104}, 217001 (2010) [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A22.00012: Origin of the variety of superconducting gap structure in iron-based superconductors: competition between orbital and spin fluctuations Seiichiro Onari, Tetsuro Saito, Hiroshi Kontani To understand the pairing mechanism in iron-based superconductors, we study the three-dimensional gap structure based on the orbital fluctuation theory. We focus on the fully-gapped state in (i) heavily electron-doped KFe$_2$Se$_2$ [1], nodal gap structure in (ii) isovalent-doped BaFe$_2$(As,P)$_2$, and strongly band-dependent gap structure in (iii) hole-doped (Ba,K)Fe$_2$As$_2$. Based on the three-dimensional ten orbital model for (i), we obtain orbital-fluctuation-mediated fully-gapped $s_{++}$ wave state without sign reversal. For (ii), we reproduce the loop-shaped nodal structure on the electron-Fermi surface, due to the competition between orbital and spin fluctuations. For (iii), we obtain a drastic change in the gap structure by hole-doping, reflecting the variation of orbital fluctuations due to the topological change of electron-pockets. These results indicate the significant role of orbital fluctuations in iron-based superconductors. [1] Saito et al., PRB 83, 140512(R) (2011) [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A22.00013: Microscopic Mechanism for a Pairing State with Time-Reversal Symmetry Breaking in Iron-Based Superconductors Werner Hanke, Shou-Cheng Zhang, Christian Platt, Ronny Thomale The multipocket Fermi surfaces of iron-based superconductors promote pairing states with both s$\pm$ -wave and dx2-y2 -wave symmetry. We argue that the competition between these two order parameters could lead to a time-reversal-symmetry breaking state with s + id-pairing symmetry in the iron-based superconductors, and propose serveral scenarios in which this phase may be found. To understand the emergence of such a pairing state on a more rigorous footing, we start from a microscopic 5-orbital description representative for the pnictides. Using a combined approach of functional renormalization group and mean-field analysis, we identify the microscopic parameters of the s + id-pairing state. There, we find the most promising region for s + id-pairing in the electron doped regime with an enhanced pnictogen height.\\[4pt] [1] arXiv:1106.5964v1 [Preview Abstract] |
Session A23: Metals Magnetic
Sponsoring Units: DCMPChair: Ralph Skomski, U. Nebraska
Room: 255
Monday, February 27, 2012 8:00AM - 8:12AM |
A23.00001: Bulk electronic structure of FeRh undergoing metamagnetic transition via hard x-ray photoemission Alexander Gray, David Cooke, Peter Kruger, Catherine Bordel, Eric Fullerton, Shigenori Ueda, Keisuke Kobayashi, Frances Hellman, Charles Fadley In this study changes in the electronic structure accompanying a temperature-induced metamagnetic transition from anti-ferromagnetic to ferromagnetic order are investigated in strained epitaxial FeRh thin films via valence-band and core-level hard x-ray photoelectron spectroscopy with a photon energy of 6 keV. At such high photon energy, the resulting inelastic mean-free paths of the photoemitted electrons and therefore the average probing depths are on the order of 60 {\AA}, corresponding to about 20 unit cells and ensuring truly bulk-sensitive measurement. Clear differences between the AFM and FM states are observed across the entire valence-band spectrum and these are well reproduced using density functional theory. Changes in the Fe 2p core-levels of Fe are also observed and interpreted using Anderson impurity model calculations. These results suggest that significant electronic structure changes are involved in this AFM-FM transition. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A23.00002: Random magnet with competing anisotropies in Fe$_{x}$Ni$_{1-x}$F$_{2}$ alloys Trent Johnson, Felio Perez, Tudor Stanescu, David Lederman A series of epitaxial (110) Fe$_{x}$Ni$_{1-x}$F$_{2}$ films were deposited on (110) MgF$_{2}$ via molecular beam epitaxy. The Fe concentration $x$ was determined by measuring the lattice parameter along the [110] direction using x-ray diffraction. The film thicknesses and the roughness of each interface were found by fitting of x-ray reflectivity data. The magnetic ordering as a function of x was analyzed by SQUID magnetometry. Enhancement of the N\'{e}el temperature in alloys as well as evidence of spontaneous magnetization along the c-axis after field-cooling were observed for samples with x$>$0.1; for samples with x$<$0.1 the magnetization was perpendicular to the c-axis. Two phase transitions were observed for alloy samples with x$>$0.1. The phase diagram of the upper transition was consistent with mean field theory of a system with competing anisotropies. The transition at lower temperatures was unaffected by the application of a magnetic field, whereas the upper temperature transition was broadened by the application of fields as small as 50 Oe. This suggests the presence of a spin-glass phase at lower temperatures, followed by melting of the spin glass prior to the main transition to paramagnetic behavior as the temperature is raised. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A23.00003: Structural and electronic origin of large magnetostrictive $Fe_{1-x}Ga_x$ alloys Hui Wang, Yanning Zhang, Ruqian Wu, Lizhi Sun, Sonsheng Xu, Zhidong Zhang $Fe_{1-x}Ga_x$ binary alloys exhibiting large magnetostriction and excellent ductility have great potentials for various applications. The origin of large magnetostriction has not been thoroughly studied, especially at high Ga concentration ($x > 18.75 \%$). We conduct extensive ab initio molecular dynamics simulation of $Fe_{1-x}Ga_x$ alloys to generate atomic structures; and find that the alloys adopt the disordered A2 structure for $x < 18.75 \%$ and the A2+$D0_3$ mixed structures for $18.75 \% < x < 23.4 \%$, respectively. The formation of $D0_3$-like structure play a key role for the decrease of magnetostriction beyond $x = 19\%$. Interestingly, the magnetostriction may greatly enhance, up to $\lambda_{001} = 850$ ppm in ternary alloys with incorporation of $3 - 5 \%$ Cu and Zn. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A23.00004: Thin film deposition of Mn$_2$Ga under various growth condition Mingyang Li, Li Gao, Xin Jiang, Mahesh Samant, Brian Hughes, Kevin Roche, Claudia Felser, Stuart Parkin The tetragonal \emph{DO}$_{22}$ phase of Mn$_{3-x}$Ga is a ferromagnetic Heusler with perpendicular magnetic anisotropy. It has high spin polarization, high Curie temperature and low magnetic moment, and thus becomes a good candidate for spin-transfer-torque magnetic random access memory. This work reports the epitaxial growth of tetragonal Mn$_2$Ga thin films using sputtering method. The effect of various substrates, buffer layers, and substrate temperature on the film roughness is presented and compared. The formation of Mn$_2$Ga islands are observed at the beginning of growth on most buffer layers by reflection-high-energy-electron-diffraction. The smoothest film with root-mean-square roughness 0.10nm is obtained by using Pt buffer layer. The magnetic properties are also compared. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A23.00005: Magneto Caloric Effect in Ni-Mn-Ga alloys: First Principles and Experimental studies Khorgolkhuu Odbadrakh, Don Nicholson, Gregory Brown, Aurelian Rusanu, Orlando Rios, Jason Hodges, Athena Safa-Sefat, Gerard Ludtka, Markus Eisenbach, Boyd Evans Understanding the Magneto-Caloric Effect (MCE) in alloys with real technological potential is important to the development of viable MCE based products. We report results of computational and experimental investigation of a candidate MCE materials Ni-Mn-Ga alloys. The Wang-Landau statistical method is used in tandem with Locally Self-consistent Multiple Scattering (LSMS) method to explore magnetic states of the system. A classical Heisenberg Hamiltonian is parametrized based on these states and used in obtaining the density of magnetic states. The Currie temperature, isothermal entropy change, and adiabatic temperature change are then calculated from the density of states. Experiments to observe the structural and magnetic phase transformations were performed at the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory (ORNL) on alloys of Ni-Mn-Ga and Fe-Ni-Mn-Ga-Cu. Data from the observations are discussed in comparison with the computational studies. This work was sponsored by the Laboratory Directed Research and Development Program (ORNL), by the Mathematical, Information, and Computational Sciences Division; Office of Advanced Scientific Computing Research (US DOE), and by the Materials Sciences and Engineering Division; Office of Basic Energy Sciences (US DOE). [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A23.00006: First principles determination of the rhombohedral magnetostriction of $Fe_{100-x}Ga_x$ and $Fe_{100-x}Al_x$ ($x<20$) alloys Ruqian Wu, Yanning Zhang Extensive efforts have been dedicated to investigate the extraordinary magnetostriction of Galfenol (Fe$_{100-x}Ga_x$) and Alfenal (Fe$_{100-x}Al_x$) alloys, which are very promising magnetostrictive materials for various applications such as sensors, transducers and spintronic devices. In contrast to the strong response of the tetragonal magnetostriction ($\lambda_{100}$) to the Ga/Al concentration (x), the rhombohedral magnetostriction,$\lambda_{111}$, vs. x curves for both FeGa and FeAl alloys show negative and steady values for $0 |
Monday, February 27, 2012 9:12AM - 9:24AM |
A23.00007: Intrinsic Magnetic Properties of $\textit{L}1_{0}$ Derivates Ralph Skomski, Priyanka Manchanda, Pankaj K. Sahota, Arti Kashyap, Balamurugan Balasubramanian, J.E. Shield, D.J. Sellmyer It is investigated how atomic substitutions modify the magnetization, exchange and anisotropy of $\textit{L}1_{0}$-ordered ferromagnets. Emphasis is on properties of interest in permanent magnetism, including the reduction of raw-materials costs by substituting iron-series transition metals for expensive heavy (4$\textit{d}$/5$\textit{d}$) transition-metal elements. In particular, VASP calculations are used to determine the magnetizations of the Fe-Co-Pt, Mn-Al-C and Fe-Ni-S systems. We perform supercell calculations to determine the moments of Fe and Co in various $\textit{L}1_{0}$ derivates with chemical disorder. The local magnetic moments exhibit a subtle dependence on the environment, not only in each Fe-Co layer but also through alternating 4$\textit{d}$/5$\textit{d}$ layers. However, the magnitude of these spatial fluctuations is not very large, and after configurational averaging, the moments exhibit very simple dependencies on the concentrations of the involved atoms. The FeNi system is also interesting because $\textit{L}1_{0}$-ordered FeNi was originally discovered in meteorites (tetrataenite), formed with cooling times in excess of one million years. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A23.00008: Growth of Mn$_{2}$TiSn alloy and its magnetic and structural characterizations Yung Huh, P. Kharel, V.R. Shah, X.Z. Li, N. Al-Aqtash, K. Tarawneh, E.S. Krage, R.F. Sabirianov, R. Skomski, D.J. Sellmyer A ternary inter-metallic alloy Mn$_{2}$TiSn is one of the candidates predicted to be a Heusler compound with high spin polarization and Curie temperature well above room temperature, which is suitable for spintronic applications. Mn$_{2}$TiSn powder samples were prepared by arc melting and annealing under controlled environment to study magnetic and structural properties. Temperature and field dependent magnetization measurements show the sample is ferromagnetically ordered with a Curie temperature above 400 K. The low temperature magnetization at 10 K is about 2.5 emu/g under 100 Oe. The coercivity increases as temperature decreases from 1 kOe at room temperature to 2.4 kOe at 10 K. Theoretical calculation from the high field data shows the anisotropy energy is 4.0 $\times$ 10$^{5}$ ergs/cm$^{3}$ at 300 K but it becomes slightly more than doubled at 10 K. TEM and XRD characterizations reveal that the compound crystallizes in the hexagonal structure (D0$_{19}$, P6$_{3}$/mmc) rather than the theoretically proposed L2$_{1}$ cubic structure, which is supported by the first-principle structure calculations where the total energy per unit cell volume is preferred for the hexagonal structure. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A23.00009: Unusual temperature dependence of the magnetic anisotropy constant in barium ferrite BaFe$_{12}$O$_{19}$ Fan Zhao, Jun Wang, Wei Wu, Guo-meng Zhao We report magnetic hysteresis loops in a wide temperature range (4-700 K) for silica-coated barium ferrite BaFe$_{12}$O$_{19}$ nanoparticles. The saturation magnetization $M_{s}$ and the first magnetic anisotropy constant $K$ are determined simultaneously from the magnetic hysteresis loop using the law of approach to saturation. It is remarkable that $K$ is linearly proportional to $M_{s}$ and varies precisely with temperature as $K(T) = K(0)[1-(T/T_{C})^{1.58}]$ in the whole temperature range below the Curie temperature $T_{C}$ (740 K). The unusual temperature dependence of the anisotropy constant and its linear relation with the saturation magnetization in BaFe$_{12}$O$_{19}$ are not predicted from the existing theoretical models. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A23.00010: First principles study of magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite Seong-Gon Kim, Laalitha Liyanage, Jeff Houze, Sungho Kim Site occupancy and magnetic properties of Zn-Sn substituted M-type Sr-hexaferrite SrFe$_{12-x}$(Zn$_{0.5}$Sn$_{0.5}$)$_x$O$_{19}$ with $x=1$ were studied using density functional theory and generalized gradient approximation (GGA). Using the GGA+U method the description of strongly correlated $3d$ electrons of Fe was improved. Our results show that Zn and Sn atoms prefer to occupy $4f_1$ and $4f_2$ sites respectively. Favorable Zn-Sn substituted configurations show an increase in saturation of magnetization (M$_s$), and a decrease in magnetic anisotropy energy (MAE), over the pure M-type Sr-hexaferrite ($x=0$). Experimental observations agree with the decrease of MAE and the increase of M$_s$ for Zn-Sn substituted Sr-hexaferrite. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A23.00011: The Origin of Secondary Hematite Phase in Non-stoichiometric Co-ferrite Prepared by Ceramic Method David Jiles, Cajetan Nlebedim, Anthony Moses Surprising results have shown the formation of a secondary phase of hematite in co-ferrite. Co-ferrite based materials have been increasingly studied for magnetoelastic and magnetoelectric applications. Enhancement of the properties for such applications can be made by cation substitution, heat treatment or processing. Often, samples whose compositions deviate from stoichiometry or targeted compositions are made. It is not yet known, how far from stoichiometric composition one would go to create other phases; which would also affect the properties. This study shows that, when samples are made by the ceramic method, a secondary hematite phase forms with the spinel Co-ferrite phase. The origin of the hematite phase is related to the processing temperature, sintering environment and deviation from stoichiometric composition. Consequently, deviation from targeted or stoichiometric compositions may explain why properties of co-ferrite reported in literature vary. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A23.00012: Magnetic Characterization of heavily proton-irradiated MnAl Hall bars Marc Costantine, Nattawut Anuniwat, Yishen Cui, Stu Wolf, Jiwei Lu, Brad Weaver $\tau -$MnAl has L1$_{0}$ structure with the perpendicular magnetic anisotropy and it attracts some interests due to the potential application in magnetic recording media and spintronics. We fabricate $\sim $2um wide Hall bar devices from 40 nm MnAl thin film to study the transport properties. Anomalous Hall Effect (AHE) resistance loops of the devices and out-of-plane magnetization loops of unpatterned films mimic one another. This correlation shows that the electrical transport of the material is strongly spin dependent at room temperature. Crystallinity and chemical ordering characterization are performed using X-ray diffraction on unpatterned films. Both patterned and unpatterned films are then exposed to 2 MeV-energy protons for 2 hours at Naval Research Laboratory for displacement damages study. The magnetic and magneto-transport properties were not degraded after the irradiation, which shows promising radiation hardness for future spintronics devices. The heavier, more energetic beam sources and larger dose radiations will be performed and discussed. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A23.00013: Structural and magnetic properties of high anisotropy HfCo$_{7-x}$Fe$_{x}$ alloys Balamurugan Balasubramanian, Bhaskar Das, Ralph Skomski, Shah Valloppilly, Jeffrey Shield, David Sellmyer An increasing demand of rare-earth elements in recent years intensifies the search for rare-earth free permanent magnetic materials with magnetocrystalline anisotropy $K_{1}$ = 1 MJ/m$^{3}$. The present study reports melt-spun HfCo$_{7}$ alloys with a high $K_{1}$ of 1.3 MJ/m$^{3}$ along with an appreciable saturation-magnetic polarization ($J_{s})$ of 8.9 kG. A substitution of Fe for Co in HfCo$_{7-x}$Fe$_{x}$ further improves $K_{1}$ and $J_{s}$ to 1.5 MJ/m$^{3}$ and 10.4 kG, respectively. XRD studies of HfCo$_{7}$ are in agreement with an orthorhombic structure and also reveal a lattice expansion on substituting Fe for Co. These results show that HfCo$_{7-x}$Fe$_{x}$ can be a promising candidate for permanent-magnet and other significant applications. In brief, the structural and magnetic properties of HfCo$_{7-x}$Fe$_{x}$ alloys (0 $\le $ x $\le $ 1) investigated using XRD, EDX, TEM, and SQUID magnetometer will be presented. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A23.00014: Calciothermic Preparation of Sm-Co From Nanostructured Precursor Oxides Brian Kelly, Gerald Poirier, Karl Unruh A calciothermic reduction/diffusion process has been developed for the preparation of Sm-Co alloys with good magnetic properties using nanostructured Co- and SmCo-oxide powders as precursors. The precursor oxides were obtained from an aqueous solution of Sm(NO$_{3})_{3}$\textbullet 6H$_{2}$O, Co(NO$_{3})_{2}$\textbullet 6H$_{2}$O, and citric acid which, after the removal of excess water, spontaneously reacts to form a fine mixture of Co$_{3}$O$_{4}$ and SmCoO$_{3}$ nanoparticles. These nanoparticles were then mixed with metallic Ca granules, sealed under inert gas in Nb tubes, and heated to temperatures between 850 and 1000 $^{o}$C for various lengths of time. The products of the reduction/diffusion reaction were studied by scanning electron microscopy with elemental analysis, vibrating sample magnetometry, and x-ray diffraction. These measurements revealed that (in addition to the oxidation product CaO) the products of the reduction/diffusion process were micron-sized particles of essentially single phase SmCo$_{5 }$with a coercivity in excess of 40 kOe and a Curie temperature of about 950 $^{o}$C. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A23.00015: Theoretical search for new permanent magnets with no rare earth atoms Liqin Ke, Vladimir Antropov, Mark van Schilfgaarde We use the density functional theory and Quasiparticle Self-Consistent GW approximation to investigate the crystal and electronic structure, magnetic moment, anisotropy, and exchange coupling of Fe$_{16}$N$_2$, Fe$_{13}$Al$_3$, Co$_7$Hf and Zr$_2$Co$_{11}$. Both methods show similar results for magnetization and electronic structure. The experimentally unknown crystal structures of Co$_7$Hf and Zr$_2$Co$_{11}$ are obtained using structural optimization. We also discuss possible usage of these materials as permanent magnets. [Preview Abstract] |
Session A24: Materials: Synthesis, Growth and Processing (Bullk and Films)
Sponsoring Units: FIAPChair: Dmitri Tenne, Boise State University
Room: 256
Monday, February 27, 2012 8:00AM - 8:12AM |
A24.00001: Epitaxial Growth of Zinc Oxide on Single Crystalline Gold Plates Kathryn Greenberg, John Joo, Mor Baram, David Clarke, Evelyn Hu Although metal-oxide interfaces are the critical components of many electronic and optical devices, it is rare to find epitaxial metal-oxide structures. We demonstrate for the first time, a method for the low temperature, epitaxial growth of zinc oxide (ZnO) on single crystalline gold plates. The gold plates, up to 100$\mu$m in width, are grown from a gold-surfactant complex. Even with the large lattice mismatch between (111) gold and (0001) ZnO, we are able to form epitaxial zinc oxide at 90$^\circ$C on top of the single crystal gold plates. This epitaxial growth is confirmed using transmission electron microscopy, electron diffraction, and electron backscatterer diffraction. Micro-photoluminescence is also performed to investigate the optical properties of the epitaxial zinc oxide. We remove the grown ZnO membranes from the gold plates using a stamping and etching process. These membranes can potentially be used to fabricate high quality microdisks and photonic crystals. The metal-oxide interfaces that we have fabricated may have the ability to be used in a number of technologically important applications, including as better electrical contacts and for improved light extraction from planar LED structures. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A24.00002: Forward Raman scattering in ZnO: observation of phonon polaritons D.A. Tenne, T. Engman, A.K. Farrar Single crystal wurtzite ZnO samples have been studied by forward Raman scattering. Spectra were measured at 295 and 75 K using 442 and 448 nm laser lines for excitation below the fundamental absorption edge. Measuring spectra in forward geometry at varied scattering angles allowed achieving small and variable magnitudes of phonon wave vector necessary to observe a polatiton effect. The observed frequencies of the polaritons formed by A$_{1}$ TO phonons will be compared to the calculated dispersion of phonon polaritons in ZnO. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A24.00003: SiGe Nanomembranes: Defect-Free Single-Crystalline Substrates for Growth of Strained Si/SiGe Heterostructures Deborah M. Paskiewicz, Boy Tanto, Donald E. Savage, Paul G. Evans, Mark A. Eriksson, Max G. Lagally Silicon-Germanium semiconductor alloys play a pivotal role in the strain engineering of heterostructures for microelectronic devices; however, high quality, single crystalline, defect free films with more than minimal Ge concentration do not exist. The lattice mismatch between Si and bulk SiGe results in biaxial tensile strain in thin Si films and leads to electronic band offsets that allow for the confinement of electrons in the strained Si layer, i.e., a two-dimensional electron gas. Many of the current techniques used to create relaxed SiGe rely on plastic relaxation of the alloy, which impose strain variations and inject crystalline defects into all epitaxial layers grown on top. These defects can significantly degrade the performance of any device in the active layer. We demonstrate the fabrication of SiGe nanomembranes (NM): fully elastically relaxed, smooth, single-crystalline sheets of SiGe alloy. These SiGe NMs can be transferred to new handling substrates, bonded, and used as templates for growth of new defect-free materials. We compare the material quality of strained Si/SiGe heterostructures grown on SiGe NMs with those created on SiGe substrates relaxed via dislocations. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A24.00004: XRD analysis of high purity germanium single crystals grown by Czochralsk method Gang Yang, Guojian Wang, Wenchang Xiang, Muhammad Khizar, Yutong Guan, Yongchen Sun, Dongming Mei, Chaoyang Jiang, Bruce Gray Two high purity germanium (HPGe) crystals were grown in argon and hydrogen atmosphere by Czochralsk method, respectively. Both XRD 2$\theta $ scanning and rocking curve ($\omega $-scanning) were used to investigate the quality of the grown crystals. XRD 2$\theta $ scanning results show that an extremely strong (400) peak at 2$\theta $ of 66.15$^{\circ}$ and a very feeble (200) peak at 31.56$^{\circ}$ were observed in three samples cut from the crystal grown in Ar atmosphere, indicating the crystals with $<$100$>$ orientation. However, there is an obvious split on (400) peak for three samples, which could be attributed to defects in the crystal. Additionally, according to the reflection and extinction law of germanium crystal with face-centered cubic lattice, the reflection of 200 should be forbidden. The presence of (200) peak at 31.56$^{\circ}$ could be resulted from XRD multiple-beams scattering. For the crystal grown in hydrogen atmosphere, only very strong and non-split (400) peak at 2$\theta $ of 66.15$^{o}$ was observed in all three samples. The $\omega $-scanning results of two crystals at the fixed 2$\theta $ of 66.15$^{\circ}$ show that the crystal grown in hydrogen atmosphere has much more highly symmetric rocking curve with narrower FWHM, which exhibits that the crystal grown in hydrogen atmosphere has very high quality. This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A24.00005: Growth and characterization of room temperature antiferromagnetic I-Mn-V semiconductors X. Marti, T. Jungwirth, P. Wadley, H. Reichlova, V. Novak, O. Stelmakhovych, K. Uhlirova, P. Beran, M. Cukr, F. Maca, A.B. Shick, J. Masek, P. Horodysk\'a, P. Nemec, V. Holy, J. Zemek, P. Kuzel, I. Nemec, B. Gallagher, R. Campion, C.T. Foxon, J. Wunderlich The integration of ferromagnetism and semiconductors has been studied extensively, but devices operate well below room temperature. Recent theoretical and experimental works have opened a new route for spintronics based on antiferromagnets. Remarkably, high-temperature antiferromagnetic order is much more compatible with semiconductors than the ferromagnetic order. In our work we focus on the family of I-Mn-V antiferromagnetic semiconductor. We report on our synthesis of bulk and thin-film epilayers of the I-Mn-V compounds and on their basic electrical and magnetic properties. We will discuss the utility of these materials for designing antiferromagnetic semiconductor spintronic devices. \\[4pt] [1] Phys. Rev. B 83, 035321 (2010) \\[0pt] [2] http://arxiv.org/abs/1102.5373 [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A24.00006: Nonequlibrium growth of GaInAsSb on GaSb across the immiscibility region by molecular beam epitaxy for midinfrared materials Asli Yildirim, John Prineas GaInAsSb is a potentially important mid-infrared material, because alloys can in theory be grown with cut-off wavelengths from 1.7 to 4.9 ums. That potential has been hampered in the past by the present of large alloy immiscibility regions. In this work, 2 $\mu$m bulk GaInAsSb layers lattice-matched to GaSb substrates were grown across the entire compositional range, including the immiscibility region, by molecular beam epitaxy. Lower than typical growth temperatures (410-450 C) were used to limit the adatom diffusion length and move growth conditions further from equilibrium. Using a variety of techniques to characterize the optical, structural, and morphological quality of films, no phase separation was observed to occur for any alloy concentration. High resolution X-ray scans showed one narrow peak, and a single bright photoluminescence peak was observed for all samples. Smooth GaInAsSb surfaces were observed for all samples by AFM measurements, and XTEM images also show GaInAsSb layers to be homogeneous and defect free, with no sign of phase separation. EDS studies have been done, and results show low alloy scatter. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A24.00007: Electronic and Structural Properties of InGaZnO Thin Films D.S. Williams, S. Sallis, L.F.J. Piper, B.E. White We examine the effects of oxygen partial pressure during deposition and the structural changes resulting from a post-deposition anneal on the transport properties of InGaZnO. Amorphous oxygen-deficient samples sputter-deposited from a target with an atomic ratio of 2:2:1:7 at 50 watts DC in a 3 mTorr argon atmosphere have a resistivity of 0.16 ohm-centimeters. Amorphous oxygen-rich samples deposited similarly, except for a 10{\%} oxygen partial pressure, are insulating. For both samples, the as-deposited surfaces show a consistent grain size of approximately 30 nm. A subsequent rapid thermal anneal at 600C for 10 seconds leads to the coalescing and vertical growth of the grains with a resultant thinning of the background matrix. After anneal, the resistivity of the oxygen-deficient sample is decreased to 0.003 ohm-centimeters and 0.005 ohm-centimeters for the oxygen-rich sample. X-ray diffraction, scanning electron microscopy, atomic force microscopy and x-ray photoelectron spectroscopy data are presented to explain these changes and suggest possible methods of tuning the properties of InGaZnO for future use in thin film transistors, flexible electronics, transparent conductors and thermoelectric materials. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A24.00008: Low temperature atomic layer deposition of $\alpha$-Fe$_2$O$_3$ Jeffrey Klug, Thomas Proslier, Nicholas Becker, Jeffrey Elam, Michael Pellin There is significant interest in the use of $\alpha$-Fe$_2$O$_3$ (hematite) as a semiconducting thin film in a variety of applications including solar energy conversion, water oxidation, and gas sensing. In many such applications, devices may depend on non-planar geometries where traditional thin film deposition techniques are limited by line-of-sight constraints. Atomic layer deposition (ALD) is a gas-phase synthesis technique utilizing sequential self-saturating surface chemical reactions to produce uniform coatings with atomic scale control on substrates with arbitrary shape. However, ALD processes explored for Fe$_2$O$_3$ to date generally suffer from either extremely low growth rates, narrow temperature windows for self-saturating growth, or precursors with limited reactivity. In this respect, we will present a detailed study of a new, previously unexplored process for ALD of $\alpha$-Fe$_2$O$_3$ at technologically relevant temperatures between 200-300$^{\circ}$C. Self-limiting growth at $\sim$0.7 \AA/cycle was confirmed via situ quartz crystal microbalance. The results of in situ process characterization and ex situ analysis of film structure, morphology, composition, and electrical properties will be presented. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A24.00009: Impact of atmosphere on HPGe crystal growth Guojian Wang, Gang Yang, Wenchang Xiang, Jayesh Govani, Muhammad Khizar, Yutong Guan, Dongming Mei The growth of high-purity germanium crystals for radiation detectors is being developed at the University of South Dakota. High-purity germanium crystals were grown in argon and hydrogen atmosphere, individually. The growth parameters were compared and analyzed. The relationship between thermal field and crystal quality was discussed. Based on the thermal properties of argon and hydrogen gases, different thermal fields were designed to grow lower dislocation density of high-purity germanium crystals. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A24.00010: Performance optimization of diffused Li on Ga/In eutectic, In/Sn solder and eutectic Ga/In Ohmic contacts to n-high purity-crystalline (100) Ge Khizar Khan, Yang Gang, Guojian Wang, Wenchang Xiang, Yutong Guan, Dongming Mei Performance optimization study of novel contacts such as diffused lithium on Ga/In eutectic (75.5/24.5 wt{\%}), In/Sn solder (95.0/5.0 wt{\%}) and Ga/In eutectic (75.5/24.5 wt{\%}) to n-high purity-crystalline $<$100$>$ Ge (HP-SC-Ge) has been presented. Ultrasonically clean samples taken from same substrate were used to process the contacts followed by their characterization utilizing current--voltage (I--V), Hall-effect and AFM measurements. Extreme care was introduced to minimize the effect of parasitic oxide layers. Contacts such as diffused Li on eutectic Ga/In and In/Sn solder were processed in an inert glove box and characterized at 305 K (RT) and 77 K (LN) respectively. Comparative study revealed that Ga/In eutectics contacts behave throughout linear and stable, showing strong hall-effect to that of its counter parts. This was attributed due to the high adsorption behavior of anions at liquid (Ga--In) contacts and improved wettability. Whereas, for In/Sn solder case, the contacts processing considerations were substantially different, mainly because of its poor solder flow, excessive void formation, and heterogeneous phase distribution responsible for process yield loss. For diffused Li on Ga/In eutectic contacts, the linearity of the obtained Ohmic profiles was not consistent due to the high reactivity of the Li with HP-SC-Ge substrate. This work is supported by DOE grant DE-FG02-10ER46709 and the state of South Dakota. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A24.00011: Crystal-growth Underground Breeding Extra-sensitive Detectors Dongming Mei CUBED (Center for Ultra-Low Background Experiments at DUSEL) collaborators from USD, SDSMT, SDSU, Sanford Lab, and Lawrence Berkeley National Laboratory are working on the development of techniques to manufacture crystals with unprecedented purity levels in an underground environment that may be used by experiments proposed for DUSEL. The collaboration continues to make significant progress toward its goal of producing high purity germanium crystals. High quality crystals are being pulled on a weekly basis at the temporary surface growth facility located on the USD campus. The characterization of the grown crystals demonstrates that the impurity levels are nearly in the range of the needed impurity level for detector-grade crystals. Currently, the crystals are being grown in high-purity hydrogen atmosphere. With an increase in purity due to the zone refining, the group expects to grow high-purity crystals by the end of 2011. The one third of the grown crystals will be manufactured to be detectors; the remaining will be fabricated in to wafers that have large applications in electro and optical devices as well as solar panels. This would allow the research to be connected to market and create more than 30 jobs and multi millions revenues in a few years. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A24.00012: Lattice matched quaternary alloy, BGaAsBi: growth and characterisation Daniel Beaton, Aaron Ptak, Kirstin Alberi, Angelo Mascarenhas The ternary semiconductor alloy GaAs$_{1\rm -x}$Bi$_{\rm x}$ has been been the focus of many recent studies due to the large decrease in the fundamental bang gap, $\Delta E_g~\simeq~-85~meV/$\% for small incorporated amounts. However, the large size of the bismuth atom relative to the arsenic it replaces results in significant lattice mismatch to GaAs substrates. We now report on the lattice matched quaternary alloy, B$_{\rm y}$Ga$_{1\rm -y}$As$_{1\rm -x}$Bi$_{\rm x}$. Incorporating a smaller atom (boron) along with the larger atom (bismuth) allows for a reduction of the epi-layer strain and lattice matched compositions, [B]:[Bi]$\simeq1$. The benefit of the choice of boron is that it does not effect the band structure of the host GaAs; no change in the band gap is observed with increasing boron content. Samples were grown by molecular beam epitaxy under conditions conducive to bismuth incorporation: low growth temperatures and low V:III ratios. Both high-resolution X-ray diffraction (XRD) and secondary ion mass spectroscopy were used to verify material composition and photoluminescence used to measure the band gap, and these results will be presented. The increasing observation of a distribution of shallow, in-gap boron related defects will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A24.00013: Optimization of high purity germanium (HPGe) crystals growth rate through the simulation and modeling of growth system geometry Jayesh Govani, Dongming Mei, Guojian Wang, Gang Yang The growth rate and quality of high-purity germanium (HPGe) single crystals depend largely on the control of the thermal field such as the temperature profile and heat transfer. The control parameters of the thermal field can only be regulated externally through the growth system geometry, hydrogen and argon gas pressure, flow rate, pulling rate, and power and frequency of a RF heater. Since quantitative determination of the control parameters is exceptionally challenging and expensive, computer modeling and simulation of C$_{Z}$ growth processes play an imperative role in the advances of innovative pulling procedures and augmentation of Ge crystal quality. We present a detailed modeling and simulation study of radial and vertical temperature gradient, radial and vertical heat flux, temperature profile, thermo-elastic stresses, and defect density analysis for different crystal positions and diverse growth system geometry. We also virtually studied the consequences of targeted growth rate on temperature gradient and induction heating. A comparative analysis of simulated and available experimental results is also presented. In this effort, we have demonstrated the importance of simulation and modeling as it helps reducing the number of growth experiments significantly for the optimization of crystal quality and targeted growth rate. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A24.00014: Passivation Layer by SiC Thin Film Deposition for High Efficiency Solar Cells Mahdi Haghzedeh, Daniel Schmidt, Joel Therrien Deposition of a polymer derived SiC thin film as a novel, chemically and physically stable passivation layer to enhance the efficiency of solar cells by way of reducing surface recombination was studied. Starfire Matrix Polymer number 10 (SMP-10) is used to produce thin films of SiC on ion implanted silicon wafers. To ascertain the best method to deposit, three methods were tested: spin coating, spray coating, and dip coating are used. Various concentrations of SMP-10 diluted in xylene as an appropriate solvent are examined. To test the films, a contactless inductive coupling method is used. The thinner layers of SiC are grown by a lower percentage of SMP-10 (5{\%}), higher spin speed in spin coating (3000 RPM), and lower pulling out speed in dip coating (50 mm/minute). All of the methods yield controllable, repeatable, and uniform thin films. Although eliminating oxygen as an impurity in the passivation layer remains a challenge, the described approach has promise as a simple, low-cost passivation layers for higher efficiency solar cells. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A24.00015: Growth of ZnO Nanowire Arrays for Advanced Ultraviolet Detectors John Zeller, Tariq Manzur, A.F. Mehdi Anwar, Ashok K. Sood Zinc oxide (ZnO) provides a unique wide bandgap biocompatible material system exhibiting both semiconducting and piezoelectric properties. Bulk ZnO has a bandgap of 3.37 eV that corresponds to emissions in the solar blind ultraviolet (UV) spectral band (240-280 nm). We have grown highly ordered vertical arrays of ZnO nanowires using the metal organic chemical vapor deposition (MOCVD) technique on Si, silicon dioxide, c-plane sapphire, and GaN epitaxial substrates. UV detectors based on ZnO nanowires offer the highest UV sensitivity and lowest visible sensitivity for applications such as missile plume detection and threat warning. The development of UV detectors based on vertical nanowire arrays requires an innovative fabrication approach involving precise deposition of metal contacts, where UV sensor performance depends to a large extent on the growth conditions as well as on the substrate used. We will present experimental results on the structural, electrical, and optical properties of ZnO nanowires grown for UV sensing applications. [Preview Abstract] |
Session A25: Focus Session: Simulation of Matter at Extreme Conditions - Energetic Materials
Sponsoring Units: DCOMP GSCCM DMPChair: Nir Goldman, Lawrence Livermore National Laboratory
Room: 257A
Monday, February 27, 2012 8:00AM - 8:12AM |
A25.00001: Instability of planar detonation front in energetic materials Mikalai Budzevich, Vasily Zhakhovsky, Aaron Landerville, Carter White, Ivan Oleynik Detonation wave propagation in solid energetic materials (EMs), as described by the standard AB model, was studied using a novel moving window molecular dynamics (MW-MD) technique. Parameters of the AB model were modified to investigate the mechanisms of detonation propagation in EMs as a function of the activation barrier for the chemical reaction AB+B -$>$ A+BB + 3 eV. For barriers below 0.2 eV, the detonation front structure remained planar irregardless of the cross-section of the sample. For higher activation barriers, the one-dimensional planar detonation evolves into a cellular detonation upon increase of one of the transverse dimensions of the sample. The cellular detonation transforms into a stable three-dimensional turbulent-like detonation upon simultaneous increase of both transverse dimensions of the sample. These various instabilities of the planar detonation front in solid EMs observed in our MW-MD simulations mirror the major regimes of gas-phase detonation, thus confirming the universal nature of detonation phenomena. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A25.00002: Shock-induced chemical reactions in organic materials and explosives Dana Dattelbaum, Stephen Sheffield, Shawn McGrane, Peter Goodwin Interrogating chemical reactions behind a shock front is immensely difficult and, as a result, the details of shock-induced chemistry remain poorly understood. Previous research has shown that dimerizations, polymerizations, ring-opening and decomposition reactions can occur under shock compression, depending on molecular structure. Questions regarding the thresholds for incipient reaction, the nature of first and subsequent reaction steps, and the influence of shock input conditions on reaction kinetics remain to be answered. Here, we have applied \textit{in-situ} electromagnetic gauging at multiple Lagrangian positions to elucidate the evolution of multiple-wave structures associated with shock-induced reactions of several simple functional groups: carbon-carbon double (-C=C-) and triple bonds, and nitriles. The relative order of group reactivity under single shock conditions for these simple molecules is discussed. From measurements of the reactive flow, we have obtained detailed information about the temporal evolution of the waves, and global kinetic rates associated with transformation(s) between partially- and fully-reacted states. Near the reactive thresholds, evolution in particle velocities point to reaction timescales on the order of tens-to-hundreds of nanoseconds. We further compare evidence of reaction from gas gun-driven experiments to recent results using laser-driven shocks. Spectroscopic details will be presented from both types of experiments. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A25.00003: Molecular dissociation under extreme conditions Igor Schweigert Molecular dissociation under extreme temperatures and pressures is the first step towards thermal or shock initiated decomposition of energetic materials. Fast dissociation rates are challenging to measure, but amenable to first principles calculations. We combine transition-state theory with molecular dynamics simulations based on density-functional theory to predict the temperature-dependent dissociation rates in the gas and the condensed phase. Current applications focus on gas-, solution-, and liquid-phase thermal dissociation of nitramines. These studies will be discussed in the context of developing mesoscale models of initiation of energetic materials. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A25.00004: Thermodynamic Properties of energetic materials from density functional theory with van der Waals corrections Aaron Landerville, Michael Conroy, Mikalai Budzevich, You Lin, Carter White, Ivan Oleynik The calculation of thermodynamic properties for energetic materials from first-principles offers the promise to provide key parameters for mesoscopic and continuum-level simulations of explosives performance for a wide range of pressures and temperatures. While density functional theory with empirical van der Waals corrections, together with corrections for temperature and zero-point effects, can give excellent agreement between calculated and experimentally determined equations of state, quantities such as heat capacities and coefficients of thermal expansion suffer from inaccuracies in the lower frequencies of the calculated vibration spectrum. Additional approaches are discussed to account for the lowest intermolecular modes to increase the accuracy in prediction of thermal properties. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A25.00005: Relating polymorphism and decomposition of RDX under static and dynamic compression Zbigniew Dreger, Yogendra Gupta Knowledge of the reactive behavior of energetic crystals at static high pressures and high temperatures (HP-HT) is an important step toward understanding the shock wave initiation of these crystals. Vibrational spectroscopy in a diamond anvil cell was used to examine the behavior of RDX crystals at the pressures and temperatures relevant to shock wave initiation. Phase boundaries between three RDX polymorphs ($\alpha $, $\gamma $, and $\varepsilon )$ were determined up to 12 GPa and 600 K. Decomposition kinetics for the $\varepsilon $- and $\gamma $-phases were examined at various pressures and temperatures, and were found to have positive volumes of activation. CO$_{2}$, N$_{2}$O and H$_{2}$O were identified as the main decomposition species. Static HP-HT results were used to identify and understand the following processes in shocked RDX: $\alpha -\gamma $ phase transition, identification of the crystal phase at decomposition, and the role of pressure and temperature in accelerating the RDX decomposition under shock compression. This work demonstrated that static HP-HT results provide an important complementary route to elucidate the physical and chemical processes in shocked RDX crystals. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A25.00006: Atomistic Simulations of Orientation and Shock Velocity Dependences on Pentaerythritol Tetranitrate Detonation Tzu-Ray Shan, Aidan Thompson, Ryan Wixom, Ann Mattsson Predicting the behavior of energetic materials requires a detailed description of how chemical reaction, energy and pressure fronts propagate during initial stages of detonation. In this talk, classical molecular dynamics (MD) simulations are used to examine orientation and shock velocity dependences in single crystal pentaerythritol tetranitrate (PETN). This work utilizes an empirical, variable charge reactive force field (ReaxFF) that is implemented in the LAMMPS package with a time-averaged bond-order method for on-the-fly chemical species identification. The accuracy of ReaxFF is validated by comparisons of activation barriers for dissociation of a single PETN molecule along various dissociation channels with higher-fidelity, but more expensive, density functional theory (DFT) calculations. The response of single-crystal PETN to shock compression is simulated using the multi-scale shock technique (MSST) along the insensitive (100) directions, as well as the sensitive (001) and (110) directions, at steady shock velocities ranging from 6-10 km/s. Hugoniot curves, particle velocities of shocked molecules, and evolution of reaction products with time from MD simulations with ReaxFF will be discussed and compared to that from DFT calculations. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A25.00007: Atomic-Scale Theoretical Studies of Energy Transfer, Inelastic Deformation, and Void Collapse in Molecular Crystals and Polymers Invited Speaker: Thomas D. Sewell Recent atomic-scale theoretical studies of shock waves in polyatomic molecular crystals and polymers will be presented, with an emphasis on the results and interpretation of molecular dynamics simulations for pentaerythritol tetranitrate (PETN), hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX), nitromethane, and poly(butadiene) (PBD). The effects of structural and mechanical anisotropy on the material response are of particular interest. Among the topics to be discussed are orientation dependent energy transfer pathways and inelastic deformation mechanisms subsequent to shock wave passage in initially defect-free nitromethane and PETN crystals, shock-induced collapse of variously shaped voids in crystalline RDX, and details of shock wave propagation and energy localization in bulk PBD and at the PBD/RDX interface. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A25.00008: Explosive initiation of pentaerythritol tetranitrate (PETN) by laser irradiation Roman Tsyshevskiy, Onise Sharia, Maija Kuklja Understanding of explosive decomposition of energetic materials triggered by laser irradiation is of great importance for design of new economical formulations with high performance and tunable sensitivity. Earlier, laser irradiation was only considered as a source of heat. Nowadays, it is realized that optical excitation may set off initiation of rapid chemical reactions and govern further decomposition in energetic materials. However, mechanisms of this phenomenon are yet to be established. We present quantum-chemical calculations of the electronic structure of molecular and crystalline PETN to explore the effect of common impurities on its optical properties. We found that charged or excited PETN molecules exhibit significantly different electronic, optical, and chemical behavior. For example, new decomposition pathways that were not available in the ground state become favorable in the charged state of PETN. Three calculated lowest excitation energies of ionized PETN require 0.6, 0.7 and 1.13 eV, which is considerably lower than those for equilibrium PETN. The activation energy of the rate limiting decomposition stage is within 9-12 kcal/mole, while it is 35 kcal/mole in the ground state. We discuss possible ways that originate charge transfer in PETN and presented results in the context of recent experimental data. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A25.00009: Condensation of carbon during high explosive detonation Laurence Fried, Sorin Bastea, Raul Garza The formation of nano-carbon clusters is believed to be responsible for the non-ideal detonation behavior of carbon-rich explosives, such as triamino-trinitrobenzene (TATB). We have developed a new model of carbon formation during detonation. The model is based on the assumption that carbon cluster growth has features of both activated Arrhenius kinetics and diffusion controlled kinetics. In our model the variation of temperature, density, and viscosity throughout the high explosive reaction zone and expansion is calculated using a thermochemical model linked to a hydrodynamic code. We compare our model to new experimental results on the size scaling of detonations in TATB-based explosives. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A25.00010: Surface-induced effect on sensitivity of beta and delta HMX crystals Onise Sharia, Maija Kuklja It is accepted that sensitivity of energetic materials depends on many factors, including presence of defects, surfaces, interfaces, or voids. However, details of atomistic mechanisms that govern sensitivity to initiation of detonation and correlations between structure, morphology, and degradation of chemical bonds are far from being understood. In this talk, we present quantum chemical calculations combined with transition state theory to analyze chemical decomposition reactions in beta and delta HMX crystals. We calculate the activation barriers and reaction rates in the ideal crystals and materials containing internal surfaces, vacancies, and voids. We show that N-NO$_2$ homolysis is the most favorable decomposition reaction in all cases. We discuss whether a large space available in the vicinity of voids facilitates the N-NO$_2$ break in comparisons to an ideal crystal, and if this effect is enhanced in the delta phase in comparison to beta phase. The conclusions and revealed trends are presented in the context of experimental data. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A25.00011: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 10:36AM - 10:48AM |
A25.00012: 1,1-Diamino-2,2-Dinitroethylene Under High-Pressure-High-Temperature Matthew Bishop, Nenad Velisavljevic, Zhenxian Liu, Matrin Galley 1,1-Diamino-2,2-dinitroethylene (FOX-7) is an insensitive high explosive (IHE) which shows promise for use in low vulnerability ammunitions. With performance comparable to RDX and HMX, there is a growing interest in understanding the behavior under denotation conditions. Through the use of diamond anvil cell (DAC) technology and electrical resistive heating, the vibrational behavior of FOX-7, in both the mid and far-IR, were recorded at multiple isotherms under elevated pressure-temperature (PT). Energy-dispersive x-ray diffraction (XRD) was also employed along with a multi-anvil press for further investigating pressure-temperature phase space. Future planned experiments will focus on using high-resolution angular-dispersive XRD and neutron diffraction techniques to resolve high pressure-temperature structural information and obtain P-V-T data. The experiments on FOX-7 have revealed previously uninvestigated knowledge on the elevated-PT decomposition and phase boundaries allowing for a more developed basis for the behavior of FOX-7 under detonation conditions. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A25.00013: Study on the deflagration-to-detonation transition course of porous energetic material Lan Wei, Pengcheng Hao, Hefei Dong, Xiaomian Hu, Jianshi Zhu The deflagration-to-detonation transition (DDT) course of energetic material with different porosity ratio was studied utilizing a one-dimensional two-phase flow code. The equations were numerically solved by space-time conservation element and solution element (CE/SE) method. The distribution of physical quantities such as pressure and temperature were obtained together with their evolution history. The physical rules before detonation were mainly analyzed and the effect of convection on the chemical reaction of energetic material was emphasized on. [Preview Abstract] |
Session A26: Focus Session: Physics of Energy Storage Materials - Materials, Stability and Transport
Sponsoring Units: DCOMP DMPChair: David Singh, Oak Ridge National Laboratory
Room: 257B
Monday, February 27, 2012 8:00AM - 8:36AM |
A26.00001: Nanostructured Materials for Portable and Stationary Energy Storage Invited Speaker: Yi Cui Storing energy electrochemically involves electronic and ionic processes and chemical transformation inside and at the interface of materials. The ability to understand and design nanostructures and their interfaces afford the great opportunities for controlling these fundamental processes, which can ultimately lead to high performance energy storage devices. Here I will present several exciting examples on designing nanostructures and their interfaces to realize high performance energy storage devices. One example is on designing nanowires and heterostructured nanowires for ultrahigh capacity storage of lithium ions in silicon anodes and sulfur cathodes. The challenges associated with large volume expansion, electron and ion transport, and solid-electrolyte-interphase (SEI) have been addressed. Another example is to design open framework stucture of nanocrystals, which facility facile insertion of sodium and potassium ions. The high power, high energy efficiency and low-cost aqueous batteries can be enabled for grid scale stationary storage. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A26.00002: Enhanced electrochemical performance in LiFePO4/graphene nanocomposite cathode material for lithium ion batteries Kulwinder Dhindsa, B. Mandal, M.W. Lin, M. Nazri, P. Vaishnava, V. Naik, G.A. Nazri, R. Naik, Z.X. Zhou We synthesized LiFePO$_{4}$/graphene nano-composite employing a sol-gel method, where graphene oxide solution was added to the LiFePO$_{4}$ precursors during the synthesis. Electrical measurement reveals that the addition of 10{\%} graphene (by weight) to LiFePO$_{4}$ increases its conductivity by 5 orders of magnitude. SEM images of the composite show that the material consists of LiFePO$_{4}$ nanoparticles (with a mean particle size $\sim $ 50 nm) homogeneously mixed with graphene sheets; the latter provides a three-dimensional conducting network for Li+ ion and electron transport. A large specific capacity of 170 mAh/g was observed at a discharge rate of C/2. To further increase the conductivity and inhibit particle size growth of LiFePO$_{4}$ (thus to increase the rate capacity), we coated the nanoparticles with a thin carbon layer by adding 0.25M lauric acid as precursor in addition to graphene oxide during the synthesis. The respective roles of graphene and lauric-acid-induced carbon coating in the specific capacity and charge-discharge rate of the LiFePO$_{4}$ cathode material will be discussed. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A26.00003: Simultaneous enhancement of electronic and Li+ ion conductivity in LiFePO4 Jaekwang Lee, Stephen J. Pennycook, Sokrates T. Pantelides The electronic conductivity is generally highly sensitive to the electronic properties of a material while the ionic conductivity is mainly sensitive to the structural properties. Thus, the simultaneous control of both is very challenging. Furthermore, many electrochemical systems for advanced energy technologies require materials in which both ionic and electronic conductivities are optimized. Here we explore the influence of biaxial strain on the electronic and Li$^{+}$ ion conductivities of the battery cathode material LiFePO$_{4}$ by performing first-principles calculations. We show that biaxial tensile strain leads to simultaneous increases in electronic and ionic conductivities of LiFePO$_{4}$. The results unveil a generic mechanism that should be present in other materials with polaronic transport and provide a promising new approach to enhance the performance of energy-related materials. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A26.00004: Optimization of the surface stability of the LiMn$_{2}$O$_{4}$ spinel by employing DFT calculations Altaf Karim, Kristin Persson One of the most important materials for the lithium batteries electrodes is LiMn2O4. We used GGA+U method to calculate the bulk and surface properties of LiMn2O4. Our calculations show that the both correct AFM and electron localization (GGA+U) are necessary to obtain the semiconducting, Jahn-Teller distorted electronic ground state of LiMn2O4. Further we calculated energies of different surfaces such as (100), (110), and (111) to study their stability. Our calculations show that (111) surface has the lowest energy which makes it more stable than other surfaces and it also confirms the experimental results, whereas (101) and (001) similar energies. Absolute surface energies change with +U value, but the ratios between the energies are very similar. Based on these calculations we constructed the equilibrium (Wulff) shape of LiMn2O4 particle, which is similar to the cubo-octahedral shape with predominant {\{}1~1~1{\}} facets as it was found in experiments. Our density of states calculations show that the bulk and (100) are semiconducting, whereas (110) and (111) surfaces exhibit metallic behavior. We also calculated the LiMn2O4 bulk and surface potentials as a function of lithium concentration. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A26.00005: Atomistic Simulation Studies of Nanostructured TiO$_{2}$ Phuti Ngoepe, Malili Matshaba, Dean Sayle TiO$_{2}$ has been confirmed as a safe anode material in lithium ion batteries due to its higher Li-insertion potential, ($\sim $1.5V) in comparison with commercialised carbon anode materials. Recently in order to attain high rate capabilities of TiO$_{2}$ anode, for application in lithium ion batteries with both high power and high energy density, intensive attention has been paid to various TiO$_{2}$ nanostructures, such as nanoparticles, nanowires and mesoporous structures. In the current study, amorphisation recrystallization method is used to produce nano- porous, sheets and bulk structures for TiO$_{2}$ which have been extensively studied experimentally. Simulated X-ray diffraction patterns are produced from such structures and compared with the experimental XRDs. The simulated microstructures are analysed and compared with available high resolution transmission experimental results. Lithiation of TiO$_{2}$ nanostructures is considered and discussed in the context of current investigations and concentration profiles of different ions are shown in the structures. Semi-empirical models based on DFT tightbinding methods, are introduced for TiO$_{2}$ and lithiated its forms. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A26.00006: First-principles study of electronic structures and phase transitions of lithiated molybdenum disulphide Jun Li, Xiaobo Chen By first-principles calculations, electronic structures of MoS2, intercalation-induced 2H to 1T phase transition and reversibility are investigated. It is revealed that change of interlayer stacking from 2H to 3R imposes negligible influence on the band structure and stability of MoS2. In contrast, the change of intralayer stacking from 2H to 1T changes the character of p-d repulsion, resulting in a semiconductor-to-metal transition. We demonstrate that the Kohn-Sham band energy, rather than the coulomb repulsion energy, plays dominant roles in both the phase stabilization and transition during Li intercalation. It is found that the evolution of 1T phase is crucially determined by chemical hardness, which underlies the cycle irreversibility. Due to the charge-density-wave (CDW) phase, Li extraction is impeded by the enhancement of Li-host binding. It is indicated that the cycle reversibility can be improved by electron-donor doping in MoS2, because the resultant pre-reduction of Mo and S eliminates the electron transfer from Li to host. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A26.00007: Simulating Dendritic Formation on the Cathode of Lithium-ion Batteries Ning Sun, Dilip Gersappe The Lattice-Boltzmann method was used to simulate the process of dendrite formation on the cathode of lithium-ion batteries. ``In-Situ'' maps of the electrodeposition process under constant charging current conditions were obtained. The results showed preferential dendrites formation on higher curvature spots of the cathode, because of the higher electrical field intensity. Different morphologies were obtained due to different initial roughness of the electrode. A mossy-like electrode can be observed after deposition on a rough electrode with randomly generated initial conditions. A tree-like dendrite can be observed when depositing on a single small dendrite. A smooth surface can be obtained when the initial electrode is ideally smooth. The influence of current density was also studied. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A26.00008: Density functional and molecular dynamics studies of solid electrolyte Li$_7$La$_3$Zr$_2$O$_{12}$ Michelle Johannes, Khang Hoang, Noam Bernstein Garnet-type structured Li$_7$La$_3$Zr$_2$O$_{12}$(LLZO) is considered as a promising candidate for Li-ion battery solid electrolytes because of its high ionic conductivity and electrochemical and chemical stability. We use first-principles density-functional theory calculations and molecular dynamics simulations to reveal the underlying mechanism that drives a tetragonal to cubic transition at elevated temperatures, and also to explain why the cubic phase can be stabilized with the incorporation of a certain amount of impurities such as Al. We show that the relationship between the observance of a cubic phase and the measurement of a substantially higher ionic conductivity is a secondary effect not directly attributable to the presence of Al in the crystal structure. Suggestions for enhancing the ionic conductivity in LLZO will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A26.00009: First principles investigation of the superionic electrolyte Li$_7$P$_3$S$_{11}$ Nicholas Lepley, N.A.W. Holzwarth Li$_7$P$_3$S$_{11}$ has been shown to be a promising superionic conductor for solid state rechargeable batteries with a room temperature conductivity as high as $10^{-3}$ S/cm and a thermal activation energy as small as $E_A$=0.12 eV.\footnote {F. Mizuno et al., {\em{Solid State Ionics}} {\bf{177}}, 2721 (2006).} We have performed first principles modeling studies\footnote{N. A. W. Holzwarth, N. D. Lepley, Y. A. Du, {\em{J. Power Sources}} {\bf{196}}, 6870 (2011).} on this material in order to explain its stability and Li ion migration properties. Our investigation considers optimized crystal structures, migration involving both vacancy and interstitial mechanisms, as well as related materials. We find optimized crystal structures in reasonable agreement with experiment,\footnote{H. Yamane et al., {\em{Solid State Ionics}} {\bf{178}}, 1162 (2007); Y. Onodera et al., {\em{J. Phys. Soc. Jpn.}} {\bf{79}}, 87 (2010), suppl. A. } and the lowest calculated activation energy barrier was found to be $E_A$=0.15 eV in good agreement with the experimental value. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A26.00010: Solid Electrolyte for Advanced Lithium Batteries Gholam-Abbas Nazri, Balaji P Mandal, Maryam Nazri, Vaman Naik, Prem Vashinava, Ratna Naik Lithium battery is a promising energy storage system due to its high energy density and high rate capability and its application ranges from micro to large scale megawatt batteries. The current technology is using liquid electrolyte that limits its application due to flammable nature of the electrolyte, particularly at high temperatures, and difficulty in fabrication and miniaturization of the device. We report a novel solid electrolyte with high lithium ion conductivity as a replacement for the current liquid electrolyte, particularly for electronic applications. The solid state lithium ion conductor is based on lithium germanium phosphorous sulfide compound. The compound is prepared by solid state reaction at 500 \r{ }C. The crystallinity and phase purity of the sample is checked by XRD. We also measured ionic conductivity of the sample using both 4-probe and impedance techniques. High lithium ion conductivity at room temperature is observed. In this study we have investigated the dynamics of ion conduction, XRD, and Raman spectra of the super ion conductor. Electrochemical performance of the solid electrolyte in a lithium cell and its stability against high voltage cathodes and lithium anode will also be presented. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A26.00011: First principles computer simulations of Li$_{10}$GeP$_2$S$_{12}$ and related lithium superionic conductors N.A.W. Holzwarth A recent paper by Kamaya {\em{et al.}}\footnote{ N. Kamaya {\em{et al.}}, {\em{Nature Materials}} {\bf{10}}, 682 (2011).} reported a new crystalline superionic conductor having a compact tetrahedral structure and a stoichiometry of Li$_{10}$GeP$_2$S$_{12}$. The room temperature conductivity was reported to be 0.01 S/cm, comparable to liquid electrolyte conductivies and five times higher the compositionally related thio-LISICON material Li$_{3.25}$Ge$_{0.25}$P$_{0.75}$S$_4$ developed earlier.\footnote{R. Kanno {\em{et al.}}, {\em{J. Electrochem. Soc.}} {\bf{148}}, A742 (2001).} This talk will present a progress report on our work to perform first principles computer simulations for these materials focussing on the structural, stability, and Li ion mobility properties of idealized crystalline models. From the perspective of our previous studies of Li ion conductivity in lithium thiophosphate electrolytes,\footnote{ N. A. W. Holzwarth {\em{et al.}}, {\em{J. Power Sources}} {\bf{196}}, 6870 (2011).} the effects of introducing Ge can be assessed. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A26.00012: Exploring Li+ Potential Energy Surface in Poly(ethylene oxide)-based Sulfonate Ionomers Huai-Suen Shiau, Michael J. Janik, Ralph H. Colby Ion-containing polymers are of interest as single-ion conductors for use as electrolytes in electrochemical devices, including lithium ion batteries. Current ion conductivities of the best ionomers are roughly 100X too small for practical applications and have a small fraction of their Li+ counterions participating in conduction. We are using ab initio methods to investigate the Li+ conduction mechanism, and specifically the role of transient positive triple ions (Li+A-Li+) in the conduction process. The positive triple ion has a lower energy separated state that allows for facile transport, if there is a pair within 1.4 nm. We will discuss the competition between cation solvation with ether oxygen atoms and cation-anion interaction. The importance of anion-anion separation in altering Li+ hopping barriers will be examined, as well as the variation in hopping rates with solvent identity. Ab initio calculations are used to evaluate the relative energy of ion states (contact and separated states), and this analysis is used to explain experimental phenomena of Li+ mobility in ionomers. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A26.00013: Ab initio molecular dynamics simulations of organic electrolytes, electrodes, and lithium ion transport for Li-ion batteries P.R.C. Kent, P. Ganesh, De-en Jiang, O. Borodin Optimizing the choice of electrolyte in lithium ion batteries and an understanding of the solid-electrolyte interphase (SEI) is required to optimize the balance between high-energy storage, high rate capability, and lifetime. We perform accurate ab initio molecular-dynamics simulations of common cyclic carbonates and LiPF6 to build solvation models which explain available Neutron and NMR spectroscopies. Our results corroborate why ethylene carbonate is a preferred choice for battery applications over propylene carbonate and how mixtures with dimethyl carbonate improve Li-ion diffusion. We study the role of functionalization of graphite-anode edges on the reducibility of the electrolyte and the ease of Li-ion intercalation at the initial stages of SEI formation. We find that oxygen terminated edges readily act as strong reductive sites, while hydrogen terminated edges are less reactive and allow faster Li diffusion. Orientational ordering of the solvent molecules precedes reduction at the interphase. Inorganic reductive components are seen to readily migrate to the anode edges, leading to increased surface passivation of the anode. We are currently quantifying Li-intercalation barriers across realistic SEI models, and progress along these lines will be presented. [Preview Abstract] |
Session A27: Invited Session: Excitonic and Correlation Effects in Single-Layer Graphene
Sponsoring Units: DCMPChair: Bruno Uchoa, University of Oklahoma
Room: 258AB
Monday, February 27, 2012 8:00AM - 8:36AM |
A27.00001: Dynamic screening and the effective fine structure constant of graphene Invited Speaker: Peter Abbamonte Electrons in graphene behave, in the low energy sector, like massless Dirac fermions. The degree to which Coulomb correlations influence these fermions is still subject to debate. In this talk I will describe inelastic x-ray scattering experiments on crystals of graphite, to which we have applied newly developed reconstruction algorithms to image the dynamical screening of charge in (effectively) a freestanding graphene sheet. We found that the polarizability of the Dirac fermions in graphene is amplified by excitonic effects in the particle-hole spectrum, which enhances screening of interactions among low energy quasiparticles. I will argue that interactions should be characterized by an effective, screened fine-structure constant, $\alpha_g^*({\bf k},\omega)$, which our measurements suggest converges to the value $\alpha_g^*=0.14 \pm 0.092$ in the small wave vector limit. This value is smaller than the bare $\alpha_g=2.2$, and suggests that the strength of interactions in graphene is similar to that in band semiconductors like Si or GaAs. I will discuss the implications of this result for other Dirac systems, such as nodal quasiparticles in cuprates and topological insulator surface states. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A27.00002: Observation of strong excitonic effects in the optical spectrum of graphene Invited Speaker: Tony Heinz . [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A27.00003: Electron-electron interaction and excitonic effects in graphene systems Invited Speaker: Steven G. Louie Owing to the unique electronic structure of graphene and enhanced electron-electron and electron-hole interactions in lower dimensions, graphene structures exhibit interesting and novel electronic and optical properties. We discuss in this talk results from recent theoretical studies on several graphene systems. First-principles calculations, based on the GW-Bethe-Salpeter Equation approach, have predicted very strong features in the optical absorption spectra of single-layer graphene and multi-layer graphene, arising from resonant excitons. Many of these features have since been observed experimentally. We also explore the effects of charge carrier doping and of having an external electric field on the absorbance of graphene and bilayer graphene. Graphene nanoribbons are semiconductors that, owing to electron-electron interactions, also exhibit extraordinarily magnetic and excitonic effects. Another intriguing phenomenon is one in the magneto-optical response of graphene, which involves resonant transitions of absorption of a photon together with the simultaneous creation of an intervalley, intra-Landau level exciton and a K-phonon. Finally, first-principles results on the plasmon satellite structures in the spectral function of graphene computed with the GW method and beyond are presented. We discuss the origin of these phenomena and make comparison with experiments. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A27.00004: Graphene Update Invited Speaker: Andre Geim I will overview the latest experimental progress made by our group in Manchester. This will cover several subjects with particular attention being paid to interaction effects that have been observed in graphene and its double layer heterostructures. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A27.00005: Electron-electron interactions in doped graphene sheets Invited Speaker: Marco Polini In this talk I will review some of the most important electronic properties of graphene. I will first discuss the appearance of plasmaron satellite bands in both angle-resolved photoemission [1] and STM spectra [2,3], emphasizing the important role of the sublattice pseudospin degree of freedom. I will then illustrate some unusual features, which appear only beyond the widely used Random Phase Approximation, characterizing plasmons and Drude weight of the electron gas in this material~[4].\\[4pt] [1] A. Bostwick et al., Science 328, 999 (2010).\\[0pt] [2] V.W. Brar et al., Phys. Rev. Lett. 104, 036805 (2010).\\[0pt] [3] A. Principi, M. Polini, and A.H. MacDonald, to be submitted\\[0pt] [4] S.H. Abedinpour et al., Phys. Rev. B 84, 045429 (2011). [Preview Abstract] |
Session A28: Focus Session: Dopants and Defects in Semiconductors - ZnO
Sponsoring Units: DMPChair: Anderson Janotti, University of California Santa Barbara
Room: 258C
Monday, February 27, 2012 8:00AM - 8:12AM |
A28.00001: The trends of oxygen vacancy levels in metal oxides Wanjian Yin, Suhuai Wei, Mowafak Al-Jassim, Yanfa Yan Most of the $d$ or $d^{10}$ oxides such as ZnO, SnO$_{2}$, In$_{2}$O$_{3}$, and TiO$_{2 }$are wide-bandgap $n$-type semiconductors even though they are not intentionally doped. For quite a long time, it was commonly believed that oxygen vacancies (V$_{O})$ in metal oxides are the electron donors because the formation energy of V$_{O}$ in metal oxides is low and the electrical conductivity of the $n$-type oxides is closely linked to the formation of V$_{O}$ However, recent theoretical and experimental studies have put this point of view in question especially with different calculation methods involved. We present a detailed analysis of the wavefunction characters of oxygen vacancy in conventional metal oxides and unveil the chemical trend of oxygen vacancy transition energy levels with respect to the conduction-band minimum (CBM). We show that in the type-$s$ and type-$p$ metal oxides, where the character of an oxygen vacancy level is similar to that of the CBM, the oxygen vacancy levels are generally deep and become deeper when the cation size decreases. In type-$d$ metal oxides, the oxygen vacancy levels are generally shallow and can sometimes even be above the CBM. Our analysis is confirmed by the calculated trends of oxygen vacancy levels in representative metal oxides using hybrid density functional analysis. It also provides guidelines to search a metal oxide that has shallow V$_{O}$ donor levels, such as the one found in BiVO$_{4}$ [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A28.00002: Highly Efficient Defect Emission from ZnO and ZnO:S Jay Simmons, John Foreman, Jie Liu, Henry Everitt Bulk Zinc Oxide (ZnO) is a wide, direct band gap semiconductor with an energy of 3.4 eV that contains two emission bands: the UV band-edge emission and the green defect emission band. We have shown that the external quantum efficiency of the green band can exceed 50{\%}. To investigate the mechanism of efficient defect emission, vacuum annealed (ZnO:Zn) and sulfur-doped (ZnO:S) ZnO were investigated because of their strong defect emission and suppressed UV band edge emission. Continuous wave low temperature photoluminescence (PL) and PL excitation (PLE) spectra were measured for the two compounds. It was found that bound excitons, not free photo-excited carriers, mediate the defect emission in ZnO:Zn, while the defect emission from ZnO:S seems to originate from a Zn-S complex formed in the crystal lattice. Temperature-dependent PLE spectra for the defect and band edge emission were measured to estimate trapping and activation energies of the bound excitons. XPS and X-Ray diffraction studies were also performed to ascertain the concentration and nature of sulfur doping in the ZnO lattice. The results presented here offer hope that engineering defects in ZnO materials may significantly improve the quantum efficiency for white light phosphor applications. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 9:00AM |
A28.00003: Acceptors in bulk and nanoscale ZnO Invited Speaker: M.D. McCluskey Zinc oxide (ZnO) is a semiconductor that emits bright UV light, with little wasted heat. This intrinsic feature makes it a promising material for energy-efficient white lighting, nano-lasers, and other optical applications. For devices to be competitive, however, it is necessary to develop reliable p-type doping. Although substitutional nitrogen has been considered as a potential p-type dopant for ZnO, theoretical and experimental work indicates that nitrogen is a deep acceptor and will not lead to p-type conductivity. This talk will highlight recent experiments on ZnO:N at low temperatures. A red/near-IR photoluminescence (PL) band is correlated with the presence of deep nitrogen acceptors. PL excitation (PLE) measurements show an absorption threshold of 2.26 eV, in good agreement with theory. Magnetic resonance experiments provide further evidence for this assignment. The results of these studies seem to rule out group-V elements as shallow acceptors in ZnO, contradicting numerous reports in the literature. If these acceptors do not work as advertised, is there a viable alternative? Optical studies on ZnO nanocrystals show some intriguing leads. At liquid-helium temperatures, a series of sharp IR absorption peaks arise from an unknown acceptor impurity. The data are consistent with a hydrogenic acceptor 0.46 eV above the valence band edge. While this binding energy is still too deep for many practical applications, it represents a significant improvement over the $\sim$ 1.3 eV binding energy for nitrogen acceptors. Nanocrystals present another twist. Due to their high surface-to-volume ratio, surface states are especially important. Specifically, electron-hole recombination at the surface give rises to a red luminescence band. From our PL and IR experiments, we have developed a ``unified'' model that attempts to explain acceptor and surface states in ZnO nanocrystals. This model could provide a useful framework for designing future nanoscale ZnO devices. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A28.00004: Origin of the ``Red'' Luminescence Band in Bulk N-doped ZnO E.R. Glaser, N.Y. Garces, M.C. Tarun, M.D. McCluskey Optically detected magnetic resonance (ODMR) at 24 GHz was performed on bulk ZnO crystals doped with nitrogen impurities (of high interest for p-type conductivity) to provide more details on the origin of a recently reported red/near-IR photoluminescence (PL) band.\footnote{M.C. Tarun et al., AIP Advances \textbf{1}, 022105 (2011).} PL at 7K revealed strong bandedge excitonic recombination at 3.364 eV, a broad ``green'' emission band at 2.45 eV, and a broad ``red'' PL band near 1.7 eV. Two luminescence-increasing ODMR signals were found on this ``red'' emission. The first was a sharp feature with g-value of 1.957 and FWHM of 1 mT and is attributed to shallow donors based on electron spin resonance (ESR) of n-type ZnO. The second feature exhibited a g-value near 2 and a broad, asymmetric lineshape with FWHM of $\sim $ 10 mT. A simulation of the spectrum showed that the broad resonance could be fit as the sum of three equally spaced lines with magnetic field splitting value and relative intensities in close agreement to those observed for deep nitrogen acceptors as identified from previous ESR studies. Thus, the ODMR results strongly suggest that the ``red'' PL is due to radiative recombination involving residual shallow donors and deep nitrogen acceptor centers. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A28.00005: Elevated Temperature Dependent Transport Properties of As- and P-doped Zinc Oxide B. Cai, M. L. Nakarmi, T. Oder, M. McMaster, A. Smith, N. Velpukonda Achieving highly conductive p-type zinc oxide (ZnO) is desired for the development of ZnO based optoelectronic devices. Understanding electrical properties of ZnO, doped with p-type dopants, is necessary for improving p-type conductivity. We employed temperature dependent Hall effect measurement to study the electrical transport properties of As- and P-doped ZnO epilayers. The samples were grown on sapphire substrates by magnetron sputtering technique. From the Hall effect measurements performed at elevated temperatures ranging from 20 to 750 K, we observed double activation processes in both As- and P-doped ZnO epilayers. We will compare the results of uniform doped and delta-doped samples. Correlation between electrical properties from these Hall effect measurements and optical properties from low temperature photoluminescence measurements will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A28.00006: Impurity complexes and conductivity of Ga-doped ZnO Denis Demchenko Using hybrid functional theory compared with experimental measurements, we investigate the in?uence of gallium impurities and their complexes on electrical properties of ZnO. In contrast to the behavior of isolated Ga impurities and native defects, the calculated formation energies of Ga complexes are consistent with experimental data. We show that for high levels of Ga doping the acceptor behavior of (Ga$_{Zn}-V_{Zn}$) and (Ga$_{Zn}-O_i$) complexes explains the conductivity measurements and compensation levels in ZnO. The computed binding energies of these complexes are in agreement with the binding energies obtained from the measurements of the temperature dependence of carrier mobility. The binding energy dependence on the Fermi level, as well as the computed barrier heights for the formation of complexes are also consistent with the latest experiments on annealing of Ga doped ZnO samples. Our results show that the formation of defect complexes is essential for capturing the physics Ga defects in ZnO. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A28.00007: Cu-Doping of ZnO by Nuclear Transmutation and Electrical and Optical Characterization of Cu Acceptors Farida Selim, Marianne Taurn, Jianfeng Ji, Donald Wall, Lynn Boatner, Matthew McCluskey Cu doping is known to have a large effect on the electrical and optical properties of ZnO, its role is different from other dopants, and a fundamental understanding of this role is lacking. One problem of ZnO doping is arising from the difficulty in controlling dopant locations in conventional doping methods. In this work we doped Zn single crystals with copper acceptors by means of the nuclear transmutation doping (NTD) method, which gives highly uniform dopant distributions and has a much higher probability of controlling the dopant locations in the lattice. The Cu doping was confirmed by the infrared absorption signature of Cu$^{2+}$ at 5780 cm$^{-1}$. Hall-effect measurements indicated that the Cu acceptor level lies 0.160 eV below the conduction band minimum. With respect to optical properties, an interesting low-temperature thermal stimulated luminescence has been observed in as-grown and doped ZnO single crystals. This low-temperature luminescence can reveal the density of donors and acceptors in ZnO and their location in the band gap. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A28.00008: The G0W0 band gap of ZnO: effects of plasmon-pole models Gabriel Antonius, Martin Stankovski, David Waroquiers, Anna Miglio, Hemant Dixit, Kiroubanand Sankaran, Matteo Giantomassi, Xavier Gonze, Michel Cote, Gian-Marco Rignanese Carefully converged calculations are performed for the band gap of ZnO within the G0W0 approximation. The results obtained using four different well-established plasmon-pole models are compared with those of explicit calculations without such models (the contour-deformation approach). We evaluate the difference between plasmon-pole models that enforce the f-sum rule and those that are fitted to reproduce the low energy response. In the case of ZnO, plasmon-pole models enforcing the f -sum rule underestimate the low-frequency region of the dielectric function probably because of the presence of semi-core states in these calculations. Our results confirm that the band gap of ZnO is underestimated in the G0W0 approach as compared to experiment. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A28.00009: Evidenece for surface states in ZnO nanostructures using non-linear optical spectroscopy Benoy Anand, Martin Egblewogbe, Ramakrishna Podila, Reji Philip, Apparao Rao An unexpected presence of ferromagnetic (FM) ordering in nanostructured ZnO has been reported previously. Recently, from our detailed magnetization studies and \textit{ab initio} calculations, we attributed this FM ordering in nanostructured ZnO to the presence of surface states, and a direct correlation between the magnetic properties and crystallinity of ZnO was observed. Such defect induced surface states appear as green/yellow emission in the photoluminescence spectrum of ZnO nanostructures. In this study, through a systematic sample preparation of both pristine and Co-doped ZnO nanostructures, and detailed PL and nonlinear optical measurements, we confirm that the observed FM ordering is due to the presence of surface states. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A28.00010: Terahertz properties of Tm (Tm=Cu, Ag) doped ZnO thin films Mi He, Xingquan Zou, Tom Wu, Elbert Chia Optical properties of Zn$_{0.95}$Tm$_{0.05}$O (Tm=Cu, Ag) thin films are studied by terahertz time-domain spectroscopy (THz-TDS) at different temperatures (10K -- 300K) in the frequency range extending from 0.22 -- 3 THz. The measured complex dielectric response and conductivity are well fitted by a Drude-based model. Comparing with undoped ZnO thin films, the doping effect of Ag and Cu is investigated. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A28.00011: The growth of non-polar ZnO and ZnO/Mg$_{0.25}$Zn$_{0.75}$O epi-films by radio-frequency magnetron sputtering Bi-Hsuan Lin, Wei-Rein Liu, Shao-Ting Hsu, Chia-Hung Hsu, Wen-Feng Hsieh High quality non-polar ZnO and ZnO/Mg$_{x}$Zn$_{1-x}$O (x = 0.15, 0.25) epi-films have been successfully grown on m-plane and r-plane sapphire by using RF magnetron sputtering. The structural properties, including crystalline quality, strain state, and defect structures, of the ZnO and ZnO/Mg$_{x}$Zn$_{1-x}$O layers are thoroughly examined by synchrotron x-ray scattering, transmission electron microscopy and atomic force microscopy. We found the surface morphology of the a-plane oriented ZnO epi-films is smoother than that of m-plane oriented one. Moreover, the surface of Mg$_{0.25}$Zn$_{0.75}$O epi-layer is significantly better than the ZnO epi-film and exhibits the same orientation dependence. The optical properties of these samples are also investigated by temperature, polarization and power dependent photoluminescence, and polarization dependent Raman spectroscopy. The results reveal the a-plane ZnO epi-film grown on r-sapphire with a Mg$_{0.25}$Zn$_{0.75}$O buffer layer is promising for bright UV emission application. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A28.00012: Excitonic Energy Shifts in Isotopically Controlled $I-III-VI_2$ Chalcopyrites: $CuGaS_2$ and $AgGaS_2$ J.S. Bhosale, H. Alawadhi, I. Miotkowski, A.K. Ramdas, R. Lauck, M. Cardona $CuGaS_2$ and $AgGaS_2$ tetrahedrally co-ordinated chalcopyrites are ``genealogically related" to $II-VI$ semiconductors like ZnS. We have investigated the shifts in their excitonic signatures by controlling the isotopic mass of the $I$, $III$ or $VI_2$ constituent in the crystals grown by physical vapor deposition. The excitonic signatures are observed in wavelength modulated reflectivity employing a high S/N, LED based technique.\footnote{J. S. Bhosale, Rev. Sci. Instrum. 82, 093103 (2011)} For example it reveals a 3.9 meV shift for the A exciton in $Ag^{71}GaS_2$ with respect to that of natural $AgGaS_2$; a smaller increase occurs in ZnS\footnote{M. Cardona and M.L.W. Thewalt, Rev. Mod. Phys. 77, 575 (2002)}. These effects have been related to electron-phonon interaction caused by the zero-point vibrations. Similar effects, but with an opposite sign, have been observed for Cu-isotopes in $CuGaS_2$ as well as in the Cu-monohalides CuCl, CuBr, and CuI\footnote{D.Olguin et al., Solid State Commun. 122, 575 (2002)}; their origin is receiving considerable attention at present though not yet understood. In this context the excitonic temperature dependence\footnote{Cardona, op. cit}$^,$\footnote{H.Alawadhi et al., Phys. Rev.B 75, 205207 (2007)} will be discussed. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A28.00013: Hybrid Hartree-Fock density functional study of transition-metal doped ZnO Jesuan Betancourt, Alan Kalitsov, Julian Velev Dilute magnetic semiconductors (DMS) obtained by doping semiconductors with transition metals (TM) hold much promise for spintronics. Transition metal doped ZnO (ZnO:TM) has been investigated for a possible room-temperature DMS. Density functional theory gives incorrect prediction for the band gap of ZnO which leads to diverging interpretations for the magnetic behavior of ZnO:TM [1,2]. Here we report Heyd-Scuseria-Ernzerhof (HSE) hybrid functional study of the electronic structure of ZnO:TM (TM=Cu, Ni, Co, Fe, Mn). The hybrid functional corrects for both the bandgap problem on the host and the lack of correlation in the impurity, without the use of \textit{ad-hoc }intra-atomic potentials. Our results show although the HSE opens the band gap of the host, the Hubbard splitting of the impurity levels makes the empty impurity levels reside in the host conduction band. This leaves open the possibility for spin polarized carriers. We discuss the validity of the results and explore their implications for the magnetic behavior of ZnO:TM. [1] H. Raebiger, S. Lany, and A. Zunger, Physical Review B 79, 165202 (2009). [2] P. Gopal and N. A. Spaldin, Phys.l Review B 74, 094418 (2006). [Preview Abstract] |
Session A29: Focus Session: Quantum Optics with Superconducting Circuits: Hybrid Systems and Other Quantum Optics
Sponsoring Units: GQIChair: Alexandre Blais, University of Sherbrooke
Room: 259A
Monday, February 27, 2012 8:00AM - 8:36AM |
A29.00001: Atomic physics and quantum optics using superconducting circuits: from the Dynamical Casimir effect to Majorana fermions Invited Speaker: Franco Nori This talk will present an overview of some of our recent results on atomic physics and quantum optics using superconducting circuits. Particular emphasis will be given to photons interacting with qubits, interferometry, the Dynamical Casimir effect, and also studying Majorana fermions using superconducting circuits.\\[4pt] References available online at our web site:\\[0pt] J.Q. You, Z.D. Wang, W. Zhang, F. Nori, \textit{Manipulating and probing Majorana fermions using superconducting circuits,} (2011). Arxiv. J.R. Johansson, G. Johansson, C.M. Wilson, F. Nori, \textit{Dynamical Casimir effect in a superconducting coplanar waveguide,} Phys. Rev. Lett. \textbf{103}, 147003 (2009). \\[0pt] J.R. Johansson, G. Johansson, C.M. Wilson, F. Nori, \textit{Dynamical Casimir effect in superconducting microwave circuits,} Phys. Rev. A \textbf{82}, 052509 (2010). \\[0pt] C.M. Wilson, G. Johansson, A. Pourkabirian, J.R. Johansson, T. Duty, F. Nori, P. Delsing, \textit{Observation of the Dynamical Casimir Effect in a superconducting circuit. }Nature, in press (Nov. 2011). P.D. Nation, J.R. Johansson, M.P. Blencowe, F. Nori, \textit{Stimulating uncertainty: Amplifying the quantum vacuum with superconducting circuits,} Rev. Mod. Phys., in press (2011). \\[0pt] J.Q. You, F. Nori, \textit{Atomic physics and quantum optics using superconducting circuits,} Nature \textbf{474}, 589 (2011). \\[0pt] S.N. Shevchenko, S. Ashhab, F. Nori, \textit{Landau-Zener-Stuckelberg interferometry,} Phys. Reports \textbf{492}, 1 (2010). \\[0pt] I. Buluta, S. Ashhab, F. Nori. \textit{Natural and artificial atoms for quantum computation, } Reports on Progress in Physics \textbf{74}, 104401 (2011). \\[0pt] I.Buluta, F. Nori, \textit{Quantum Simulators, }Science \textbf{326}, 108 (2009). \\[0pt] L.F. Wei, K. Maruyama, X.B. Wang, J.Q. You, F. Nori, \textit{Testing quantum contextuality with macroscopic superconducting circuits, } Phys. Rev. B \textbf{81}, 174513 (2010). \\[0pt] J.Q. You, X.-F. Shi, X. Hu, F. Nori, \textit{Quantum emulation of a spin system with topologically protected ground states using superconducting quantum circuit, } Phys. Rev. A \textbf{81}, 063823 (2010). [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A29.00002: Wigner Tomography of Classical and Non-Classical States in a Superconducting Anharmonic Oscillator Nadav Katz, Roi Resh, Ofer Fogel, Radoslaw Bialzcak, John Martinis, Yoni Shalibo The Wigner quasi-probability distribution is a powerful tool for characterizing a quantum state and understanding the state dynamics in oscillators. Until now, there have been numerous measurements of this function in harmonic oscillators, and in particular in superconducting devices. However no similar measurement on anharmonic systems has been reported. We utilize the wide-range energy tunability in the multi-level Josephson phase qubit, biased in the small anharmonicity regime, to directly measure the Wigner function of various states. We measure non-classical superpositions of Fock-type states, as well as coherent-like states in this anharmonic system. This method provides an alternative to standard state tomography techniques which usually involve a long calibration process and have limited scalability for multi-level states. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A29.00003: Geometric Phases, Noise and Non-adiabatic Effects in Multi-level Superconducting Systems S. Berger, M. Pechal, A.A. Abdumalikov, L. Steffen, A. Fedorov, A. Wallraff, S. Filipp Geometric phases depend neither on time nor on energy, but only on the trajectory of the quantum system in state space. In previous studies [1], we have observed them in a Cooper pair box qubit, a system with large anharmonicity. We now make use of a superconducting transmon-type qubit with low anharmonicity to study geometric phases in a multi-level system. We measure the contribution of the second excited state to the geometric phase and find very good agreement with theory treating higher levels perturbatively. Furthermore, we quantify non-adiabatic corrections by decreasing the manipulation time in order to optimize our geometric gate. Geometric phases have also been shown to be resilient against adiabatic field fluctuations [2]. Here, we analyze the effect of artificially added noise on the geometric phase for different system trajectories. \newline [1] P.~J.~Leek \emph{et al.}, \emph{Science} \textbf{318}, 1889 (2007) \newline [2] S.~Filipp \emph{et al.}, \emph{Phys.~Rev.~Lett.~}\textbf{102}, 030404 (2009) [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A29.00004: Observing the geometric phase of a superconducting harmonic oscillator M. Pechal, S. Berger, A.A. Abdumalikov, J.M. Fink, J.A. Mlynek, L. Steffen, A. Wallraff, S. Filipp Steering a quantum harmonic oscillator state along cyclic trajectories leads to a path-dependent geometric phase [1]. However, the linearity of the system precludes its observation without a non-linear quantum probe. We therefore make use of a superconducting qubit serving as an interferometer to measure the adiabatic geometric phase of a harmonic oscillator realized as an on-chip resonant circuit [2]. We study the geometric phase for a variety of trajectories and show that, in agreement with theory, it is proportional to the area enclosed by the trajectory in the space of coherent states. At the transition to the non-adiabatic regime, oscillatory dephasing effects caused by residual qubit-resonator entanglement are observed and analyzed. We also discuss the possibility of using the harmonic oscillator geometric phase to implement two-qubit phase gates. \\[4pt] [1] S.~Chaturvedi, M.~S.~Sriram, V.~Srinivasan, J.~Phys.~A: Math.~Gen.~{\bf 20}, L1071 (1987).\newline [2] M.~Pechal {\em et al.}, arXiv:1109.1157v1 [quant-ph]. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A29.00005: All-resonant control of superconducting resonator qudits Frederick Strauch Quantum information processing with using superpositions of Fock states in superconducting resonators holds great promise for multi-level (i.e. qudit) quantum logic. Previous theoretical work has shown that a combination of dispersive and resonant interactions allow for general qudit logic operations. Here I introduce an all-resonant approach to control resonator qudits. This scheme allows for faster logic operations and will be compared to previous methods for Fock state generation and entangled state synthesis. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A29.00006: Readout of Spin Systems with Superconducting Circuits Natania Antler, Kater Murch, R. Vijay, Steven Weber, Eli Levenson-Falk, I. Siddiqi We present progress in coupling superconducting circuitry, in particular linear resonators and dispersive magnetometers, to an ensemble of spins. Species with a zero-field splitting (ZFS), such as bismuth doped silicon or NV centers in diamond, are particularly attractive as the absence of a strong magnetic bias field facilitates compatibility with superconducting devices. We present studies of the spin linewidth, and progress towards the observation of collective strong coupling. Furthermore, we will present data on the operation of a highly sensitive nanobridge SQUID magnetometer with a flux sensitivity of 26 $n\Phi_0/\sqrt{Hz}$ and tens of MHz of signal bandwidth. We also discuss the resilience of our superconducting measurement circuitry to in-plane magnetic fields, which can be used to tune the spin splitting. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A29.00007: Hybrid quantum circuit with a superconducting qubit coupled to a spin ensemble Yuimaru Kubo, Cecile Grezes, Andreas Dewes, Denis Vion, Junichi Isoya, Vincent Jacques, Anais Dreau, Jean-Francois Roch, Igor Diniz, Alexia Auffeves, Daniel Esteve, Patrice Bertet We report the experimental realization of a hybrid quantum circuit combining a superconducting qubit and an ensemble of electronic spins. The qubit, of the transmon type, is coherently coupled to the spin ensemble consisting of nitrogen-vacancy (NV) centers in a diamond crystal via a frequency-tunable superconducting resonator acting as a quantum bus [1,2]. Using this circuit, we prepare arbitrary superpositions of the qubit states that we store into collective excitations of the spin ensemble and retrieve back later on into the qubit [3]. These results constitute a first proof of concept of spin-ensemble based quantum memory for superconducting qubits.\\[4pt] [1] Y. Kubo \textit{et al.}, Phys. Rev. Lett. \textbf{105}, 140502 (2010).\\[0pt] [2] Y. Kubo \textit{et al.}, arXiv: 1109.3960.\\[0pt] [3] Y. Kubo \textit{et al.}, arXiv: 1110.2978. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A29.00008: Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond Xiaobo Zhu, Shiro Saito, Alexander Kemp, Kosuke Kakuyanagi, Shin-ichi Karimoto, Hayato Nakano, William J. Munro, Yasuhiro Tokura, Mark S. Everitt, Kae Nemoto, Makoto Kasu, Norikazu Mizuochi, Kouichi Semba We have experimentally demonstrated coherent strong coupling between a single macroscopic superconducting artificial atom (a gap tunable flux qubit [1]) and an ensemble of electron spins in the form of nitrogen--vacancy color centres in diamond. We have observed coherent exchange of a single quantum of energy between a flux qubit and a macroscopic ensemble consisting of about 3.0*10$^{7}$ NV- centers [2]. This is the first step towards the realization of a long-lived quantum memory and hybrid devices coupling microwave and optical systems. [1] \textit{Coherent operation of a gap-tunable flux qubit} X. B. Zhu, A. Kemp, S. Saito, K. Semba, APPLIED PHYSICS LETTERS, Volume: 97, Issue: 10 pp. 102503 (2010) [2] \textit{Coherent coupling of a superconducting flux qubit to an electron spin ensemble in diamond} Xiaobo Zhu, Shiro Saito, Alexander Kemp, Kosuke Kakuyanagi, Shin-ichi Karimoto, Hayato Nakano, William J. Munro, Yasuhiro Tokura, Mark S. Everitt, Kae Nemoto, Makoto Kasu, Norikazu Mizuochi, and Kouichi Semba, Nature, Volume: 478, 221-224 (2011) [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A29.00009: Hybrid qubit-resonator systems: From strong to ultrastrong coupling, from equilibrium to non equilibrium phases David Zueco, Thomas H\"ummer, Mark Jenkins, Mar\'Ia Jos\'e Mart\'Inez-P\'erez, Georg Reuther, Juan Jos\'e Garc\'Ia-Ripoll, Fernando Luis, Peter H\"anggi Hybrid systems : spin ensembles coupled to superconducting circuits have received a lot of attention recently. In a seminal experiment it has been demonstrated strong coupling between a NV-center spins ensemble and a flux qubit. The ensemble maps to a bosonic mode, thus this setup is a realization of a qubit-resonator model. In this talk we propose molecular magnets, instead of NV centers, as ensembles and we show that with them it is possible to reach the ultra strong coupling (the coupling is around $ 40 \% $ of the qubit-resonator frequencies). Finally, we emphasize the potentiality of these architectures for exploring many body physics by building arrays made of flux qubits and crystal spins. We will discuss non equilibrium signatures of Mott insulator - Superfluid phases and its feasibility within current technology. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A29.00010: Quantum transducer in circuit optomechanics Nicolas Didier, Rosario Fazio Mechanical resonators are becoming macroscopic quantum objects with great potential. It is however difficult to measure and manipulate the phonon state due to the tiny motion in the quantum regime. We show that a superconducting microwave resonator linearly coupled to the mechanical mode constitutes a powerful probe and an interesting quantum source. This coupling is rendered much stronger than the usual radiation pressure interaction by adjusting a gate voltage and gives rise to coherent oscillations between phonons and photons. The phenomenon of phonon blockade is detected from the statistics of the light field [1] and a quantum tomography of the mechanical resonator is obtained after transferring the state to the microwave cavity. Quantum phonon states can also be synthesized from the cavity and hybrid entanglement can be engineered between phonons and photons. Mechanical resonators can furthermore be coupled to a large variety of quantum systems such as spins, optical photons, cold atoms, and Bose Einstein condensates. They act as a quantum transducer between an auxiliary quantum system and a microwave resonator, which is used as a quantum bus. The quantum communications are controlled with the individual gate voltages.\\[0pt] [1] N. Didier et al., Phys. Rev. B 84, 054503 (2011). [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A29.00011: Quantum control of mechanical oscillators Invited Speaker: Konrad Lehnert |
Session A30: Focus Session: Qubits in Diamond I
Sponsoring Units: GQIChair: Fedor Jelezko, University of Ulm
Room: 259B
Monday, February 27, 2012 8:00AM - 8:12AM |
A30.00001: Stark tuning spin qubits in diamond for quantum optical networks Victor Acosta, Charles Santori, Andrei Faraon, Zhihong Huang, Kai-Mei Fu, Alastair Stacey, David Simpson, Timothy Karle, Brant Gibson, Liam McGuiness, Kumaravelu Ganesan, Snjezana Tomljenovic-hanic, Andrew Greentree, Steven Prawer, Raymond Beausoleil Integrated diamond networks based on cavity-coupled spin impurities offer a promising platform for scalable quantum computing. A key ingredient for this technology involves heralding entanglement by interfering indistinguishable photons emitted by pairs of identical spin qubits. Here we demonstrate the required control over the internal level structure of nitrogen-vacancy (NV) centers located within 100 nm of the diamond surface using the DC Stark effect. By varying the voltages applied to lithographically-defined metal electrodes, we tune the zero-phonon emission wavelength of a single NV center over a range of $\sim $0.5 nm. Using high-resolution emission spectroscopy, we directly observe electrical tuning of the relative strengths of spin-altering lambda transitions to arbitrary values. Under resonant excitation, we apply dynamic feedback to stabilize the optical transition against spectral diffusion. Progress on application of gated control to single NV centers coupled to single-crystal diamond photonic crystal cavities and other nanophotonic structures will be presented. This work was supported by DARPA and the UC Regents. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A30.00002: Decoherence-protected quantum gates for a hybrid spin register in diamond V.V. Dobrovitski, T. van der Sar, Z.-H. Wang, M.S. Blok, H. Bernien, T.H. Taminiau, D.M. Toyli, D.A. Lidar, D.D. Awschalom, R. Hanson Protecting the dynamics of coupled quantum systems from decoherence by the environment is a key challenge for solid-state quantum information processing. An idle qubit can be efficiently insulated from the environment via dynamical decoupling, but quantum gate operations are, in general, disrupted by the decoupling. This problem is particularly salient for hybrid systems, where different types of qubits evolve and decohere at vastly different rates. Here we present an efficient scheme for combining the dynamical decoupling with the quantum gate operation, using the internal resonance in the coupled-spin system. We theoretically demonstrate and experimentally achieve high-fidelity operation of a two-spin register made of the [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A30.00003: Quantum Optics with Spins and Photons in Diamond Yiwen Chu, Emre Togan, Mikhail Lukin Quantum control of interactions between photons and solid-state systems has important applications in quantum information, metrology, and the study of material properties. The nitrogen-vacancy (NV) color center in diamond is one such solid-state system that has shown great promise as an optically addressable spin qubit and highly sensitive magnetometer. We present recent work on coherent control of spin-photon interactions in a complex solid-state environment using coherent population trapping (CPT). The intrinsic magnetic field sensitivity of our CPT scheme allows us to measure the instantaneous Overhauser field associated with the $^{13}$C bath present in the diamond crystal. We show that this quantum measurement technique can be used to prepare a state of the $^{13}$C bath that has much reduced uncertainty in the associated Overhauser field. Such a state is verified by observing a modification and narrowing of the transmission window. The preparation of a more well-defined configuration of the nuclear spin environment could lead to an increase in the coherence times of the NV electronic spin qubit, which in turn has applications in increasing the sensitivity of NV-based magnetometers. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A30.00004: Substitutional nickel impurities in diamond: decoherence-free subspace for quantum information processing Thomas Chanier, Craig Pryor, Michael E. Flatte' Magnetic color centers in diamond have received interest as qubits for quantum information processing due to diamond's wide band gap and long spin lifetimes which offer the possibility to initialize, manipulate and readout the quantum state of the qubit. Ni-related impurities have been known to form various color centers in diamond and here we propose the use of a substitutional Ni impurity as a qubit. The electronic and magnetic properties of a neutral substitutional nickel impurity in diamond are studied using density functional theory in the generalized gradient approximation. The spin-one ground state consists of two electrons with parallel spins, one located on the nickel ion in the 3d9 configuration and the other distributed among the nearest-neighbor carbons. The exchange interaction between these spins is due to p-d hybridization and is controllable with compressive hydrostatic or uniaxial strain. For sufficient strain the antiparallel spin configuration becomes the ground state. Hence, the Ni impurity forms a controllable two-electron exchange-coupled system that should be a robust qubit for solid-state quantum information processing. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A30.00005: Cooling Nuclear Spins in Diamond via Dark State Spectroscopy Adi Pick, Michael Gullans, Emre Togan, Yiwen Chu, Susanne Yelin, Mikhail Lukin Optical cooling methods in atomic physics, developed over the last half century, enable reaching temperatures as low as a few nK. Some of these methods can be applied for cooling spin ensembles in solid state systems. We describe a method for cooling the nuclear spins of$^{ 13}$C impurities in diamond, via optical manipulation of the electronic spin associated with an NV$^{-}$~center. We present the physical mechanism which leads to optical pumping of the nuclear spin ensemble into particular nuclear states. The method relies on optically driving three electronic levels in the $\Lambda $ configuration, and on using the formation of dark states under the conditions of Coherent Population Trapping, (CPT). The dynamics of the nuclear ensemble during this cooling process can be described analytically by using statistical tools, including anomalous random walk models and Levy flights. I survey the theoretical results of the model and discuss some predictions for experimental signatures of Levy flights in this system. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A30.00006: Decoherence imaging of spin ensembles by a scannable single nitrogen-vacancy center in diamond Lan Luan, Michael Grinolds, Sungkun Hong, Patrick Maletinsky, Ronald Walsworth, Mikhail Lukin, Amir Yacoby Measuring the decoherence of the spin state of a single nitrogen-vacancy (NV) center in diamond has been proposed as a sensitive method for detecting ensembles of electron or nuclear spins. Using a scanning NV center magnetometer with a single NV residing about 10 nm from the scanning device surface, we explore the effect of T$_{2}$ as the device is brought in close proximity to a sample surface. We observe that the spin coherence of the NV center is strongly reduced when it comes into contact with the sample. We are able to restore the coherence by performing dynamic decoupling schemes on the NV spin, suggesting that the sample-induced decoherence originates from the fluctuating magnetic field of a surface spin ensemble. The decoherence diminishes when we increase the NV to sample surface distance by 10nm either vertically or laterally. These experiments demonstrate the potential for using the coherence of a single NV spin to locally detect and spatially map spin ensembles. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A30.00007: Investigating spin decoherence in nanodiamonds using a multi-frequency electron spin resonance A. Das, V. Stepanov, Z. Kobos, Z.H. Wang, S. Takahashi Nitrogen-vacancy (NV) centers in nanodiamonds (NDs) are extremely useful for applications of nanoscale magnetic sensing as well as for conducting fundamental science because of their unique spin properties including capability to initialize the NV spin state and their long decoherence time even at room temperature. Various sizes of nanodiamonds are commercially available. Spin properties of NV centers in NDs are often quite different from those of bulk diamonds. Possible reasons for the difference are surface defects and surface distortions and strains. In this presentation, we will discuss spin properties and spin decoherence in various NDs studied by X-band (10 GHz) and high field (230 and 115 GHz) spectrometers. We will also study properties of environmental noises in NDs using dynamical decoupling techniques. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A30.00008: Deep Level Tight-Binding Model for Transition Metal Dopant States in Diamond Victoria Kortan, Cuneyt Sahin, Michael Flatt\'e Diamond is a promising system for quantum information processing [1], providing the possibility of single-spin-photon entanglement, as well as the potential for high-speed spin manipulation at room temperature (such as has been demonstrated for the electronic spin associated with an NV center [2]). Ion implantation has been demonstrated for controllable positioning of NV centers; in principle other dopants could be so implanted as well. For example, transition-metal dopants could potentially be used as optically and electrically active single spin qubits [3]. Here we use a deep level tight binding model to study the electronic trends and defect wave functions of transition-metal dopants in diamond. Starting with the Green's functions of homogeneous diamond (within an spds* tight-binding model), a Koster-Slater approach is used to evaluate the defect state. This work is supported by an AFOSR MURI.\\[4pt] [1] A. M. Stoneham, A. H. Harker and G. W. Morley, J. Phys.: Condns. Matter 21, 364222 (2009).\\[0pt] [2] R. Hanson, O. Gywat and D. D. Awschalom, Phys. Rev. B. 74, 161203(r) (2006).\\[0pt] [3] R. Larico, et. al., Phys. Rev. B. 79, 115202 (2009). [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A30.00009: A method to measure hyperfine interaction beyond standard statistical limit Kilhyun Bang, Wen Yang, L. J. Sham We propose a method to measure the hyperfine interaction between a single electron spin and a nuclear spin and apply it to the trapped electron and the $^{15}$N nuclear spin in a diamond nitrogen-vacancy (NV) center. The electron spin is prepared in a pure quantum state and the nuclear spin acquires an unknown partial polarization as a consequence of the preparation of the electron state. The proposed quantum measurement protocol is independent of the incoherence of the initial nuclear spin state. The model utilizes the time $\tau$ of a sequence of quantum operations as well as the number $N$ of repeated sequences to increase the accuracy and the precision of the estimation. The deleterious effect of the electron spin decoherence on the precision during time $\tau$ is included in the simulation. While in the statistical limit the standard deviation (measure of imprecision) of the estimation is proportional to $1/\sqrt{N}$, the quantum operations in time $\tau$ enables the imprecision to decrease faster as $1/\tau$ instead of the equivalent statistical limit of $1/\sqrt{\tau}$. Thus, the net imprecision of the estimation dips below the statistical limit. The robustness test of our simulation shows that experimental implementation of such a precision measurement is possible. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A30.00010: Towards Probing Living Cell Function with NV Centers in Nanodiamonds Alexander Sushkov, Igor Lovchinsky, Nicholas Chisholm, David Hunger, Alexey Akimov, Peggy Lo, Amy Sutton, Jacob Robinson, Norman Yao, Steven Bennett, Hongkun Park, Mikhail Lukin We report on recent progress in using the nitrogen-vacancy (NV) center in nanodiamonds as a local probe of paramagnetic free radical concentrations in living cells. The ability to monitor the local magnetic environment within the cell provides us a new tool to study organelle function during normal operation or in response to applied stimuli. Our approach involving biologically inert, robust sensor of local magnetic fields with nanoscale resolution opens up a new interface between quantum and biological sciences. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A30.00011: Anomalous decoherence of a nitrogen-vacancy center in a nuclear spin bath Ren-Bao Liu, Nan Zhao, Zhen-Yu Wang A spin loses it coherence in its noisy environment. It is generally believed that stronger noise causes faster decoherence. Here we show that in a quantum bath, the case can be the opposite. We predict that the double-transition of a nitrogen-vacancy center spin-1 in diamond can have longer coherence time than the single transitions, even though the former suffers twice stronger noise from the nuclear spin bath than the latter. This anomalous decoherence effect is due to manipulation of the bath evolution via flips of the center spin. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A30.00012: Manipulation of $^{13}$C nuclear spins in diamond via dynamical decoupling control of the electron spin Hoi-Chun Po, Ren-Bao Liu Utilizing the anisotropic nature of the hyperfine coupling between a negatively charged nitrogen-vacancy (NV) center spin and a moderately separated $^{13} $C nuclear spin, we present a scheme to efficiently control the $^{13}$C spin in 3 to 4 pulse cycles. This scheme uses only microwave pulses tuned to swap the NV center spin between the $m_s = 0$ and $m_s = 1$ states. With a strong magnetic field of the order of $10^{3}$ G along the NV center symmetry axis, the nuclear spin can be flipped in approximately $10~\mu s$. We also numerically study the effect of various sources of errors in realistic scenario and demonstrate that the fidelity of the scheme is satisfactory. The pulse sequences can be readily generalized to perform any single qubit operation on the nuclear spin. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A30.00013: Narrowing Overhauser field by coherent population trapping Zhan-Feng Jiang, Ren-Bao Liu Jiang By optical coupling to an excited state, an electron spin may be trapped to a coherent dark state when the Overhauser field has a proper value. At the same time, the fluctuation of nuclear spins is suppressed. Although the electron may jump out of the dark state due to the Fermi-contact hyperfine interaction between the electron and the nuclei, it still has a predominant probability to stay at the dark state. The narrowing of the Overhauser field prolongs the coherent time of the electron spin. The Overhauser field is always suppressed as the external magnetic field varies in a large range. A window with a long coherent time forms when we scan the external magnetic field. This work is supported by Hong Kong RGC/GRF Project CUHK402209 and NSFC/RGC Joint Project N{\_}CUHK403/11. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A30.00014: Spin echo probe of quantum criticality in the one-dimensional transverse-field Ising model Shaowen Chen, Ke Chen, Renbao Liu We study the detection of quantum phase transition in the one dimensional transverse-field Ising model. In the model, we attach a spin-1/2 to the Ising spin chain as a probe and study the decoherence behavior of the probing spin with spin echo techniques. We evaluate the effective spectrum density of the local field that the Ising spin chain impart to the probing spin and identify the contributions from the thermal fluctuations and quantum fluctuations to the probe spin decoherence. The output of our work provides a better understanding of the quantum phase transition of this model. This work is supported by Hong Kong RGC/GRF Project CUHK 402410 and NSFC/RGC Joint Project N{\_}CUHK403/11. [Preview Abstract] |
Session A31: Focus Session: Topological Insulators - Exotic Phases and Phase Transitions
Sponsoring Units: GMAG DMPChair: Ashvin Vishwanath, University of California, Berkeley
Room: 260
Monday, February 27, 2012 8:00AM - 8:12AM |
A31.00001: Quantum Anomalous Hall Effect in Magnetic Topological Insulator GdBiTe3 Hai-Jun Zhang, Xiao Zhang, Shou-Cheng Zhang The quantum anomalous Hall (QAH) state is a two-dimensional bulk insulator with a non-zero Chern number in absence of external magnetic fields. Protected gapless chiral edge states enable dissipationless current transport in electronic devices. Doping topological insulators with random magnetic impurities could realize the QAH state, but magnetic order is difficult to establish experimentally in the bulk insulating limit. Here we predict that the single quintuple layer of GdBiTe$_3$ film could be a stoichiometric QAH insulator based on \emph{ab-initio} calculations, which explicitly demonstrate ferromagnetic order and chiral edge states inside the bulk gap. We further investigate the topological quantum phase transition by tuning the lattice constant and interactions. A simple low-energy effective model is presented to capture the salient physical feature of this topological material. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A31.00002: First-principles study of magnetic ion doping in thin film Bi$_2$Se$_3$: electronic structure and topological phase Jino Im, Hosub Jin, Arthur J. Freeman We study the quantum anomalous Hall state in magnetic ion-doped Bi$_2$Se$_3$ thin films. By using first-principles density functional theory, we investigate this electronic structure and identify its topological phase. We find that magnetic ion doping induces the exchange field splitting and changes the spin-orbit coupling strength. As the doping concentration increases, the exchange field splitting strength increases and the spin-orbit coupling strength may decrease depending on the type of magnetic ion. Based on these results, we show that the quantum anomalous Hall state in the doped Bi$_2$Se$_3$ thin film emerges at a certain range of doping concentration. The Hall conductance of the doped Bi$_2$Se$_3$ thin film will also be discussed with various doping concentrations. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A31.00003: Charge 4e Superconductivity from Quantum Spin Hall phase Eun Gook Moon, Cenke Xu We present how to achieve charge 4e superconductor as a ground state studying non-relativistically induced quantum numbers of Skyrmions. It is shown that induced charge of Skyrmions interacting with fermion with quadratic band touching dispersion is twice bigger than one of Skyrmions with Dirac-like fermion. We also show that the former Skyrmions are always bosons while the latter ones are determined by the number of fermion flavors. Possible physical realization is discussed focusing on Skyrmions of quantum spin Hall order parameter in bi-layer graphene. Properties of quantum phase transition between charge $4e$ superconductor and quantum spin Hall phase are also discussed. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A31.00004: Majorana fermions in semiconductor nanowires with realistic physical parameters Invited Speaker: Tudor Stanescu Semiconductor nanowires proximity coupled to s-wave superconductors represent a unique solid state platform for realizing and observing the elusive Majorana fermion. The existence and stability of the Majorana bound states localized at the ends of the wire depend on a set of parameters that includes the chemical potential, the external magnetic field, the spin-orbit coupling, the strength of the semiconductor-superconductor coupling, and the strengths of various types of disorder. It is critical to determine whether or not the parameter regimes that ensure the stability of the Majoranas are consistent with realistic experimental conditions. In this talk I will summarize the results of a theoretical study of multiband semiconductor nanowires that focuses on understanding the key experimental conditions required for the realization and detection of Majorana fermions. I will show that multiband occupancy not only lifts the stringent constraint of one-dimensionality, but also allows having higher carrier density in the nanowire. This significantly enhances the stability of the topologically nontrivial phase against various types of disorder, such as short-range impurities in the bulk superconductor, disorder at the semiconductor-superconductor interface, and disorder in the semiconductor nanowire. The detailed study of the parameter space for multiband semiconductor nanowires establishes the realistic likelihood of the existence of zero-energy Majorana modes within laboratory conditions. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A31.00005: Topological States and Adiabatic Pumping in Quasicrystals Yaakov Kraus, Yoav Lahini, Zohar Ringel, Mor Verbin, Oded Zilberberg We find a connection between quasicrystals and topological matter, namely that quasicrystals exhibit non-trivial topological phases attributed to dimensions higher than their own [1]. Quasicrystals are materials which are neither ordered nor disordered, i.e. they exhibit only long-range order [2]. This long-range order is usually expressed as a projection from a higher dimensional ordered system. Recently, the unrelated discovery of Topological Insulators [3] defined a new type of materials classified by their topology. We show theoretically and experimentally using photonic lattices, that one-dimensional quasicrystals exhibit topologically-protected boundary states equivalent to the edge states of the two-dimensional Integer Quantum Hall Effect. We harness this property to adiabatically pump light across the quasicrystal, and generalize our results to higher dimensional systems. Hence, quasicrystals offer a new platform for the study of topological phases while their topology may better explain their surface properties.\\[4pt] [1] Y.~E. Kraus, Y. Lahini, Z. Ringel, M. Verbin, and O. Zilberberg, arXiv:1109.5983 (2011).\\[0pt] [2] C. Janot, \textit{Quasicrystals} (Clarendon, Oxford, 1994), 2nd ed.\\[0pt] [3] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. \textbf{82}, 3045 (2010). [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A31.00006: Skyrmion quantum numbers and quantized pumping in two dimensional topological chiral magnets Bohm-Jung Yang, Naoto Nagaosa We investigate the general conditions to achieve the adiabatic charge and spin polarizations and quantized pumping in 2D magnetic insulators possessing inhomogeneous spin structures. In particular, we focus on the chiral ferrimagnetic insulators which are generated via spontaneous symmetry breaking from correlated two dimensional topological insulators. Adiabatic deformation of the inhomogeneous spin structure generates the spin gauge flux, which induces adiabatic charge and spin polarization currents. The unit pumped charge/spin are determined by the product of two topological invariants which are defined in momentum and real spaces, respectively. The same topological invariants determine the charge and spin quantum numbers of skyrmion textures. It is found that in noncentrosymmetric systems, a new topological phase, dubbed the topological chiral magnetic insulator, exists in which a skyrmion defect is a spin-1/2 fermion with electric charge $e$. Considering the adiabatic current responses of generic inhomogeneous systems, it is shown that the quantized topological response of chiral magnetic insulators is endowed with the second Chern number. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A31.00007: Beyond Floquet theory: new paradigms for robust topological phenomena in strongly driven systems Mark Rudner, Michael Levin, Erez Berg The discoveries of the quantized Hall effect [1] and Thouless' quantized adiabatic pumping [2] revealed the existence of a new class of extremely robust quantum phenomena which can be observed with high fidelity, largely independent of sample details. The immunity of these remarkable effects to a variety of perturbations can be understood in terms of a topological structure associated with the systems' wave functions. The rapid development of powerful tools for controlling solid state and atomic systems over the last decade has motivated the exploration of topological phenomena in driven systems. Recently, Kitagawa and coworkers [3] discussed analogs of previously known topological phenomena in terms of the Floquet operators of periodically-driven systems. Intriguingly, this work also revealed new robust phenomena, such as the existence of robust chiral edge modes in a 2D system with vanishing Chern numbers for all bulk Floquet bands. Here we construct the topological invariant which distinguishes phases with and without chiral edge modes, and discuss generalizations to other 1D and 2D systems. \\[4pt] [1] K. von Klitzing, G. Dorda, and M. Pepper, Phys. Rev. Lett. 45, 494 (1980).\\[0pt] [2] D. J. Thouless, Phys. Rev. B 27, 6083 (1983).\\[0pt] [3] T. Kitagawa, et al., Phys. Rev. B 82, 235114 (2010). [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A31.00008: Emergent Topological Phases with Multiple Bands and Artificial Gauge Fields Alexandru Petrescu, Karyn Le Hur Particles on a two-dimensional Kagome lattice have attracted growing attention recently in relation to topological insulators and topological phases. Such Kagome structures can be engineered in optical lattices and can also be loaded with bosons. Similar tight-binding models might also be realized in photonic QED circuits. In this work, we investigate in detail the 3-band model emerging from such a tight-binding model on the two-dimensional Kagome lattice in the presence of artificial gauge fields and identify novel phases of matter such as bulk metals with helical edge states. We investigate the stability of edge modes inside a topological metallic phase and the role of lattice anisotropies and disorder, as well as relation to current experiments. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A31.00009: Possible interaction driven topological phases in (111) bilayers of LaNiO$_3$ Ying Ran, Kai-Yu Yang, Wenguang Zhu, Di Xiao, Satoshi Okamoto, Ziqiang Wang We use the variational mean-field approach to systematically study the phase diagram of a bilayer heterostructure of the correlated transition metal oxide LaNiO$_3$, grown along the (111) direction. The Ni$^{3+}$ ions with $d^7$ (or $e_g^1$) configuration form a buckled honeycomb lattice. We show that as a function of the strength of the on-site interactions, various topological phases emerge. In the presence of a reasonable size of the Hund's coupling, as the correlation is tuned from intermediate to strong, the following sequence of phases is found: (1) a Dirac half-semimetal phase, (2) a quantum anomalous Hall insulator (QAHI) phase with Chern number one, and (3) a ferromagnetic nematic phase breaking the lattice point group symmetry. The spin-orbit couplings and magnetism are both dynamically generated in the QAHI phase. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A31.00010: Quantum Phase Transitions in the Kane-Mele-Hubbard Model Zi Yang Meng, Martin Hohenadler, Thomas C. Lang, Stefan Wessel, Alejandro Muramatsu, Fakher F. Assaad We study the ground state phase diagram of the Kane-Mele-Hubbard model on the two-dimensional honeycomb lattice. At half-filling the phase diagram is mapped out using projective auxiliary field quantum Monte Carlo simulations. We present a refined phase boundary for the quantum spin liquid. The topological (quantum spin-Hall) insulator at finite Hubbard interaction strength is adiabatically connected to the ground state of the Kane-Mele model. For the magnetic phase at large Hubbard interaction strength, we show that the magnetic order is restricted to the transverse direction. The transition from the topological band insulator to the antiferromagnetic Mott insulator is in the universality class of the three-dimensional XY model. The numerical data also suggest that the spin liquid to topological insulator and spin liquid to Mott insulator transitions are both continuous. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A31.00011: Topological Phase Transitions for Interacting Finite Systems Kai Sun, Christopher Varney, Marcos Rigol, Victor Galitski We investigate topological phase transitions in interacting systems via the observation of a topologically protected level crossing. This level crossing is robust and sharply defines a topological transition even in finite-size systems. For Chern insulators, this technique gives the same topological transition point as obtained in the Chern number calculation (via flux insertion). However, in the presence of space inversion symmetry, we proved that if the topological index changes by an odd integer at the topological transition, the level crossing can only arise under (some of) the four high-symmetry boundary conditions. This discovery provides a very efficient way to detect topological phase transitions, which reduces the computational load dramatically. In contrast to the standard Chern number technique, which requires to compute the ground state wave function for hundreds of different boundary conditions, our technique achieves the same result by calculating the excitation gap for only four different boundary conditions. We demonstrate this technique in the Haldane-Fermi-Hubbard model utilizing exact diagonalization. Generalization to time-reversal invariant Z$_2$topological insulators will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A31.00012: Topological phases and phase transitions in a two-dimensional fermionic lattice Cristiane Morais Smith, Wouter Beugeling, Nathan Goldman A topological state of matter is characterized by a topological invariant, which is protected against disorder effects. For the quantum Hall effect, the Hall conductivity is protected since it is carried by chiral edge states, induced by a magnetic field. Systems with large spin-orbit coupling exhibit the quantum spin Hall effect, where the protected quantity is the spin Hall conductivity, carried by helical edge states. In our theoretical study of a fermionic tight-binding model we show that the interplay between the magnetic field and the spin-orbit coupling generates spin-imbalanced chiral phases and exotic phase transitions between helical and chiral spin textures. We explore the experimental possibilities to observe these phase transitions in cold-atom systems, for which the necessary strengths of the magnetic field and spin-orbit coupling are accessible. As a second application, we investigate the spectrum of topological phases in HgTe quantum wells doped with Mn ions (in collaboration with Molenkamp's group). We show that this doping leads to interesting reentrant effects of both the Hall and spin Hall conductivities. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A31.00013: Phase Structure of the Topological Anderson Insulator Dongwei Xu, Vincent E. Sacksteder, Junjie Qi, Jie Liu, Hua Jiang, X. C. Xie We report the phase structure of disordered HgTe topological Anderson insulator in a 2-D geometry. We use exact diagonalization to calculate the spectrum and eigenstate structure, and recursive green's functions to calculate the conductance. All observables are measured at several system sizes, allowing us to determine phase transitions and two critical points. The quantized-conductance TAI phase contains two phases: TAI-I lying in a bulk band gap, and TAI-II where bulk states exist but are localized. We find that the TAI-II phase persists at disorder strengths where there is no bulk band gap; a bulk band gap is not necessary to obtain conductance quantization. In a previous work the weak-disorder edge of the TAI phase was explained as a transition into the bulk gap (TAI-I), but we find also a direct transition into the ungapped (TAI-II) quantized phase. Effective medium theory (SCBA) predicts well the boundaries and interior of the TAI-I phase, but fails at larger disorders including the interior of the TAI-II phase. When the system size is smaller than the bulk localization length, the quantized TAI region is bounded by either the bulk band edge or the localization length, but when the system size is large it is bounded by a transition of edge states. [Preview Abstract] |
Session A32: Topological Insulators: Quantum Hall Effect
Sponsoring Units: DCMPChair: Ion Garate, Yale University
Room: 261
Monday, February 27, 2012 8:00AM - 8:12AM |
A32.00001: Weak antilocalization in HgTe quantum wells and topological surface states: Massive versus massless Dirac fermions Ewelina Hankiewicz, Grigory Tkachov HgTe quantum wells and surfaces of three-dimensional topological insulators support Dirac fermions with a single-valley band dispersion. In this work we conduct a comparative theoretical study of the weak antilocalization in HgTe quantum wells (QWs) and topological surface states. The difference between these two single-valley systems comes from a finite band gap (effective Dirac mass) in HgTe quantum wells in contrast to gapless (massless) surface states in topological insulators. The finite effective Dirac mass implies a broken internal symmetry, leading to suppression of the weak antilocalization in HgTe quantum wells and transition to the weak localization regime as a function of the gap or carrier density. In particular, we show how the difference in the behavior of the weak localization corrections for HgTe QWs allows to distinguish topological versus normal insulators. Further for the topological surface states we predict specific weak-antilocalization magnetoconductivity in a parallel magnetic field due to their exponential decay in the bulk. The relevant experiments will be discussed. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A32.00002: Topological insulators in magnetic fields: Quantum Hall effect and edge channels with non-quantized $\theta$-term Lars Fritz, Matthias Sitte, Achim Rosch, Ehud Altman We investigate how a magnetic field induces one-dimensional edge channels when the two-dimensional surface states of three-dimensional topological insulators become gapped. The Hall effect, measured by contacting those channels, remains quantized even in situations, where the $\theta$-term in the bulk and the associated surface Hall conductivities, $\sigma_{xy}^S$, are not quantized due to the breaking of time-reversal symmetry. The quantization arises as the $\theta$-term changes by $\pm 2 \pi n$ along a loop around $n$ edge channels. Model calculations show how an interplay of orbital and Zeeman effects leads to quantum Hall transitions, where channels get redistributed along the edges of the crystal. The network of edges opens new possibilities to investigate the coupling of edge channels. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A32.00003: The topological Hubbard model and its high-temperature quantum Hall e Titus Neupert, Luiz Santos, Shinsei Ryu, Claudio Chamon, Christopher Mudry The quintessential two-dimensional lattice model that describes the competition between the kinetic energy of electrons and their short-range repulsive interactions is the repulsive Hubbard model. We study a time-reversal symmetric variant of the repulsive Hubbard model defined on a planar lattice: Whereas the interaction is unchanged, any fully occupied band supports a quantized spin Hall effect. We show that at 1/2 filling of this band, the ground state develops spontaneously and simultaneously Ising ferromagnetic long-range order and a quantized charge Hall effect when the interaction is sufficiently strong. We ponder on the possible practical applications, beyond metrology, that the quantized charge Hall effect might have if it could be realized at high temperatures and without external magnetic fields in strongly correlated materials. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A32.00004: Magnetic field dependence of the edge channels in HgTe quantum wells Benedikt Scharf, Alex Matos-Abiague, Jaroslav Fabian In recent years much attention has been devoted to topological insulators, that is, materials which are insulating in the bulk, but have conducting states at their surface. This new class of topological states has first been observed experimentally in HgTe quantum wells. In such a two-dimensional topological insulator, which is also synonymously referred to as a quantum spin Hall insulator, these surface states are one-dimensional, helical edge channels. Here we study a HgTe quantum well in the presence of a constant perpendicular magnetic field. Using tight-binding calculations as well as deriving an analytical expression to determine the electron dispersion and states, we solve the effective low-energy Hamiltonian of this system in a finite-strip geometry. Our main focus is on the behavior of these solutions with increasing magnetic field. In particular, we describe the evolution of the edge states as the system crosses from the quantum spin Hall to the quantum Hall regime. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A32.00005: Phase Diagram of Massive Dirac Fermions with Tunable Interactions in High Magnetic Fields R.N. Bhatt, Z. Papic, Y. Barlas, D.A. Abanin We study the strongly correlated states of massive fermions in two dimensions with Berry's phase $\pi$ and $2\pi$, in the limit of high magnetic fields. Due to the chiral band structure and massive carriers, the effective Coulomb interactions depend on the external magnetic field, and lead to a number of phases within a single low-lying Landau level. The tunability of the interactions allows the study of the transitions between phases in a more direct manner than in GaAs-based systems where the form of the interactions is independent of the magnetic field. We map the phase diagram at partial fillings $\nu = 1/3, 1/2, 3/5$ of the low-lying Landau levels, and find transitions between fractional quantum Hall states, compressible Fermi-liquid-like states, as well as charge-density-waves. We also find a new, broad regime of the effective interactions which favor the paired non-Abelian states. Our study identifies the strongly correlated phases expected in high-mobility graphene, bilayer graphene, topological insulators, and other materials with the non-trivial Berry phases, and provides a realistic method for studying the phase transitions between them. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A32.00006: Isotropic Landau levels of relativistic and non-relativistic fermions in 3D flat space Yi Li, Congjun Wu The usual Landau level quantization, as demonstrated in the 2D quantum Hall effect, is crucially based on the planar structure. In this talk, we explore its 3D counterpart possessing the full 3D rotational symmetry as well as the time reversal symmetry. We construct the Landau level Hamiltonians in 3 and higher dimensional flat space for both relativistic and non-relativistic fermions. The 3D cases with integer fillings are Z$_{2}$ topological insulators. The non-relativistic version describes spin-1/2 fermions coupling to the Aharonov-Casher SU(2) gauge field. This system exhibits flat Landau levels in which the orbital angular momentum and the spin are coupled with a fixed helicity. Each filled Landau level contributes one 2D helical Dirac Fermi surface at an open boundary, which demonstrates the Z$_{2}$ topological nature. A natural generalization to Dirac fermions is found as a square root problem of the above non-relativistic version, which can also be viewed as the Dirac equation defined on the phase space. All these Landau level problems can be generalized to arbitrary high dimensions systematically. \\[4pt] [1] Yi Li and Congjun Wu, arXiv:1103.5422.\\[0pt] [2] Yi Li, Ken Intriligator, Yue Yu and Congjun Wu, arXiv:1108.5650. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A32.00007: Topological property for magnetic flux tubes in a two-dimenaionl electron gas Lu-Yao Wang, Zhiguo Lv, Chon-Saar Chu We have studied the energy spectrum in the presence of square magnetic flux-tubes in a two-dimensional electron gas. Our finding is that the Dirac-like dispersion can be formed out between the third and the forth magnetic subbands without including the Zeeman interaction. This Dirac-like dispersion is not band --inverted type with a global gap. This Dirac-like dispersion becomes band-inverted when the Zeeman interaction is included. We expect that the inverted magnetic subbands due to the Zeeman interaction can be recognized as the topological insulator. This topology property can be supported by the Chern number from the magnetic subbands. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A32.00008: Pi-flux as a universal probe of two-dimensional topological insulators Vladimir Juricic, Andrej Mesaros, Robert-Jan Slager, Jan Zaanen We show that the existence of a Kramers pair of zero-energy modes bound to a vortex carrying $\pi$-flux is a generic feature of topologically nontrivial phases in the $M-B$ model, describing HgTe quantum wells, and therefore this vortex represents the bulk probe of the band topology [1]. We explicitly find the form of the zero-energy states by analytically solving Dirac equation which contains a momentum-dependent Schr\" odinger mass, besides the usual Dirac mass term. A particular regularization of the vortex potential yields the modes exponentially localized and regular at the origin that carry nontrivial charge or spin quantum number. \\[4pt] [1] V. Juricic, A. Mesaros, R.-J. Slager, and J. Zaanen, arXiv:1108.3337. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A32.00009: Strongly driven Floquet topological insulator in semiconductor quantum wells Xueda Wen, Ching-Kai Chiu Floquet topological insulator in a weak-field driven semiconductor quantum well was proposed most recently. In this article we extend the situation to strongly driving field, which can generate high-order harmonic resonances. With appropriate form of driving field, it is found that whether topological transition can happen depends on the number of resonances N we can observe. If N is odd, topological transition can happen; if N is even, topological transition cannot happen. This phenomenon may be observed in semiconductor quantum wells by applying a strongly oscillating magnetic field. In addition, our discussion can be extended to other systems such as p-wave superconductors and spin chains. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A32.00010: Quantum Hall effect in a one-dimensional dynamical system Jonathan Edge, Jan Dahlhaus, Jakub Tworzydlo, Carlo Beenakker We construct a periodically time-dependent Hamiltonian with a phase transition in the quantum Hall universality class [1]. This Hamiltonian is closely related to that of a discrete time quantum walker, but additionally it allows us to study effects of disorder. A particular choice for the form of the Hamiltonian enables us to determine the time evolution of the system in one of the dimensions exactly. Simulations of the system can thus be performed in one dimension, thereby reducing the computational effort required. We investigate the topological phase transition associated with tuning between different quantum Hall plateaux and determine the critical exponent for the divergence of the localisation length. Our scheme can in principle also be implemented in cold atoms experiments, opening the doors to investigating the quantum Hall phase transition in a one-dimensional cold atoms set up. \\[4pt] [1] J.~P. and Edge, J.~M. and Tworzydlo, J. and Beenakker, C.~W.~J., PRB 84 115133 (2011). [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A32.00011: Quarter-Filled Honeycomb Lattice with a Quantized Hall Conductance Efrat Shimshoni, Ganpathy Murthy, Ramamurti Shankar, Herbert Fertig We study a generic two-dimensional hopping model on a honeycomb lattice with strong spin-orbit coupling, without the requirement that the half-filled lattice be a Topological Insulator. For quarter-(or three-quarter) filling, we show that a state with a quantized Hall conductance generically arises in the presence of a Zeeman field of sufficient strength. We discuss the influence of Hubbard interactions and argue that spontaneous ferromagnetism (which breaks time-reversal) will occur, leading to a quantized anomalous Hall effect. G. Murthy, E. Shimshoni, R. Shankar, and H. A. Fertig, arxiv:1108.2010[cond-mat.mes-hall] [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A32.00012: Berry Curvature and Phonon Hall Effect Tao Qin, Junren Shi Abstract We establish the general phonon dynamics of magnetic solids by incorporating the Mead-Truhlar correction in the Born-Oppenheimer approximation. The effective magnetic-field acting on the phonons naturally emerges, giving rise to the phonon Hall effect. A general formula of the intrinsic phonon Hall conductivity is obtained by using the corrected Kubo formula with the energy magnetization contribution properly incorporated. The resulting phonon Hall conductivity is fully determined by the phonon Berry curvature and the dispersions. Based on the formula, the topological phonon system could be rigorously defined. In the low temperature regime, we predict that the phonon Hall conductivity is proportional to $T^{3}$ for the ordinary phonon systems, while that for the topological phonon systems has the linear $T$ dependence with the quantized temperature coefficient. \\[4pt] [1] Tao Qin and Junren Shi, arXiv:1111.1322 (2011) \\[0pt] [2] Tao Qin, Qian Niu and Junren Shi, Phys. Rev. Lett., Accepted, (2011). [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A32.00013: Dissipationless Phonon Hall Viscosity Suk-Bum Chung, Maissam Barkeshli, Xiao-Liang Qi We study the acoustic phonon response of crystals hosting a gapped time-reversal symmetry breaking electronic state. The phonon effective action can in general acquire a dissipationless ``Hall'' viscosity, which is determined by the adiabatic Berry curvature of the electron wave function. This Hall viscosity endows the system with a characteristic frequency, $\omega_v$; for phonons of frequency $\omega$, it shifts the phonon spectrum by an amount of order $(\omega/\omega_v)^2$ and it mixes the longitudinal and transverse sound waves with a relative amplitude ratio of $\omega/\omega_v$ and with a phase shift of $\pm \pi/2$, to lowest order in $\omega/\omega_v$. We study several examples, including the integer quantum Hall states, the quantum anomalous Hall state in Hg$_{1-y}$Mn$_{y}$Te quantum wells, and a mean-field model for $p_x + i p_y$ superconductors. We discuss situations in which the phonon response is directly related to the gravitational response, for which striking predictions have been made. When the electron-phonon system is viewed as a whole, this provides an example where measurements of Goldstone modes may serve as a probe of adiabatic curvature of the gapped sector of a system. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A32.00014: 3D Topological Insulators in the Continuum with Nearly Flat Bands Congjun Wu, Yi Li, Xiangfa Zhou We propose a three-dimensional topological insulating state in the harmonic potential with a strong spin-orbit coupling breaking the inversion symmetry. The system gives rise to Landau-level like quantization with the full 3D rotational symmetry and time-reversal symmetry. The radial quantization generates the energy gap between neighboring bands. The states inside each band are characterized by their angular momentum over which the dispersions are suppressed by spin-orbit coupling and thus nearly flat and without Bloch-wave states. Surface states exhibit helical Dirac Fermi surfaces which are described by the Z2 index. Similar analysis in 2D shows the existence of topological insulators in the harmonic potential with strong Rashba spin-orbit coupling. These topological insulating states can be achieved from the dimensional reduction of the quantum Hall states in 3D and 4D flat space. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A32.00015: Photo-induced quantum Hall insulators without Landau levels Takuya Kitagawa, Takashi Oka, Liang Fu, Arne Brataas, Eugene Demler In this talk, we demonstrate that non-equilibrium transport of graphene under the application of off-resonant light displays a near quantization of Hall conductance. While previous studies of topological phenomena under coherent drives focused on the effective topological band structure, electron occupations in the non-equilibrium systems, which are so far neglected, play a crucial role to determine the transport properties. Using the formalism that consistently take this into account, we show that the topological band structures can directly manifest themselves in the transport properties in Landauer-type configurations under the application of off-resonant light. We give an intuitive explanation of the induction of Chern numbers in the band structure, by showing that the virtual photon absorption/emissions of electrons produces an effective second-order hopping with phase accumulation, leading to the effective Haldane model. Our proposal opens the perspective to realize so-called quantum Hall systems without Landau levels in materials such as graphene and three dimensional topological insulators under coherent drives. [Preview Abstract] |
Session A33: Physics of Photovoltaics
Sponsoring Units: GERAChair: Jeffrey Nelson, Sandia National Laboratories
Room: 106
Monday, February 27, 2012 8:00AM - 8:12AM |
A33.00001: Search of novel photovoltaic absorbers from first-principles spectroscopic screening of hundreds of materials Liping Yu, Alex Zunger Screening for candidate PV absorbers from numerous existing materials would require a good selection criterion. Initial selection criteria generally rest on the intrinsic material properties and abundance, postponing defect and contact issues to after the list of candidates has been narrowed down. The currently available Shockley-Queisser efficiency formula gives a universal efficiency vs band gap curve (no matter whether the gap is direct or indirect) and ignore all radiative recombination loss, and has proven over the years to be insufficient. Here we propose a calculable selection criterion of ``spectroscopic limited maximum efficiency (SLME)'' which considers the type of band gap (direct allowed, direct forbidden and indirect), the shape of absorption spectra and material-dependent radiative recombination loss by a simple model. First-principles quasiparticle calculations of SLME for $\sim$300 generalized I-III-VI and $\sim$500 I-V-VI materials identify over 40 candidates with higher SLME than currently used best thin-film absorbers. Analysis of the electronic structure of the top candidates reveals an interesting mechanism for high absorptivity and shows that some indirect gap materials can even be better than direct gap materials. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A33.00002: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 8:24AM - 8:36AM |
A33.00003: First-principles electronic structure of $\beta $-FeSi$_{2}$ and FeS$_{2}$ Pengxiang Xu, Timo Schena, Stefan Bl\"{u}gel, Gustav Bihlmayer Applying density functional theory in the framework of the full-potential linearized augmented plane-wave (FLAPW) method [1], we investigated electronic structure of potential future photovoltaic materials -$\beta $-FeSi$_{2}$ and FeS$_{2}$ in their bulk phases and for selected surface orientations and terminations. Their band gaps are examined using hybrid functionals as well as many-body perturbation theory in the GW-approximation to get insight of their photovoltaic performance. The gap nature in $\beta $-FeSi$_{2}$ changes from direct to indirect as suitable stain field is induced in the structure by epitaxially matching with Si substrate. Furthermore, we also studied the atomic and electronic structure of $\beta $-FeSi$_{2}$ and FeS$_{2}$ thin films for different orientations with different terminations. The most stable orientations are determined by comparing their cohesive energy. Detailed electronic structure calculations show that surface states originating from Fe play an important role and might determine their photovoltaic properties. \\[4pt] [1] www.flapw.de [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A33.00004: The effect of surface stoichiometry on the band gap of the pyrite FeS$_2$(100) surface Yanning Zhang, Jun Hu, Matt Law, Ruqian Wu Iron pyrite ($FeS_2$) is experiencing a resurgence of interest for use in solar photovoltaic and photoelectrochemical cells. The main hurdle to the use of pyrite is the low open-circuit voltage of pyrite devices, which may result from gap states created by surface and bulk defects. Recently, systematic spin-polarized DFT calculations were performed for a series of pyrite $FeS_2$(100) surfaces to clarify the effect of surface stoichiometry on stability, electronic structure, and band gap. It was found that while stoichiometric and S-poor $FeS_2$(100) surfaces are semiconductors with band gaps of 0.56-0.72 eV, S-rich surfaces are small-gap semiconductors ($E_g < 0.3$ eV) or metals. The stoichiometric FeS2(100) surface is spin polarized in the topmost layer (2 $\mu_B$ per Fe) and displays a band of Fe $d_{z^2}$ gap states centered $~0.2$ eV above the valence band edge. Our calculations suggest that the low open-circuit voltage of pyrite solar cells may result from a narrowed surface band gap. S-poor surfaces may provide larger photovoltages than S-rich surfaces. The segregation process of sulfur vacancy under different surface conditions are also being studied, so as to provide useful guidelines for the design and fabrication of better pyrite photovoltaic materials and devices. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A33.00005: Band gap engineering of Zn based II-VI semiconductors through uniaxial strain Satyesh Yadav, Rampi Ramprasad The electronic structure of bulk wurtzitic Zn$X$ ($X$=O, S, Se, and Te) under uniaxial strain along the [0001] direction is investigated using hybrid density functional theory calculations and many-body perturbation theory. It is found that uniaxial tensile and large compressive strains decrease the band gap, similar to what has been predicted by semilocal density functional theory (DFT) calculations [Yadav et. al, Phys. Rev. B, \textbf{81}, 144120 (2010)]. Moreover, the change in the band gap under uniaxial strains predicted by semilocal DFT is in good quantitative agreement with the present results at all strains considered, thereby bringing a measure of redemption to conventional (semi)local DFT descriptions of the electronic structure of at least this class of insulators. The present results have important implications for band gap engineering through strain, especially for complex systems containing a large number of atoms (e.g., nanowires) for which higher-level calculations may be too computationally intensive. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A33.00006: Searching Stable CuxS Structure for Photovoltaic Application Qiang Xu, Bing Huang, Yufeng Zhao, Yangfa Yan, Rommel Noufi, Su-Huai Wei Employing the density-functional theory (DFT) methods, we systematically search for the most energetically favorable CuxS compounds in a wide range of 1.2 $<$ x $\le $ 2.0 guided by the experimentally identified four minerals, i.e., the chalcocite (x = 2), djurleite (x = 1.94), digenite (x = 1.80), and and anilite (x = 1.75) compounds. For the chalcocite Cu2S, all its three phases have direct band gaps of 1.3-1.4 eV, with the chalcocite low phase being more stable than other two phases, i.e., the chalcocite high and the chalcocite cubic. According to our calculation, the poor durability of the Cu2S is mainly due to the energetically favorable formation of Cu vacancies. The calculated formation heat as a function of x shows that the anilite Cu1.75S is the most stable structure. Unfortunately, this material is not a good light absorber because of its metallic feature. We propose that doping of the anilite Cu1.75S with interstitial Sn atoms may result in a compound Cu1.75Sn0.125S with an optimum direct band gap of 1.37 eV. Such a material has the ability of light absorption similar to the chalcocite Cu2S. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A33.00007: Si Nanoparticles embedded in solid matrices for solar energy conversion: electronic and optical properties from first principle calculations Stefan Wippermann, Tianshu Li, Dario Rocca, Gergely Zimanyi, Giulia Galli In device applications for solar energy conversion, nanoparticles are often embedded in a solid matrix, either crystalline or amorphous. At present a detailed understanding of the influence exerted by the embedding matrix on absorption of sunlight by the nanoparticle, and the role of the nanoparticle-matrix interface remains elusive. Building on a previous study of Si nanoparticles embedded in SiO2 [1], we investigate Si nanoparticles embedded in ZnS, used in recent experiments as a charge transport layer. A realistic model of the nanoparticle-matrix interface is created by performing ab-initio molecular dynamics simulations, and electronic and optical properties of the embedded Si nanocrystals, are obtained by first principles.\\[4pt] [1] T.Li, F.Gygi, G.Galli, Phys, Rev. Lett. 107, 206805 (2011) [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A33.00008: Multiple exciton generation in silicon nanoparticles Marton Voros, Adam Gali, Dario Rocca, Giulia Galli, Gergely Zimanyi Multiple Exciton Generation (MEG) in semiconductor nanocrystals is considered to be a promising path to improve the efficiency of solar energy conversion. Recent experimental and theoretical studies indicate that MEG is more efficient in nanoparticles (NPs) than in the bulk only on a relative energy scales in units of the gap. The primary cause of this is that quantum confinement increases the energy gap substantially in NPs. MEG will be a true breakthrough when nanocrystals are identified whose impact ionization rate is enhanced even on the absolute energy scale. For this search, we calculated impact ionization rates of silicon NPs with diameters up to 2 nm using density functional theory. Our calculations clearly demonstrate that surface reconstruction creates classes of new states at low energies, de facto lowering the NP gap and thus holding the promise of inreasing MEG even on the absolute energy scale. Our calculations include static screening within the random-phase approximation. We show that a full treatment of the transition matrix elements is essential to obtain correct results due their strong energy dependence. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A33.00009: All-Carbon Photovoltaics Marco Bernardi, Priyank Kumar, Nicola Ferralis, Shenqiang Ren, Jeffrey C. Grossman We present an alternative scheme for nanostructured solar cells, where carbon nanomaterials are the only constituents of a bulk-heterojunction acrive layer and fulfill the role of absorbers, donors and acceptors, in the absence of conjugated polymers. Ab-initio simulations were employed to calculate the band alignment for interfaces between carbon nanotubes, fullerene derivative PCBM and reduced graphene oxide, showing the presence of Type-II and Schottky heterojunctions useful for charge separation in the active layer. Accordingly, we prepared all-carbon solar cells with optimized proportions of these three components that achieved AM1.5 efficiencies up to 1.5{\%}, with fill factors up to 70{\%} and increased thermal stability and lifetime compared to polymer-based devices. Our results show the potential of all-carbon solar cells as an alternative to polymer based ones: the key combination of high carrier mobility, visible and IR absorption and stability under illumination makes them suitable for next-generation flexible photovoltaics. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A33.00010: Porous Silicon Applications for Photovoltaic Solar Cell Devices Gulsen Kosoglu, Mehmet Yumak, Ugur Dinc, Ozhan Ozatay, Yani Skarlatos, Carlos Carcia Carcia The photovoltaic industry searches for low cost, energy competitive solar cell modules, and the usual material used is crystalline silicon, which has been covering 90{\%} of the solar module market for a long time. Porous silicon (P-Si) is used in photovoltaic applications as an ultra efficient anti-reflection coating, while a graded layer with an expanded band gap offers increased absorption in the visible spectrum regions. We have built P-Si solar cell devices having considered different physical Si wafer parameters such as crystal orientation, resistivity, and doping levels; which crucially affect the device efficiency. Porous Si wafers, prepared after etching crystalline silicon in high HF concentrations, exhibit fluorescence in the purple wavelength region of the visible spectrum under UV illumination. We are now in the process of improving the efficiency of the device by modulating the structure of the P-Si wafer, and studying its photovoltaic characteristics. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A33.00011: Quantum-dot nanostructures for effective harvesting, detection, and conversion of IR radiation Nizami Vagidov, Andrei Sergeev, Andrei Antipov, Kimberly Sablon, John Little, Vladimir Mitin Our novel approach to improve harvesting, detection, and conversion of IR radiation is based on engineering of three-dimensional potential barriers introduced by quantum dots with built-in charge due to inter-dot doping. The barriers around dots exponentially suppress capture processes and increase the photoelectron lifetime. The built-in-dot charge also strongly enhances the coupling of QD structures to IR radiation. Both effects radically improve the responsivity of IR photodetectors and photovoltaic efficiency of quantum-dot solar cells. Here we report a 50{\%} increase in photovoltaic efficiency in quantum-dot solar cells as well as 25 times increase of the photoresponse of quantum-dot infrared photodetectors when the built-in-dot charge increases up to six electrons per dot. We also present results of modeling of photoelectron kinetics and discuss perspectives of IR photodetectors and solar cells based on quantum dots with built-in charge. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A33.00012: ABSTRACT HAS BEEN MOVED TO K1.00149 |
Monday, February 27, 2012 10:24AM - 10:36AM |
A33.00013: Device analysis of vertically aligned single-core nanocoaxial solar cells T. Kirkpatrick, C. Andronache, M.J. Burns, M.J. Naughton Analytical expressions for device transport are derived and numerically calculated for an array of vertically aligned single-core nanocoaxial solar cells. Total current of the device is derived as a function of the geometrical configuration of the photovoltaic junction, and expressions for rectifying current behavior are subsequently solved for and analyzed. Fundamental differences and similarities in the physics of device performance are inferred based on the analytical expressions for planar, nanowire, and nanocoaxial solar cells. To emphasize the physical difference of device performance pertaining to geometrical configuration, a comparison between planar and nanocoaxial device performance is analyzed for an amorphous silicon p-i-n junction solar cell. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A33.00014: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 10:48AM - 11:00AM |
A33.00015: ABSTRACT WITHDRAWN |
Session A34: Focus Session: Impact of Ultrafast Lasers I: X-rays and THz
Sponsoring Units: DCPChair: Nancy Levinger and Amber Kummel, Colorado State University
Room: 107A
Monday, February 27, 2012 8:00AM - 8:36AM |
A34.00001: Ultrafast X-ray Laser Studies of Chemical Dynamics Invited Speaker: Kelly Gaffney First light at the LCLS x-ray free electron laser at the SLAC National Accelerator Laboratory marked the beginning of hard x-ray laser science 2009. With pulse energies in excess of a milliJoule and pulse durations as short as 5 femtoseconds in duration, the LCLS provides a novel and potentially transformative approach for investigating chemical dynamics in complex systems. Understanding the coupled evolution of electrons and nuclei during chemical transformations remains the central and vexing challenge in the study of chemical reaction dynamics. Ultrafast optical electronic spectroscopy can monitor both the nuclear and the electronic evolution that occurs during a chemical reaction, but this joint sensitivity often impedes the robust interpretation of experimental measurement. The LCLS provides the opportunity to simultaneously measure electronic dynamics with x-ray fluorescence and nuclear dynamics with elastic x-ray scattering, providing a robust means for disentangling the coupled motions of electrons and nuclei during excited state internal conversion and intersystem crossing. These exciting new opportunities will be discussed in the context of recent studies of photo-induced spin crossover dynamics in iron(II) tris-bipyridine. [Fe(bpy)$_{3}$]$^{2+}$. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A34.00002: Coherently controlled orientation and alignment of molecules in the gas phase by intense THz fields Sharly Fleischer, Yan Zhou, Robert W. Field, Keith A. Nelson We report the use of intense terahertz (THz) pulses to orient polar molecules in the gas phase. Short THz fields exert torques that drive coherent molecular rotational motion, and multiple interactions with strong THz fields can yield multiple-quantum rotational coherences with the prospect of high degrees of orientation (dipoles pointing in the same direction in space) and alignment (molecular axes parallel to each other regardless of dipole orientation). THz-induced molecular orientation offers new possibilities in gas-phase x-ray diffraction, molecular orbital mapping through high harmonic generation and photoelectron angular distribution imaging, and other applications. We demonstrate significantly enhanced coherent control using two THz pulses with an optimized relative time delay. We show in the case of atmospheric water that a short, strong THz field induces long-lived coherent THz emission (free induction decay) that drives significant further rotational responses in a pre-excited polar gas sample. This class of experiments enables broad new capabilities for molecular spectroscopy and control beyond those afforded through molecular alignment by intense optical fields, which do not produce any net orientation. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:24AM |
A34.00003: Capturing the Coupled Dynamics of Electrons, Atoms and Spins in Molecules and Materials using Ultrafast X-Rays Invited Speaker: Margaret Murnane |
Monday, February 27, 2012 9:24AM - 9:36AM |
A34.00004: Using time-resolved Ru L-edge X-ray absorption spectroscopy to capture photoinduced transient electronic structure in a solar cell dye molecule Benjamin Van Kuiken, Michael Lynch, Munira Khalil, Nils Huse, Hana Cho, Matthew Strader, Robert Schoenlein Understanding the electronic structure of transition metal dye molecules used in dye-sensitized solar cells (DSSC) is critical for determining the functioning of these devices. Ru$^{II}$ dyes such as Ru(dcbpy)$_{2}$(NCS)$_{2}$ (termed RuN3) have been components in some of the most effective DSSCs. We use synchrotron-based picosecond Ru L-edge X-ray absorption spectroscopy (XAS) to monitor changes the changes in the electronic structure of RuN3$^{4-}$ that accompany the $^{1}$A$_{1}$ to $^{3}$MLCT conversion initiated with a 400 nm light pulse. The results are interpreted by simulating the Ru L$_{3}$ X-ray absorption spectra of the $^{1}$A$_{1}$ and $^{3}$MLCT states with time-dependent density functional theory (TD-DFT). We observe the formation of the Ru$^{III}$ oxidation state of RuN3$^{4-}$ within the 70 ps time resolution of our experiment. The TD-DFT simulation allows us to assign a spectral feature in the Ru L-edge spectrum as a probe of the electronic structure of the NCS ligands due to overlap between Ru 4d and NCS $\pi $* orbitals. A 1.2 eV blue shift in this feature between ground and $^{3}$MLCT state corresponds to depletion in charge density on the NCS ligands in the excited state. We will discuss in detail the local electronic structure around the Ru atom in the transient $^{3}$MLCT state measured by time-resolved XAS. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 10:12AM |
A34.00005: Ultrafast molecular processes mapped by femtosecond x-ray diffraction Invited Speaker: Thomas Elsaesser X-ray diffraction with a femtosecond time resolution allows for mapping photoinduced structural dynamics on the length scale of a chemical bond and in the time domain of atomic and molecular motion. In a pump-probe approach, a femtosecond excitation pulse induces structural changes which are probed by diffracting a femtosecond hard x-ray pulse from the excited sample. The transient angular positions and intensities of diffraction peaks give insight into the momentary atomic or molecular positions and into the distribution of electronic charge density. The simultaneous measurement of changes on different diffraction peaks is essential for determining atom positions and charge density maps with high accuracy. Recent progress in the generation of ultrashort hard x-ray pulses (Cu K$_{\alpha}$, wavelength $\lambda=0.154$ nm) in laser-driven plasma sources has led to the implementation of the powder diffraction and the rotating crystal method with a time resolution of 100 fs. In this contribution, we report new results from powder diffraction studies of molecular materials. A first series of experiments gives evidence of a so far unknown concerted transfer of electrons and protons in ammonium sulfate [(NH$_4$)$_2$SO$_4$], a centrosymmetric structure. Charge transfer from the sulfate groups results in the sub-100 fs generation of a confined electron channel along the c-axis of the unit cell which is stabilized by transferring protons from the adjacent ammonium groups into the channel. Time-dependent charge density maps display a periodic modulation of the channel's charge density by low-frequency lattice motions with a concerted electron and proton motion between the channel and the initial proton binding site. A second study addresses atomic rearrangements and charge dislocations in the non-centrosymmetric potassium dihydrogen phosphate [KH$_2$PO$_4$, KDP]. Photoexcitation generates coherent low-frequency motions along the LO and TO phonon coordinates, leaving the average atomic positions unchanged. The time-dependent maps of electron density demonstrate a concomitant oscillatory relocation of electronic charge with a spatial amplitude of the order of a chemical bond length, two orders of magnitude larger than the vibrational amplitudes. The coherent phonon motions drive the charge relocation, similar to a soft mode driven phase transition between the ferro- and paraelectric phase of KDP. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A34.00006: Time-resolved X-ray fragmentation probing of molecular isomerization Vladimir Petrovic, Marco Siano, James L. White, Philip H. Bucksbaum Short intense X-ray pulses, now available at FELs, are already having a strong impact on the AMO science. In addition to scattering or resonant absorption, it is possible to use X-ray radiation to probe processes on molecular timescales through non-resonant absorption by initiating a molecular fragmentation in a time-resolved manner. Core-ionized molecules composed of light elements predominantly relax through Auger decay into multiply charged molecular ions, which subsequently fragment through Coulomb repulsion. The fragmentation patterns (ion time-of-flight spectra, ion-kinetic energy release, Auger spectra, etc.) encode information about instantaneous nuclear geometry and momenta. Unlike intense IR laser field fragmentation, X-ray fragmentation occurs in the weak-field regime. In order to test the potential of time-resolved X-ray fragmentation for probing isomerization, we selected the example of ring opening of 1,3-cyclohexadiene. In a time-resolved UV-pump - X-ray fragmentation-probe experiment performed at Linac Coherent Light Source at SLAC we observed an increase in the average ion-KER and an increase in the number of lighter fragments upon photoexcitation. We discuss how the evolving fragmentation patterns reflect the structural change that the molecule is undergoing. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A34.00007: A Femtosecond Time-Resolved Transient X-ray Absorption Study of Light-Induced Coupling and Transparency in Xenon Ming-Fu Lin, Adrian Pfeiffer, Daniel Neumark, Stephen Leone, Oliver Gessner We have performed a femtosecond time-resolved transient x-ray absorption spectroscopy study to monitor the light induced coupling between bright and dark states in xenon exposed to infrared (780 nm) laser pulses with intensities up to 5$\times$10$^{13}$ W/cm$^{2}$. Significant transient variations in the inner-shell absorption spectra between $\sim $63 eV and $\sim $69 eV photon energy are observed. Near time zero, the transmission at 67 eV increases from 6(1) {\%} (field-free) to 14(3) {\%} (laser dressed). Transient absorption and transient transparency effects are interpreted within a picture of strong-field induced coupling between core-excited 4$d^{-1}(^{2}$D$_{5/2})$6$p(^{2}$P$_{3/2})$ and 4$d^{-1}(^{2}$D$_{3/2})$6$p(^{2}$P$_{1/2})$ states at 65.1 and 67 eV, respectively, and nearby dark states with \textit{ns} and \textit{nd} characters. The major features of the transient absorption spectra can be well described within a three-level (Autler-Townes) coupling scheme. Employing this model, a dipole-moment for the 4$d^{-1}(^{2}$D$_{3/2})$6$p(^{2}$P$_{1/2})$ to 4$d^{-1}(^{2}$D$_{3/2})$6$s(^{2}$S$_{1/2})$ transition of 0.4(0.1) Debye is derived. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A34.00008: High Resolution THz Studies of Crystalline Channel Hydrates David Plusquellic, Shin Chou, Zeeshan Ahmed, Kevin Douglass, Julia Scherschligt In this study, we use vibrationally state-resolved THz spectroscopy to examine the phonon mode structure of several peptides and carbohydrates co-crystallized with water. Studies are performed before and after the water is removed under vacuum on a range of systems for water in hydrophobic and hydrophilic environments. Predictions from first principles calculations are used to model the observed spectra and to obtain detailed energetic information on the dehydration processes. A new phase-coherent heterodyne method based on chirped THz pulses may be discussed. [Preview Abstract] |
Session A35: Focus Session: DFT I: Strongly Correlated Systems; GW and Many-Body Perturbation Theory
Sponsoring Units: DCPChair: Carsten Ullrich, University of Missouri
Room: 107B
Monday, February 27, 2012 8:00AM - 8:36AM |
A35.00001: Real-Space DFT Models for Strong Correlation Invited Speaker: Erin Johnson Accurate treatment of strongly-correlated electrons remains an important challenge for density-functional theory. Most functionals underestimate the energy lowering arising from non-dynamical electron correlation, as in stretched covalent bonds, open-shell singlet states, and many transition-metal compounds, including semiconductors. We develop a new density-functional approach combining physical insight from chemical structure with real-space modeling of the exchange-correlation hole, based on the Becke-Roussel exchange functional. The method is capable of predicting correct dissociation limits and describing strong correlation in many-electron systems. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A35.00002: DFT+DMFT versus DFT+U description of Mott insulators: DMFT restores spin and orbital symmetry and removes metastables states Bernard Amadon In the last twenty years, the developpement of methods based on the coupling of Density Functional Theory in the Local Density Approximation and Hubbard-like terms has led to a successfull description of many strongly correlated systems. These methods include DFT+U which contains a static description of interaction and the combination of DFT with Dynamical Mean Field Theory (DFT+DMFT) which adds fluctutations to the description of interactions. We present implementations of DFT+U and DFT+DMFT in the same framework, and with the same approximations. We show, in agreement with previous results in the litterature, and even in the simple Hubbard I approximation to DMFT,that DFT+DMFT is able to describe Mott insulator without any breaking of spin and orbital symmetry. We show that this improvement simply remove the appearance of metastable states, and thus solve a practical and physical problem encountered in particular in the description of actinides oxydes in DFT+U calculations. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A35.00003: Renormalized second-oder perturbation theory for the electron correlation energy: concepts and benchmarks Patrick Rinke, Xinguo Ren, Matthias Scheffler, Gustavo Scuseria We present a renormalized second-oder perturbation theory (R2PT) for the electron correlation energy that combines the random-phase approximation (RPA), second-order screened exchange (SOSEX) [1], and renormalized single excitations (rSE) [2]. These three terms all involve a summation of certain types of diagrams to infinite order, and can be viewed as a ``renormalization" of the direct, the exchange and the single excitation (SE) term of 2nd-order Rayleigh-Schr{\"o}rdinger perturbation theory based on an (approximate) Kohn-Sham reference state. A preliminary version of R2PT has been benchmarked for covalently-bonded molecular systems and chemical reaction barrier heights [3] and shows an overall well balanced performance. We have extended this, by including ``off-diagonal'' diagrams into the rSE term and expect this refined version of R2PT to be more generally applicable to electronic systems of different bonding characteristics. Extended benchmarks of van-der-Waals-bonded molecules and crystalline solids will be presented. [1] A. Gr\"uneis {\it et al.}, J. Chem. Phys. \textbf{131}, 154115 (2009). [2] X. Ren {\it et al.}, Phys. Rev. Lett. \textbf{106}, 153003 (2011). [3] J. Paier {\it et al.}, arXiv:cond-mat/1111.0173. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A35.00004: Efficient GW methods implemented in molecular orbital space: Ionization energy and electron affinity of conjugated molecules San-Huang Ke An efficient all-electron non-selfconsistent GW (G$^0$W$^0$) method is proposed, which is based on the full random-phase approximation and implemented in the molecular-orbital space with algorithms for reducing the error coming from the incompleteness of the basis set. The convergence of its result with regard to the size of the basis set is examined. Based on this, we further implement a quasiparticle self-consistent GW (QSGW) approach with Gaussian basis functions. The high computational efficiency allows us to deal with larger molecules from the first principles, and we applied our methods to calculate the ionization energy (IE) and electron affinity (EA) of ten conjugated molecules with up to 32 atoms. The G$^0$W$^0$ result improves the Hartree-Fock result significantly, especially for EA, and, furthermore, the QSGW improves the G$^0$W$^0$ and gives results of both IE and EA in very good agreement with the available experimental data and also with the results from the $\Delta$SCF calculation using the B3LYP functional. This indicates that our all-electron {\it ab initio} GW calculation can describe very well molecular electronic structures, making the QSGW approach a good candidate for investigating electronic and transport properties of molecular systems. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A35.00005: Exact DFT with the density matrix renormalization group Invited Speaker: E. Miles Stoudenmire The density matrix renormalization group (DMRG) is a powerful, controlled numerical method traditionally applied to lattice models of strongly correlated electrons in 1D and 2D. By extending DMRG to simulate electronic structure models on a 1D quasi-continuum grid, we can compute exact Kohn-Sham (KS) potentials and even perform exact full KS calculations.\footnote{E.M. Stoudenmire, Lucas O. Wagner, Steven R. White, Kieron Burke, arXiv:1107.2394} On their own, the continuum DMRG calculations provide significant insight into the structure of strongly correlated many-body wavefunctions both for smaller molecules and chains exceeding one hundred atoms. Combining DMRG with the machinery of density functional theory allows us to explore the reasons for the success or failure of standard DFT approximations and to better understand which many-body effects are faithfully reproduced by the KS system. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A35.00006: Exact density functionals for 1d systems Lucas Wagner, E. Stoudenmire, Kieron Burke, Steven White The success of modern density functional theory (DFT) can be attributed to the Kohn-Sham (KS) scheme, for which density functional approximations are both practical and rather simple. We are often left in the dark, however, when trying to understand why certain approximations fail, or how well they approximate the true functional. To study such questions, an exact implementation of KS-DFT is required. Though exact KS-DFT is as difficult as solving the original many-body problem, the density matrix renormalization group (DMRG) gives us a powerful tool to do this. DMRG is a highly efficient wavefunction solver in 1d, which we use to solve model continuum systems with a long-range soft-Coulomb interaction between particles. Using DMRG, we implement exact KS-DFT and investigate its inner workings. Results for some atom chains are discussed and compared to HF and LDA calculations. Preprint at arXiv:1107.2394. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A35.00007: Exact Kohn-Sham eigenstates versus quasi-particles in simple models of strongly correlated electrons Jaime Ferrer, Diego Carrascal We present analytic expressions for the exact density functional and Kohn-Sham Hamiltonian of simple tight-binding models of correlated electrons. These are the single- and double-site versions of the Anderson, Hubbard and spinless fermion models. The exact exchange and correlation potentials are fully non-local. The analytic expressions allow to compare the Kohn-Sham eigenstates of exact density functional theory with the many-body quasi-particle states of these correlated-electron systems. The exact Kohn-Sham spectrum describes correctly many of the non-trivial features of the many-body quasi-particle spectrum, as for example the precursors of the Kondo peak. However, we find that some pieces of the quasi-particle spectrum are missing because the many-body phase-space for electron and hole excitations is richer. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A35.00008: Spectroscopic Fingerprinting of Small Molecules via Many-Body Perturbation Theory Peter Doak, Pierre Darancet, Jeffrey Neaton Quantitative understanding of the photophysics of small organic molecules is an important challenge and relevant to a range of energy conversion applications. Existing first-principles methods, such as time-dependent density functional theory, coupled cluster, and other quantum chemistry-based approaches can sometimes provide onset energies with good accuracy, but agreement at higher energies - a more complete spectral fingerprint - is frequently less adequate. Here we use DFT and many-body perturbation theory, within the GW approximation and the Bethe-Salpeter Equation approach, to compute the UV-Vis absorption spectra for a range of small molecules, comparing closely to room-temperature, solution-phase measurements of onsets and spectra. First-principles molecular dynamics is used to prepare snapshots of finite temperature conformations. The effects of continuum and explicit solvation models are considered. The importance of dynamic disorder, delocalized unoccupied states, and solvation are thoroughly discussed in the context of experiments. Support: DOE via the Molecular Foundry and Helios SERC, and NSF via NCN. Computational support provided by NERSC. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A35.00009: Going beyond DFT for Organic/Titanium Dioxide interfaces Leonardo M. Marion Jorge, Marcelo Alves-Santos, Mar\'{I}lia J. Caldas There is increasing interest in organic/oxide interfaces, particularly for light harvesting and light-emitting devices, and it is important to obtain theoretical information for basic quantities such as the energy-level alignment across the interface. Accurate descriptions of the electronic structure of the composite system, as e.g. [1], are however scarce. The method used should give reliable results for both organic and oxide materials, to guarantee a good description of the hybrid system. In this work we have explored different DFT functionals (PZ-LDA, and those that include a fraction of Exact-Exchange as PBE0, HSE and B3LYP [2]) and compared the results with those obtained by Many-Body Perturbation theory with the GW approximation[3]. We have chosen as prototype systems TiO2, both bulk crystal and model surface, and Thiophene. We have found that none of the used DFT schemes give optimal results for both organic and inorganic systems at the same time, so moving beyond DFT is mandatory.\\[4pt] [1] C.D. Valentin et.al., Phys. Rev. Lett. 97, 166803 (2006)\\[0pt] [2] J.P. Perdew et.al., J. Chem. Phys. 105, 9982 (1996); J. Heyd et.al., J. Chem. Phys. 118, 8207 (2003); P.J. Stephens et.al., J. Phys. Chem. 98, 11623 (1994)\\[0pt] [3] A. Marini et.al., Comp. Phys. Comm. 180, 1392 (2009) [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A35.00010: Accurate potential energy surfaces for transition-metal complexes with DFT+U(R) Heather Kulik We introduce an improvement to the Hubbard U augmented density functional approach known as DFT+$U$ that incorporates variations in the value of self-consistently calculated, linear-response $U$ with changes in geometry. This approach overcomes the one major shortcoming of previous DFT+$U$ studies, i.e. the use of an averaged Hubbard U when comparing energies for different points along a potential energy surface is no longer required. While DFT+$U$ is quite successful at providing accurate descriptions of localized electrons (e.g. $d$ or $f$) by correcting self-interaction errors of standard exchange correlation functionals, we show several examples from diatomic molecules to porphyrins to surface science applications where this position-dependent DFT+$U(\mathbf{R})$ provides a significant two- to four-fold improvement over DFT+$U$ predictions. DFT+$U(\mathbf{R})$ reduces errors in binding energies, frequencies, and equilibrium bond lengths by applying the linear-response, position-dependent $U(\mathbf{R})$ at each point. We also propose a metric for whether a standard DFT+$U$ approach is sufficient by determining the strength of the dependence of $U$ on changes in coordinates. [Preview Abstract] |
Session A36: Focus Session: New Energy I
Sponsoring Units: DCPChair: Bruce Garrett, Pacific Northwest Research Laboratory and Anders Nilsson, SLAC
Room: 107C
Monday, February 27, 2012 8:00AM - 8:36AM |
A36.00001: Facing our Energy Challenges in a New Era of Science Invited Speaker: Eric Rohlfing As our nation and the world face daunting challenges in energy supply and the environmental consequences of energy use, how can basic science contribute to transforming the way in which we generate and use energy? How can we use the power of modern science, notably chemical physics, to accelerate the transition to a more sustainable energy future? In this talk, I shall present some of the current strategic thinking from the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (BES). In particular, I shall present how the department is approaching the entire R{\&}D spectrum for energy technologies and how BES engages the scientific community to identify basic research needs for advancing energy technologies. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A36.00002: Semiconductor nanostructures for artificial photosynthesis Invited Speaker: Peidong Yang Nanowires, with their unique capability to bridge the nanoscopic and macroscopic worlds, have already been demonstrated as important materials for different energy conversion. One emerging and exciting direction is their application for solar to fuel conversion. The generation of fuels by the direct conversion of solar energy in a fully integrated system is an attractive goal, but no such system has been demonstrated that shows the required efficiency, is sufficiently durable, or can be manufactured at reasonable cost. One of the most critical issues in solar water splitting is the development of a suitable photoanode with high efficiency and long-term durability in an aqueous environment. Semiconductor nanowires represent an important class of nanostructure building block for direct solar-to-fuel application because of their high surface area, tunable bandgap and efficient charge transport and collection. Nanowires can be readily designed and synthesized to deterministically incorporate heterojunctions with improved light absorption, charge separation and vectorial transport. Meanwhile, it is also possible to selectively decorate different oxidation or reduction catalysts onto specific segments of the nanowires to mimic the compartmentalized reactions in natural photosynthesis. In this talk, I will highlight several recent examples in this lab using semiconductor nanowires and their heterostructures for the purpose of direct solar water splitting. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A36.00003: Oxygen Evolution Electrocatalysis on Cobalt Oxide surfaces Michal Bajdich, Jens K. Norskov, Monica Garc\'Ia-Mota, Alexis T. Bell The oxidation of water for hydrogen production using sunlight is of high importance to photo-fuel cell research. The electrochemical approach via heterogeneous catalysis to water splitting is a very promising route. The key challenge of this method lies in reduction of the loses, i.e., over-potential, for the oxygen evolution reaction (OER) on the anode. In this work, we investigate the dependence of theoretical over-potential of OER on type of anode by applying standard density functional theory (DFT). We attempt to explain recent experimental observation of enhanced activity on gold supported Cobalt Oxide surfaces [1]. We explore variety of possible CoO structures and associated surfaces which could emerge under operating conditions of catalyst. Finally, we also explore the influence of environment and admixtures of CoO with other elements. \\[4pt] [1] B.S. Yeo, A.T. Bell, AT, J. Am. Chem. Soc., 133, 5587-5593 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A36.00004: High-throughput synthesis and screening of photon absorbers and photocatalysts for solar fuel cells Slobodan Mitrovic, Martin Marcin, Sean Lin, Jian Jin Joint Center for Artificial Photosynthesis is a D.O.E. Energy Innovation Hub conceived to develop solar fuel cell technologies by bringing together the critical mass of scientist and engineers nationwide. The High-Throughput Experimentation group at JCAP is developing pipelines for accelerated discovery of new materials - photon absorbers, photoelectrochemical and electrochemical catalysts - using combinatorial approaches (ink-jet, sol-gel, physical vapor deposition). Thin films of semiconducting metal-oxides, sulfides, nitrides and phosphides are synthesized and screened in high-throughput according to their optical and photoelectrochemical properties, as well as structure and phase. Vast libraries of materials and data are generated and made available to inside and outside research groups. Here we present data on binary, ternary and quaternary metal-oxide systems prepared by the ink-jet technology. The systems include tungsten-based photo-absorbers and nickel-iron-based catalysts for water splitting. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 10:12AM |
A36.00005: Transport in and chemistry on transition metal oxides for energy conversion Invited Speaker: Emily Carter We use quantum mechanics techniques to search for robust, efficient, and inexpensive materials for solid oxide fuel cells (SOFCs) that convert fuels to electricity, photovoltaics (PVs) that convert sunlight to electricity, and photo-catalytic electrodes (PCEs) that convert sunlight, CO$_{2}$, and H$_{2}$O into fuels. In our SOFC research, we focus on cathode optimization, often considered the limiting factor. If oxide ion diffusion and electron transport can be enhanced, along with rapid dissociative adsorption of dioxygen, lower temperatures can be used, which would facilitate wider deployment. In the solar energy conversion arena, the cost-efficiency tradeoff for PV materials motivates new options. I will discuss why it is difficult to find effective PCE materials; in particular I will enumerate the very significant constraints beyond those on PVs that they must satisfy to achieve high efficiency. Limiting oneself to abundant elements further constrains the design space. As a result, we are focusing primarily on first row transition metal oxide materials. Key properties of conventional and novel materials, along with some new design principles, will be discussed. The work is revealing which dopants or mixed oxides are likely to provide the most efficient energy conversion materials. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A36.00006: A comparison in activity between transition-metal oxides and transition metals Aleksandra Vojvodic, Felix Studt, Frank Abild-Pedersen, Thomas Bligaard, Jens N{\O}rskov Transition-metal oxides are widely used materials in catalysis as substrates and promoters, but also as the active catalyst materials themselves. We compare the reactivity of transition-metal oxides with the one of transition metals. The comparison is exemplified for the ammonia synthesis reaction. First we show that there exist characteristic Br{\o}nsted-Evans Polanyi (BEP) relations (linear relations between transition state and dissociation energies) for dissociation of molecules on transition-metal oxides in the rutile and perovskite structure. It is well-known that the (211) metal surface is several orders of magnitude more reactive than the (111) metal surface due to the lower BEP line for the 211 facet. We find that both rutiles and perovskites follow BEP relations that are lower than the one of the 211 facet. Second we utilize the established BEP relations together with calculated adsorption energetics in a micro-kinetic model to obtain a volcano plot for the catalytic activity. We find that oxides have a higher turn over frequency as compared with metals. Hence, oxides intrinsically have a great advantage in terms of catalytic activity which opens up for catalyst design. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A36.00007: Energy conversion and fuel production from electrochemical interfaces Invited Speaker: Nenad Markovic Design and synthesis of energy efficient and stable electrochemical interfaces (materials and double layer components) with tailor properties for accelerating and directing chemical transformations is the key to developing new alternative energy systems -- fuel cells, electrolizers and batteries. In aqueous electrolytes, depending on the nature of the reacting species, the supporting electrolyte, and the metal electrodes, two types of interactions have traditionally been considered: (i) direct -- covalent {\-} bond formation between adsorbates and electrodes, involving chemisorption, electron transfer, and release of the ion hydration shell; and (ii) relatively weak non-covalent metal-ion forces that may affect the concentration of ions in the vicinity of the electrode but do not involve direct metal-adsorbate bonding. The range of physical phenomena associated with these two classes of bonds is unusually broad, and are of paramount importance to understand activity of both metal-electrolyte two phase interfaces and metal-Nafion-electrolyte three phase interfaces. Furthermore, in the past, researcher working in the field of fuel cells (converting hydrogen and oxygen into water) and electrolyzers (splitting water back to H$_{2}$ and O$_{2})$ ) seldom focused on understanding the electrochemical compliments of these reactions in battery systems, e.g., the lithium-air system. In this lecture, we address the importance of both covalent and non-covalent interactions in controlling catalytic activity at the two-phase and three-phase interfaces. Although the field is still in its infancy, a great deal has already been learned and trends are beginning to emerge that give new insight into the relationship between the nature of bonding interactions and catalytic activity/stability of electrochemical interfaces. In addition, to bridge the gap between the ``water battery'' (fuel cell $\leftrightarrow $ electrolyzer) and the Li-air battery systems we demonstrate that this would require fundamentally new knowledge in several critical areas. We conclude that understanding the complexity (simplicity) of electrochemical interfaces would open new avenues for design and deployment of alternative energy systems. [Preview Abstract] |
Session A37: Research Collaboration between Mentors and Undergraduate Students
Sponsoring Units: FEd SPSChair: Richard Peterson, Bethel University
Room: 108
Monday, February 27, 2012 8:00AM - 8:12AM |
A37.00001: Atomic, molecular and optical physics at Bethel Chad Hoyt, Dan Klemme An example of the close connection between research and advanced labs at Bethel University is the recent realization of cold lithium atoms in a magneto-optical trap (MOT). Several aspects of the cooling and trapping research took root in the laboratory components of the Optics and Lasers upper-level courses. These included a wavelength meter with sub-picometer accuracy and precision, stabilized laser diodes and molecular and atomic spectroscopy. Work on the MOT began in 2008 and has involved students (a total of 12, including several post-General Physics sophomores) working during summers, course projects and senior research. Lithium MOTs offer challenges (e.g.\ low vapor pressure) and advantages in an undergraduate lab with respect to the more common rubidium systems. Lasers for lithium are at 671~nm, a more practical red color that can still take advantage of inexpensive laser diodes and broadband optical coatings. Its relatively simple atomic structure makes lithium amenable for stringent comparisons between theory and experiment. Recent high precision absolute frequency measurements using an atomic beam disagree. Cold-atom spectroscopy of lithium could help resolve questions about the atomic structure of lithium. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A37.00002: Lithium in a Magneto-Optical Trap Dan Klemme, Chad Hoyt We recently cooled and trapped $\sim$10$^7$ neutral $^7$Li atoms in a magneto-optical trap. Our laser source is a home-built external cavity diode laser at 671~nm and a semiconductor tapered amplifier. Acousto-optic modulators are used to generate five different laser detunings that are necessary for repumping between hyperfine states in the trapping and slowing laser beams. The laser is locked by phase-sensitive detection of fluorescence produced by a frequency-modulated laser beam normally incident to the lithium atomic beam. The laser is tuned to the $2S_{1/2} (F=2)\rightarrow2P_{3/2}(F')$ D2 transition. Two coils of wire with $\sim$100~A of current flowing through them in an anti-Helmholtz orientation generate the magnetic field gradient whose magnitude increases from zero with distance from the center of the trap. We describe a few preliminary measurements on the trapped atoms such as temperature, atom number, and loading/unloading times. Eventually, the cold lithium will be used to demonstrate single-photon cooling in an optical dipole trap. Additionally, we plan to use the trap to do high-resolution, cold atom spectroscopy. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A37.00003: Undergraduate Research in Theoretical Physics at Goucher College: Thermal Properties of Extreme Type-II Superconductors in High Magnetic Fields Sasha Dukan, Julian Irwin The subject matter of this faculty/student collaborative research involves the complex theoretical task of calculating the thermodynamic properties of strongly coupled extreme type-II superconductors starting from the high-field limit of the Landau level pairing scheme. In these systems, the low temperature and high magnetic field regime of the phase diagram fundamentally differs from the familiar low-field mixed phase, primarily by the appearance of gapless quasiparticle excitations in the energy spectrum. This theoretical/computational research was performed over the past three years in collaboration with Goucher College undergraduate students majoring in physics, mathematics and computer science. These students used their analytical and computational skills to develop computer programs and to calculate numerically the thermal properties of realistic superconducting materials. I will describe how this project created an intellectual environment where students developed an awareness of theoretical physics research and its impact on emerging technologies as well as its possible contribution to the solutions for the global energy challenge. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A37.00004: Thermal properties of Extreme Type-II Superconductors in High Magnetic Fields Julian Irwin, Sasha Dukan The subject matter of our investigation involves the complex theoretical task of calculating the specific heat of a two-gap extreme type-II superconductor. In extreme type-II superconductors, at low temperatures and high magnetic fields, Landau level quantization of electronic energies results in the appearance of gapless excitations at highly symmetrical points on the Fermi surface. The careful measurements of thermal properties like specific heat in the superconducting mixed state at low temperatures and high magnetic fields can reveal this novel gapless behavior. We present a detailed theoretical and numerical study of a realistic, disordered superconductor in high magnetic field and compare our calculated specific heat to available experimental data for the well-known two-gap superconductors NbSe$_{2}$ and LuNi$_{2}$B$_{2}$C. We also discuss the alterations to the location and number of gapless excitations caused by the inclusion of off-diagonal electronic pairing. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A37.00005: Undergraduate Studies on Compressible Flows and Shock Waves Keith Stein, Jennifer Schommer, Benjamin Heppner The Bethel University physics department believes advanced lab projects and undergraduate research experiences are crucial in the development of our students, particularly those that pursue graduate studies in physics, engineering and other applied fields. Open-ended advanced lab projects are key components in several upper level physics courses. Student project work in a specific course is often enhanced by student experiences in other upper level physics courses or other research experiences. For example, projects in {\em Fluid Mechanics} (PHY420) are often enriched by experiences that students bring from projects in {\em Optics} (PHY330) and {\em Computer Methods in Physics} (PHY350). We present examples from recent undergraduate projects on compressible flows and shock waves. Special attention is given to a project involving the design, construction, and initial testing of a small supersonic blowdown tunnel. This facility was initially constructed as part of a project in {\em Fluid Mechanics} (fall 2010). Subsequent student research projects have included high-speed video shadowgraph imaging (summer 2011) and the development of a MATLAB GUI to allow for side-by-side comparisons between simulation and ongoing experiments with the tunnel (fall 2011). [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A37.00006: Graphical User Interface for Supersonic Flow and Shock Waves in a Converging-Diverging Nozzle Jennifer Schommer, Keith Stein, Benjamin Heppner A graphical user interface (GUI) is developed to study the compressible flow in a converging-diverging (CD) nozzle. Related experiments are carried out with a small supersonic blowdown tunnel in the Bethel University physics department. The tunnel is constructed with two 5-gallon pressure tanks which are connected by a CD nozzle. Flow in the nozzle goes through three stages during the operation of the blowdown tunnel. The first stage is highly transient and culminates with the development of the quasi-static flow condition throughout the nozzle. In the second stage, flow is fully developed with sufficient driving pressure to sustain supersonic flow in the entire divergent section of the nozzle. In the final stage, the driving pressure is no longer sufficient and a normal shock recedes from the exit of the nozzle to the throat. The GUI is created with MATLAB and focuses on modeling the second and third stage of the flow in the nozzle. Modeling in stage two is based on a 1D isentropic flow assumption, whereas stage three is based on 1D isentropic flow along with normal shock relations. Additional functions of the GUI are being implemented to allow for side-by-side comparisons between simulation and ongoing experiments. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A37.00007: Characterization of Gold Nanoparticles for Radiotherapy Applications Yoshi Araki, James Rooney, Tabbetha Dobbins Gold nanoparticles (AuNPs) are able to conjoin with biological molecules and efficiently absorb light for conversion into heat energy. Thus, they are being investigated for treatment of near surface carcinomas. In this research, we synthesize AuNPs using two different approaches to yield variation in particle size and monodispersity. Transmission electron microscopy was used to characterize particle size and shape while UV-Visible spectrophotometry characterized their absorption wavelength. Presently, fibroblast cells are being used to establish protocols for cell growth, exposure to nanoparticles, irradiation, and cell viability. The future direction of this work is to synthesize a variety of nanoparticles in order to determine the optimal shape, size and composition for photothermal radiotherapy treatment. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A37.00008: Absorption and Agglomeration of Gold Nanoparticles as a means to Probe Cell Metabolic Activity for Radiotherapy Applications Adam Stefankiewicz, Frank Holder, Tabbetha Dobbins Polylactic-co-glycolic acid (PLGA) surface treatment of gold nanoparticles (AuNPs) enable those particles to cross the cellular wall. Once within the cell, absorption spectral shifts, predicted by the Mie Theory equation, occur because the dielectric constant of the matrix differs from that surrounding the cell. This phenomena will lead to particle agglomeration (due to changes in surface energy of the particles) and shifts in the Mie absorption spectra. Both phenomena are being explored in fibroblast cells as a means to track cell type and cellular metabolic activity. UV-Vis spectrophotometry and scanning electron microscopy (SEM) are incorporated to analyze the resulting particle/cell mixtures. Early results will be presented. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A37.00009: Research in growth, characterization, and applications of wide bandgap materials at a Primarily Undergraduate Institution Christopher Moore, Robert Louder Zinc oxide (ZnO) is a wide bandgap semiconductor that has attracted a great deal of attention with demonstrated applications in ultraviolet (UV) light detection, air-quality monitoring, missile warning systems, gas detection, and utilization as light-emitting diodes. Our undergraduate research group has been characterizing the growth of various ZnO film and nanowire systems, and we have fabricated and characterized ZnO-based devices, such as UV photodetectors, gas sensors, and photocatalysts. The materials and characterization systems with which we are working and our small niche within the broader field combine to address many of the challenges associated with undergraduate research. In this talk, we will discuss these challenges and how we have overcome them. We will also discuss how we have taken small amounts of money and crumbling facilities and produced a strong research group that involves 3-5 undergraduate students per semester, publishes approximately two peer-reviewed articles per year with undergraduate co-authors, and has achieved a steady stream of external funding. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A37.00010: Photocatalytic activity of ZnO thin films prepared by DC sputter deposition and thermal oxidation Robert Louder, Christopher Moore Zinc oxide (ZnO) is a wide-bandgap semiconductor with a broad range of applications, such as photocatalysis. The photocatalytic properties of ZnO result from the highly-oxidizing holes and reducing electrons that are induced on the ZnO surface by ultraviolet (UV) light. The efficiency of electron-hole pair formation is therefore critical for photocatalysis, and thus the optical quality of the films in the UV region is of critical importance. ZnO thin films have been fabricated using DC sputter deposition of Zn-metal films followed by thermal oxidation at different temperatures (300, 600, and 900$^{\circ}$C). Characterization of the optical properties of the resulting ZnO thin films through photoluminescence indicates that increasing oxidation temperature leads to reduced UV excitonic emission. The photocatalytic activities of the films were also characterized by measuring the efficiency of degradation of Rhodamine B dye in solution. The photocatalytic efficiency of the film annealed at a temperature of 300$^{\circ}$C was higher compared to those of the films annealed at temperatures of 600$^{\circ}$C and 900$^{\circ}$C. The increased photocatalytic efficiency is attributed to the increased optical quality of the films that results from lower oxidation temperatures. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A37.00011: Investigating the Role of Disorder in the Two-Dimensional Superfluid Transition D.R. Luhman, L.R. Wadleigh, P.G. Bumcrot The superfluid transition in two-dimensional helium films is an example of a Berezinskii-Kosterlitz-Thouless transition. Characteristic features of the superfluid transition, such as the abrupt onset of superfluidity, have been observed to be significantly altered in disordered two-dimensional systems. We are in the initial stages of experiments aimed at understanding the role of disorder in the two-dimensional superfluid transition. Disorder is introduced into the system by adsorbing helium films to CaF$_2$ surfaces with varying roughness. Quantitative characterization of the roughness of these films is crucial to understanding the superfluid transition in a disordered environment and is our current focus. The overall scope and goals of our experiments will be presented along with some of the challenges of doing low temperature physics in the undergraduate setting. The important role that undergraduates have played in these experiments will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A37.00012: Characterization of CaF$_{2}$ surfaces using Adsorption-Desorption Isotherms and Atomic Force Microscopy L.R. Wadleigh, D.R. Luhman, P.G. Bumcrot We are interested in using rough CaF$_{2}$ films to study the superfluid transition in two-dimensional helium systems. These experiments require quantitative information regarding the topography of the CaF$_{2}$ surfaces. The surface roughness of CaF$_{2}$ films is known to increase with film thickness as has been shown with previous atomic force microscopy (AFM) measurements [1]. We have fabricated a series of CaF$_{2 }$samples of different film thicknesses and thus different surface roughnesses. These surfaces were studied using AFM and adsorption-desorption isotherm measurements with liquid nitrogen at T=77 K. The isotherm measurements allow us to determine the pore size distribution of each CaF$_{2 }$film thickness. We find the emergence of hysteretic capillary condensation due to deep pores in the CaF$_{2}$ as the film thickness increases. The development of these deep pores is also seen in our AFM measurements. Our combined results provide a detailed description of CaF$_{2}$ surface roughness which can be utilized in the planned superfluid experiment. [1] D.R. Luhman and R.B. Hallock, Phys Rev. E \textbf{70}, 051606 (2004). [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A37.00013: Surface plasmon microscopy with low-cost metallic nanostructures for biosensing I Nathan Lindquist, Sang-Hyun Oh, Lauren Otto The field of plasmonics aims to manipulate light over dimensions smaller than the optical wavelength by exploiting surface plasmon resonances in metallic films. Typically, surface plasmons are excited by illuminating metallic nanostructures. For meaningful research in this exciting area, the fabrication of high-quality nanostructures is critical, and in an undergraduate setting, low-cost methods are desirable. Careful optical characterization of the metallic nanostructures is also required. Here, we present the use of novel, inexpensive nanofabrication techniques and the development of a customized surface plasmon microscopy setup for interdisciplinary undergraduate experiments in biosensing, surface-enhanced Raman spectroscopy, and surface plasmon imaging. A Bethel undergraduate student performs the nanofabrication in collaboration with the University of Minnesota. The rewards of mentoring undergraduate students in cooperation with a large research university are numerous, exposing them to a wide variety of opportunities. This research also interacts with upper-level, open-ended laboratory projects, summer research, a semester-long senior research experience, and will enable a large range of experiments into the future. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A37.00014: Surface plasmon microscopy with low-cost metallic nanostructures for biosensing II Lauren Otto, Sang-Hyun Oh, Nathan Lindquist Due to diffraction, traditional far-field optical microscopy cannot achieve the resolution necessary for applications in nano-scale imaging, sensing, and spectroscopy. However, the manipulation of surface plasmon waves in metallic nanostructures offers a solution. Surface plasmons are evanescent electromagnetic waves sustained by the oscillations of free electrons at the surface of a metal film. As plasmons propagate on the surface, they will probe the surface with high sensitivity and large field intensities. Furthermore, since plasmons are sensitive to within only 10-100 nm from the metallic surface and exhibit large electric field intensities, they have been explored for applications in biosensing and surface-enhanced Raman spectroscopy. We present the development of a microscopy setup for surface plasmon biosensing and the use of reliable, repeatable, low-cost nanofabrication techniques based on template stripping. High-quality nano-patterns are fabricated and used for proof-of-concept biosensing and surface-enhanced Raman spectroscopy experiments. In our customized microscopy setup, holographic illumination with arbitrary laser patterns is made possible by a spatial light modulator. This aids in the careful optical characterization of our nanofabricated samples. [Preview Abstract] |
Session A39: Electronic Structure: Theory and Numerical Methods
Sponsoring Units: DCOMPChair: Juergen Eckert, University of California, Santa Barbara
Room: 109B
Monday, February 27, 2012 8:00AM - 8:12AM |
A39.00001: Ab initio calculation of atomic level stress in intermetallic compounds and glasses Madhusudan Ojha, Don M. Nicholson, Takeshi Egami The atomic level stress is largely unexplored as a characterization tool that is sensitive to the local atomic environment. Local quantities, such as magnetic moment and volume, are directly related to the local pressure. For example the local Voronoi volume and pressure have the expected inverse relationship and magnetic moments are reduced due to reduced volume associated with pressure. For a simple system with one atom per unit cell at equilibrium the local stresses are zero. An atom in a multicomponent system can find itself under pressure that results from its cage of surrounding atoms. The atomic level stress is calculated with the Locally Self-consistent Multiple scattering (LSMS) method for Al-Au and Cu-Zr compounds and glasses, and trends are compared to the results for simple B2 compounds with atoms of different sizes. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A39.00002: Towards ab initio simulation of solid state materials at 10 nm scale Yang Wang, G. Malcolm Stocks, Don M. Nicholson, Odbadrakh Khorgolkhuu, Aurelian Rusanu For the simulation of nanostructured materials and materials with structural defects, it is necessary to consider large size unit cells consisting of thousands or more atoms. It is especially true for our investigation of radiation damage effects on the structural materials. We found that for a reasonably high energy radiation, large unit cell samples at least at 10 nm scale are needed in order for allowing the thermal energy introduced into the sample by the radiation to have sufficient space to dissipate. In this presentation, we point out that, for ab initio electronic structure calculation of a system with a unit cell at such a large scale, it is essential to pay careful attention to numerical stability. We show some numerical pitfalls that will arise as a result of large number of atoms getting involved. The round-off errors accumulated in the calculation of long-range Coulomb interactions, in particular, can lead to divergence of self-consistent iterations. We discuss our approach for circumventing these numerical difficulties. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A39.00003: Distributed Network-Based Verification and Validation of Electronic Structure Computations using ESTEST Gary Yuan, Francois Gygi ESTEST is a web-based framework for verification and validation of electronic structure computations [1,2]. It enables automatic comparison and post-processing of simulation data obtained using Abinit, Quantum-Espresso, Siesta, Exciting, Qbox and VASP. We present new features of ESTEST that extend its operation to a distributed network of servers. This capability enables sharing, verification, validation, comparison, and post-processing of simulations across a decentralized network of ESTEST servers hosted by different institutions. Examples of cross-server operations including multiple servers will be demonstrated. \\[4pt] [1] G. Yuan and F. Gygi, Computational Science \& Discovery 3, 015004 (2010) doi:10.1088/1749-4699/3/1/015004G\\[0pt] [2] http://estest.ucdavis.edu [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A39.00004: Calculation of quasi particle energies using a spectral decomposition of the static dielectric matrix: application to molecules and nanoparticles T. Anh Pham, Huy-Viet Nguyen, Dario Rocca, Giulia Galli We present a novel approach to evaluate quasi particle energies within many body perturbation theory, that substantially improves both the computational efficiency and the numerical accuracy of existing techniques.\footnote{Huy-Viet Nguyen, T. Anh Pham, D. Rocca and G. Galli (preprint).} We use a spectral decomposition of the static dielectric matrix as a basis for the frequency dependent density-density response function, and density functional perturbation theory to avoid the explicit calculation of empty electronic states. A Lanczos-chain algorithm is employed that allows for the evaluation of spectra over a wide frequency range. The numerical accuracy of computed quasi particle energies is controlled by a single parameter. The efficiency and accuracy of our approach are demonstrated by computing vertical ionization potentials and electron affinities of several molecules and diamondoids. Our results are in good agreement with experiment and those reported in the literature using Quantum Monte Carlo calculations. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A39.00005: First-principles calculations of quasiparticle energies of open-shell condensed matter systems Johannes Lischner, Jack Deslippe, Manish Jain, Steven G. Louie We present a Green's function approach to quasiparticle excitations of open-shell systems within the GW approximation. It is shown that accurate calculations of the characteristic multiplet structure require a precise knowledge of the self energy and, in particular, its poles. We achieve this by constructing the self energy from appropriately chosen mean-field theories on a fine frequency grid. We present results for the nitrogen dioxide molecule and the negatively charged nitrogen-vacancy defect in diamond, which are in good agreement with experiment and other high-level theories. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A39.00006: On the accuracy of the G$^0$W$^0$ method: From Si to ZnO Peihong Zhang, Zhijun Yi, Bi-Ching Shih, Jack Deslippe, Steven G. Louie The \textit{ab initio} GW method has been recognized as one of the most powerful theories in predicting quasiparticle excitations in solids. Due to computational limitations, earlier GW calculations usually made use of certain generalized plasmon-pole approximations to carry out frequency integration of the electron self-energy. In addition, often the convergence of the calculated results with respect to various cutoff parameters (such as the number of conduction bands and the size of the dielectric matrix) was not investigated in details. With advances in computational methodology and technology, fully converged GW calculations with explicit frequency integration become possible. In this talk, we will discuss fully converged G$^0$W$^0$ results for a range of materials, ranging from the most ``theory friendly'' system Si to the widely discussed and controversial system ZnO. We will compare explicit frequency integration vs various generalized plasmon-pole models. We will also discuss effects of various cutoff parameters used in GW calculations. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A39.00007: A Comparison of Different Treatments of Spin-Orbit Coupling within the GW Approach Brad Barker, Jack Deslippe, Oleg V. Yazyev, Steven G. Louie Spin-orbit coupling is responsible for the unique electronic properties of many fascinating materials such as topological insulators. We have developed and implemented an approach in which the effects of spin-orbit interactions to the quasiparticle band structure are incorporated within the GW approach, employing spinor wavefunctions computed at the DFT level with fully relativistic pseudopotentials. We compare these results to separate calculations where spin-orbit coupling is applied as a perturbation either before or after the GW calculation of the band structure. We apply these methods to the properties of materials with heavy ion cores to determine possible differences from the different schemes. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A39.00008: Dielectric screening : Effects of Core Polarization on Phonons and Quasiparticle Bands Amandeep Kaur, Erik Ylvisaker, Deyu Lu, Giulia Galli, Warren Pickett We investigate the influence of core polarization on the dielectric screening of atoms, molecules and solids with focus on non metallic systems. We compare results for dielectric band structures and for the eigenvalues of the dielectric matrix obtained by varying the number of valence electrons included in our calculations. We show that (semi)-core electronic states may substantially influence the dielectric screening, even if they lie very deep in energy compared to the outermost valence electrons. We then discuss how the changes in dielectric screening observed when including (semi)-core electrons affect computed quasi particle energies at the GW level, and phonon frequencies, e.g. the LO-TO splitting. We focus on closed shell atoms, including Be,Mg, Ca, Ar, Zn, simple diatomic molecules and simple ionic solids, e.g. LiH and NaH. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A39.00009: Novel approach to electron partitioning and definitive oxidation state assignment in solids Lai Jiang, Sergey Levchenko, Andrew Rappe Oxidation state of a atom is usually defined by partitioning electrons to the nucleus based on charge density distribution, which inherits uncertainty from the probabilistic nature of wavefunctions. Here we propose a first principle approach to electron partitioning in insulating solids based on wavefunction topology. By calculating polarization change upon shifting an atomic sublattice to its periodic image, the charge transferred during nuclei displacement can be derived. To rationalize, in Wannier representation the Berry's phase polarization is directly related to the position of Wannier Center (WC) of each band, therefore a quantized charge flow is determined by the number of WCs that move together with (\textit{i.\ e.} belong to ) the nucleus. We provide both rigorous mathematical definition of oxidation states in this scheme and results from calculations of various sample systems that corroborate with oxidation states assigned by conventional chemical insight. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A39.00010: On the Formation of XLi$_{3}$N$_{2}$ Compounds (X = Sc-Zn) Jan Herbst, Louis Hector, Jr. Ternary lithium nitrides XLi$_{3}$N$_{2}$ are known to form for the 3d transition elements X = Sc, Fe. We explore the formation of such compounds for other 3d elements by means of density functional theory using the crystal structures of ScLi$_{3}$N$_{2}$ and FeLi$_{3}$N$_{2}$ as templates. Enthalpies of formation including electronic and phonon contributions are calculated for the most stable structures. Thermodynamic stability with respect to known binary and ternary compounds is investigated in order to assess prospects for phase formation. In the case of FeLi$_{3}$N$_{2}$ we find an antiferromagnetic state lower in energy than the ferromagnetic state previously identified. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A39.00011: Surface Studies with Combined Free Energy Functionals of Electronic and Liquid Densities Kendra Letchworth Weaver, Ravishankar Sundararaman, Tom\'as Arias The microscopic structure of both a solid surface and a contacting liquid can be dramatically affected by the interaction between the two systems, particularly at the interface between a polar surface and a polar liquid. We present a study of oxide and metallic surfaces in an aqueous electrolyte environment with Joint Density Functional Theory (JDFT), a computationally efficient alternative to molecular dynamics simulations which replaces thermal sampling with a single variational principle for the free energy of the full system. Within the rigorous framework of JDFT, we introduce classical density-functionals for ionic species and describe how to couple them with existing functionals for liquid water and traditional electronic density-functionals. Calculations employ a liquid water functional, which captures bulk properties and microscopic structure over the entire phase diagram of the liquid, and a density-only coupling functional between the electronic and liquid systems, which can reproduce solvation free energies of small molecules to within chemical accuracy. With this microscopically accurate description of the liquid-solid interface structure, we gain physical insight which could direct future studies of catalysis and electrode stability in electrochemical systems. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A39.00012: Origin of magnetic interactions and their influence on the structural properties of Ni2MnGa and related compounds Burak Himmetoglu, Matteo Cococcioni In this work, we perform first principles DFT calculations to investigate the interplay between magnetic and structural properties in Ni$_2$MnGa. We demonstrate that the relative stability of austenite (cubic) and non-modulated martensite (tetragonal) phases depend critically on magnetic interactions between Mn atoms. While standard approximate DFT functionals stabilize the latter phase, a more accurate treatment of electronic localization and magnetism obtained with DFT+U suppresses the non-modulated tetragonal structure for the stoichiometric compound, in better agreement with the experiments. This observation can be explained using the Anderson impurity model, where Mn atoms are treated as periodic magnetic impurities embedded in Ga $p$ and Ni $d$ conduction electrons that mediate RKKY type magnetic interactions between Mn $d$ electrons. Using this picture we show that the structural properties of the material are determined by the competition between super-exchange interactions mediated through Ni $d$ and Ga $p$ states. Finally, we show that off-stoichiometric compositions with excess Mn promote transition to a non-modulated tetragonal structure, in agreement with experiments. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A39.00013: Redshift of excitons in carbon nanotubes caused by the environment polarizability: A BSE study Michael Rohlfing Optical excitations of molecular systems can be modified by their physical environment. We analyze the underlying mechanisms within many-body perturbation theory (GW approximation and Bethe-Salpeter equation, BSE), which is particularly suited to study non-local polarizability effects on the electronic structure. Here we focus on the example of a semiconducting carbon nanotube, which observes redshifts of its excitons when the tube is touched by another nanotube or other physisorbates. We show that the redshifts mostly result from the polarizability of the attached ad-system. Electronic coupling may enhance the redshifts, but depends very sensitively on the structural details of the contact. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A39.00014: Vibrational properties of correlated systems from ab-initio calculations Matteo Cococcioni, Burak Himmetoglu, Andrea Floris In this talk I will present a recent extension of Density Functional Perturbation Theory to the DFT+U energy functional that allows to compute the vibrational properties of materials from their correlated ground state. The new computational tool, named DFPT+U, is used to investigate the phonon spectrum of MnO and NiO. The more accurate account of electronic correlation through the Hubbard-corrected functional results in a significant improvement in the agreement between the computed phonon frequencies and available experiments. In particular, we obtain a significant reduction in the splitting between the center-zone optical modes (due to the antiferromagnetic order) that confirms the importance of electronic localization in the description of magnetic couplings between metal ions. The order of the split optical modes is also shown to correlate with the occupation of the d states of the transition metal atoms opening for the possibility to use measurements of the vibrational frequencies to investigate certain aspects of the electronic structure of these compounds. In the last part of the talk I will also present the application of DFPT+U to Cu compounds and will discuss the possibility to use it to compute the electron-phonon coupling of high T$_c$ superconductors. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A39.00015: Effect of strong correlations on the electronic and transport properties of porphyrin wires Victor Manuel Garcia Suarez, Ruben Ferradas, Jaime Ferrer Recent transport experiments performed on porphyrin molecules show that wires based on these molecules could be used to design future nanoscale elements with low attenunation as a function of distance and able to work at room temperature. Apart from their rather good transport properties, which come from the conjugation of their molecular backbone, porphyrins are also very interesting due to the fact that they have a metallic element in the middle that affects their electronic properties and could also introduce magnetic effects. In this talk I will show first-principles simulations based on density functional on the electronic and transport properties of porphyrin wires between gold leads, paying special attention to the effect of using different metallic elements in the central part of the molecule (Fe, Co, Ni, Zn and Cu). Strong correlations are introduced in the form of LDA+U. The results show that the metallic element plays a crucial role in the electronic structure of the molecule around the HOMO and LUMO orbitals and introduces magnetic effects that affect the transport properties. [Preview Abstract] |
Session A40: Focus Session: Protein Association I: Phase Separation, Crystallization, Self-Assembly
Sponsoring Units: DBIO DPOLYChair: Steve Whitelam, Lawrence Berkeley National Lab
Room: 156A
Monday, February 27, 2012 8:00AM - 8:36AM |
A40.00001: TBD Invited Speaker: Robert Hayre |
Monday, February 27, 2012 8:36AM - 8:48AM |
A40.00002: Phase Separation in Solutions of Monoclonal Antibodies George Benedek, Ying Wang, Aleksey Lomakin, Ramil Latypov We report the observation of liquid-liquid phase separation (LLPS) in a solution of humanized monoclonal antibodies, IgG2, and the effects of human serum albumin, a major blood protein, on this phase separation. We find a significant reduction of phase separation temperature in the presence of albumin, and a preferential partitioning of the albumin into the antibody-rich phase. We provide a general thermodynamic analysis of the antibody-albumin mixture phase diagram and relate its features to the magnitude of the effective inter-protein interactions. Our analysis suggests that additives (HSA in this report), which have moderate attraction with antibody molecules, may be used to forestall undesirable protein condensation in antibody solutions. Our findings are relevant to understanding the stability of pharmaceutical solutions of antibodies and the mechanisms of cryoglobulinemia. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A40.00003: Concentrated dispersions of equilibrium protein nanoclusters that reversibly dissociate into active monomers Thomas M. Truskett, Keith Johnston, Jennifer Maynard, Ameya Borwankar, Maria Miller, Brian Wilson, Aileen Dinin, Tarik Khan, Kevin Kaczorowski Stabilizing concentrated protein solutions is of wide interest in drug delivery. However, a major challenge is how to reliably formulate concentrated, low viscosity (i.e., syringeable) solutions of biologically active proteins. Unfortunately, proteins typically undergo irreversible aggregation at intermediate concentrations of 100-200 mg/ml. In this talk, I describe how they can effectively avoid these intermediate concentrations by reversibly assembling into nanoclusters. Nanocluster assembly is achieved by balancing short-ranged, cosolute-induced attractions with weak, longer-ranger electrostatic repulsions near the isoelectric point. Theory predicts that native proteins are stabilized by a self-crowding mechanism within the concentrated environment of the nanoclusters, while weak cluster-cluster interactions can result in colloidally-stable dispersions with moderate viscosities. I present experimental results where this strategy is used to create concentrated antibody dispersions (up to 260 mg/ml) comprising nanoclusters of proteins [monoclonal antibody 1B7, polyclonal sheep Immunoglobin G and bovine serum albumin], which upon dilution in vitro or administration in vivo, are conformationally stable and retain activity. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A40.00004: Aggregation behavior of bovine and porcine insulin in presence of inhibitors Kiersten Batzli, Brian Love Insulin aggregation can be problematic when the insulin is in pharmaceutical solution, as well as when aggregates form in insulin pumps or subcutaneously at port sites. Pharmaceutical insulins often include an added stabilizer, such as metacresol, but even with this added compound the protein may degrade, aggregate and become less effective. With greater understanding of the kinetics of aggregation associated with insulin aggregation, a more effective stabilizer may be identified to inhibit aggregation. Bovine and porcine insulin in pharmaceutically relevant concentrations of ~5 mg/ml were induced to denature and aggregate at elevated temperature and low pH and the aggregation behavior was characterized as a function of time and temperature by rheology and small angle x-ray scattering (SAXS). Aggregation behavior was probed in both neat solution and with the addition of known inhibiting compounds. The efficacy of the inhibiting compounds at retarding the protein aggregation was found to be related to the hydrophobicity of the compounds and potential inhibitors of interest were identified. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A40.00005: Design rules for the self-assembly of a protein crystal Thomas Haxton, Stephen Whitelam The need to crystallize proteins for X-ray studies has motivated theories of protein crystallization. These theories, based largely on the behavior of isotropic spheres that form close-packed crystals, predict that assembly is enhanced near the metastable critical point associated with phase separating into a vapor of proteins (protein-poor solution) and a liquid of proteins (protein-rich solution). However, most protein crystals are open structures stabilized by anisotropic interactions, and many assemble best above the critical point. Using theory and simulation, we show that optimal assembly of one such crystal, a model surface-layer protein crystal, is not predicted by the critical point but can be predicted by a combination of two design rules: the thermodynamic driving force must be on the order of the thermal energy, and interactions must be made as nonspecific as possible without promoting liquid-vapor phase separation. In experimental terms, our results suggest adjusting solution conditions in order to impose a defined supersaturation at the liquid-vapor critical point. Our findings suggest that self-assembly of open crystals is more akin to viral capsid self-assembly than to the crystallization of spherical colloids. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A40.00006: Protein Crystal Nucleation and Growth Sathish Akella, Seth Fraden We have developed a microfluidic emulsion based technique to determine the homogeneous and heterogeneous nucleation rates of protein crystallization under conditions of high supersaturation. We utilize the fact that the nucleation rate is constant if no crystal nucleus is formed and count the number of protein droplets with no crystals with time, which decays exponentially with decay constant inversely proportional to nucleation rate and drop volume. We report results of experiments on nucleation and growth rates of lysozyme crystallization. The emulsions are placed on a temperature gradient stage allowing simultaneous measurement of rates as a function of temperature. We routinely scan 30,000 drops in each experiment. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A40.00007: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 9:48AM - 10:00AM |
A40.00008: Water dynamics during the association of hiv capsid proteins studied by all-atom simulations Naiyin Yu, Michael Hagan The C-terminal domain of the HIV-1 capsid protein (CA-C) plays an important role in the assembly of the mature capsid. We have used molecular dynamics simulations combined with enhanced sampling methods to study the association of two CA-C proteins in atomistic detail. In this talk we will discuss the dynamics of water during the association process. In particular, we will show that that water in the interfacial region does not undergo a liquid-vapor transition (de-wetting) during association of wild type CA-C. However, mutation of some hydrophilic residues does lead to a dewetting transition. We discuss the relationship between the arrangement of hydrophilic and hydrophobic residues and dewetting during protein association. For the HIV capsid protein, the arrangement of hydrophilic residues contributes to maintaining weak interactions, which are crucial for successful assembly. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A40.00009: Interfacial Microrheology of Lysozyme Layers During Formation at the Air-Water Interface Daniel Allan, Daniel Reich, Robert Leheny Proteins can adsorb to the air-water interface to form viscoelastic layers. Characterizing the rheology of such layers is challenging, due to the confined geometry, the fragility of the layers, and the possibility of mesoscale spatial heterogeneity. Passive microrheology --- using the thermal motion of colloidal probes to interrogate the mechanical response of the surrounding medium --- is a suitable technique for addressing these difficulties. In particular, this approach sheds light on the properties of incipient protein layers that are characterized by modest interfacial viscosities. We describe microrheology studies of lysozyme layers at the air-water interface, in which we determine the evolving interfacial shear response through the viscoelastic transition that signifies layer formation. Spatial heterogeneity in the interfacial rheology is identified and discussed within the framework of layer formation as a gel transition. Layers formed by adsorption of protein from the aqueous subphase and by spreading protein directly onto the interface are compared and studied across a range of concentrations, demonstrating the sensitivity of layer properties to the rate and manner of protein accretion. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A40.00010: Measurements of Diffusion within Concentrated Bovine $\alpha$-Crystallin Suspensions Nuwan Karunaratne, Justin Berry, Larence Lurio, George Thurston, Janae Debartolo, Suresh Narayanan, Alec Sandy, John Weizeorick $\alpha -$Crystallin is a major protein component of the vertebrate eye lens. The chaperone-like behavior of these water soluble proteins play a key role in maintaining lens transparency by preventing condensation of other lens proteins. We report photon correlation spectroscopy measurements, both X-ray Photon Correlation Spectroscopy (XPCS) and Dynamic Light Scattering (DLS), indicating protein diffusion within suspensions of $\alpha $-Crystallin. Measurements were carried out at 2$^{\circ}$C, 10$^{\circ}$C and 35$^{\circ}$C, over a wide range of concentrations from the diluted limit to the regime close to the physiological lens concentration. In the diluted regime, DLS measurements can be modeled by a single exponential fit indicating a single relaxation mode and at higher concentrations two relaxation modes can be identified by fitting the data to a double exponential decay function, a clear indication of the ploydispersed nature of the concentrated samples. XPCS measurements show dynamics at the highest concentration but cannot resolve the faster dynamics (below 20ms) at lower concentration. We also provide estimates for the viscosity of $\alpha $-Crystallin suspensions as a function of temperature and protein volume fraction using the falling ball method. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A40.00011: Master Equation Approach to Protein Assembly -- Degradation of Protein Aggregates Georg Meisl Protein aggregation is dependent on several microscopic processes such as nucleation, elongation and fragmentation of aggregates. Applying simple chemical kinetics to these processes allows one to derive master equations describing the entire system. In almost all cases they take the form of highly non-linear coupled differential equations for which no exact analytical solutions can be derived. Nonetheless an analytical description of the problem is absolutely essential to determine the relative importance of different microscopic processes and develop a rational approach to finding cures. Using a self consistent approach, my group has recently made headway in finding approximate analytical solutions for several systems and successfully applied them to explain a wide range of experimental observations (Knowles et al., Science 326, 1533 (2009)). I have generalised this description to include degradation of aggregates by various cellular processes. These degradation processes are thought to play an important role in vivo in determining when aggregation speed becomes critical and leads to disease. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A40.00012: Intermediate-range order in protein suspensions Yun Liu, Lionel Porcar, Peter Falus, Wei-Ren Chen, Emiliano Fratini, Kunlun Hong, Piero Baglioni Intermediate-range order (IRO) has been widely observed in vitreous silica, water ice, metallic glass, and even in ionic liquids. Our recent work demonstrates that there is IRO present in a colloidal suspension, such as protein solution, when both a short-range attraction and long-range repulsion are present. We have verified this experimentally using lysozyme solutions, where a peak (IRO peak) seen in small angle neutron scattering (SANS) has been mistakenly called a cluster peak as it has once been considered an indication of a cluster rich phase in solution. By combining both SANS and neutron spin echo (NSE) techniques, we clearly show that there is no direct relation between cluster formation and the presence of an IRO peak. By investigating the short time dynamics using NSE, we show that the formation of clusters is still indeed possible at high concentrations. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A40.00013: An energy landscape approach to protein aggregation Alexander Buell, Tuomas Knowles Protein aggregation into ordered fibrillar structures is the hallmark of a class of diseases, the most prominent examples of which are Alzheimer's and Parkinson's disease. Recent results (e.g. Baldwin et al. J. Am. Chem. Soc. 2011) suggest that the aggregated state of a protein is in many cases thermodynamically more stable than the soluble state. Therefore the solubility of proteins in a cellular context appears to be to a large extent under kinetic control. Here, we first present a conceptual framework for the description of protein aggregation ( see AK Buell et al., Phys. Rev. Lett. 2010) that is an extension to the generally accepted energy landscape model for protein folding. Then we apply this model to analyse and interpret a large set of experimental data on the kinetics of protein aggregation, acquired mainly with a novel biosensing approach (see TPJK Knowles et al, Proc. Nat. Acad. Sc. 2007). We show how for example the effect of sequence modifications on the kinetics and thermodynamics of human lysozyme aggregation can be understood and quantified (see AK Buell et al., J. Am. Chem. Soc. 2011). These results have important implications for therapeutic strategies against protein aggregation disorders, in this case lysozyme systemic amyloidosis. [Preview Abstract] |
Session A41: Focus Session: Structure and Dynamics of Membranes
Sponsoring Units: DBIO DPOLY DMPChair: Mu-Ping Nieh, University of Connecticut
Room: 156B
Monday, February 27, 2012 8:00AM - 8:36AM |
A41.00001: A Neutron View of Proteins in Lipid Bilayers Invited Speaker: Stephen White Despite the growing number of atomic-resolution membrane protein structures, direct structural information about proteins in their native membrane environment is scarce. This problem is particularly relevant in the case of the highly-charged S1-S4 voltage- sensing domains responsible for nerve impulses, where interactions with the lipid bilayer are critical for the function of voltage-activated potassium channels. We have used neutron diffraction, solid-state nuclear magnetic resonance spectroscopy, and molecular dynamics simulations to investigate the structure and hydration of bilayer membranes containing S1-S4 voltage-sensing domains. Our results show that voltage sensors adopt transmembrane orientations, cause a modest reshaping of the surrounding lipid bilayer, and that water molecules intimately interact with the protein within the membrane. These structural findings reveal that voltage sensors have evolved to interact with the lipid membrane while keeping the energetic and structural perturbations to a minimum, and that water penetrates into the membrane to hydrate charged residues and shape the transmembrane electric field. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A41.00002: Curvature Forces in Membrane Lipid-Protein Interactions Michael F. Brown Membrane protein conformational changes, folding, and stability may all involve elastic deformation of the bilayer. Non-specific properties of the bilayer play a significant role in modulating protein conformational energetics. A flexible-surface model (FSM) describes the balance of curvature and hydrophobic forces in lipid-protein interactions. The FSM describes elastic coupling of membrane lipids to integral membrane proteins. Curvature and hydrophobic matching to the lipid bilayer entails a stress field that explains membrane protein stability. Rhodopsin provides an important example, where solid-state NMR and FTIR spectroscopy characterize the energy landscape of the dynamically activated receptor. Time-resolved UV-visible and FTIR spectroscopic studies show how membrane lipids affect the metarhodopsin equilibrium due to non-specific material properties. Influences of bilayer thickness, nonlamellar-forming lipids, detergents, and osmotic stress on rhodopsin function are all explained by the new biomembrane model. By contrast, the older fluid-mosaic model fails to account for such effects on membrane protein activity. According to the FSM proteins are regulated by membrane lipids whose spontaneous curvature most closely matches the activated state within the lipid membrane. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A41.00003: Thickness fluctuations in pure lipid bilayers Michihiro Nagao, Andrea Woodka, Paul Butler, Lionel Porcar, Bela Farago Recently neutron spin echo (NSE) revealed dynamical processes in a surfactant membrane system around the length scale of the membrane thickness where the classical Helfrich treatment breaks down. This excess dynamics (on top of the bending fluctuations) observed in a nonionic surfactant, water and oil system, was attributed to thickness fluctuations of the membrane. In the case of bilayers formed with the nonionic surfactant in water the thickness fluctuation amplitude was estimated to be a few angstroms. In the study presented here, we apply the technique to explore thickness fluctuations in lipid bilayers. The result shows clear evidence of thickness fluctuations above $T_{m}$, where the lipid tails display liquid ordering, while none are discernable below $T_{m}$. The estimated amplitude of the observed membrane thickness fluctuations is approximately 4 {\AA}. These results are consistent with theoretical expectation and recent molecular dynamics simulations. Varying the lipid tail length from 14 carbons to 18 carbons per tail does not appear to affect to the dynamics very much. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A41.00004: A Simple Analysis of Small-Angle Neutron Scattering Data to Estimate Thickness Fluctuations of the Membrane Takumi Hawa, Victor Lee Small-angle neutron scattering (SANS) and neutron spin echo (NSE) experiments are one of the most important laboratory techniques to investigate structure and dynamic properties of biological and nanotechnology-related membrane systems. Due to the sensitivity of about 1~100 nm length scales, these experimental techniques provide extensive information over a wide variety of technological and scientific applications. Recently, the author and his colleagues studied swollen lamellar structure systems consisting of nonionic surfactant, water, and oil using SANS/NSE and molecular dynamics (MD) simulation. They proposed a new experimental technique to measure the thickness fluctuations of surfactant layers and verified their approach using MD simulations. In this talk a possible simpler approach to estimate the membrane thicknesses and fluctuations directly from the isotropic scattering intensities in the two-dimensional SANS profile will be proposed. Generally, this characteristic feature is reproduced using various scattering theories to estimate the membrane thickness; however, the thickness fluctuation amplitude has never been estimated from the SANS profile. The results obtained from the approach will be compared with the experimental results obtained by Nagao and co-workers. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A41.00005: High-resolution Structures of Protein-Membrane Complexes by Neutron Reflection and MD Simulation: Membrane Association of the PTEN Tumor Suppressor Invited Speaker: Mathias Loesche The lipid matrix of biomembranes is an in-plane fluid, thermally and compositionally disordered leaflet of 5 nm thickness and notoriously difficult to characterize in structural terms. Yet, biomembranes are ubiquitous in the cell, and membrane-bound proteins are implicated in a variety of signaling pathways and intra-cellular transport. We developed methodology to study proteins associated with model membranes using neutron reflection measurements and showed recently that this approach can resolve the penetration depth and orientation of membrane proteins with {\AA}ngstrom resolution if their crystal or NMR structure is known. Here we apply this technology to determine the membrane bindung and unravel functional details of the PTEN phosphatase, a key player in the PI3K apoptosis pathway. PTEN is an important regulatory protein and tumor suppressor that performs its phosphatase activity as an interfacial enzyme at the plasma membrane-cytoplasm boundary. Acting as an antagonist to phosphoinositide-3-kinase (PI3K) in cell signaling, it is deleted in many human cancers. Despite its importance in regulating the levels of the phosphoinositoltriphosphate PI(3,4,5)P$_{3}$, there is little understanding of how PTEN binds to membranes, is activated and then acts as a phosphatase. We investigated the structure and function of PTEN by studying its membrane affinity and localization on in-plane fluid, thermally disordered synthetic membrane models. The membrane association of the protein depends strongly on membrane composition, where phosphatidylserine (PS) and phosphatidylinositol diphosphate (PI(4,5)P$_{2})$ act synergetically in attracting the enzyme to the membrane surface. Membrane affinities depend strongly on membrane fluidity, which suggests multiple binding sites on the protein for PI(4,5)P$_{2}$. Neutron reflection measurements show that the PTEN phosphatase ``scoots'' along the membrane surface (penetration $<$ 5 {\AA}) but binds the membrane tightly with its two major domains, the C2 and phosphatase domains. In the bound state, PTEN's regulatory C-terminal tail is displaced from the membrane and organized on the far side of the protein, $\sim $~60 {\AA} away from the bilayer surface, in a rather compact structure. The combination of binding studies and neutron reflection allows us to distinguish between PTEN mutant proteins and ultimately may identify the structural features required for membrane binding and activation of PTEN. Molecular dynamics simulations, currently in progress, refine this structural picture further. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A41.00006: Application of Small-Angle Neutron and X-ray Scattering in Determining Lipid Bilayer Structure Jianjun Pan, Frederick A. Heberle, Norbert Kucerka, Stephanie Tristram-Nagle, Michelle Szymanski, Mary Koepfinger, John Katsaras Accurately determining lipid structure in biologically relevant fluid bilayers is not straightforward. We have recently developed a hybrid experimental/computational technique (i.e., the scattering density profile, or SDP model), which exploits the fact that neutron and X-ray scattering are sensitive to different bilayer thicknesses - the large difference in neutron scattering length density (SLD) between proteated lipid and deuterated water defines the overall bilayer thickness, while X-ray scattering resolves the headgroup-headgroup distance due to the large scattering contrast between the electron-rich phosphate groups and the hydrocarbon/aqueous medium. A key step in the SDP analysis is the use of MD simulations to parse the lipid molecule into fragments whose volume probability distributions follow simple analytical functional forms. Given the appropriate atomic scattering lengths, these volume probabilities can simultaneously predict both the neutron and X-ray SLD profiles, and hence the scattering form factors. Structural results for commonly used phosphatidylcholine and phosphatidylglycerol lipids will be given. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A41.00007: Growth Mechanism of Lipid-Based Nanodiscs -- a Model Membrane for Studying Kinetics of Particle Coalescence Andrew Hu, Mu-Ping Nieh, Anthony Dizon, Ming Li, Tai-Hsi Fan Lipid-based nanodiscs composed of long- and short- chain lipids [namely, dimyristoyl phosphatidylcholine (DMPC), dimyristoyl phosphatidylglycerol (DMPG) and dihexanoyl phosphatidylcholine (DHPC)] constantly form at high lipid concentrations and at low temperatures (i.e., below the melting transition temperature of DMPC, T$_{M})$. The initial size of these nanodiscs (at high total lipid concentration, C$_{L}>$ 20 wt.{\%}) is relatively uniform and of similar dimension (according to dynamic light scattering and small angle neutron scattering experiments), seemingly independent of thermal history. Upon dilution, the nanodiscs slowly coalesce and grow in size with time irreversibly. Our preliminary result shows that the growth rate strongly depends on several parameters such as charge density, C$_{L}$ and temperature. We have also found that the nanodisc coalescence is a reaction limit instead of diffusion limit process through a time-resolved study. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A41.00008: Swellable Model POPC/POPG/DHPC Membrane with a Lamellar Long-Range Order Ming Li, Mu-Ping Nieh A physiological relevant biomimetic model membrane is of great necessity for the structural characterization of membrane protein. This presentation will report a small-angle neutron scattering (SANS) result on two lipid bicellar series composed of 1-palmitoyl-2-oleoyl-\textit{sn}-glycero-3-phosphocholine(POPC)/1,2-dihexanoyl-\textit{sn}-glycero-3-phosphocholine (DHPC) and POPC/DHPC/1-palmitoyl-2-oleoyl-\textit{sn}-glycero-3-phospho-(1'-\textit{rac}-glycerol) (POPG). Instead of the multi-lamellae vesicle (MLV) structure observed in zwitterionic POPC/DHPC mixture, the perforated lamellae (PL) structure is found in POPC/POPG/DHPC upon addition of small amount of charged lipid, POPG {\{}R=[POPG]/([POPC]+[POPG])=0.01{\}}. The PL phase exists from 10 to 60 degree C and the interlamellar spacing (d-spacing) varies from 12.9 to 49.0 nm as the lipid concentration changes from 25 to 7.5{\%} wt where the lamellae still indicate long-range order. The effect of temperature and charge density (R) on structural variation will be discussed in this presentation. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A41.00009: Mobility of water and selected atoms in bilayer DMPC membranes F.Y. Hansen, H. Taub, M. Bai, A. Miskowiec Molecular dynamics simulations have been used to determine the mobility of water molecules as a function of their positions in a fully hydrated free standing DMPC membrane at 303 K. In a 10 \AA{} thick water layer with bulk density just outside the membrane, the mobility of the water molecules is reduced by about a factor of two relative to bulk. For water molecules penetrating deeper into the membrane there is an increasing reduction in the mobility with up to two orders of magnitude for those deepest into the membrane. A comparison with the mobility of selected atoms in the lipid molecules shows that about 5 water molecules/lipid molecule move on the same time scale as the lipid molecules and may therefore be considered to be so tightly bound to them that they essentially follow their motion. The simulation results are quantitatively compared with quasielastic neutron scattering results on single-supported bilayer DMPC membranes. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A41.00010: Structure of single-supported DMPC lipid bilayer membranes as a function of hydration level studied by neutron reflectivity and Atomic Force Microscopy A. Miskowiec, P. Schnase, M. Bai, H. Taub, F.Y. Hansen, M. Dubey, S. Singh, J. Majewski We have recently been investigating the diffusion of water on single-supported DMPC lipid bilayer membranes at different levels of hydration, using high-resolution quasielastic neutron scattering (QNS). To aid in the interpretation of these QNS studies, we have conducted neutron reflectivity (NR) measurements on SPEAR at LANSCE to characterize the structure of similarly prepared samples. Protonated DMPC membranes were deposited onto SiO$_{2}$-coated Si(100) substrates and characterized by Atomic Force Microscopy (AFM) at different levels of hydration. We find reasonable agreement between the membrane thickness determined by NR and AFM at room temperature. We also find consistency between the scattering length density (SLD) profile in the vicinity of the upper leaflet of the supported DMPC membrane and that found in a molecular dynamics simulation of a freestanding membrane at 303 K. However, the fit to the reflectivity curve can be improved by modifying the SLD profile near the leaflet closest to the SiO$_{2}$ surface. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A41.00011: Cholesterol Flip-Flop Dynamics in a Phospholipid Bilayer: A 10 Microsecond All-Atom Molecular Dynamics Simulation Ken-ichi Nomura, Amit Choubey, Rajiv Kalia, Aiichiro Nakano, Priya Vashishta Cholesterol (CHOL) molecules play a key role in modulating the rigidity of cell membranes, and controlling intracellular transport and signal transduction. Using all-atom molecular dynamics and the parallel replica approach, we study the effect of CHOL molecules on mechanical stresses across a dipalmitoylphosphatidycholine (DPPC)-CHOL bilayer, and the mechanism by which CHOL molecules migrate from one bilayer leaflet to the other (flip-flop events). On average, we observe a CHOL flip-flop event in half-a-microsecond. Once a CHOL flip-flop event is triggered, the inter-leaflet migration occurs in about 62 nanoseconds. The energy barrier associated with flip-flop events is found to be 73 kJ/mol. Results for membrane rigidity as a function of CHOL concentration will also be presented. [Preview Abstract] |
Session A42: Focus Session: Cytoskeleton and Biomechanics - Biochemical mechanisms
Sponsoring Units: DBIO DPOLYChair: Megan Valentine, University of California, Santa Barbara
Room: 156C
Monday, February 27, 2012 8:00AM - 8:12AM |
A42.00001: Stochastic simulations of the growth dynamics and organization of lamellipodia-like actin networks Longhua Hu, Garegin Papoian Cell migration is essential to many biological processes such as embryonic development, wound healing and immune response. The crawling movement of cells is a complex process that involves the protrusion of the leading edge of a cell, adhesion to the substrate, generation of the traction force to move cell body and the subsequent release of adhesions. Lamellipodia are flat sheet-like membrane protrusions at the leading edge of the crawling cells. The dynamic remodeling of the dendritically branched actin network in lamellipodia generates force to drive the movement of cells. We have developed a simplified, three-dimensional computational model to study the growth of lamellipodia-like actin networks. Our model integrates the essential biochemical regulation processes as well as the mechanical aspect of actin polymerization, where the interactions between the semi-flexible filaments and the plasma membrane are taken into account. Using stochastic simulations, we study how membrane tension and external resistance on membrane affect the growth dynamics and organization of the actin network. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A42.00002: How ion binding affects actin filament stability and flexural rigidity Hyeran Kang, Michael Bradley, Brannon McCullough, Ana\"elle Pierre, Elena Grintsevich, Enrique De La Cruz Actin filaments are semi-flexible biopolymers essential for the mechanical support and cell motility. Ions strongly affect actin polymerization and the flexibility of actin filaments; however, the molecular basis for how ions are coupled to the mechanics of actin filaments remains elusive. Here, we demonstrate a linkage between cation binding and both actin filament polymerization and flexural rigidity. Our results show that the thermodynamic stability and flexural rigidity of actin filament increase with cation concentration in a manner that implicates specific cation binding as opposed to general electrostatic screening. Using structural bioinformatics, we identify two distinct cation-binding sites within the F-actin structure that help explain how specific cation binding is linked to actin polymerization and flexural rigidity. Site-specific substitution of a charged amino acid residue at one of the sites modulates the cation concentration-dependence of filament bending stiffness, consistent with a bound cation at this site increasing the flexural rigidity of actin filaments. Mutation of a charged amino acid at the other site causes ``polymerization incompetent'' G-actin. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A42.00003: Mechanotransduction through the plasma membrane {\&} cytoskeleton Kristina Haase, Andrew Pelling Mechanical forces initiate immediate and long-term changes in cells; however the exact mechanisms remain unclear, albeit crucial for understanding the pathology of disease. We used combined confocal and atomic force microscopy (AFM) to investigate changes in cell morphology and elasticity in response to a mechanical stimulus. The AFM was used as a nano-indentor to gauge the response of the membrane and cytoskeleton (CSK) of HeLa cells.~We observed their viscoelastic nature by probing cells transfected with a green fluorescent protein localized at the plasma membrane. Inhibition of acto-myosin contractility (AMc) resulted in a significant decrease of cellular elasticity, and a corresponding increase in mean deformation. We also investigated the rate at which the membrane and CSK deform and relax in response to a local force. The response to a local perturbation is nearly instantaneous for control cells and shows no statistical difference when compared to cells treated with CSK-inhibiting drugs. Inhibition of AMc affects the rate of recovery, in comparison to control cells which recover quite quickly (30-60s). Overall, we demonstrated short and long-term deformation and subsequent recovery of both the cell membrane and actin network in response to a local force. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A42.00004: Leading at the Front: How EB Proteins Regulate Microtubule Dynamics Invited Speaker: Taviare Hawkins Microtubules are the most rigid of the cytoskeletal filaments, they provide the cell's scaffolding, form the byways on which motor proteins transport intracellular cargo and reorganize to form the mitotic spindle when the cell needs to divide. These biopolymers are composed of alpha and beta tubulin monomers that create hollow cylindrical nanotubes with an outer diameter of 25 nm and an inner diameter of 17 nm. At steady state concentrations, microtubules undergo a process known as dynamic instability. During dynamic instability the length of individual microtubules is changing as the filament alternates between periods of growth to shrinkage (catastrophe) and shrinkage to growth (rescue). This process can be enhanced or diminished with the addition of microtubule associated proteins (MAPs). MAPs are microtubule binding proteins that stabilize, destabilize, or nucleate microtubules. We will discuss the effects of the stabilizing end-binding proteins (EB1, EB2 and EB3), on microtubule dynamics observed in vitro. The EBs are a unique family of MAPs known to tip track and enhance microtubule growth by stabilizing the ends. This is a different mechanism than those employed by structural MAPs such as tau or MAP4. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A42.00005: Redundancy and cooperativity in the mechanics of compositely crosslinked cytoskeletal networks J.M. Schwarz, D.Q. Quint, Moumita Das The cytoskeleton contains many types of crosslinkers. Some crosslinkers allow free rotations between filaments and others do not. The mechanical interplay between different crosslinkers is an open issue in cytoskeletal mechanics. Therefore, we develop a theoretical framework based on rigidity percolation to study a generic filamentous system containing both stretching and bond-bending forces to address this issue. The framework involves both analytical calculations via effective medium theory and numerical simulations on a percolating triangular lattice with very good agreement between both. We find that the introduction of angle-constraining crosslinkers to a semiflexible filamentous network with freely-rotating crosslinks can cooperatively lower the onset of rigidity to the connectivity percolation threshold---a result speculated for years but never before obtained via effective medium theory. In other words, the system can attain rigidity at the lowest concentration of material possible. We further demonstrate that introducing angle-constraining crosslinks results in mechanical behaviour similar to just freely-rotating crosslinked semflexible filaments, indicating redundancy. Our results also impact upon collagen and fibrin networks in biological and bio-engineered tissues. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A42.00006: The origins of strain stiffening in fibrin networks Louise Jawerth, Stefan Muenster, David Weitz Fibrin networks form the structural scaffold of blood clots; their non-linear mechanical properties are crucial to stem the flow of blood at a site of vascular injury. A hallmark of these networks is strain stiffening: a stiffness that increases non-linearly as a network is strained. Deformations of the fibers and the network combine to control the mechanical properties of the bulk and must lead to the strain stiffening behavior of the networks; however, the details of this process are unknown. Here, we study fibrin networks undergoing shear on a confocal microscope and compare this to bulk rheological measurements. We track individual fiber branchpoints as function of system strain. We characterize the non-affinity of the motion and show that the low strain, linear regime corresponds to highly non-affine motion while the high strain, nonlinear regime corresponds to affine motion. Moreover, we show that the non-linear bulk response can be well approximated by considering the fibers to be linear elastic elements with soft compressive behavior and, therefore, is a result of the topology of the network itself rather than nonlinearity of its constituents. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A42.00007: Semiflexible filament networks viewed as fluctuating beam frames Tianxiang Su, Prashant Purohit We present a new method combining structural and statistical mechanics to study the entropic elasticity of semiflexible filament networks. We view a filament network as a frame structure and use structural mechanics to determine its static equilibrium configuration under applied loads in the first step. To account for thermal motion around this static equilibrium state, we then approximate the potential energy of the deformed frame structure up to the second order in kinematic variables and obtaina deformation-dependent stiffness matrix characterizing the flexibility of the network. Using statistical mechanics, we then evaluate the partition function, free energy and thermo-mechanical properties of the network in terms of the stiffness matrix. We show that penalty methods commonly used in finite elements to account for constraints, are applicable even when statistical and structural mechanics are combined in our method. We apply our framework to understand the expansion, shear, uniaxial tension and compression behavior of some simple filament networks. We are able to capture the stress-stiffening behavior due to filament reorientation and stretching out of thermal fluctuations, as well as the reversible stress-softening behavior due to filament buckling. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A42.00008: Spindle Assembly and Architecture: From Laser Ablation to Microtubule Nucleation Daniel Needleman, Jan Brugues, Valeria Nuzzo, Eric Mazur Spindles are arrays of microtubules that segregate chromosomes during cell division. It has been difficult to validate models of spindle assembly due to a lack of information on the organization of microtubules in these structures. Here we present a novel method, based on femtosecond laser ablation, capable of measuring the detailed architecture of spindles. We used this method to study the metaphase spindle and find that microtubules are shortest near poles and become progressively longer towards the center of the spindle. These data, in combination with mathematical modeling, high resolution imaging, and biochemical perturbations, are sufficient to reject previously proposed mechanisms of spindle assembly. Our results support a new model of spindle assembly in which microtubule polymerization dynamics are not spatially regulated, microtubule transport locally sorts microtubules -- determining their proper organization in the spindle without moving them appreciable distances --, and the profile of microtubule nucleation controls the length of the spindle. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A42.00009: Coupling stochastic kinetics and mechanics uncovers new dynamics induced by focal adhesions in filopodia Maria Minakova, Garegin Papoian Cell motility recently became a target for physical chemists and biophysicists. It is generally understood that in real biological systems, chemical and mechanical processes are coupled, sometimes in a very complex manner. Focal adhesions (FAs) represent a biologically relevant example of multi molecular assemblies that serve a mechanical function and have intriguing biochemical properties. FAs and their role in filopodial dynamics have been extensively studied experimentally from a biological standpoint. Although there are many biochemical studies of FAs kinetics in the literature, only a few works study FA dynamics from a physical perspective. In our work we developed a robust stochastic model of the filopodia coupled to mechanical properties of FAs, retrograde flow and the substrate. We carried out extensive simulations of the filopodial stochastic growth on the timescales of minutes, as well as a detailed theoretical description of a steady state, mapping multi dimensional phase space of mechanical and kinetic parameters onto various dynamics regimes. The combination of mean field analyses with detailed microscopic simulations provides a united platform for treating mechanochemical processes underlying complex behavior of the filopodial system. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A42.00010: Growth Cone Biomechanics in Peripheral and Central Nervous System Neurons Jeffrey Urbach, Daniel Koch, Will Rosoff, Herbert Geller The growth cone, a highly motile structure at the tip of an axon, integrates information about the local environment and modulates outgrowth and guidance, but little is known about effects of external mechanical cues and internal mechanical forces on growth-cone mediated guidance. We have investigated neurite outgrowth, traction forces and cytoskeletal substrate coupling on soft elastic substrates for dorsal root ganglion (DRG) neurons (from the peripheral nervous system) and hippocampal neurons (from the central) to see how the mechanics of the microenvironment affect different populations. We find that the biomechanics of DRG neurons are dramatically different from hippocampal, with DRG neurons displaying relatively large, steady traction forces and maximal outgrowth and forces on substrates of intermediate stiffness, while hippocampal neurons display weak, intermittent forces and limited dependence of outgrowth and forces on substrate stiffness. DRG growth cones have slower rates of retrograde actin flow and higher density of localized paxillin (a protein associated with substrate adhesion complexes) compared to hippocampal neurons, suggesting that the difference in force generation is due to stronger adhesions and therefore stronger substrate coupling in DRG growth cones. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A42.00011: An Atomic Force Microscopy based investigation of specific biomechanical properties for various types of neuronal cells Elise Spedden, James White, David Kaplan, Cristian Staii Here we describe the use of Atomic Force Microscope (AFM) based techniques to characterize and explore the influence of biochemical and biomechanical cues on the growth and interaction of neuronal cells with surrounding guidance factors. Specifically, we use AFM topography and AFM force spectroscopy measurements to systematically investigate the morphology, elasticity, and real time growth of neuronal processes in the presence of different types of extracellular matrix proteins and growth factors. We therefore create a series of systems containing specified neuron densities where the type of the underlying growth promoting protein is different from sample to sample. For each system we measure key biomechanical parameters related to neuronal growth such as height and elastic modulus at multiple growth points on several types of neurons. We show that systematic measurements of these parameters yield fundamental information about the role played by substrate-plated guidance factors in determining elastic and morphological properties of neurons during growth. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A42.00012: Nuclear Physics in a biological context Dennis Discher A solid tissue can be soft like fat or brain, stiff like striated muscle and heart, or rigid like bone -- and of course every cell has a nucleus that contributes in some way small or large to tissue mechanics. Indeed, nuclei generally exhibit rheology and plasticity that reflects both the chromatin and the nuclear envelope proteins called lamins, all of which change in differentiation. Profiling of tissue nuclei shows that the nuclear intermediate filament protein Lamin-A/C varies over 30-fold between adult tissues and scales strongly with micro-elasticity of tissue, while other nuclear envelope components such as Lamin-B exhibit small variations. Lamin-A/C has been implicated in aging syndromes that affect muscle and fat but not brain, and we find nuclei in brain-derived cells are indeed dominated by Lamin-B and are much softer than nuclei derived from muscle cells with predominantly Lamin-A/C. In vitro, matrix elasticity can affect expression of nuclear envelope components in adult stem cells, and major changes in Lamin-A/C are indeed shown to direct lineage with lower levels favoring soft tissue and higher levels promoting rigid tissue lineage. Further molecular studies provide evidence that the nucleus transduces physical stress. References: (1) J.D. Pajerowski, K.N. Dahl, F.L. Zhong, P.J. Sammak, and D.E. Discher. Physical plasticity of the nucleus in stem cell differentiation. PNAS 104: 15619-15624 (2007). (2) A. Buxboim, I. Ivanova, and D.E. Discher. Matrix Elasticity, Cytoskeletal Forces, and Physics of the Nucleus: how deeply do cells `feel' outside and in? Journal of Cell Science 123: 297-308 (2010). [Preview Abstract] |
Session A43: Invited Session: Broader Impacts of Research-NSF Policy and Individual Responsibility
Sponsoring Units: FPSChair: Donald Prosnitz, Rand Corporation
Room: 157AB
Monday, February 27, 2012 8:00AM - 8:36AM |
A43.00001: Science, the Scientists and Values Invited Speaker: Alan Leshner Although individual scientists engage in research for diverse reasons, society only supports the enterprise because it benefits humankind. We cannot always predict how that will happen, or whether individual projects will have clear and direct benefits, but in the aggregate, there is widespread agreement that we are all better off because of the quality and diversity of the science that is done. However, what scientists do and how it benefits humankind is often unclear to the general public and can at times be misunderstood or misrepresented. Moreover, even when members of the public do understand what science is being done they do not always like what it is showing and feel relatively free to disregard or distort its findings. This happens most often when findings are either politically inconvenient or encroach upon issues of core human values. The origins of the universe can fit into that latter category. This array of factors contributes to the obligation of scientists to reach out to the public and share the results of their work and its implications. It also requires the scientific community to engage in genuine dialogue with the public and find common ground where possible. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A43.00002: The APS and the Impact on Physics and Society Invited Speaker: Barry C. Barish |
Monday, February 27, 2012 9:12AM - 9:48AM |
A43.00003: Why Physicists Have a Responsibility to Society Invited Speaker: Charles Ferguson The debate and controversy over the NSF criterion on broader societal impacts of NSF-funded research have served the important function of challenging the physics community to reexamine why public money should support pure and applied physics research and what is the role of physicists in society. I will argue that the criterion, while well intentioned, appears ill informed and runs the risk of creating a check list of activities that will seemingly fulfill physicists' responsibility to connect their work to larger societal issues. Moreover, I will argue in favor of having a portion of government-funded research for scientific investigations based primarily, if not solely, on the intellectual and scientific merits of the proposals. Most government-funded research is already connected to larger societal impacts such as national defense, energy research, and economic issues. While I will call for reassessment of the NSF criterion on broader societal impacts, my talk will explain why physicists, as citizens and scientists, must reenergize their efforts to positively effect society and will offer advice about how they can do so. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A43.00004: The Broader Impact Criteria- What's the solution? A panel discussion Invited Speaker: Don Prosnitz The need for social control of science---especially 20th and now 21st century science---has been debated for decades. Science is supported by society, and research results ultimately have large societal impacts. Physicists arguing for increased research budgets invariably point out the collateral benefits to society---often economic---of their discovery-based research. Congress agreed, and attempted to institutionalize this metric in the `America COMPETES Reauthorization Act of 2010' by insisting that researchers illustrate how their work will provide for (among other things) increased economic competitiveness of the United States and increased National Security. Is such a metric inevitable in these days of constrained resources? Is it desirable or even implementable? Can benefits be predicted, or are the most important societal consequences of basic research serendipitous? This talk will examine these questions, including looking at how other nations are attempting to deal with the issue, and present some suggestions for satisfying both society's insistence on a return on their investment and scientists' need for unfettered exploration. The program will conclude with a Q and A and panel discussion with all of the session's speakers. [Preview Abstract] |
Session A44: Focus Session: Hydrophobic Interactions and Hydrogen Bonding Networks in Polymeric and Soft Matter Systems
Sponsoring Units: DPOLY DBIOChair: Garegin Papoian, University of Maryland
Room: 157C
Monday, February 27, 2012 8:00AM - 8:12AM |
A44.00001: More than one dynamic crossover in protein hydration water Giancarlo Franzese, Marco G. Mazza, Kevin Stokely, Sara Pagnotta, Fabio Bruni, H. Eugene Stanley We study by theory, simulations and experiments, the dynamics of the hydrogen bond (HB) network of a percolating layer of water hydrating lysozyme powder. Using dielectric spectroscopy we measure the temperature dependence of the relaxation time of proton charge fluctuations. These fluctuations are associated with the dynamics of the HB network of water molecules adsorbed on the protein surface. Using Monte Carlo simulations and mean-field calculations, we study the dynamics and thermodynamics of a coarse-grained model that successfully reproduces the properties of hydration water. Both experimental and model analyses are consistent with the interesting possibility of two dynamic crossovers, (i) at $\approx$ 252 K, and (ii) at $\approx$ 181 K. Because the experiments agree with the model, we can relate the two crossovers to the presence at ambient pressure of two specific heat maxima. The first is caused by fluctuations in the HB formation, and the second, at a lower temperature, is due to the reordering of the HB network. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A44.00002: The effect of salt solutes on the relaxation dynamics of water from 65 to 720 GHz Nguyen Vinh, Mark Sherwin, Jim Allen, Kevin Plaxco During the past decade, a variety of measurement techniques have provided evidence that ions and other solute molecules effect the structure and dynamics of the water molecules directly surrounding them. Most of these experiments have employed infrared spectroscopy which explores vibrational relaxation of the hydration shell by observing int\textit{ra}molecular vibrations. Terahertz spectroscopy, in contrast is sensitive to int\textit{er}molecular dynamics. Here we use a vector network analyzer based\textit{ terahertz dielectric relaxation spectrometer} operating over the frequency range from 65 to 720 GHz. The literature on relaxation dynamics of water is extensive and variable. But these measurements clarify the situation and confirm that the dynamics of water over this regime are best described in terms of three Debye relaxation processes with the characteristic times of 8.56, 1.1 ps and 179 fs (at 25.0\r{ }C). Remarkably, while the relaxation times themselves are not sensitive to salt concentration, the relative strength of the relaxation modes depends in a systematic way on the solute molarity. We discuss these results by relating the salt concentration dependent strength of the three processes to the dynamics and structure of first three hydration shells. Our measurements shed light on the dynamics of hydration shells around solute molecules in a biologically relevant environment. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A44.00003: Investigating How Contact Angle Effects the Interaction between Water and a Hydrophobic Surface Adele Poynor, Caitlyn Neidig By definition hydrophobic substances hate water. What happens when water is forced into contact with a hydrophobic surface? One theory is that an ultra-thin low-density region forms near the surface. Contact angle is a measure of how hydrophobic a surface is. We have employed an automated home-built Surface Plasmon Resonance (SPR) apparatus to investigate the effect of varying the contact angle on the depletion layer [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A44.00004: Mesoscale inhomogeneities in an aqueous ternary system Deepa Subramanian, Stephen Hayward, Elia Altabet, Peter Collings, Mikhail Anisimov Aqueous solutions of certain low-molecular-weight organic compounds, such as alcohols, amines, or ethers, which are considered macroscopically homogeneous, show the presence of mysterious mesoscale inhomogeneities, order of a hundred nm in size. We have performed static and dynamic light scattering experiments in an aqueous ternary system consisting of tertiary butyl alcohol and propylene oxide. Tertiary butyl alcohol is completely soluble in water and in propylene oxide, and forms strong hydrogen bonds with water molecules. Based on results of the study, we hypothesize that the mesoscale inhomogeneities are akin to a micro phase separation, resulting from a competition between water molecules and propylene oxide molecules, wanting to be adjacent to amphiphilic tertiary butyl alcohol molecules. Coupling between two competing order parameters, super-lattice binary-alloy-like (``antiferromagnetic'' type) and demixing (``ferromagnetic'' type) may explain the formation of these inhomogeneities. Long-term stability investigation of this supramolecular structure has revealed that these inhomogeneities are exceptionally long-lived non-equilibrium structures that persist for weeks or even months. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A44.00005: Statistical Mechanics of Thermosensitive Nanoparticle Binding: Quantyfing Hydrophobic Interactions in Bulk Solution Alessio Zaccone, Jerome Crassous, Benjamin Beri, Matthias Ballauff We present a novel method which allows one to quantify the binding energy between complex hydrophobic nanoparticles in bulk aqueous solutions by means of light scattering. We tested the method on the case of thermosensitive nanoparticles made of a solid polymeric core onto which a thermosensitive p-NIPAM microgel shell is grafted. The microgel shrinks above a critical T at which the hydrophobic attraction sets in. By means of a novel statistical mechanics model to interpret the data, we manage to demonstrate that the binding energy as a function of T of thermosensitive nanoparticles behaves like in the case of neat two-level systems, with a rather sharp transition from hard-sphere (hydrophilic) to attractive (hydrophobic) at the critical temperature. The model allows us to make a clear quantitative connection between the binding energy and the entropy change of the grafted microgel upon going from hydrophilic to hydrophobic (in turn related to the microgel structure). The methods presented in this work can be applied to quantify the binding energy of complex biomolecules in bulk solution, which is a major challenge in biophysics nowadays. Reference: A. Zaccone, J.J. Crassous, B. Beri, and M. Ballauff, Phys. Rev. Lett. 107, 168303 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A44.00006: Effect of Valence of Counterions on the Structure of Charged Membranes, a Computer Simulation Study Baofu Qiao, Monica Olvera de la Cruz Phospholipids have been investigated for a long period, due to its ability of self-assembling into bilayer structures which resemble biological membranes. But most of the studies have been limited on the neutral phosphatidylcholine based lipids. The understanding of charged membranes (e.g., phosphatidylserine) is very limited due to the repulsion between the charged groups on lipids. In the present work, we investigated the effect of different counter-ions on the structures of charged membranes formed by 1,2-dilauroyl-sn-glycoro-3-phospho-L-serine. Three kinds of counterions were investigated, from monovalent, to divalent, to trivalent ions. Molecular dynamics simulations were performed at all-atom level. We have calculated the area per lipid. And the interaction between counterions and COO$^{-}$ groups was found to dominate over that between counterions and PO$_{4}^{-}$ groups. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A44.00007: Deconstructing Classical Water Models at Interfaces and in Bulk: Hydrophobic Interactions and Hydrogen Bonding Invited Speaker: John D. Weeks Using concepts from perturbation and local molecular field theories of liquids we divide the potential of the SPC/E water model into short and long ranged parts. The short ranged parts define a minimal reference network model that captures very well the structure of the local hydrogen bond network in bulk water while ignoring effects of the remaining long ranged interactions. This deconstruction can provide insight into the different roles that the local hydrogen bond network, dispersion forces, and long ranged dipolar interactions play in determining a variety of solvation and other properties of SPC/E and related classical models of water. We use these short ranged models along with local molecular field theory to quantify the influence of these interactions on the structure of hydrophobic interfaces and the crossover from small to large scale hydration behavior. The implications of our findings for theories of hydrophobicity and possible refinements of classical water models will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A44.00008: A SAFT-based classical density functional for water David Roundy, Jessica Hughes, Eric Krebs We present a new classical density functional for water based on a combination of Statistical Associating Fluid Theory (SAFT-VR) with the Fundamental Measure Theory (FMT) functional for the hard-sphere fluid. In the homogeneous limit, our functional reduces to the the published optimal SAFT model of Clark \emph{et al} [1]. By adding a single fitting parameter, we reproduce the bulk surface tension of water within a wide temperature range. We will present results for hydrophobic hard rods and spheres, including the temperature dependence of the hydrophobic interaction. \\[4pt] [1] G. Clak, A. Haslam, A. Galindo, and G. Jackson, Molecular Physics \textbf{104}, 3561 (2006). [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A44.00009: Morphology and chirality control self-assembly of sickle hemoglobin inside red blood cells Xuejin Li, Huan Lei, Bruce Caswell, George Karniadakis Sickle cells exhibit abnormal morphology and membrane mechanics in the deoxygenated state due to the polymerization of the interior sickle hemoglobin (HbS). In this study, the dynamics of self-assembly behavior of HbS in solution and corresponding induced cell morphologies have been investigated by dissipative particle dynamics approach. A coarse-grained HbS model, which contains hydrophilic and hydrophobic particles, is constructed to match the structural properties and physical description (including crowding effects) of HbS. The hydrophobic interactions are shown to be necessary with chirality being the main driver for the formation of HbS fibers. In the absence of chain chirality, only the self-assembled small aggregates are observed whereas self-assembled elongated step-like bundle microstructures appear when we consider the chain chirality. Several typical cell morphologies (sickle, granular, elongated shapes), induced by the growth of HbS fibers, are revealed and their deviations from the biconcave shape are quantified by the asphericity and elliptical shape factors. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A44.00010: Secondary structure formation in peptide amphiphile micelles Matthew Tirrell Peptide amphiphiles (PAs) are capable of self-assembly into micelles for use in the targeted delivery of peptide therapeutics and diagnostics. PA micelles exhibit a structural resemblance to proteins by having folded bioactive peptides displayed on the exterior of a hydrophobic core. We have studied two factors that influence PA secondary structure in micellar assemblies: the length of the peptide headgroup and amino acids closest to the micelle core. Peptide length was systematically varied using a heptad repeat PA. For all PAs the addition of a C12 tail induced micellization and secondary structure. PAs with 9 amino acids formed beta-sheet interactions upon aggregation, whereas the 23 and 30 residue peptides were displayed in an apha-helical conformation. The 16 amino acid PA experienced a structural transition from helix to sheet, indicating that kinetics play a role in secondary structure formation. A p53 peptide was conjugated to a C16 tail via various linkers to study the effect of linker chemistry on PA headgroup conformation. With no linker the p53 headgroup was predominantly alpha helix and a four alanine linker drastically changed the structure of the peptide headgroup to beta-sheet, highlighting the importance of hydrogen boding potential near the micelle core. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A44.00011: Friction and Hydration Repulsion Between Hydrogen-Bonding Surfaces Invited Speaker: Roland Netz The dynamics and statics of polar surfaces are governed by the hydrogen-bonding network and the interfacial water layer properties. Insight can be gained from all-atomistic simulations with explicit water that reach the experimentally relevant length and time scales. Two connected lines of work will be discussed: 1) On surfaces, the friction coefficient of bound peptides is very low on hydrophobic substrates, which is traced back to the presence of a depletion layer between substrate and water that forms a lubrication layer. Conversely, friction forces on hydrophilic substrates are large. A general friction law is presented and describes the dynamics of hydrogen-bonded matter in the viscous limit. 2) The so-called hydration repulsion between polar surfaces in water is studied using a novel simulation technique that allows to efficiently determine the interaction pressure at constant water chemical potential. The hydration repulsion is shown to be caused by a mixture of water polarization effects and the desorption of interfacial water. [Preview Abstract] |
Session A45: Focus Session: Thin Film Block Copolymers - Directed Assembly
Sponsoring Units: DPOLYChair: Thomas Epps, University of Delaware
Room: 159
Monday, February 27, 2012 8:00AM - 8:36AM |
A45.00001: Directed Ordering of Block Copolymer Thin Films with Flexible Interfaces for Functional Materials Invited Speaker: Alamgir Karim Orientation control of block copolymer (BCP) films is important for advanced technological applications. We present studies on directed ordering of block copolymer thin films on rigid substrates such as quartz to elastomeric PDMS and flexible Kapton substrates for tunable orientation of microphase separated poly (styrene) -- block -poly (methylmethacrylate) (PS-PMMA) cylinder and lamellae forming BCP films. Although the crosslinked PDMS has low surface energy, its surface energy can be tuned by exposing to UV-Ozone (UVO) that presents an opportunity to change BCP-PDMS interfacial energy to control BCP orientation across full range of orientation and film wettability. On the other hand, Kapton offers a near neutral surface for PS-PMMA without surface modification. Via a modified version of a dynamic thermal processing termed cold zone annealing-sharp (CZA-S), we obtain a wide range of orientations of the block copolymer films in unfilled and nanoparticle filled systems with an interest in photovoltaic systems. With CZA-S, vertical orientation of PS-PMMA can be obtained in films as thick as 1 micron with etchable PMMA domains for membrane applications. GISAXS characterization of these etched BCP membranes reveals up to 5 orders of diffraction indicating hexagonally packed vertical nanopores that extend throughout the film. Under similar thermal gradient, but static conditions, temporally stable vertical cylinders form only within a narrow zone of maximum temperature gradient. Primary CZA-S ordering mechanism thus involves propagating this narrow vertically oriented zone of BCP cylinders created at the maximum thermal gradient section, across the film. An optimal speed is needed since the process competes with preferential surface wetting dynamics that favors parallel orientation. These results are reproduced on large area flexible films on a prototype dynamic R2R assembly platform with incorporated multi-CZA gradient for thin (100 nm) BCP films currently. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A45.00002: Facile Route to Vertically Aligned High-Aspect Ratio Block Copolymer Films via Dynamic Zone Annealing Gurpreet Singh, Manish Kulkarni, Kevin Yager, Detlef Smilgies, David Bucknall, Alamgir Karim Directed assembly of block copolymers (BCP) can be used to fabricate a diversity of nanostructures useful for nanotech applications. The ability to vertically orient etchable high aspect ratio ($\sim $30) ordered BCP domains on flexible substrates via continuous processing methods are particularly attractive for nanomanufacturing. We apply sharp dynamic cold zone annealing (CZA-S) to create etchable, and predominantly vertically oriented 30nm cylindrical domains in 1 $\mu $m thick poly(styrene-b-methylmethacrylate) films on low thermal conductivity rigid (quartz) and flexible (PDMS {\&} Kapton) substrates. Under similar static conditions, temporally stable vertical cylinders form within a narrow zone above a critical temperature gradient. Primary ordering mechanism of CZA-S involves sweeping this vertically orienting zone created at maximum thermal gradient. An optimal speed is needed since the process competes with preferential surface wetting dynamics that favors parallel orientation. GISAXS of etched BCP films confirms internal morphology. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A45.00003: Controlling Domain Orientation in Thin Films of Lamellar AB and ABA Block Copolymers Nikhila Mahadevapuram, Thai Vu, Gila Stein Thin films of block copolymers are very popular for low cost, large area nanopatterning. To generate a nanoporous template, the domains must be oriented perpendicular to the substrate. This talk will discuss the effects of copolymer architecture on surface energetics and domain orientations. We consider lamellar copolymers based on polystyrene (PS) and poly(methyl methacrylate) (PMMA) blocks. All films are cast on ``neutral'' substrates, and the resulting structures are evaluated with microscopy and grazing-incidence small-angle X-ray scattering. For PS-PMMA diblock copolymers, domain orientations are very sensitive to process conditions. The desired perpendicular orientation is most reliably obtained at high annealing temperatures where PS and PMMA have similar melt surface tensions. For PMMA-PS-PMMA triblock copolymers, the perpendicular domain orientation is stable for all film thicknesses and annealing temperatures that were studied, consistent with recent works that consider architectural effects when calculating the copolymer surface tension. These data suggest that triblocks are easier to use for nanopatterning. However, we also find that diblock and triblock films contain a high density of tilted domains, and such defects should be minimized for most applications. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A45.00004: Diblock and Triblock Copolymer Thin Films on a Substrate with Controlled Selectivity Yong-Biao Yang, Jaeup Kim, Junhan Cho We study the morphology developments in linear ABC triblock and also AB diblock copolymer films on neutral and selective substrates, using a self-consistent field theory (SCFT). For the ABC copolymer films, various nanopatterns with tunable square morphologies evolved due to the effects of the substrate preferable to interior (B) block. The domain patterns became diversified from those parallel to the substrate with substrate selectivity for end-block or those vertical to the substrate without substrate selectivity. Furthermore, in order to figure out an economical and efficient way to fabricate useful passive pattern transfer layers potentially applicable to microelectronic processes and ultrahigh density storage media, we scrutinized conditions for generating square symmetries using symmetric AB diblock copolymers deposited on substrates created from ABC triblock copolymer films. It was found that a thinner film with relatively weak incompatibility can produce square patterns. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A45.00005: Microdomain Orientation of PS-b-PMMA Thin Films on the PS grafted Substrates Du Yeol Ryu, Rui Guo, Eunhye Kim The phase transitions and morphologies, specifically the orientation of lamellar microdomains of symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) were investigated with grazing incidence small-angle x-ray scattering (GISAXS) and transmission electron microscopy (TEM). The microdomain orientation in thin films of PS-b-PMMAs on the substrates with brushed (or grafted) polymers was influenced by the interfacial interactions, where grafting density of underlying PS layers are controlled by the reaction parameters such as time and temperature during grafting to the substrates. When grafting density range of PS brush is adjusted, such a simple and facile route provides the perpendicular orientation of cylindrical and lamellar microdomains in PS-b-PMMA films. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A45.00006: Orientation control of cold zone annealed Block copolymer films on tunable gradient surface energy substrates using combinatorial methods Manish Kulkarni, Gurpreet Singh, Alamgir Karim Microphase morphologies of poly(styrene)-block-poly(methylmethacrylate) (PS-PMMA) block co-polymer (BCP) films coated on various tunable surface energy gradient (SEG) substrates were compared. Substrates were prepared by coating silane self assembled monolayer (SAM) and hydrophobic sol-gel based layer of silica (xerogel) on quartz and exposed to UV-ozone radiation by placing them on an accelerating stage that oxidizes the surface to generate SEG. The combinatorial thickness gradient samples of BCP film were prepared by flow coating the BCP solution orthogonal to the SEG. Samples were annealed using novel cold zone annealing (CZA) method with a sharp thermal gradient (50 $^{o}$C/mm) to obtain highly ordered BCP morphologies. Effect of CZA annealing rate and film thickness on BCP morphologies of the SAM treated and untreated quartz as well as xerogel substrates were compared. It was observed that BCP films coated on the untreated quartz substrates exhibited hexagonally packed perpendicular cylindrical morphologies whereas higher area fraction of parallel cylinders was observed for SEG xerogel substrates for higher surface energies ($>$40 mJ/m$^{2})$. BCP 2D surface morphologies studied using AFM, were confirmed to extend to the interior of the film (3D) by GISAXS. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A45.00007: Block Copolymer Ordering on Soft, Patternable and Flexible Substrates Arzu Hayirlioglu, Gurpreet Singh, Alamgir Karim Directed assembly of cylinder and lamellae forming block copolymer films via flexible PDMS substrate is examined to investigate the ordering properties of polystyrene-$b$-poly(methyl methacrylate) (PS-$b$-PMMA) films. We study the cases where the PS-$b$-PMMA films are either directly coated on patterned PDMS flexible substrates, or coated on a flat PDMS substrate with a top patterned and flexible PDMS confinement. The surface energy of the PDMS substrates was modified to vary from 20 to 68 mJ/m$^{2}$ by exposing them to UV-ozone (UVO) for controlled wettability and orientation control. We replicated different patterned media and observed perpendicular lamellar orientation and parallel cylindrical orientation on patterned flexible substrate at higher surface energies in preliminary measurements. Characterization of orientation was investigated with Grazing-Incidence Small Angle X-ray Scattering (GISAXS) measurement as well as with Atomic Force Microscope (AFM) results. Optical Microscope (OM) was also used to study of the surface morphology of the BCP films. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A45.00008: Controlling the alignment and the morphology of bilayer BCP films using templated self-assembly Amir Tavakkoli K. G., Kevin W. Gotrik, Adam F. Hannon, Alfredo Alexander-Katz, Caroline A. Ross, Karl K. Berggren Templated self-assembly of block copolymer (BCP) thin films can control the alignment of BCP microdomains in a single layer using chemical and topographical methods. However, controlling the alignment and the morphology of BCP microdomains in two different layers simultaneously and fabricating complicated three-dimensional (3D) structures is relatively unexplored. This control is useful for the fabrication of multilevel thin film devices. Also, the forces and energetics governing BCP self-assembly are better understood at the bulk scale. This paper discusses how to control the BCP in two different layers by using a majority-block-functionalized post template. We showed by using an array of majority block functionalized posts, we could fabricate very complicated three-dimensional structures and we were able to control the BCP in two different layers. We fabricated three dimensional junctions and bends in two different levels of the BCP, bottom and top. Moreover, we showed we could fabricate periodic superstructures as well as changing the morphology of the BCP in one of the layers from the original cylinders to ellipsoids, spheres, and bicontinuous cylinders and having two different morphologies on top of each other. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A45.00009: Controlling the Block Copolymer Patterns via Combining Graphoepitaxy and Surface Guidance Hyunjung Jung, Sumi Lee, Eun Ae Kwak, Sanghoon Woo, Frank Leifarth, Craig Hawker, June Huh, Du Yeol Ryu, Joona Bang One of the key issues for the block copolymer patterning is controlling the pattern array. Using directed assembly of block copolymer on the chemically patterned surfaces guarantee highly uniform array or controlled non-regular array. In this system, the chemically patterned surface can induce well resisted array over arbitrary large areas. However, this method requires expensive and complicated e-beam lithography and thus is not readily applicable to mass production process. Alternatively, another method is the graphoepitaxy method. In this case, the self-assembly of block copolymer is guided by the topographical wall of lithographically pre-patterned substrates. But, it has limitation on the pitch size of wall to obtain the highly ordered patterns. In this work, we demonstrate a new type of fabrication method to achieve highly controlled and uniform block copolymer patterns. Our approach is to combine the graphoepitaxy method and hexagonally surface guiding patterns from crosslinkable block copolymers. When the lamellar forming block copolymers were prepared on hexagonal patterns, a highly aligned stripe was obtained and the alignment was significantly improved comparing to the case when no hexagonal surface patterns were used. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A45.00010: Macroscopic Ordering of Block Copolymers into Sequenced Patterns on Topographically Corrugated Surface Sungjune Park, Larisa Tsarkova, Stephanie Hiltl, Stefan Roitsch, Joachim Mayer, Alexander B\"oker For the guided block copolymer assembly we used corrugated SiCN ceramic substrates which were fabricated by a facile replication process using non-lithographic PDMS masters. During thermal annealing of polystyrene-b-polybutadiene diblock copolymer, the material transport was guided by the wrinkled substrate to form regular modulations in the film thickness. As a consequence of the thickness-dependent morphological behavior of block copolymers, the film surface appears as sequenced patterns of alternative microphase separated structures. The ordering process is attributed to the formation of reverse terraces which match the substrate topography, so that the resulting surface patterns are free from the surface relief structures within macroscopically-large areas. The issues of the film thickness, the substrate surface energy and the pattern geometry are addressed. Our approach demonstrates an effective synergism of external confinement and internal polymorphism of block copolymers towards complex hierarchically-structured patterned surfaces. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A45.00011: Block copolymer thin films with long-range lateral order Ilja Gunkel, Ting Xu, Alexander Hexemer, Thomas Russell We investigated thin block copolymer films on silicon wafers as well as faceted surfaces of sapphire. Thin films were prepared by spin-coating block copolymer solutions on the corresponding substrates. Subsequent annealing in organic solvent vapors served as a means to induce lateral long-range order in the thin films. The resulting block copolymer structures were analyzed by AFM and GISAXS. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A45.00012: Combining block copolymer lithography with nano-imprinting lithography: Mass-producing sub-20nm unidirectional line patterns Xiaodan Gu, Sung Woo Hong, Thomas P. Russell A facile, simple pathway for mass production of highly aligned sub 20 nm unidirectional line patterns over arbitrarily large areas is reported. The directed self-assembly of block copolymers and nano-imprint lithography technique are combined together for this purpose. Polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) was self-assembled on the faceted sapphire substrates to generate unidirectionally aligned cylindrical microdomains oriented parallel to the substrate. The nanoporous trench patterns were achieved by selective reconstruction of the cylindrical P4VP microdomains followed by oxygen plasma etching. The resulting 1D trench nanopatterns were characterized by scanning force microscopy, scanning electron microscopy, and grazing incident small angle x-ray scattering (GISAXS), yielding an orientation parameter of 0.974. A cyclic-siloxane was cast onto the polymeric trench nanopatterns and cured to make a negative replica. After demolding, the cross-linked siloxane replica was used as a second master to mass-produce unidirectionally aligned nanolines on various substrates with high fidelity, quality, and yield by nano-imprinting lithography technique. Importantly, this strategy works for both thermal and UV-assisted imprinting. The methodology may afford an easy approach for mass production of ideal templates for the fabrication of magnetic storage media, optical devices, and arrays of conducting wires. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A45.00013: Offset block copolymer printing using brushless substrates and elastomeric stamps Hyo Seon Suh, Hyunsik Yoon, Do Kyeong Kwon, Kookheon Char, M. Serdar Onses, Jeong In Lee, Paul Nealey The block copolymer (BCP) films have been utilized as the useful templates for the nanopatterning because of their ability to form quite small feature size with a relatively easy and cheap method. Recently, the control of orientation as well as the perfect registration with long range order of nanosturctures in BCP films has been achieved by directing the assembly of BCP films with the chemical or topographical templates. However, such control of nanostructures and utilization of BCP films are limited on Si wafers since the templates are usually prepared by the semiconductor fabrication process. In this study, we will introduce a technique that overcomes such limitation on the application of BCP films. The chemical pattern prepared by e-beam lithography on the substrate without polymer brushes or mats was used as a template for the directed assembly of BCP films. Because the adhesion between this template and a BCP film is fairly weak, the directed assembled BCP films on this template could be easily transferred to the other substrates with adhesion-controlled elastomeric stamps. We believe that this transfer method, or printing method, would expand the applications of BCP films to areas where the nanotemplates are required. [Preview Abstract] |
Session A46: Invited Session: Inherently Strained Polymers and Soft Materials
Sponsoring Units: DPOLYChair: Sergei Sheiko, University of North Carolina
Room: 160AB
Monday, February 27, 2012 8:00AM - 8:36AM |
A46.00001: Switching Shape of Nematic Elastomers Invited Speaker: Kenji Urayama Nematic elastomers (NEs) are a novel class of materials. NEs possess both the elastic properties of rubbers and the orientational properties of liquid crystals. The combination of these two properties makes the shape of NEs very sensitive to external stimuli. We focus on the thermally induced deformation of the NE films inherently possessing the two types of inhomogeneous director alignments, i.e., hybrid and twist alignments. In the NEs with hybrid alignments (HNEs), the director continuously changes by 90 degree from planar alignment to vertical alignment between the top and bottom surfaces. In the twist NEs, the director parallel to the surfaces smoothly rotates by 90 degree around the thickness axis, and the director at the mid-plane is parallel to the long or short axis of the film. In the HNEs and TNEs, the director change along the normal of the films causes the planes at different depth to respond differently to temperature variation, and the films are thus expected to change shape. We experimentally demonstrate that (i) depending on the width/thickness ratio, the TNE ribbons form the spiral ribbons or helicoids whose spiral or helical pitch markedly depends on temperature [1], and (ii) the HNE ribbons exhibit giant bending in response to temperature variation [2]. We theoretically interpret these experimental observations on the basis of the elastic models with the data of thermally induced uniaxial deformation of the corresponding NEs with globally planar alignment.\\[4pt] [1] Sawa, Ye, Urayama, Takigawa, Gimenez-Pinto, Selinger, R., Selinger, J., Proc. Natl. Acad. Sci., USA, 108, 6364 (2011).\\[0pt] [2] Sawa, Urayama, Takigawa, DeSimone, Teresi, Macromolecules, 43, 4362 (2010). [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A46.00002: Surface shape memory in polymers Invited Speaker: Patrick Mather Many crosslinked polymers exhibit a shape memory effect wherein a permanent shape can be prescribed during crosslinking and arbitrary temporary shapes may be set through network chain immobilization. Researchers have extensively investigated such shape memory polymers in bulk form (bars, films, foams), revealing a multitude of approaches. Applications abound for such materials and a significant fraction of the studies in this area concern application-specific characterization. Recently, we have turned our attention to surface shape memory in polymers as a means to miniaturization of the effect, largely motivated to study the interaction of biological cells with shape memory polymers. In this presentation, attention will be given to several approaches we have taken to prepare and study surface shape memory phenomenon. First, a reversible embossing study involving a glassy, crosslinked shape memory material will be presented. Here, the permanent shape was flat while the temporary state consisted of embossed parallel groves. Further the fixing mechanism was vitrification, with Tg adjusted to accommodate experiments with cells. We observed that the orientation and spreading of adherent cells could be triggered to change by the topographical switch from grooved to flat. Second, a functionally graded shape memory polymer will be presented, the grading being a variation in glass transition temperature in one direction along the length of films. Characterization of the shape fixing and recovery of such films utilized an indentation technique that, along with polarizing microscopy, allowed visualization of stress distribution in proximity to the indentations. Finally, very recent research concerning shape memory induced wrinkle formation on polymer surfaces will be presented. A transformation from smooth to wrinkled surfaces at physiological temperatures has been observed to have a dramatic effect on the behavior of adherent cells. A look to the future in research and applications for surface shape memory in polymers will round out the talk. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:48AM |
A46.00003: Cell Forces and Cytoskeletal Order Parameters Invited Speaker: Dennis Discher Nematic, Smectic and Isotropic Order parameters have found wide-spread use in characterizing all manner of soft matter systems, but have not yet been applied to characterize and understand the structures within living cells, particularly cytoskeletal structures. Several examples will be used to illustrate the utility of such analyses, ranging from experiments on stem cells attached to or in various elastic matrices to embryonic heart tissue and simulations of membrane cytoskeletons under all manner of stressing. Recently developed theory will be shown to apply in general with account of cell contractility, matrix elasticity and dimensionality as well as cell shape and a newly defined ``cytoskeletal polarizability.'' The latter property of cells is likely different between different cell types due to different amounts of key cytoskeletal components with some types of stem cells being more polarizable than others. Evidence of coupling to the nucleus as a viscoelastic inclusion will also be presented. \\[4pt] References: (1) P. Dalhaimer, D.E. Discher, T. Lubensky. Crosslinked actin networks exhibit liquid crystal elastomer behavior, including soft-mode elasticity. Nature Physics 3: 354-360 (2007). (2) A. Zemel, F.Rehfeldt, A.E.X. Brown, D.E. Discher, and S.A. Safran. Optimal matrix rigidity in the self-polarization of stem cells. Nature Physics 6: 468 - 473 (2010). [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:24AM |
A46.00004: Tension in Highly Branched Polymers Invited Speaker: Michael Rubinstein We propose a systematic method of designing branched macromolecules capable of building up high tension in their covalent bonds, which can be controlled by changing solvent quality. This tension is achieved exclusively due to intramolecular interactions by focusing lower tensions from its numerous branches to a particular section of the designed molecule. The simplest molecular architecture, which allows this tension amplification, is a so-called pom-pom macromolecule consisting of a relatively short linear spacer and two $z$-arm stars at its ends. Tension developed in the stars due to crowding of their branches is amplified by a factor of $z$ and focused to the spacer. There are other highly branched macromolecules, such as molecular brushes - comb polymers with high density of side branches, that have similar focusing and amplification properties. In addition molecular brushes transmit tension along their backbone. Adsorption or grafting of these branched molecules on a substrate results in further increase in tension as compared to molecules in solution. Molecular architectures similar to pom-pom and molecular brushes with a high tension amplification parts can be used in numerous sensor applications. Unique conformations of molecular brushes in a pre-wetting layer allow direct visualization by atomic force microscope. Detailed images of individual molecules spreading along the surface enable critical evaluation of theories of chain dynamics in polymer monolayer. Strong spreading of densely branched macromolecules on a planar substrate can lead to high tension in the molecular backbone sufficient to break covalent bonds. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 11:00AM |
A46.00005: New strategies in adaptive polymer gels Invited Speaker: Eugenia Kumacheva Plants possess a remarkable ability to change their shape in response to the changes in ambient conditions. These transitions are believed to be governed by the non-uniform accumulation of elastic energy and the release of localized stresses. The self-shaping behaviour of plants offers a new paradigm for creating adaptable materials by-design, however currently prediction of three-dimensional transformations in soft matter remain a challenge. Here we report on the nature-inspired strategy for the generation of complex three-dimensional structures by programming stimuli-responsive deformations of a composite planar polymer gel sheet. This work constitutes a major step towards the preprogrammed design of adaptable soft materials with applications in sensing and actuation. [Preview Abstract] |
Session A47: Focus Session: Gelation and Glass Transition in Colloids and Soft Matter Systems I
Sponsoring Units: DPOLYChair: Mohammad Islam, Carnegie Mellon University
Room: 160C
Monday, February 27, 2012 8:00AM - 8:12AM |
A47.00001: Gelation kinetics of gelatin using particle tracking microrheology Joseph Hardcastle, Rama Bansil Previous studies with gelatin have observed four distinct stages during the physical gelation process [Normand et al. Macromolecules, 2000, 33, 1063]. In this presentation we report measurements of microrheology in an effort to examine the time evolution of the gel on short length scales and time scales. By tracking latex particles in gelatin solution at different temperatures we can follow the microrheological changes and kinetics of the gelation process. Using the generalized Stokes-Einstein relation viscoelastic properties of these quasi-static gel states the evolution of the storage and loss moduli, G' and G'', are examined as functions of both time and temperature. The data show that both G' and G'' exhibit power law scaling versus frequency with the same exponent. The temperature and concentration dependence of the frequency at which the system crosses over from viscous to elastic behavior will be presented. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A47.00002: Temperature Dependent Dynamics and Structure of Soft Colloids Praveen Agarwal, Samanvaya Srivastava, Lynden Archer We have investigated the effect of temperature on the structure and dynamics of a particular class of soft colloids created by densely grafting polymer chains to the nanoparticle surface. These materials are able to display fluid like properties even in the absence of any external solvent and are termed as self-suspended fluids. Temperature dependent rheology of these materials has displayed several interesting features including increased solid like response with increase in temperature. Tethered polymer chain in this particular system is cis 1-4 polyisoprene, which is a type-A dielectric and allows the effect of temperature change on the global dynamics of the tethered chain to be separately investigated with broadband dielectric spectroscopy. Furthermore, we have investigated the effect of temperature change on the nanoparticle structure and dynamics with the help of small angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS). Based on our finding, we have provided an alternative description of the jamming phase diagram applicable to this system. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A47.00003: Confinement-induced solidification of attractive colloidal suspensions Jacinta Conrad, Melissa Spannuth Using a model colloidal-polymer suspension, we show that confinement induces solidification in attractive colloidal suspensions via a fundamentally different route from that active in hard sphere colloidal suspensions. For a range of polymer concentrations, the suspensions undergo a phase transition from a colloidal fluid with clusters to a colloidal gel with increasing confinement while polymer and particle concentration are held constant. Surprisingly, the effects of confinement appear at much larger thicknesses in attractive colloidal suspensions than in hard sphere suspensions. We find that solidification in confined attractive suspensions is not driven by structuring of the colloids at the walls or by shear-induced migration or clustering. Instead, by analyzing the cluster size distributions in the fluid phase as a function of confinement, we find that the strength of the interparticle attraction increases as the samples are confined. We further demonstrate that this change in the strength of attraction leads to increasingly arrested particle dynamics as colloidal gels are confined. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A47.00004: Gelation of self-assembed bile acid-PEG conjugates Satu Strandman, Frantz Le Devedec, X.X. Zhu The aggregation of macromolecules and low-molar-mass compounds into elongated self-assemblies such as wormlike micelles, fibers, or tubules increases the viscosity of the solutions and often leads to gelation due to network formation, even in organic solvents. Such one-dimensional nanostructures are promising candidates for drug delivery vehicles, packing materials for separation, templates for metal nanowires, biocides, and photo- or biocatalysis. An interesting group of compounds capable of this type of self-organization are bile acids, which are endogeneous steroids known to form gels at high concentrations and appropriate pH conditions. Grafting poly(ethylene oxide) on bile acids via anionic polymerization brings along thermoresponsiveness represented by lower critical solution temperature (LCST), while self-assembling occurs below another threshold temperature leading to a gelation at high concentrations, as shown by rheological experiments. The latter transition is assigned to the nanotube formation of pegylated bile acids, visualized by electron microscopy. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A47.00005: The Role of Charge Interactions in Colloidal Gelation Emily Russell, Joris Sprakel, Thomas Kodger, David Weitz We demonstrate the gelation of a novel system of oppositely charged colloidal particles. The particles are charged by grafting a polyelectrolyte brush from the surface, and suspended in a polar solvent with added monovalent salt. Confocal microscopy allows us to study in detail the three-dimensional structure and dynamics of these binary gels as we vary the particle volume fraction, interaction strength, and relative number ratio of the two particle species, and we find a transition between a gel and a fluid state with each of these parameters. We find that the mean contact number of particles in the gel decreases as we approach the gel line, in contrast to what has been reported in the literature for depletion gels. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A47.00006: Anomalous Phase Transitions in Soft Colloid-Polymer Binary Mixtures Jaydeep Basu, Ajoy Kandar, Suresh Narayanan, Alec Sandy We have shown earlier [1] that these PGNPs resemble star polymers or spherical brushes in terms of their morphology in the melt. However, these particles show dynamics in melt which is quite different from other soft colloidal particles. Since most of the work on soft colloidal particles have been performed in solutions we have now explored the phase behavior of the PGNPs in good solvent using microscopic structural and dynamical measurements on binary mixtures of homopolymers and soft colloids consisting of polymer grafted nanoparticles. We observe anomalous structural and dynamical phase transitions of these binary mixtures, including appearance of spontaneous orientational alignment and logarithmic structural relaxations, as a function of added homopolymers of different molecular weights. Our experiments points to the possibility of exploiting the phase space in density and homopolymer size, of such hybrid systems, to create new materials with unique properties. Reference: 1. Sivasurender Chandran, Sarika C. K., A. K. Kandar, J. K. Basu, S. Narayanan, and A. Sandy, J. Chem. Phys. \textbf{135}, 134901 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A47.00007: Injectable Solid Peptide Hydrogel as Cell Carrier: Effects of Shear Flow on Hydrogel and Cell Payload Darrin Pochan Peptides were designed to intramolecularly fold into $\beta $-hairpins once they are exposed to physiological conditions and then consequently self-assemble into a rigid hydrogel with a network structure of branched and entangled, 3nm-wide fibrils. These physical hydrogels can be injected as preformed solids, because they can shear-thin and consequently flow under an appropriate shear stress but immediately recover back into solids on removal of the stress with gel stiffness restoring over time. In this work, mechanisms of gel shear-thinning and immediate recovery were elucidated by investigating gel behavior during and after flow via mechanical and structural characterizations. Importantly, hydrogel flow behavior was studied in a capillary geometry that mimicked the actual situation of syringe injection. Hydrogel flow profiles were obtained via fluorescent particle tracking and the profile shape was found dependent on flow rate and gel stiffness. Hydrogel nanostructure was probed with small angle neutron and x-ray scattering. The results demonstrate that these hydrogels can be excellent candidates for tissue regeneration substrates and injectable therapeutic delivery vehicles. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A47.00008: Shear-driven gelation of dilute colloidal suspensions Emanuela Del Gado, Alessio Zaccone, Daniele Gentili, Hua Wu, Massimo Morbidelli Shear-driven solidification of dilute colloidal suspensions has dramatic impact on their applications, ranging from industrial making of paints to artificial or natural microfluidic devices and is a prototype of far from equilibrium transitions. In a set of experiments on a dilute charge-stabilized colloidal suspension, we have monitored shear-induced aggregation in a fully controlled way and rationalized the effect of the shear stress from the initially liquid suspension to the final solid. By combining light scattering, rheology and microscopy images, we show that the suspension changes, under shear, into a suspension of non-Brownian aggregates whose packing fraction increases with the shearing time. Upon flow cessation, these aggregates can eventually form cohesive random packings where each inter-aggregate bond involve a large number of colloidal bonds. Such solidification mechanism is thus a hybrid between colloidal gelation and the packing-driven jamming of non-Brownian suspensions. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A47.00009: Aggregation and network formation in aqueous methylcellulose near the sol-gel transition Sara Arvidson, John McAllister, Joseph Lott, Frank Bates, Timothy Lodge Methylcellulose (MC) is a semi-flexible polymer which can be soluble in water at low temperatures, depending on the average number of methyoxyl groups on each repeat unit. Upon heating, soluble MCs pass through a lower critical solution temperature (LCST) and undergo thermoreversible gelation, which is well described by Winter-Chambon critical gelation theory. The relaxation exponent ($n$) exhibits a smooth variation with concentration, approaching $n = 1$ at low concentration and $n = 0.5$ high concentration. We selected a set of commercial MC for materials with similar degrees of substitution, but known for their significant variations in gelation temperature in water. MCs which gel at higher temperatures also exhibit a plateau in elastic modulus at low frequencies, which indicates two relevant length scales coexist just below the gel point. Scattering experiments (static, dynamic, and small angle x-ray and neutron) are compared to rheological measurements to reveal the MC chain structures and aggregation associated with phase separation and gelation and enable a mechanistic understanding of these phenomena. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A47.00010: Effect of particle stiffness on glassy dynamics of dense colloidal liquids Raymond Seekell, Prasad Sarangapani, Y. Elaine Zhu ``Fragile'' glassy materials show a non-Arrhenius dependence of relaxation time with temperature close to the glass transition and have been extensively studied for molecular glass formers as model ``hard-sphere'' colloidal suspensions, but we lack a complete understanding of ``strong'' glass formers which show an Arrhenius dependence on temperature approaching the glass transition. In this work, we investigate the glassy dynamics of microgels of varied particle stiffness in dense aqueous suspensions using confocal microscopy. Poly(N-isopropylacrylamide) (PNIPAM) microgel particles of variable stiffness in aqueous media are synthesized by precipitation polymerization varying the cross-linking density to resemble ``strong'' glass forming liquids owing to their directional elastic interparticle interactions at increased microgel volume fraction. The fragility effect on the glassy dynamics in dense colloidal suspension is investigated as we tune the behavior from ``soft-sphere'' to ``hard-sphere'' limits. We find that dynamic heterogeneity, specifically string-like motion, is more pronounced as stiffness increases. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A47.00011: Thermo-adjustable mechanical properties of polymer, lipid-based complex fluids Millicent Firestone, Sungwon Lee Combined rheology (oscillatory and steady shear) and SAXS studies reveal details on the temperature-dependent, reversible mechanical properties of nonionic polymer, lipid-based complex fluids. Compositions prepared by introduction of the polymer as either a lipid conjugate or a triblock copolymer form an elastic gel as the temperature is increased to 18 C. The network is produced from PEO chain entanglement and physical crosslinks confined within the intervening aqueous layers of a multilamellar structured lyotropic mesophase. Although the complex fluids are weak gels, tuning of the gel strength can be achieved by temperature adjustment. The sol state formed at reduced temperature arises as a consequence of the well-solvated PEO chains adopting a non-interacting, conformational state. Complex fluids prepared with the triblock copolymers exhibit greater tunability in viscoelasticity than those containing the PEGylated-lipid conjugate because of the temperature-dependent water solubility of the central PPO block. The water solubility of PPO at reduced temperatures results in the polymer being expelled from the self-assembled amphiphilic bilayer, causing collapse of the swollen lamellar structure and loss of the PEO network. At elevated temperatures, the triblock reinserts into the bilayer producing an elastic gel. These studies identify macromolecular architectures for the facile preparation of dynamic soft materials possessing a range of mechanical properties that can be tuned by modest temperature control. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A47.00012: Mechanics of networks of aliphatic fibers in aqueous surfactant media Giuliano Zanchetta, Marco Caggioni, Vincenzo Guida, Veronique Trappe We investigate the structural and rheological properties of aliphatic fibers dispersed in aqueous solutions of anionic surfactants, typically used in liquid detergents to provide yield stress. This system displays an onset to solid-like properties that depends on fiber concentration. In this contribution we will discuss how tuning the state of the surfactant background influences the fiber-fiber interactions and the mechanical properties of the gel. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A47.00013: Elimination of branching in self- assembled beta hairpin peptide fibrils Sameer Sathaye, Darrin Pochan Hydrophobic collapse of amphiphilic -hairpin peptides (e.g. MAX1 VKVKVKVKV$^{D}$PPTKVKVKVKV-NH$_{2})$ into fibrils and their hierarchical assembly into branched, hydrogel networks has been extensively studied. A physically crosslinked hydrogel network is formed due to fibrillar entanglement and branched defects in hydrophobic collapse during fibril formation. Alternating valine residues with side chains of the same size are responsible for the hydrophobic collapse of the molecule into a b-hairpin and fibril nanostructure with branching. In a new sequence LNK1 (LNK1 (Nal)K(Nal)KAKAKV$^{D}$PPTKAKAK(Nal)K(Nal)-NH$_{2})$ the non-beta turn valines were replaced with Napthylalanine and alanine amino acid residues, with hydrophobic side chains of larger and smaller volume, respectively, than valine. Thus, formation of a ``lock and key'' type structure was attempted in the hydrophobic core of the peptide fibrils that would eliminate fibril branching. The folding and network formation of LNK1 has been studied by Circular Dichroism spectroscopy (CD), Transmission Electron Microscopy (TEM) and Oscillatory Rheology. Preliminary rheological characterization suggests the elimination of branching in the fibrils and also a possibility that LNK1 networks, unlike MAX1, are just nanofibrillar suspensions rather than permanently physically crosslinked hydrogels. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A47.00014: Bond orientational order in randomly-packed colloidal spheres Eli Sloutskin, Alexander Butenko Systems of jammed particles are abundant, yet poorly understood. These systems are often naively assumed to be disordered, such that only short-range correlations are present and all spatial directions are equivalent. Yet, the mechanical stability of these materials implies that a network of mechanical forces percolates through the sample, which may give rise to long-range correlations and symmetry breaking. We directly measure, by confocal microscopy, the positions of hard colloids, which are sedimented by centrifugation, to form a jammed matter. We follow the centrifugation process in motion, measuring the density profile of our particles along the sample. Strikingly, while only short-range positional order exists in our system, both in the fluid and in the jammed state, the orientations of the bonds between the nearest neighbors are correlated in the jammed state, throughout the system. This breaks the rotational symmetry of the jammed state. Moreover, the rotational symmetry is correlated with the direction of gravity, suggesting that the mechanical network of forces plays an important role in our system. This breaking of rotational symmetry, observed in our system, must have an impact on a wide range of properties in other, more complex, randomly packed systems. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A47.00015: Anharmonicity in Amino Acids Herculano Martinho, Thamires Lima, Mariana Ishikawa Two special dynamical transitions of universal character have been recently observed in macromolecules (lysozyme, myoglobin, bacteriorhodopsin, DNA, and RNA) at $T^{*}\sim 100 - 150$ K and $T_{D}\sim 180 - 220$ K. The underlying mechanisms governing these transitions have been subject of debate. In the present work it is reported a survey on the temperature dependence of structural, vibrational and thermodynamical properties of a nearly anhydrous amino acid (orthorhombic polymorph of the amino acids L-cysteine and L-proline at a hydration level of $3.5\%$). The temperature dependence of X-Ray diffraction, Raman spectroscopy, and specific heat were considered. The data were analyzed considering amino acid-amino acid, amino acid-water, and water-water phonon-phonon interactions, and molecular rotors activation. Our results indicated that the two referred temperatures define the triggering of very simple and specific events that govern all the interactions of the biomolecule: activation of CH$_{2}$ rigid rotors ($T |
Session A48: Charged and Ion-Containing Polymers
Sponsoring Units: DPOLYChair: Thomas Seery, University of Connecticut
Room: 161
Monday, February 27, 2012 8:00AM - 8:12AM |
A48.00001: Influence of Plasticizer on Ion Aggregation in Single-ion Polymer Conductors Hanqing Zhao, Dan King, Paul Painter, Ralph Colby, James Runt In this study, we add a miscible small molecule plasticizer to a polyester copolymer ionomer. The latter is synthesized from oligomeric polyethylene oxide (molecular weight = 600) separated by the lithium salt of 85 percent sulfonated dimethyl isophthalate units. Materials with different plasticizer contents are systematically investigated by FTIR spectroscopy, X-ray scattering and dielectric spectroscopy. Decreased Tg and a corresponding Increase in cation mobility are expected in these plasticized systems. Ionic conductivity depends on both the number of ions and their mobility, which in turn depends on the relationship between ion states (free ions, ion pairs and ion aggregates). With FTIR, we characterize and quantify the ionic structures in order to investigate how their ratio changes with plasticizer content. X-ray scattering reveals the change in ion aggregation. Further, dielectric spectroscopy is used to study ion conductivity and polymer dynamics of these materials. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A48.00002: Atomistic molecular dynamics simulations of model ionomers Dan Bolintineanu, Mark Stevens, Amalie Firschknecht Ionomers are polymers that contain a small fraction of ionic groups. Due to their unique electrical properties, ionomers are being investigated as potential solid electrolytes in mobile battery applications. However, a lack of fundamental understanding of the relationship between ionomer chemistry, morphology and ion transport have hindered such efforts. To this end, we report atomistic molecular dynamics (MD) simulations of a model ionomer (polyethylene-co-acrylic acid) neutralized with different ions at various neutralization levels. The structure factor computed from the simulations is in good agreement with experimental X-ray scattering data, which provides strong validation of the simulation methods. Our simulations provide additional insight into the shape and size distribution of ionic clusters; in particular, we observe large networks of string-like clusters, and report quantitative features of these structures as a function of ionic group spacing in the polymer backbone, counterion type and neutralization level. We also investigate several features of ion transport in these systems. Since ion diffusion is slow relative to the time scales accessible to our simulations, we limit discussion to local, qualitative features of the ion transport mechanism. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A48.00003: Weak polyelectrolytes grafted to nanoparticles and flat surfaces Stephen Barr, Athanassios Panagiotopoulos The charge distribution on polyelectrolytes is a key factor which controls their conformation and interactions. In weak polyelectrolytes, this distribution is determined by a number of factors, including the solvent conditions and the local environment. We investigate this using grand canonical titration Monte Carlo simulations of a coarse grained polymer model. In this method, each polymer bead is able to change its ionization state based on its dissociation constant, the pH of the solution, and interactions with other particles in the system. We focus on a system of polymers with one end tethered to the surface of a nanoparticle and determine both the charge and the polymer conformation as the pH and solvent quality are varied. We compare the results to both a fixed charge model and to polyelectrolytes grafted to a flat surface. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A48.00004: Large scale molecular dynamics study of polymer-surfactant complex Monojoy Goswami, Bobby Sumpter In this work, we study the self-assembly of cationic polyelectrolytes mediated by anionic surfactants in dilute or semi-dilute and gel states. The understanding of the dilute system is a requirement for the understanding of gel states. The importance of polyelectrolyte with oppositely charged colloidal particles can be found in biological systems, such as immobilization of enzymes in polyelectrolyte complexes or nonspecific association of DNA with protein. With the same understanding, interaction of surfactants with polyelectrolytes shows intriguing phenomena that are important for both in academic research as well as industrial applications. Many useful properties of PE surfactant complexes come from the highly ordered structures of surfactant self-assembly inside the PE aggregate. We do large scale molecular dynamics simulation using LAMMPS to understand the structure and dynamics of PE-surfactant systems. Our investigation shows highly ordered ring-string structures that have been observed experimentally in biological systems. We will investigate many different properties of PE-surfactant complexation which will be helpful for pharmaceutical, engineering and biological applications. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A48.00005: Near Edge X-ray Absorption Fine Structure Studies of Cu Ion-Containing PAMAMOS Dendrimer Networks Robert Bubeck, Petar Dvornic, Daniel Fischer There is continuing interest in the development of nanocomposites containing metal ions based on the use of dendrimers as the host matrix. One may utilize functionalized dendrimer interiors to complex the added constituent and serve as a template for the organization of the resulting nanoscale structures. Potential applications of include: catalysts, biotechnology, functional membranes, molecular sensors, etc. We report on recent results obtained using near edge X-ray absorption fine structure (NEXAFS) to characterize Cu(2+) covalent interactions in three-dimensionally cross linked dendrimer networks. These networks were made from radially-layered poly(amidoamine-organosilicon), PAMAMOS, dendrimers having generation 4 (G4) polyamidoamine (PAMAM) interiors surrounded by one layer of organosilicon, OS, exterior branch cells. Lower generation homologues (i.e., G1 through G3) were also examined on a more limited basis. The nitrogen and carbonyl moieties contained in the PAMAM dendrimer interior were shown to be highly interactive with metallic cations, in large measure because of the dendrimer geometry. Similar measurements on chemically similar but much less physically constraining hyperbranched polymers indicated very limited interaction with the amine and carbonyl moieties. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A48.00006: Mean-Field Modeling of the Encapsulation of Weakly Acidic Particles in Polyelectrolyte Dendrimers Thomas Lewis, Venkat Ganesan The unique architecture of dendrimers has attracted interest in a wide-variety of biomedical applications such as drug delivery. Dendrimers act as covalent micelles and have been shown experimentally to internalize hydrophobic molecules inside their cavities. Moreover, many drugs of low water solubility are weakly acidic and have been shown to form complexes with polybasic dendrimers, with the encapsulation ability being dependent upon the solution pH. Furthermore, the grafting of neutral water soluble chains such as polyethylene glycol (PEG) have shown to increase the encapsulation of poorly soluble drug molecules. In order to gain insight into the equilibrium behavior of drug-dendrimer complexes, we have developed and numerically solved a Self-Consistent Field Theory approach for both grafted and non-grafted annealed charged dendrimer molecules in the presences of drug molecules. Specifically, this work examines the effect of drug size, dendrimer generation, grafting chain length, and solution pH upon dendrimer encapsulation abilities. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A48.00007: Structure-Property Relationships in Precise Acid-Containing Polymers Francisco Buitrago, Kathleen Opper, Kenneth Wagener, Karen Winey Acid-containing polymers have specific interactions that produce complex and hierarchical morphologies providing a remarkable combination of mechanical properties, namely being both dissipative and resilient. Despite the widespread industrial use of such materials, rigorous structure-property relationships remain elusive due to structural heterogeneity in the available copolymers. Recently, linear polyethylenes with pendent acid groups separated by a precisely controlled number of carbon atoms have been synthesized by acyclic diene metathesis (ADMET) polymerization. X-ray scattering shows that the molecular uniformity of these acid copolymers results in morphologies with nearly monodisperse acid aggregates and polyethylene crystals assembled in highly organized hierarchical structures. Taking advantage of these ordered morphologies to obtain well-defined mechanical data, we probe the elastic modulus, yield stress and second yield stress of precise acid-containing polymers as a function of the number of carbon atoms between acid groups, acid type (acrylic and phosphonic), and mono- or geminal acid functionalization. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A48.00008: ABSTRACT HAS BEEN MOVED TO S1.00320 |
Monday, February 27, 2012 9:36AM - 9:48AM |
A48.00009: Block copolymer ion gels for gas separation Yuanyan Gu, Timothy Lodge Carbon dioxide removal from light gases (eg. N$_{2}$, CH$_{4}$, and H$_{2})$ is a very important technology for industrial applications such as natural gas sweetening, CO$_{2}$ capture from coal-fire power plant exhausts and hydrogen production. Current CO$_{2}$ separation method uses amine-absorption, which is energy-intensive and requires frequent maintenance. Membrane separation is a cost-effective solution to this problem, especially in small-scale applications. Ionic liquids have recently received increasing interest in this area because of their selective solubility for CO$_{2}$ and non-volatility. However, ionic liquid itself lacks the persistent structure and mechanical integrity to withstand the high pressure for gas separation. Here, we report the development and gas separation performances of physically crosslinked ion gels based on self-assembly of ABA-triblock copolymers in ionic liquids. Three different types of polymers was used to achieve gelation in ionic liquids. Specifically, a triblock copolymer ion gel with a polymerized ionic liquid mid-block shows performances higher than the upper bound of well-known ``Robeson Plot'' for CO$_{2}$/N$_{2}$. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A48.00010: Transport and phase behavior in anion-conducting diblock copolymers Guillaume Sudre, Sebnem Inceoglu, Nitash Balsara Anion-exchange membranes can be used in various applications such as direct methanol fuel cells or devices for artificial photosynthesis. Consequently, these membranes have to conduct the anions efficiently, remain insoluble in water or methanol, and be impermeable to various gases. Block copolymers are good candidates to reach these aims. Their ability to self-assemble, particularly into bi-continuous phases, makes it possible to use one polycationic block that would conduct the anions, while a second neutral block can be designed as a structural block to insure the mechanical stability of the system. Our study focuses on the relationship between phase behavior and anion conductivity of diblock copolymers as a function of molecular weight, composition and cationic groups. We have chosen to use a model system made of styrene for the neutral block, and of chloromethylstyrene for the second block since it is easily cationizable with various functionalities, reacting easily with e.g. trimethylamine or n-butylimidazole. This model system is synthesized by nitroxide-mediated radical polymerization with molecular weights between 2 to 40 kg/mol and fractions of chloromethylstyrene in the range of 15-40 mol{\%}. The results obtained from small-angle X-ray scattering showed lamellar morphologies for most systems. The temperature-dependence of the conductivity was assessed by performing measurements on membranes that were either immersed in water or in a controlled atmosphere at 98{\%} of relative humidity. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A48.00011: Polyelectrolyte Uptake by PEMs at High Salt David Hoagland, Zhaohui Su, Bo Peng, Xingjie Zan Upon a jump in salt concentration, a polyelectrolyte multilayer (PEM) constructed by the layer-by-layer process will swell, and in consequence, uptake from solution a large additional mass of the capping polyelectrolyte. Here, swelling and uptake are monitored in time by the quartz crystal microbalance with dissipation (QCM-D) method as a function of elevated salt concentration (0.75M$<$[NaCl]$<$2.5 M) during the uptake of poly(styrene sulfonate) (PSS, MW$\sim $70,000 g/mol) by poly(diallyldimethylammonium chloride)/PSS PEMs made at [NaCl]=0.5M. For [NaCl] less than $\sim $1 M, PSS adds only at/near the PEM surface, while for higher [NaCl], PSS fully permeates the PEM, contributing a PSS mass approaching, even exceeding, that already present; higher salt concentration leads to faster and greater PSS uptake. Above [NaCl]=1.0 M, uptake is diffusive, characterized by surprisingly large and sharply [NaCl]-dependent diffusion coefficients ($\sim $10$^{-14} - 10^{-12}$ cm$^{2}$/s). This uptake process opens a general opportunity for facile bulk and surface modifications of PEMs. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A48.00012: Polyelectrolytes in Salt Solutions: Molecular Dynamics Simulations Andrey Dobrynin, Jan-Michael Carrillo We performed MD simulations of polyelectrolyte solutions in the presence of salt. Polyelectrolyte solutions were modeled as an ensemble of chains of charged LJ particles with explicit counterions and salt ions. Our simulations have shown that in dilute and semidilute polyelectrolyte solutions the electrostatic chain persistence length scales with ionic strength as $I^{-1/2}$. This is due to counterion condensation on the polymer backbone. In dilute polyelectrolyte solutions the chain size decreases with increasing salt concentration as $R\propto I^{-1/5}$, which is in line with the scaling of the persistence length on the ionic strength, $l_{p}\propto I^{-1/2}$. In semidilute solution at low salt concentrations the chain size decreases with increasing polymer concentration, $R\propto c_{p}^{-1/4}$, while at high salt concentrations it is, $R\propto I^{-1/8}$. Our simulations confirmed that the peak position in the polymer scattering function scales with the polymer concentration in dilute polyelectrolyte solutions as $c_{p}^{1/3}$. In semidilute polyelectrolyte solutions at low salt concentrations the peak shifts towards larger values of $q^{\ast }\propto c_{p}^{1/2}$ while at high salt concentrations the peak location depends on the ionic strength as $I^{-1/4}$. The simulations confirmed a general scaling relations between a quantity $X(I)$ in salt solutions and corresponding quantity $X(I_{0})$ in salt-free solutions,$ X(I)= X(I_{0})(I$/$I_{0})^{\beta }$ with exponents; \textit{$\beta $}=-1/2 for persistence length $l_{p}$, \textit{$\beta $}=1/4 for solution correlation length, \textit{$\beta $}=-1/5 and \textit{$\beta $}=-1/8 for chain size $R$ in dilute and semidilute solution, respectively. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A48.00013: Electrostatic persistence length of flexible polyelectrolytes: resolving the controversy between theory and experiments Wei Qu, Erik Luijten The salt concentration dependence of the electrostatic persistence length $l_{e}$ of flexible polyelectrolytes has been the subject of extensive debate over the past two decades. Although theoretically a consensus has been reached regarding the correctness of the extension by Khokhlov and Khachaturian (KK) of the well-known Odijk--Skolnick--Fixman (OSF) theory to flexible polyelectrolytes, one crucial question remains: the strong disagreement between the OSF--KK prediction and various experimental observations. We present the results of extensive simulations of a flexible polyelectrolyte in solution to elucidate the origin of this discrepancy, and demonstrate that it originates from neglecting ionic excluded volume in the theoretical treatment. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A48.00014: Layer-by-Layer Assembly of Charged Nanoparticles on Porous Substrates: Molecular Dynamics Simulations Jan-Michael Carrillo, Andrey Dobrynin We performed molecular dynamics simulations of multilayer assembly of oppositely charged nanoparticles on porous substrates with cylindrical pores. The film was constructed by sequential adsorption of oppositely charged nanoparticles in layer-by-layer fashion from dilute solutions. The multilayer assembly proceeds through surface overcharging after completion of each deposition step. There is almost linear growth in the surface coverage and film thickness during the deposition process. The multilayer assembly also occurs inside cylindrical pores. The adsorption of nanoparticles inside pores is hindered by the electrostatic interactions of newly adsorbing nanoparticles with the multilayer film forming inside the pores and on the substrate. This is manifested in saturation of the average thickness of the nanoparticle layers formed on the pore walls with increasing number of deposition steps. The distribution of nanoparticles inside cylindrical pore was nonuniform with significant excess of nanoparticles at the pore entrance. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A48.00015: Diffusion of Polyelectrolyte Chains within Multilayer Films Svetlana Sukhishvili, Li Xu, Aliaksandr Zhuk, John Ankner Using a series of polycations synthesized by atom transfer radical polymerization, we investigate the relative importance of the effects of hydrophobicity, polymer charge density, and steric hindrance to charge pairing on chain dynamics within polyelectrolyte complexes (PECs) and within polyelectrolyte multilayer (PEM) films. First, by applying fluorescence correlation spectroscopy (FCS), ellipsometry and fluorescence recovery after photobleaching (FRAP), we found that the dynamics of chain exchange within PECs is directly correlated with the mode (linear \textit{vs}. exponential) of PEM film growth. Second, through a combination of neutron reflectometry (NR) and FRAP techniques to the same PEM types, we found that diffusion of polyelectrolyte chains within multilayer films is highly anisotropic, with diffusion coefficients being 10$^{4}$-10$^{5}$ higher in a direction parallel to the substrate compared to that perpendicular. Chain mobility was also controlled by ionic strength of annealing solutions and steric hindrance to ionic pairing of interacting polyelectrolytes.\\[4pt] This work was supported by the National Science Foundation under Award DMR-0906474 (S.S.). Neutron measurements were performed at the Spallation Neutron Source at the Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the DOE under contract No. DE-AC05-00OR22725. [Preview Abstract] |
Session A49: Focus Session: Long-time, Entangled Dynamics in Polymers - Linear, Transient, Non-linear Rheology, Tubes
Sponsoring Units: DPOLYChair: Gergory McKenna, Texas Tech University
Room: 162A
Monday, February 27, 2012 8:00AM - 8:12AM |
A49.00001: Microscopic theory of the tube confinement potential and relaxation of entangled needle liquids under stress Daniel Sussman, Ken Schweizer We have developed a first-principles theory of the transverse confinement potential in an entangled needle fluid based on exactly enforcing uncrossability at the two-rod level while self-consistently renormalizing many-particle effects [Sussman \& Schweizer PRL 107, 078102 (2011); J. Chem. Phys. 135, 131104 (2011)]. The predicted tube radius and long-time diffusion constant are consistent with the asymptotic reptation scaling laws under quiescent conditions, but in contrast with the usual tube model strong anharmonicities soften the confinement potential in a manner that quantitatively agrees with experiments on heavily entangled F-actin solutions. This weakening of entanglement constraints has multiple dramatic consequences under applied deformation: tube dilation, accelerated reptation, reduction of the transverse entropic barrier, and a critical stress or strain beyond which tube localization is destroyed. The degree-of-entanglement-dependent competition between reptative and transverse-hopping relaxation is established as a function of stress and strain. A mapping between rigid rods and flexible chain systems is also proposed, allowing predictions to be made for the tube diameter, entanglement onset, and transport properties of chain polymer liquids. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A49.00002: Rheology of linear monodisperse polyethylene melts from atomistic Molecular Dynamics simulations Nikos Karayiannis, Jorge Ramirez, Alexei Likhtman In our contribution we present results from very long, Molecular Dynamics simulations of linear monodisperse polyethylene (PE) chains with lengths ranging from 0.5 to 18 entanglements. We adopt a hierarchical modeling approach: in the first step we employ Monte Carlo simulations consisting of chain-connectivity altering algorithms to ensure full scale equilibration. Secondly, massive parallel MD simulations are conducted in the canonical ensemble. Besides the standard dynamical information, the stress relaxation curves are calculated for all PE systems. By bridging present atomistic results with the tube theory through the newly-introduced slip-spring model [Likhtman, Macromolecules 38, 6128 (2005)] we are able to calculate the plateau modulus and viscosity for well entangled, industrially relevant PE melts. In all cases, comparison between available experimental data and present simulation findings reveals a very good to excellent agreement. The proposed multi-scale methodology is generally applicable and can be extended to polymers of different molecular architecture and chemical constitution, as well as blends and more complex interfacial systems. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A49.00003: Intermolecular constraints in the dynamics of semiflexible entangled polymer melts Marina Guenza We present a Langevin equation for the contemporary dynamics of a group of interpenetrating semiflexible entangled polymer chains. The theory explicitly accounts for the intermolecular intermonomer repulsion between a pair of chains, generated by their inability to cross each other, i.e. the phenomenon of entanglements. The ``effective'' potential experienced by the chains arises from the repulsion between two monomers belonging to different chains, propagating through the chain connectivity, and the dynamics of chain interdiffusion and relaxation. With time the local hard-core potential is overcome by the relative motion of the polymers and the system relaxes. The same formalism applies to both unentangled and entangled melts. Short chains do not experience entanglements, because their relaxation process is faster than the average time that is necessary for the chain to diffuse a distance comparable to the mesh size, or length between two entanglements. Finally no a priori hypothesis has to be made about the processes that drive relaxation as the formalism is simply the conventional Rouse approach, generalized to treat the motion of interacting macromolecules, whose chains cannot cross each other. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 9:12AM |
A49.00004: Long-Time Dynamics in Polymers: Experimental Results Invited Speaker: Sindee Simon The long-time dynamics in several polymeric systems have been investigated with a focus on the relationship between the bulk and shear viscoelastic responses. Materials studied include polystyrene, a three-arm star polystyrene, and two polycyanurates of different crosslink densities. A custom-built pressurizable dilatometer has been used to measure the time-dependent bulk modulus, as well as the pressure-volume-temperature behavior in these materials. The temperature-dependent shift factors are used to test the TV$^{\gamma}$ thermodynamic scaling law proposed in the literature for segmental relaxation times. The thermodynamic scaling law successfully reduces the data for all of the materials; however, T - Tg scaling also successfully reduces the data and differences in implications of the two scaling approaches will be discussed. Comparison of the retardation spectra for the bulk and shear responses shows that at extremely long times, the chain mechanisms available to the shear response are not available to the bulk; for times related to the glassy dynamics, the two responses have similar slopes, indicating that they may have similar underlying molecular mechanisms, but the magnitudes are different, a finding that remains to be explained. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A49.00005: Tube Dynamics of Mildly Entangled Polymers: Semiflexibility Effects Jian Qin, Scott Milner, Pavlos Stephanou, Vlasis Mavrantzas The prevailing theory of polymer rheology rests on a careful analysis of tube dynamics, tested by comparing predicted rheological response functions to experimental measurements. We provide a direct test of this theory by analyzing the tube dynamics of recently simulated mildly entangled polyethylene melt. The tube dynamics is obtained by defining the tube primitive path, \textit{i.e.}, the tube center line, as the short-time average of molecular dynamics trajectories, and by monitoring how the tangent-tangent correlations evolve with time. It was found that the tube is semiflexible, and that the tube relaxation rate obtained from simulation results cannot be accounted for by the prevailing theory, since the effect of contour length fluctuations built into the theory is too strong. This discrepancy is particularly relevant to the mildly entangled system. To fix this, we incorporated semiflexibility into the theory, which was originally designed for flexible tubes, and found that the corrected theory describes the simulation results nearly quantitatively. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A49.00006: A critical analysis of typical assumptions in the theory of entangled polymer dynamics in elongational flows Marat Andreev, Renat Khaliullin, Jay Schieber The discrete slip-link model (DSM) was developed to describe the dynamics of entangled polymer melts of arbitrary chain architecture in arbitrary deformation. The model is able to predict linear viscoelasticity of monodisperse linear, polydisperse linear and star-branched systems. The model also shows good agreement with dielectric relaxation experiments. In this work we apply DSM to non-linear flows of monodisperse linear polystyrene and polyisoprene melts without any adjustable parameters. Model predictions for shear flow agree very well with experimental results. The DSM is able to capture the transient response as well as the steady state viscosity. However, for elongational flow, agreement is unsatisfactory at large strains. We explore a number of simplifications of the model and their effect on flow predictions, including: finite extensibility, convective constraint release and activation of dangling ends. Only after discarding all approximations and assumptions as a source of discrepancy between DSM predictions and experimental data can we conclude whether additional physics concepts are necessary to describe non-linear rheology of entangled polymer melts. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A49.00007: Exploring the role of long-chain branching in large deformation of entangled melts Gengxin Liu, Kwstas Ntetsikas, Apostolos Avgeropoulos, Shi-Qing Wang Most of our past studies have focused on nonlinear responses to large deformation of entangled polymers made of linear flexible chains. Little is known about nonlinear rheological behavior of entangled polymers containing long-chain branching (LCB), apart from the literature work on low-density polyethylene (LDPE). In this work, we present a first study to compare linear polyisoprene with a well-defined dendritic polyisoprene. Consistent with the extensional rheological behavior of LDPE, we find LCB to impede shear yielding so that the entanglement network could extend significantly more before failure during uniaxial extension. This study also investigated its shear deformation behavior to explain the absence of necking like failure in uniaxial extension. The research is funded, in part, by a grant from the National Science Foundation (DMR-1105135). [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A49.00008: Similarity and difference between simple shear and uniaxial extension of entangled polymers Hao Sun, Shi-Qing Wang There is ample evidence to show that the essential physics governing yielding of entangled polymers is the same, independent of the mode of deformation, e.g., shear versus extension. In either of these two most commonly studied forms of deformation, the elastic retraction force associated with the chain deformation cannot grow without bound during continuous deformation. In practice, a transition from the initial dominantly elastic deformation to flow (irreversible deformation) inevitably takes place. Such yielding can produce strain localization in large deformation of well entangled polymer melts. Apart from the superficial difference related to the confusion about the ``strain hardening'' behavior, a true difference in the respective responses of entangled melts to shear and extension arises when the strain rate is sufficiently high. The entanglement network can still yield on its path to the eventual flow state upon startup shear. However, startup extension could cause the entanglements to lock in, and the melt undergoes rubber-like rupture instead of yielding. This presentation raises the question of whether shear is an intrinsically different deformation from uniaxial extension in the extremely high rate limit. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A49.00009: Large-deformation and long-time behavior of entangled melts in complex geometries Xiangyang Zhu, Shi-Qing Wang Recent particle-tracking velocimetric (PTV) observations have revealed strain localization either during startup shear beyond the stress overshoot or after a large step shear of entangled polymers [e.g., Macromolecules, \textbf{42}, 6261 (2009)]. The physical pictures leading to these decohesion events have been put forward [J. Chem. Phys. \textbf{127}, 064903(2007); J. Rheol. \textbf{53}, 1389 (2009)]. In this presentation we apply the particle-tracking velocimetric method [Macromol. Mater. Engr. \textbf{292}, 15 (2007)] to study similar strain localization phenomena originating from yielding of the entanglement network in other forms of deformation including uniaxial extension, ``squeeze flow'' and extrusion of polymers from a wide open space into a narrow opening. The striking discontinuities in the velocity profile can all be understood in terms of a shear yielding criterion. The research is funded, in part, by a grant from the National Science Foundation (CMMI-0926522) [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A49.00010: Barrier to chain retraction: where we are six years after the first report of shear inhomogeneity in entangled polymers? Shi-Qing Wang, Pouyan Boukany, Yangyang Wang, Shiwang Cheng At APS2006, we reported the first PTV observations of macroscopic motions after shear cessation from step strain on an entangled polybutadiene solution (Macromolecules \textbf{2007}, $40$, 8031). Since then we have shown that the classical polystyrene solutions display similar non-quiescent relaxation, invalidating the agreement between the data based on PS solutions and the Doi-Edwards damping function. Based on polymer melts we found that this network breakup phenomenon also occurs after a step strain produced with a rate that according to the tube model is too low to generate chain stretching (Macromolecules \textbf{2009}, $42$, 6261). Does the current tube model possess the necessary ingredients to depict these findings? Here we present new experimental data that further supports the concept of a finite cohesion level for the entanglement network: There is a finite confining force that keeps chains engaged in the network, ensures the structural integrity and allows linear response behavior to take place. In contrast, the tube model perceives barrier-free chain retraction on the Rouse time for any amount of imposed strain, which would necessarily lead to destruction of the original network. Our experiments show that this does not appear to be the case. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A49.00011: Neutron Reflectivity Study of Interdiffusion of Ionomers into Van der Waals Polymer Thin Films Thusitha Etampawala, Dilru Ratnaweera, Sidath Wijesinghe, Dvora Perahia, Jaroslaw Majewski The slow dynamic processes in amorphous ionic polymers are affected by physical cross-links resulting from clustering of the ionic groups. Therefore in addition to entanglement barriers, the motion of the polymers is coupled to the dynamics of the ionic clusters where the resulting dynamics is an interplay between the effects of the two types of barriers. Using neutron reflectometry we have probed a model system where interfacial diffusion of a Van der Waals polymer, polystyrene, into its sulfonated analogs. Results controlling the molecular weights that determine the overall number of entanglements as well as the degree of sulfonation which affects the strength and number of the ionic clusters will be presented. Comparison to the diffusion of polystyrene into polystyrene will resolve the effects of the ionic clusters from those of entanglements. The presence of the physical cross-links slows down the dynamics significantly with respect to that of polystyrene and an asymmetric process where the non-ionic blocks migrate into the ionic one is observed. Further rearrangements take place at a later stage. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A49.00012: Development of Interfacial Strength and Entanglements During Welding of Polymers Ting Ge, Mark O. Robbins, Flint Pierce, Gary S. Grest, Dvora Perahia Thermal welding is a common means of joining polymer parts. Interfacial
strength increases with welding time $t_w $ as polymer chains diffuse across
the interface. The microscopic origin of this interfacial strength
enhancement was investigated with large scale molecular simulations
employing a coarse-grained bead-spring model. Polymer surfaces were held
together at a temperature well above the glass transition temperature $T_g$. States at $t_w $ up to $10^9$ time steps were then quenched to a
temperature below $T_g $ for mechanical tests. We test the interfacial
strength by shearing the weld along a direction parallel to the interface.
The maximum shear stress $\sigma _{\max} $before failure is used to
characterize the interfacial strength. We find that $\sigma _{\max} $
increases as $t_w ^{1/4}$ before saturating to its bulk value. This agrees
with previous experiments by a lap-joint shear method [1]. In addition, our
analysis shows that the dominant shear failure mode changes from chain
pull-out at the interface for small $t_w $, to chain scission for large $t_w
$. We examine the average contour length $ |
Monday, February 27, 2012 10:48AM - 11:00AM |
A49.00013: Using the parallel plates geometry for nonlinear rheological measurements Zahra Fahimi, Chase P. Broedersz, Hans M. Wyss Conventional wisdom dictates that studying the mechanical response of viscoelastic materials in the nonlinear regime should be done either with a cone-plate or a Couette geometry, where the applied strain is homogenous in the measuring volume. However, the use of parallel plates would have important advantages in a wide range of applications. For instance solid-like hydrogel materials can often be processed readily into flat films. We show that the nonlinear viscoelastic behavior can also be obtained from measurements in a parallel plate geometry. By tracing the torque response and its derivative with respect to the applied strain, we obtain a general stress strain relation, which indeed captures the proper material behavior. The approach does not require any assumptions for the material's viscoelastic behavior. We show practical examples different classes of soft materials to illustrate that our approach enables access to the full nonlinear response of these materials, including the detailed shape of the stress response in large amplitude oscillatory shear measurements. Our approach should be applicable to a wide range of soft materials, including hydrogels, colloidal suspensions, or biological tissues. [Preview Abstract] |
Session A50: Liquid Crystalline Order in Polymers, Soft Matter, and Complex Fluids
Sponsoring Units: DPOLYChair: Peter Shibayev, Fordham University
Room: 162B
Monday, February 27, 2012 8:00AM - 8:12AM |
A50.00001: Shape Memory as a Process: Optimizing Polymer Design for Shape Recovery Richard Vaia, Hilmar Koerner, Kyungmin Lee, Robert Strong, Mattew Smith, Huabin Wang, Tim White, Loon-Seng Tan Shape memory is a process that enables the reversible storage and recovery of mechanical energy through a change in shape. Polymers provide a unique alternative to kinematic designs and other materials (e.g. metallic alloys) for applications requiring large deformation and novel control options. The effect control of storage and relaxation of strain energy associated with chain deformation depends on the nonlinear visco-elasitc behavior and glassy dynamics of the polymer network. Considering the molecular understanding of rubbery elasticity, chain entanglements in concentrated polymer liquids, affine deformation of networks, and glass fragility, heuristic guidelines can be formulated to optimize the molecular design of a polymer for shape memory. These are applied to the development of a polymer system for shape memory processes at high-temperature (200$^{o}$C). The low-crosslink density polyimide exhibits very rapid shape recovery, excellent fixity, high creep resistance, and good cyclability. Furthermore, the molecular design affords a very narrow temperature range for programming and triggering shape change that can also be accessed by photo-isomerization of the cross-link nodes. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A50.00002: Shape and Memory in Liquid Crystalline Elastomers Anselm Griffin, Wanting Ren, Philip McMullan, Whitney Kline, Mohan Srinivasarao As part of an ongoing effort to understand the origins of shape memory in liquid crystalline elastomers (LCE), we have synthesized and examined a series of SmC main-chain LCEs. Uniaxial stretching of these polydomain films at room temperature produces a monodomain structure that can, upon removal of load, retain the monodomain and a significant level of strain. Although these films show ordinary elastic response at temperatures near the isotropization (clearing) temperature, at room temperature -- far below the clearing temperature -- the mechanical response is anelastic. Experimental studies of isothermal strain recovery vs time after unloading will be presented along with details of the temperature profile for strain recovery of these LCEs. A rationale for the shape memory behavior is proposed that involves moving of crosslink points in the smectic lamellar arrangement during the stretching event and trapping of these crosslinks in different positions at low temperatures. This trapping is driven by the chemical segregation of the crosslink points from the mesogenic unit which can be thermally overcome at elevated temperatures allowing full elastic recovery. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A50.00003: Phase Behavior of Disk-Coil Macromolecules YongJoo Kim, Edward Ha, Alfredo Alexander-Katz We explore the self-assembly of disk-coil macromolecules using Monte Carlo simulations in the NPT ensemble. Our study focuses on the role that coil length compared to the size of the disk has on the phase behavior of the system as well as the effect of stacking interactions between the disks. As a function of temperature T, we find a disordered phase at high T and lamellar, perforated lamellar, and cylinder phases at intermediate T. If we further lower the temperature, the disk-rich regions spontaneously order, and we find ordered lamellar, ordered perforated lamellar, and ordered cylinder phases depending on the strength of the stacking interactions. The appearance of any of these phases is, however, strongly dependent on the length of the coil. In addition to constructing a comprehensive phase diagram, we further analyze the correlations in the system, as well as the director vector field of the disks, and use it to construct an order parameter. We show that the latter changes drastically at the ordering transition points. We find that the ordered cylinder phase has a high degree of parallel packing. Our results are important to understand the self-assembly of supramolecular structures of disk-coil amphiphiles that are ubiquitous in nature, such as the chlorophyll molecule. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A50.00004: Liquid Crystal Phases of Semiflexible Polymers Ian MacKay, Don Sullivan Liquid crystal polymers exhibit orientational order (nematic phase) and position order (smectic phase). Previous work on semiflexible polymers using self consistent field theory studied the isotropic-nematic and nematic-smectic transition for homogenous and diblock copolymers. The nematic phase is stabilized by excluded-volume effects between wormlike cylindrical segments. The smectic phase is further stabilized by excluded-volume effects between terminal end segments. Because models of semiflexible polymers include orientational degrees of freedom, in addition to the usual positional degrees of freedom, they are computationally more demanding to study. Spectral decomposition applied to segment orientations has previously been used to make computation feasible. However this method does not converge well for strongly ordered states, which arise in many real systems. I describe a Crank-Nicolson finite difference method applied to the orientations which is expected to converge well for highly ordered systems. This method also exhibits better numerical stability and accuracy and may thus serve as a better foundation for further studies of highly ordered systems. I also describe a modification to the spectral method which can compute the tilted Smectic C phase. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A50.00005: Photomechanical mechanism and structure-property considerations in the generation of photomechanical work in glassy, azobenzene liquid crystal polymer networks Kyung Min Lee, Nelson Tabiryan, Timothy Bunning, Timothy White Azobenzene-containing polymeric materials have shown shape adaptive responses when irradiated with light. We contrast the photogenerated mechanical response of glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCN) upon exposure to either UV or blue-green irradiation. The profound differences in the fundamental photomechanical response to exposure to light in these wavelength regimes are dictated by distinctive photochemical mechanisms, elucidated through UV/VIS spectroscopic examination of the materials before and after irradiation with UV and blue-green light. The glassy, photoresponsive polymeric materials were subjected to structure-property examination to ascertain the role of crosslink density, azobenzene concentration, and azobenzene connectivity (crosslinked or pendant) on the photomechanical output. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A50.00006: Surface wrinkling in liquid crystal elastomer bilayers Aditya Agrawal, Paul Luchette, Walter Chapman, Peter Muhoray, Rafael Verduzco Mechanically-induced surface wrinkling patterns, also known as strain-induced elastic buckling instability for mechanical measurements (SIEBIMM), represent a versatile and high throughput technique for thin film metrology. However, the technique requires clamping and mechanically straining bilayer samples, which can introduce errors and present challenges with small samples. Here, we present a modified approach in which thin films are deposited on top of a stimuli-responsive liquid crystal elastomer (LCE). Temperature changes induce a spontaneous and controllable shape-change in the LCE substrate, without the need for clamping or mechanically straining. We show that LCE bilayers can be used to accurately measure the modulus of nanoscale poly(styrene) films, down to 20 nm thick films. Furthermore, the surface wrinkle orientation can be controlled using different preparation methods and reoriented in a single sample with temperature changes only. In the case of thick (over 500 nm) PS films, the bilayer flexes in response to temperature changes. This work shows that LCE bilayers are useful systems for thin film metrology and controlled assembly using well-defined surface structures. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A50.00007: Field-induced orientational order of liquid crystals in random environments Lena Lopatina, Jonathan Selinger Over the last twenty years, there has been extensive theoretical and experimental work on liquid crystals in disordered polymer networks and other random environments. It was shown that the disordered environment disrupts the long-range order of the liquid crystal. Recently, D.-K. Yang has performed new experiments, in which an electric field is applied to the polymer-disordered liquid crystal, leading to a large Kerr effect, i.e. field-induced long-range orientational order [1]. This experimental approach offers new opportunities for liquid-crystal displays. To understand the experiments and improve the applications, we perform Monte Carlo simulations of a nematic liquid crystal in a disordered polymer network. These simulations show the formation of randomly oriented domains of uniform directors. We study the response to an applied field by calculating the Kerr coefficient for variable system parameters. Furthermore, using an Imry-Ma-like approach we predict the domain size as a function of temperature and material properties of the system, and estimate the induced orientational order parameter due to an electric field. The simulations and analytic results agree well with each other and with the experiments. \\[4pt] [1] Y.-C. Yang and D.-K. Yang, Appl. Phys. Lett. 98, 023502 (2011). [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A50.00008: Highly viscous liquid crystalline mixtures: the alternative to liquid crystalline elastomers Petr Shibaev, Cristina Schlesier, Leah Newman, Scott McDonald Novel highly viscous liquid crystalline materials based on mixtures of glass forming oligomers and low molar mass liquid crystals were recently designed [1, 2] and studied. In this communication the novel data are presented, the analysis and discussion are extended. It is shown that viscoelastic properties of the materials are due to the physical entanglements between cyclic oligomers and low molar mass mesogens, not due to the chemical crosslinks between molecular moities. However, the mechanical properties of these viscoelastic materials resemble those of chemically crosslinked elastomers (elasticity and reversibility of deformations). The properties of chiral and non-chiral materials loaded with ferromagnetic nanoparticles are discussed in detail. Cholesteric materials undergo gigantic color changes in the wide spectral range under the deformation that allows distant detection of deformation and determination the anisotropy of deformation and its type. The materials doped with laser dyes become mechanically tunable lasers themselves and emit coherent light while pumped by external laser. A simple model is suggested to account for the observed effects; physical properties of the novel materials and liquid crystalline elastomers are compared and discussed. \\[4pt] [1] P.V. Shibaev, C. Schlesier, R. Uhrlass, S. Woodward, E. Hanelt, Liquid Crystals, 37:12, 1601-1604 \\[0pt] [2] P.V. Shibaev, P. Riverra, D. Teter, S. Marsico, M. Sanzari, V. Ramakrishnan, E. Hanelt, Optics Express, 16, 2965 (2008) [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A50.00009: Linear aggregation and liquid crystalline ordering: from semi-flexible polymers to rigid rods Tatiana Kuriabova, Zach Kost-Smith, M.D. Betterton, Matthew A. Glaser Reversible self-assembly of filamentous aggregates is ubiquitous in soft matter and biophysics. Examples include worm-like micelles, patchy colloids, chromonic liquid crystals, DNA and RNA, and protein polymers and fibrils. These rod-like aggregates can form liquid-crystal (LC) phases; the liquid-crystal order then couples to the aggregation, promoting the formation of longer aggregates in the LC phases. We study the coupled aggregation and liquid crystalline ordering of a minimal system of sticky cylinders that interact primarily by hard-core interactions but can stack and bind end to end, making use of both analytic theory and Monte Carlo simulation. Accurate treatment of aggregate flexibility is essential for quantitative comparison of theoretical and experimental phase diagrams and other properties. Our analytic model describes aggregates as wormlike chains in an effective aligning nematic field, and allows self-consistent determination of this field within a density functional theory formalism over a broad range of aggregate flexibilities. We compare the analytic results for isotropic-nematic phase equilibrium with simulations performed in our group as well as similar work from the Sciortino group [De Michele et al., arXiv:1108.6135]. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A50.00010: Liquid Crystal Elastomer Motors Peter Palffy-Muhoray, Xiaoyu Zheng Motors produce motion due to the transfer of energy, but not of momentum, to the device. In LCE motors, motion arises due to changes in the shapes of solid samples. Here we consider motors where the shape change is a bend, rather than an elongation or contraction. We focus on the light-driven motor of Ikeda et al.; we analyze in detail the physical mechanisms which bring about the motion, and discuss the momentum current which is generated. We present the results of numerical simulations, and compare these with experimental observations. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A50.00011: Orientational fluctuations of amorphous nematogenic solids Fangfu Ye, Bing Lu, Paul Goldbart Amorphous nematogenic solids (ANS) are media comprising rod-like nematogens that have been randomly linked to form elastically deformable macroscopic networks. ~Classes of ANS include chemical nematogen gels (i.e., networks of small molecules) and liquid crystalline elastomers (built from crosslinked nematogen-containing macromolecules), as well as biophysical networks such as those composed of actin filaments. ~We use a method inspired by the cavity approach to construct a replica free energy for these random systems, and investigate the correlations of the thermal fluctuations of the orientational alignment of the nematogens at spatially separated points. ~We identify two qualitatively distinct regimes: (a) a weakly localized regime, in which the correlations decay exponentially with separation; and (b) a strongly localized regime, characterized by correlations that also decay but oscillate as they do. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A50.00012: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 10:24AM - 10:36AM |
A50.00013: Non-Monotonic Concentration Effects in the Phase Behavior and Nematic Orders: Mixtures of Side-Chain Liquid Crystalline Polymers and Low-Molecular-Weight Liquid Crystals Bilin Zhuang, Zhen-Gang Wang Mixtures of side-chain liquid crystal polymers (SCLCPs) and low-molecular-weight liquid crystals (LMWLCs) are novel materials with applications such as optical data storage, non-linear optics, solid polymer electrolytes, chromatography and display materials. Recent experiments showed that the nematic-isotropic transition temperature and the nematic orders of each component vary non-monotonically with concentration. Existing theories, which combine the Flory-Huggins theory for isotropic mixing and the Maier-Saupe theory for nematic order, cannot explain such non-monotonicity. Here, we extend the existing theories by, first, incorporating the local steric constraints between the side-chain and the polymer backbone on the SCLCPs, and second, accounting for the crowding effects at high SCLCP concentrations. The new extended theory is able to resolve the discrepancies between the predictions of existing theories and the experimental observations. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A50.00014: Effect of blending on nematic order in semiflexible polymers Kiran Khanal, Jutta Luettmer-Strathmann Semiflexible polymers of sufficient stiffness exhibit liquid crystalline order at sufficient polymer concentrations. In this work, we investigate blends of flexible and semiflexible polymers with the aid of Monte Carlo simulations of a bond-fluctuation model. The model is an extension of Shaffer's bond-fluctuation model, where chain stiffness is controlled by including different forms of bending penalties, and includes attractive interactions between monomers. From simulations for a range of values of the bending energy, density, and temperature, we determine the effect of concentration of the flexible polymer on liquid crystalline order and domain formation. [Preview Abstract] |
Session A51: Colloids I: Beyond Hard Spheres
Sponsoring Units: DCMP DFDChair: Anthony Dinsmore, University of Massachusetts, Amherst
Room: Boston Convention Center 154
Monday, February 27, 2012 8:00AM - 8:12AM |
A51.00001: Photonic Droplets Containing Transparent Aqueous Colloidal Suspensions with Optimal Scattering Properties Jin-Gyu Park, Sofia Magkiriadou, Young-Seok Kim, Vinothan Manoharan In recent years, there has been a growing interest in quasi-ordered structures that generate non-iridescent colors. Such structures have only short-range order and are isotropic, making colors invariant with viewing angle under natural lighting conditions. Our recent simulation suggests that colloidal particles with independently controlled diameter and scattering cross section can realize the structural colors with angular independence. In this presentation, we are exploiting depletion-induced assembly of colloidal particles to create isotropic structures in a milimeter-scale droplet. As a model colloidal particle, we have designed and synthesized core-shell particles with a large, low refractive index shell and a small, high refractive index core. The remarkable feature of these particles is that the total cross section for the entire core-shell particle is nearly the same as that of the core particle alone. By varying the characteristic length scales of the sub-units of such `photonic' droplet we aim to tune wavelength selectivity and enhance color contrast and viewing angle. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A51.00002: Curvature-Induced Capillary Interaction between Spherical Particles at a Liquid Interface Nesrin Senbil, Chuan Zeng, Benny Davidovitch, Anthony D. Dinsmore Capillary interactions among particles adsorbed at a fluid interface are important in a variety of natural and technological systems but still pose many mysteries. Capillary interactions induced by buoyancy, referred to as the''Cheerios ''effect, have been studied for years. Here, we experimentally investigate how anisotropic interfacial shape affects capillary forces among millimeter-sized spheres. The Cheerios model predicts that particles with densities that are higher and lower compared to the fluids adsorbed at an initially flat interface will repel. Our experiments, however, clearly show that they can attract one another at the short range. We explain our results with a model, in which each sphere creates an anisotropic curvature at the position of the other sphere. To satisfy the constant contact-angle boundary condition, the interface is deformed with quadrupolar symmetry around each sphere. This quadrupolar deformation creates a short-ranged, attractive capillary force. The range of size and density ratios at which we observe a dominant short-range attraction is consistent with the model. Our results show how interfacial shape may be used to direct the assembly of interfacial particles. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A51.00003: Using Micron-Sized Ellipsoids as a New Tool for Microrheology David C. Kilgore, Kenneth W. Desmond, Eric R. Weeks Microrheology is a well-established technique, and in its simplest form it allows you to measure the viscosity of a fluid by examining the diffusion of microspheres, provided the diameter of the microspheres is known. We are developing a similar technique using ellipsoids, where the viscosity can be calculated without prior knowledge of the length and width of the ellipsoid. The asymmetry of ellipsoids provides a distinct advantage, allowing for the diffusion to be decomposed into two translational motions and one rotational motion. For each of these diffusive motions, we can measure a diffusion constant and relate the constant to the three unknowns: the length and width of the ellipsoid, and the viscosity. By measuring the three diffusion constants, we can determine the three unknowns. To verify this technique, we produce ellipsoids in the lab and suspend them in a viscous solution for three-dimensional imaging of the diffusion with a confocal microscope. ~We are able to get good agreement between the microrheological measurements and macroscopic viscosity measurements. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A51.00004: Non-capillary binding of colloidal particles to liquid interfaces David Kaz, Ryan McGorty, Vinothan Manoharan We observe colloidal polystyrene particles binding reversibly to an oil-water interface through the combination of a repulsive electrostatic force and an attractive van der Waals force. Previously studied interactions of an aqueous colloidal particle and a liquid interface have generally fallen into two categories: 1) electrostatic repulsion indicated by the dependence on salt and 2) capillary adsorption where surface tension brings the particle in contact with both phases and is indicated by practically irreversible binding. With our technique of pushing individual colloidal particles towards a planar oil-water interface and observing their motion in three-dimensions with holographic microscopy we have observed both interactions. However, our observations indicate that under certain conditions the electrostatic repulsion, which is due to repulsive image charges, is weak enough for a particle to experience a van der Waals attraction while strong enough to prevent a particle from penetrating the interface and becoming bound through capillary action. We observe individual particles transition between repulsive and attractive interactions with the interface suggesting that these colloidal particles have a heterogeneous surface charge. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A51.00005: Memory effects in soap film arrangements Nicolas Vandewalle, Stephane Dorbolo, Geoffroy Lumay, Julien Schockmel, Martial Noirhomme We report experiments on soap film configurations in a triangular prism for which the shape factor can be changed continuously. Two stable configurations can be observed for a range of the shape factor $h$. A hysteretic behaviour is found, due to the occurence of another local minima in the free energy. Experiments demonstrate that soap films can be trapped in a particular configuration being different from a global surface minimization. This metastability can be evidenced from a geometrical model based on idealized structures. Depending on the configuration, providing clues on the structural relaxations taking place into 3D foams, such as T1 rearrangements. The composition of the liquid is also investigated leading to dynamical picture of the transition. (Phys. Rev. E 83, 021403 (2011)) [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A51.00006: Droplet-based microfluidics and the dynamics of emulsions Jean-Christophe Baret, Quentin Brosseau, Benoit Semin, Xiaopeng Qu Emulsions are complex fluids already involved for a long time in a wide-range of industrial processes, such as, for example, food, cosmetics or materials synthesis [1]. More recently, applications of emulsions have been extended to new fields like biotechnology or biochemistry where the compartmentalization of compounds in emulsion droplets is used to parallelise (bio-) chemical reactions [2]. Interestingly, these applications pinpoint to fundamental questions dealing with surfactant dynamics, dynamic surface tension, hydrodynamic interactions and electrohydrodynamics. Droplet-based microfluidics is a very powerful tool to quantitatively study the dynamics of emulsions at the single droplet level or even at the single interface level: well-controlled emulsions are produced and manipulated using hydrodynamics, electrical forces, optical actuation and combination of these effects. We will describe here how droplet-based microfluidics is used to extract quantitative informations on the physical-chemistry of emulsions for a better understanding and control of the dynamics of these systems [3].\\[4pt] [1] J. Bibette et al. Rep. Prog. Phys., 62, 969-1033 (1999)\\[0pt] [2] A. Theberge et al., Angewandte Chemie Int. Ed. 49, 5846 (2010)\\[0pt] [3] J.-C. Baret et al., Langmuir, 25, 6088 (2009) [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A51.00007: Pseudo-Steady Liquid Transport in Aqueous Foams during Filling of a Container Michael Conroy, Ramagopal Ananth Various applications of aqueous foams involve filling a container or a column (e.g., fractionation), where the foam is formed and processed. However, existing studies in the literature do not treat the filling stage and only describe liquid transport within a static foam bed. We developed a theory that predicts liquid loss from the foam and the liquid distribution within its interior during the filling and post-filling stages. During the filling stage, the theory predicts that the foam reaches a pseudo-steady state characterized by a time-independent drainage rate and liquid fraction. The pseudo-steady-state liquid fraction appears above a thin, liquid-saturated boundary layer that exists at the bottom of the foam bed. During the post-filling stage, the theory predicts that the drainage rate decreases with time, similar to static foams beds studied by others. The theory compares well with our previously reported volume-averaged macroscopic model and drainage measurements for dry (high-expansion) foams. We will show that drainage during the filling stage is significant when the fill time is comparable to the intrinsic drainage time scale of the foam. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A51.00008: Coarsening of Two Dimensional Foams on a Curved Surface Adam Roth, Chris Jones, Doug Durian We report on foam coarsening and statistics of bubble distributions in a closed, two dimensional, hemispheric cell of constant curvature. Using this cell it is possible to observe individual bubbles and measure their coarsening rates. Our results are consistent with the modification to von Neumann's law predicted by Avron and Levine. We observed the relative frequencies of bubbles with a given number of sides and found a shortage of bubbles with few sides as compared to a flat two dimensional cell. We also measured the value of $m(n)$, the average number of sides of an $n$ sided bubble, and found general agreement with the Aboav-Weaire law, although there was greater deviation than for a flat cell. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A51.00009: Analysis of emulsion stability in acrylic dispersions Suresh Ahuja Emulsions either micro or nano permit transport or solubilization of hydrophobic substances within a water-based phase. Different methods have been introduced at laboratory and industrial scales: mechanical stirring, high-pressure homogenization, or ultrasonics. In digital imaging, toners may be formed by aggregating a colorant with a latex polymer formed by batch or semi-continuous emulsion polymerization. Latex emulsions are prepared by making a monomer emulsion with monomer like Beta-carboxy ethyl acrylate ($\beta $-CEA) and stirring at high speed with an anionic surfactant like branched sodium dodecyl benzene sulfonates , aqueous solution until an emulsion is formed. Initiator for emulsion polymerization is 2-2'- azobis isobutyramide dehydrate with chain transfer agent are used to make the latex. If the latex emulsion is unstable, the resulting latexes produce a toner with larger particle size, broader particle size distribution with relatively higher latex sedimentation, and broader molecular weight distribution. Oswald ripening and coalescence cause droplet size to increase and can result in destabilization of emulsions. Shear thinning and elasticity of emulsions are applied to determine emulsion stability. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A51.00010: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 10:00AM - 10:12AM |
A51.00011: Dynamics of charged particles in nonpolar solvent in response to an electric field Tina Lin, David Weitz In nonpolar solvent, surfactant molecules aggregate to form charge-stabilizing reverse micelles. This enables surface charging of colloidal particles suspended in nonpolar solvent. We investigate the dynamics of such charged particles in response to an externally applied electric field. By combining microfluidics and confocal microscopy, we directly visualize the transport of particles between two parallel electrodes. We use direct visualization to measure the electrophoretic mobility of each particle and determine the effect of added surfactant on the measured mobility. In addition, we find that the presence of surfactant has a significant effect on the transport dynamics of the charged particles. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A51.00012: Controlling Aggregation in Non-Polar Asphaltene Suspensions Through Electrostatics Sara Hashmi, Abbas Firoozabadi Asphaltenes, the most aromatic and largest molecular weight components of petroleum fluids, can undergo a liquid-liquid phase transition in conditions including highly non-polar environments. Phase separation begins with molecular association and proceeds to and through the colloidal length-scale until complete sedimentation or deposition. Non-ionic polymeric dispersants can stabilize asphaltenes at the colloidal scale in non-polar suspensions. We perform a variety of experiments which suggest that stabilization occurs by adsorption of dispersant onto the asphaltenes, truncating the progress of precipitation. In particular, dynamic light scattering (DLS) and phase-analysis light scattering (PALS) measurements indicate that electrostatic repulsion is responsible for stabilizing asphaltene colloids against further aggregation. Aggregation time increases exponentially with dispersant concentration, as expected for particles interacting through a combination of attractive dispersion forces and repulsive electrostatics. However, contrary to current understandings of electrostatic stabilization in non-polar systems, the charges in colloidal asphaltene suspensions seem to arise from the asphaltene colloids themselves rather than from dispersant micelles. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A51.00013: The Electric Double Layer Structure Around Charged Spherical Interfaces Zhenwei Yao, Mark Bowick, Xu Ma We derive a formally simple approximate analytical solution to the Poisson-Boltzmann equation for the spherical system via a geometric mapping. Its regime of applicability in the parameter space of the spherical radius and the surface potential is determined, and its superiority over the linearized solution is demonstrated. In addition, the influence of nonuniform surface potential on the electric double layer structure is studied for large spheres in the weak potential limit. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A51.00014: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 10:48AM - 11:00AM |
A51.00015: First experimental determination of permanent and induced electric dipolar moments of colloidal cellulose nanocrystals dispersed in apolar solvents Bruno Frka-Petesic, Bruno Jean, Laurent Heux Scientists and industrialists show a growing interest for cellulose nanocrystals (CNCs) since these rod-like nanoparticles display excellent mechanical properties that make them perfect candidates for the design of high performance biobased composites. Furthermore, CNCs can be obtained as colloidal suspensions in apolar solvents that form chiral nematic (cholesteric) liquid crystals. Our aim is to obtain homogeneous unidimensionnal structures to enhance the optical and/or mechanical properties of CNCs-based architectures at a macroscopic scale. Using electric fields, CNCs suspensions from either cotton or tunicate were succesfully oriented in the direction of an electric field, in both AC and DC configurations. To probe the electric field induced orientation of the CNCs, a birefringence experimental set-up has been developped. While applying short electric DC field pulses, static and transient birefringence has been measured in diluted isotropic suspensions. From these measurements, we determined both the permanent and induced electric dipolar moments of the CNCs, whose effects appeared to be of the same order of magnitude. The results are discussed regarding to the CNC type and the apolar solvent used. [Preview Abstract] |
Session A52: Using Chaos to Control Quantum Systems
Sponsoring Units: GSNPChair: Stephen Anlage, University of Maryland
Room: 153C
Monday, February 27, 2012 8:00AM - 8:36AM |
A52.00001: Regularization of tunneling rates in quantum chaotic systems Invited Speaker: Louis Pecora Prototypical systems of two potential wells separated by a tunneling barrier exhibit the unexpected and counter-intuitive results that regular, non-chaotic systems have tunneling rates that fluctuate with energy dramatically over several orders of magnitude whereas the fully chaotic wells have orders of magnitude smaller fluctuations. All calculations were found from the Schrodinger equation using the Boundary Element Method. A random, plane wave theory explains the magnitude of the average tunneling rates as well as their fluctuations. We show that we can tune the amount of variance in tunneling rates by changing the shape of the quantum wells implying possible device design capabilities for nanodevices that operate in the electron ballistic regime. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A52.00002: Relativistic quantum Darwinism in Dirac fermion and graphene systems Xuan Ni, Liang Huang, Ying-Cheng Lai, Louis Pecora We solve the Dirac equation in two spatial dimensions in the setting of resonant tunneling, where the system consists of two symmetric cavities connected by a finite potential barrier. The shape of the cavities can be chosen to yield both regular and chaotic dynamics in the classical limit. We find that certain pointer states about classical periodic orbits can exist, which are signatures of relativistic quantum Darwinism (RQD). These localized states suppress quantum tunneling, and the effect becomes less severe as the underlying classical dynamics in the cavity is chaotic, leading to regularization of quantum tunneling. Qualitatively similar phenomena have been observed in graphene. A physical theory is developed to explain relativistic quantum Darwinism and its effects based on the spectrum of complex eigenenergies of the non-Hermitian Hamiltonian describing the open cavity system. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A52.00003: Relativistic quantum chaos----an analytic Dirac equation approach Liang Huang, Hongya Xu, Ying-Cheng Lai Relativistic quantum chaos has attracted much attention since the discovery of graphene in 2004. Using graphene billiard as an apparatus of relativistic quantum particles, relativistic quantum scars, level spacing statistics, and relativistic quantum scattering have been widely investigated recently. However, since graphene has two non-equivalent Dirac points which can be coupled together by various processes, it has been wondered that whether the observed phenomena are inherent to the relativistic movement or caused by the discrete graphene lattice structure and boundary terminations. Based on Berry et al.'s work on neutrino billiards, we developed a conformal transformation method to solve the 2D Dirac equation in a confined region resembling chaotic billiards in the classical limit. This method solves both the eigen-energies and the eigen-wavefunctions of the 2D massless Dirac fermions. Level spacing statistics and relativistic quantum scars for a heart-shaped billiard are investigated and comparisons with graphene billiards are made. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A52.00004: Using Local Perturbations To Manipulate and Control Pointer States in Quantum Dot Systems Richard Akis, Gil Speyer, David Ferry, Roland Brunner Recently, scanning gate microscopy (SGM) was used to image scarred wave functions in an open InAs quantum dot[1]. The SGM tip provides a local potential perturbation and imaging is performed by measuring changes in conductance. Scarred wave functions, long associated with quantum chaos, have been shown in open dots to correspond to pointer states[2], eigenstates that survive the decoherence process that occurs via coupling to the environment. Pointer states modulate the conductance, yielding periodic fluctuations and the scars, normally thought unstable, are stabilized by quantum Darwinism [3]. We shall show that, beyond probing, pointer states can be manipulated by local perturbations. Particularly interesting effects occur in coupled quantum dot arrays, where a pointer state localized in one dot can be shifted over into another with a perturbation in a completely different part of the system. These nonlocal effects may perhaps be exploited to give such systems an exotic functionality. [1] A. M. Burke, R. Akis, T. E. Day, Gil Speyer, D. K. Ferry, and B. R. Bennett, Phys. Rev. Lett. 104, 176801 (2010). [2] D. K. Ferry, R. Akis, and J. P. Bird, Phys. Rev. Lett. 104, 176801 (2004). [3] R. Brunner, R. Akis,D. K. Ferry, F. Kuchar,and R. Meisels, Phys. Rev. Lett. 101, 024102 (2008). [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A52.00005: Periodic Orbit Scar in Propagation of Wave Packet Mitsuyoshi Tomiya, Hiroyoshi Tsuyuki, Shoichi Sakamoto, Eric Heller The scar-like enhancement is found in the accumulation of the time-evolving wave packet in stadium billiard. The time-average of the absolute square of the time-evolving wave functions in the stadium billiard is investigated numerically and semiclassically. Nowadays nano- or subnano-sized devices are getting more and more available. This kind of dynamical properties is essential, when the devices actually work. The enhancement appears along an unstable periodic orbit, when the Gaussian wave packet is launched as the initial state along the orbit. Introducing the window function which is closely related to the eigenfunction expansion coefficients of the wave packet, the localization around the periodic orbit is clarified by the semiclassical approximation that it is due to essentially the same mechanism of the scar states in stationary states. The ``smooothed'' window function is well estimated by the intensity spectrum in Prof. Heller's theory of the long-time semiclassical dynamics. The key parameters that determine its shape are actually classical quantities: the size of the initial wave packet and the Lyapunov exponent. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A52.00006: Chaotic Ionization of Bidirectionally Kicked Rydberg Atoms Korana Burke, Kevin Mitchell, Shuzhen Ye, F. Barry Dunning, Brendan Wyker A highly excited quasi one-dimensional Rydberg atom exposed to periodic alternating external electric ?eld pulses exhibits chaotic behavior. The ionization of this system is governed by a geometric structure of phase space called a homoclinic tangle and its turnstile. We present and explain the results from an experiment designed to probe the structure of the phase space turnstile. We create time-dependent Rydberg wave packets, subject them to alternating applied electric fields (kicks), and measure the survival probability. We show that the survival probability of the electron depends not only on the initial electron energy, but also on the phase space position of the electron with respect to the turnstile--the portion of the electron wave packet inside the turnstile ionizes quickly, after one period of the applied field, while that portion outside the turnstile ionizes after multiple kicking periods. Finally, we use the turnstile geometry to explain the dependence of ionization on the kicking period. This procedure describes a very robust yet simple way to control chaotic ionization of an atomic system. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A52.00007: Quantized Intrinsically Localized Modes Peter Riseborough We have calculated the quantized $n=2$ breather spectra of both the $\beta$ and the $\alpha$ Fermi-Pasta-Ulam lattices. The breather spectra are composed of resonances in the two-phonon continuum and branches of infinitely long-lived excitations. The non-linear attributes of these excitations become more pronounced at elevated temperatures. The calculated $n=2$ breather and the resonance of the $\beta$-lattice hybridize and exchange identity at the zone boundary, and are in reasonable agreement with the results of previous calculations using the number conserving approximation. However, by contrast the breather spectrum the $\alpha$-lattice couples resonantly with the single-phonon spectrum and cannot be calculated within a number conserving approximation. Furthermore we show that, for sufficiently strong non-linearity, the $\alpha$-lattice breathers can be observed directly through the single-phonon inelastic neutron scattering spectrum. As the temperature is increased, the single-phonon dispersion relation for the $\alpha$-lattice becomes progressively softer as the lattice instability is approached. We compare our theoretical results with the recent experimental observation of breathers in NaI by Manley {\it et al}. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A52.00008: Using quantum chaos to control the production of bipartite entangled states Lock Chew, Ning Chung Recently, we have shown that it is possible for the entanglement dynamics to depend on the global classical dynamical regime instead of the local classical behavior. We observe that as the corresponding classical system becomes more chaotic, the rate of entanglement production increases with the emergence of larger entanglement entropy in the steady state. This suggests that quantum chaos can be used to control the generation of highly entangled quantum states, which are typically more robust against the effects of decoherence from the environment. Furthermore, the dependence of our system on the global classical dynamical regime indicates that the mode of production is insensitive to errors in the preparation of the initial separable coherent states. In this talk, I will present our recent results of using the additional control of quantum squeezing to further enhance the entanglement of the dynamically generated quantum states. I will show that the concomitant application of quantum squeezing and quantum chaos leads to a more entangled state at a faster production rate relative to squeezing without quantum chaos. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A52.00009: Entanglement Entropy and Entanglement Spectrum for Two-Dimensional Classical Spin Configuration Hiroaki Matsueda In quantum spin chains at criticality, two types of scaling for the entanglement entropy exist: one comes from conformal field theory (CFT), and the other is for entanglement support of matrix product state (MPS) approximation. On the other hand, quantum spin-chain models can be mapped onto two-dimensional (2D) classical ones. Motivated by the scaling and the mapping, we introduce new entanglement entropy for 2D classical spin configuration as well as entanglement spectrum, and examine their basic properties in the Ising and the three-state Potts models on the square lattice. They are defined by the singular values of the reduced density matrix for a Monte Carlo snapshot. We find scaling relations of the entropy analogous to the CFT and the MPS results. At criticality, the spin configuration is fractal, and various sizes of ordered clusters coexist. Then, the original snapshot can be decomposed into a set of images, and they have different length scales, respectively. This is the origin of the scaling. Based on these observations as well as calculation of the entanglement spectrum, we conclude that the amount of information of only one snapshot at criticality is equal to that of 1D quantum critical systems. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A52.00010: Fluctuation theorem for a double quantum dot coupled to a point-contact electrometer Yasuhiro Utsumi, Dmitri Golubev, Michael Marthaler, Gerd Schoen We study the fluctuation theorem in single-electron sequential tunneling regime. We consider single-electron transport through a double quantum dot (DQD) monitored by a capacitively coupled quantum point-contact (QPC) electrometer. In this setup it is possible to perform a direction resolved real-time electron counting experiment. We derive the full counting statistics for the coupled DQD - QPC system and obtain the joint probability distribution of the charges transferred through the DQD and the QPC. We show that the joint probability distribution satisfies the fluctuation theorem for 4-terminal system. For two-terminal DQD, the effective temperature should be introduced to recover the fluctuation theorem. The system can be described by a master equation with tunneling rates depending of the counting fields and satisfying a generalized local detailed-balance relation. Furthermore, we derive universal relations between the non-linear corrections to the current and noise, which can be verified in experiment. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A52.00011: Electromagnetic fluctuations in non-equilibrium: Casimir forces and heat transfer Matthias Kruger, Thorsten Emig, Giuseppe Bimonte, Vladyslav Golyk, Alexander McCauley, Mehran Kardar It is well known that quantum Casimir forces play an important role in micro- or nanostructures. Recently, the role of temperature in thermal non-equilibrium raised theoretical as well as experimental interest. If the objects are held at different temperatures, the interactions depend on all temperatures in the system, and show many effects which are absent in equilibrium. Additionally, the objects exchange thermal energy by electromagnetic fields, known as radiative heat transfer, which is fundamentally different from macroscopic cases described by the well known laws of Planck or Stefan-Boltzmann. We discuss recent theoretical progress describing such effects, and illustrate the dependence of both quantities on the shapes as well as the distances of the objects. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A52.00012: A First-Principles Method of Determining van der Waals Forces in Dissipative Media Yi Zheng, Arvind Narayanaswamy Lifshitz theory of van der Waals (vdW) force and energy between two planar objects is strictly valid when the medium separating two planar objects is vacuum. Generalization of Lifshitz theory to the case when intervening medium is a dissipative material, as opposed to vacuum, is a surprisingly difficult undertaking because there is no expression for the electromagnetic stress tensor in dissipative materials. Here, we derive the expression for vdW energy and pressure in planar multilayered dissipative media by computing the work done in assembling the multilayered structure from its constituent thin films. In doing so, we avoid any calculations of the Maxwell stress tensor in any medium but vacuum. Even though this work has proven to be a corroboration of Dzyaloshinskii, Lifshitz, and Pitaevskii, it has thrown new light on our understanding of vdW forces and suggests that it should be possible to achieve the similar result for objects with arbitrary shapes. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A52.00013: Heat radiation from long cylindrical objects Vladyslav Golyk, Matthias Kruger, Mehran Kardar The heat radiated by objects small or comparable to the thermal wavelength can be very different from the classical blackbody radiation as described by the laws of Planck and Stefan-Boltzmann. We derive methods based on scattering of electromagnetic waves to explore the dependence on size, shape, as well as material properties. In particular, we discuss the radiation from a long cylinder at uniform temperature, describing in detail the degree of polarization of the emitted radiation by nanowires and carbon nanotubes. [Preview Abstract] |
Session A53: Disordered and Glassy Systems I
Sponsoring Units: GSNPChair: Horacio Castillo, Ohio University
Room: 153B
Monday, February 27, 2012 8:00AM - 8:12AM |
A53.00001: The dependence of fraglity of glass forming liquids on interparticle interactions and density Srikanth Sastry, Shiladitya Sengupta, Frederic Affouard, Filipe Vasconcelos The fragility of a glass forming liquid quantifies the rapidity of the change in viscosity and relaxation times with temperature and is an important material property. We study the influence of interparticle interactions on the fragility of a set of model glass formers using computer simulations. We consider both the kinetic fragility, given by the temperature variation of relaxation times, and the thermodynamic fragility obtained by the temperature variation of the configurational entropy. The Adam-Gibbs relation describes the temperature variation of relaxation times in terms of the variation of the configurational entropy, and thus we expect the kinetic and thermodynamic fragilities to be consistent with each other. We however find that the kinetic fragility increases with increasing softness of the interaction potential, with thermodynamic fragility showing the opposite trend. We rationalize our results by considering the full form of the Adam-Gibbs relation, which requires knowledge in addition of the high temperature activation energies, and explore the role of recent ideas on the scaling of temperature and density in systems exhibiting behavior akin to those with inverse power law interactions. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A53.00002: Frequency and Wavevector Dependence of the Atomic Level Stress-Stress Correlation Function in a Model Supercooled Liquid Valentin A. Levashov, James R. Morris, Takeshi Egami Temporal and spatial correlations among the local atomic level shear stresses were studied for a model liquid iron by molecular dynamics simulation [PRL 106,115703]. Integration over time and space of the shear stress correlation function $F(r,t)$ yields viscosity via Green-Kubo relation. The stress correlation function in time and space $F(r,t)$ was Fourier transformed to study the dependence on frequency, $E$, and wave vector, $Q$. The results, $F(Q,E)$, showed damped shear stress waves propagating in the liquid for small $Q$ at high and low temperatures. We also observed additional diffuse feature that appears as temperature is reduced below crossover temperature of potential energy landscape at relatively low frequencies at small $Q$. We suggest that this additional feature might be related to dynamic heterogeneity and boson peaks. We also discuss a relation between the time-scale of the stress-stress correlation function and the alpha-relaxation time of the intermediate self-scattering function $S(Q,E)$. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A53.00003: Measuring Dynamical Facilitation in Supercooled Liquids and Related Materials Yael Elmatad, Aaron Keys We provide a physical interpretation for excitation dynamics in kinetically constrained lattice models in the context of supercooled liquids and granular materials. Several physical quantities such as instanton times, onset temperatures, and particle displacement fields are derived. These quantities are used to interpret measurements of dynamical facilitation previously performed for atomistic and molecular supercooled liquids and granular materials. We show that these previous measurements provide strong evidence that dynamical facilitation plays a key role in glassy materials. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A53.00004: Excitations are localized and relaxation is hierarchical in glass-forming liquids Aaron Keys, Lester Hedges, Juan Garrahan, Sharon Glotzer, David Chandler For several atomistic models of glass formers, at conditions below their glassy dynamics onset temperatures, $T_{\mathrm{o}}$, we use importance sampling of trajectory space to study the structure, statistics and dynamics of excitations responsible for structural relaxation. Excitations are detected in terms of persistent particle displacements of length $a$. At supercooled conditions, for $a$ of the order of or smaller than a particle diameter, we find that excitations are associated with correlated particle motions that are sparse and localized, occupying a volume with an average radius that is temperature independent and no larger than a few particle diameters. We show that the statistics and dynamics of these excitations are facilitated and hierarchical. Excitation energy scales grow logarithmically with $a$. Excitations at one point in space facilitate the birth and death of excitations at neighboring locations, and space-time excitation structures are microcosms of heterogeneous dynamics at larger scales. This nature of dynamics becomes increasingly dominant as temperature $T$ is lowered. We show that slowing of dynamics upon decreasing temperature below $T_{\mathrm{o}}$ is the result of a decreasing concentration of excitations and concomitant growing hierarchical leng [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A53.00005: Anomalous properties of liquids for a family of models with short range tetrahedral interactions Sergey Buldyrev, Giancarlo Franzese Liquids with tetrahedral symmetry of the first coordination shell often display anomalous thermodynamic and dynamic behavior. The main reason for these anomalies is that pressurizing such liquids leads to the disordering of this local symmetry by the particles migrating from the second to the first coordination shell. This in some case may lead to the increase of entropy upon pressurizing and consequently to the volume increase upon cooling. Molecular simulations of various models with tetrahedral symmetry are able to reproduce this anomalous behavior. We study a family of simple models in which we can gradually change the degree of tetrahedrality and investigate the associated changes of the phase diagram by discrete molecular dynamics. A molecule in these models consist of a hard sphere and four point particles attached to the center of the hard sphere by directional bonds arranged in tetrahedral geometry. Each of these four particles has a narrow attractive square well so that the particles belonging to different molecules can attract to each other. We also impose a condition which does not allow a point particle in one molecule to include in its attractive well more than one point particle belonging to different molecules. We investigate how the phase diagram of the system depends on the parameters of the models. None of these models has a liquid -liquid phase transition in the accessible region of the phase. However, adding weak attractive square well to the hard sphere, or wider weak attractive square wells to the point particles can create a liquid-liquid critical point. A comparison with other simple models of the anomalous liquids is made. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A53.00006: Stretched-exponential relaxation and hidden power laws in a solidifying 2D liquid Alexander Patashinski, Rafal Orlik, Antoni Mitus, Bartosz Grzybowski, Mark Ratner In a 2D Lennard-Jones liquid, the number of particles keeping their memorized nearest neighbors is found to decay stretched-exponentially; the probability for a particle to keep the same 6 nearest neighbors for a time t can be fitted with a power law. Using the lists of nearest neighbors (\textit{nn}-lists) as a topological order parameter, we studied the dynamics of the structure underlying these signature features of complexity in materials. The \textit{nn}-changes randomly appear along the boundaries of better ordered blocks at a time scale of the order of particles vibration period; these boundaries, and the shapes of the blocks, perform a next time-scale random motion. Particles diffusion includes periods of slow and fast diffusion. We discuss the feed-back interactions between nn-changes, block boundaries motion, and orientation relaxation in the system. [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A53.00007: Sheared athermal soft-particle suspensions near jamming: dependence of effective diffusion on packing density and system size Kamran Karimi, Craig Maloney We perform numerical simulations to study diffusion in a model bi-disperse frictionless athermal soft-particle suspension of disks in two dimensions (2D) using the so-called ``mean field'' version of Durian's bubble model. We measure the effective transverse diffusion coefficient $D_{\rm eff}$ in shear flows at various volume fraction $\phi$ and shearing rate $\dot{\gamma}$. For $\phi>\phi_c$, where $\phi_c$ is identified with the random close packing limit, in the quasi-static limit, $D_{\rm eff}$ shows a pronounced linear system size dependence with very weak dependence on $\phi$. For $\phi<\phi_c$, $D_{\rm eff}$, in the quasi-static limit, increases with increasing $\phi$ and shows very little system size dependence. We discuss how the behavior of $D_{\rm eff}$ is related to non-trivial correlations in the spatial structure of the displacement fields \emph{at long times} in the Fickian regime. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A53.00008: Decoupling of Rotational and Translational Diffusion in Supercooled Colloidal Fluids Kazem V. Edmond, Mark T. Elsesser, Gary L. Hunter, HyunJoo Park, David J. Pine, Eric R. Weeks Using high-speed confocal microscopy, we directly observe the three-dimensional rotational dynamics of rigid clusters of microspheres suspended in dense colloidal suspensions. The clusters are highly ordered packings of fluorescently-labeled PMMA particles, fabricated using a recently developed emulsification technique. Our colloidal suspensions serve as an excellent model of hard spheres, perhaps the simplest system with a glass transition, while the clusters probe the system's local rotational and translational dynamics. Far from the colloidal liquid's glass transition, both rotational and translational motion of the clusters are purely Brownian. However, in the liquid's supercooled regime, we observe a decoupling between the two types of motion: as the glass transition is approached, rotational diffusion slows down even more than translational diffusion. The nature of the decoupling is in good agreement with theoretical predictions and experiments with molecular glass formers. Our observation supports the notion that supercooled liquids are not merely liquids with large viscosities but that diffusion takes place by fundamentally changed mechanisms. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A53.00009: Thermodynamics versus network topology of network glasses Le Yan, Matthieu Wyart Under cooling, the thermodynamics and the dynamics of super-cooled liquids are strongly correlated. The thermal evolutions of these quantities, characterizing the liquid fragility, depend greatly on the specific liquid considered. To date, there is no understanding of what controls these properties at a microscopic level. In chalcogenide glasses, the coordination of the covalent network can be changed continuously by varying their composition. Experiments show that as the coordination is increased, the jump of specific heat varies non-monotonically and is minimal at coordination near the Maxwell threshold where the covalent network becomes rigid. At such a composition the liquid is strong. We introduce a simplified model for the thermal evolution of networks that captures this observation. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A53.00010: Elastic properties of compressed emulsions Ivane Jorjadze, Jasna Brujic Visualizing the packing of a dense emulsion in 3D as a function of the external pressure allows us to characterize the geometry and the local stress distribution inside this jammed system. We first test the scaling laws of the pressure and average coordination number over two orders of magnitude in density. We find deviations from theoretical exponents due to the non-affine motion of the particles. Second, we observe that the distribution of forces changes from a broad exponential at the jamming point to a narrower Gaussian-like distribution under high compression. Finally, we calculate the density of states from the measured force network in the approximation of a harmonic potential. Close to jamming, the number of low frequency modes is high, while the application of pressure shifts the distribution to higher frequencies, indicative of a rigid network. The confocal images reveal the structural features associated with the low frequency modes, as well as their localization within the packing. These data are then compared with published results from numerical simulations. [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A53.00011: Glassy Dynamics of Charge Density Waves in Chromium Hyekyung Kim, Jonathan Logan, Oleg Shpyrko, Eric Isaacs Charge-density waves provide theoretically tractable systems for exploring longstanding questions posed by the physics of elastic media in the presence of quenched disorder. Interaction of quenched pinning fields and phase elasticity of CDWs results in a complex energetic landscape of metastable states, which in turn gives rise to ``glassy'' phenomena such as aging and hysteresis. Using synchrotron x-rays we have observed aging of CDW order parameter Q in bulk chromium following thermal quench to out-of-equilibrium configuration. Although temperature stabilization occurs in under two minutes, Q relaxes exponentially over the course of hours toward metastable configurations that depend on sample history. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A53.00012: Local collapse of the atomic cage in a liquid flow Takuya Iwashita, Takeshi Egami The local structure of a model glass under steady shear was studied by molecular dynamics simulation for both high (T$>$Tg) and low (T$<$Tg) temperature ranges. The local structure was presented in terms of the anisotropic pair-density function (PDF). We found that the local structure was strained over a limited range of distances, and the length-scale of the strained region was dependent on the strain rate, extrapolating to zero at a critical strain rate. A strong correlation between the local collapse, represented by cutting of the atomic bond, and the structural strain in the PDF was found. At low temperatures local failure happens in a serrated manner, caused mechanically by shear. At high temperatures the local failure occurs more randomly, which is governed by thermal fluctuation. An anomalous behavior was observed as temperature approaches to Tg. The results suggest that except for the supercooled state above Tg local failure occurs by cutting of a single bond. Only in the supercooled state multiple bonds have to be cut for flow to occur. A possible relation to the dynamic heterogeneity is discussed. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A53.00013: Local Perturbation of Quasi Two-Dimensional Colloidal Glasses Kevin Aptowicz, Tim Still, Ke Chen, Peter Yunker, Arjun Yodh Colloids are promising and widely used model systems to investigate the phase behavior of matter at length scales and timescales accessible to optical microscopy. We utilize disordered quasi two-dimensional colloidal systems in the jammed state to study the properties of glasses. It was recently found in simulation and experiment that so-called 'soft spots', where low frequency quasi-localized modes concentrate, are the locations in glasses prone to rearrangements. Therefore, we utilize short laser pulses to perturb colloidal glasses locally. By varying the pulse intensity of the laser, the strength of the perturbation is tunable, which allows us to induce either elastic or plastic deformations in the glasses. With this experimental geometry in combination with video microscopy, we investigate the correlation between locally induced rearrangements in the colloidal glass and the location of soft spots determined from analysis of the system's vibrational eigenmodes. The dependence of the mechanical response of a glass on the local environment in terms of these dynamic heterogeneities and their persistence is discussed. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A53.00014: X-ray investigation of colloidal glasses under shear Dmitry Denisov, Triet Dang, Bernd Struth, Peter Schall Understanding glassification or dynamical arrest is one of the grand challenges of material science and is a topic of great current interest. It is a central observation in soft matter systems as well as glass forming molecular systems that -- with increasing density or decreasing temperature -- the motion of the particles or molecules slows down and eventually becomes arrested. Understanding this dynamical arrest as well as relaxations in the arrested state are fundamental problems, which to a large degree remain unanswered. We use a novel combination of rheological measurement and small angle x-ray scattering (at the synchrotron DESY in Hamburg) to study structure factor of dense suspensions under shear and during relaxation. The suspensions consist of silica particles 50nm in diameter. We observe clear changes of inter particles distances and configurations due to the different shear rates. Together with future dynamic x-ray measurements we aim to develop a universal scale-bridging understanding of dynamic arrest. [Preview Abstract] |
Monday, February 27, 2012 10:48AM - 11:00AM |
A53.00015: Boundaries Matter for Confined Colloidal Glasses Gary L. Hunter, Kazem V. Edmond, Eric R. Weeks We confine dense colloidal suspensions within emulsion droplets to examine how confinement and properties of the confining medium affect the colloidal glass transition. Samples are imaged via fast confocal microscopy. By observing a wide range of droplet sizes and varying the viscosity of the external continuous phase, we separate finite size and boundary effects on particle motions within the droplet. Suspensions are composed of binary PMMA spheres in organic solvents while the external phases are simple mixtures of water and glycerol. In analogy with molecular super-cooled liquids and thin-film polymers, we find that confinement effects in colloidal systems are not merely functions of the finite size of the system, but are strongly dependent on the viscosity of the confining medium and interactions between particles and the interface of the two phases. [Preview Abstract] |
Session A54: Focus Session: Complex and Co-evolving Networks - Cascades in Networks
Sponsoring Units: GSNPChair: Beate Schmittmann, Virginia Tech
Room: 152
Monday, February 27, 2012 8:00AM - 8:12AM |
A54.00001: Percolation of Double-Layer Networks with Different Topologies Under Random Attacks Di Zhou, Jianxi Gao, Shlomo Havlin, H.Eugene Stanley We report on the effects of topology on failure propagation models for systems consisting of two interdependent networks, which are either SF networks with different parameters, or different types of networks. Pairs of interdependent networks are known to exhibit a percolation transition upon failure accumulation. The topology of the nets significantly change the critical density of failure for the total disruption of the two net composite system. When the system is not fully coupled, the existence of a very small fraction of SF network will hold the system from fragment. When the critical threshold $p_c$ doesn't change, robustness measure R is introduced. The consequences of the study may provide insights for future network architectures and their evolution to improve their robustness and enhancing the protection of critical infrastructure. [Preview Abstract] |
Monday, February 27, 2012 8:12AM - 8:24AM |
A54.00002: Statistics of interacting networks with extreme preferred degrees: Simulation results and theoretical approaches Wenjia Liu, Beate Schmittmann, R.K.P. Zia Network studies have played a central role for understanding many systems in nature - e.g., physical, biological, and social. So far, much of the focus has been the statistics of networks in isolation. Yet, many networks in the world are coupled to each other. Recently, we considered this issue, in the context of two interacting social networks. In particular, We studied networks with two different preferred degrees, modeling, say, introverts vs. extroverts, with a variety of ``rules for engagement.'' As a first step towards an analytically accessible theory, we restrict our attention to an ``extreme scenario'': The introverts prefer zero contacts while the extroverts like to befriend everyone in the society. In this ``maximally frustrated'' system, the degree distributions, as well as the statistics of cross-links (between the two groups), can depend sensitively on how a node (individual) creates/breaks its connections. The simulation results can be reasonably well understood in terms of an approximate theory. [Preview Abstract] |
Monday, February 27, 2012 8:24AM - 8:36AM |
A54.00003: Correlated multiplexity in random and co-evolving multiplex networks Jung Yeol Kim, Kyu-Min Lee, K.-I. Goh, I.-M. Kim Nodes in a complex networked system often engage in multiple types f interactions among them; they form a {\it multiplex} network with multiple layers that can be interdependent and co-evolve. In many real-world complex systems, such multiple network layers are not randomly coupled but correlated. Such a correlated multiplexity can imprint nontrivial structural correlations in the multiplex network, which in turn can impact the dynamical processes on it. Here we present some recent results on the correlated multiplexity in multiplex networks. First we show how the correlated multiplexity can dramatically alter the giant component properties of multiplex random networks. Secondly we introduce an evolution model of co-evolving multiplex networks by generalizing the well-known Barab\'asi-Albert-type model, to show how the co-evolution of network layers can induce and modulate the degree of correlated multiplexity. [Preview Abstract] |
Monday, February 27, 2012 8:36AM - 8:48AM |
A54.00004: Cascades of overload failures in spatial networks Andrea Asztalos, Sameet Sreenivasan, Boleslaw Szymanski, Gyorgy Korniss Our daily life imposes increasing demands on infrastructural networks such as the power grid, transportation network, water supply, etc. Understanding the vulnerabilities of these systems is crucial to securing them. To this end, we study the effect of spatial constraints on network resilience against cascading overloads. Specifically, we consider distributed and shortest path flows on spatially embedded networks and study the model of cascading failures (Motter and Lai (2002)) triggered by the removal of a single or multiple nodes. We present results of intentional attacks on highly loaded and high degree nodes as well as a comparison between spatially concentrated and randomly distributed, multiple attacks. [Preview Abstract] |
Monday, February 27, 2012 8:48AM - 9:00AM |
A54.00005: Multiplexity-facilitated cascade dynamics in networks Kyu-Min Lee, Charles Brummitt, Kwang-Il Goh Most complex network studies thus far have been focused on single-layer framework. It becomes increasingly clearly, however, that many real-world complex systems are {\it multiplex} ---nodes interact with multiple types of interactions (links) which coexist, co-depend, and co-evolve. The interplay of such multiplex interactions may confer nontrivial consequences on network dynamics. Here we present recent results on how such multiplexity affects cascade dynamics on networks. We found the cascasde progression is greatly facilitated by multiplexity. Layers that are unable to achieve global cascades in isolation can cooperatively achieve them by multiplex coupling. Therefore, the multiplex network can be dramatically more vulnerable to global cascades than single-layer or simplex networks. Implication of these results on cascade prediction and control in social and economic dynamics is also discussed. [Preview Abstract] |
Monday, February 27, 2012 9:00AM - 9:12AM |
A54.00006: Cascades of failures in various models of interdependent networks Sergey Buldyrev, Benjamin Kadish, Nathaniel Shere, Mitchel Aharon, Gabriel Cwilich Complex networks appear in almost every aspect of science and technology. Recently an analytical framework for studying the percolation properties of interacting networks has been developed [1]. These studies however have several limitations. The real networks do are not randomly connected. They are often embedded into two dimensional space. The dependency links are not connecting nodes at random but have tendency to connect nodes with similar degrees, or nodes which are close to each other in Euclidian space. Moreover, the network failures may occur not only to the loss of connectivity but also due to overload of nodes with high betweennes. We have study these situations analytically and by computer simulations and found the conditions at which networks collapse in an abrupt first order like transition when the entire network becomes non-functional or fail gradually like in a second order transition as a greater fraction of nodes is removed in the initial attack or failure. \\[4pt] [1] \textbf{S. V. Buldyrev}, R. Parshani, G. Paul, H. E. Stanley, and S. Havlin, ``Catastrophic cascade of failures in interdependent networks,'' \textit{Nature} \textbf{464}, 1025-1028 (2010) [Preview Abstract] |
Monday, February 27, 2012 9:12AM - 9:24AM |
A54.00007: Robustness of a Network of Networks Jianxi Gao, Sergey V. Buldyrev, H. Eugene Stanley, Shlomo Havlin Network research has been focused on studying the properties of a single isolated network, which rarely exists. We develop a general analytical framework for studying percolation of $n$ interdependent networks. We illustrate our analytical solutions for three examples: (i) For any tree of $n$ fully dependent Erd\H{o}s-R\'{e}nyi (ER) networks, each of average degree $\bar{k}$, we find that the giant component $P_{\infty}=p[1-\exp(-\bar{k}P_{\infty})]^n$ where $1 - p$ is the initial fraction of removed nodes. This general result coincides for $n = 1$ with the known second-order phase transition for a single network. For any $n>1$ cascading failures occur and the percolation becomes an abrupt first-order transition. (ii) For a starlike network of n partially interdependent ER networks, $P_{\infty}$ depends also on the topology--in contrast to case (i). (iii) For a looplike network formed by $n$ partially dependent ER networks, $P_{\infty}$ is independent of $n$. [Preview Abstract] |
Monday, February 27, 2012 9:24AM - 9:36AM |
A54.00008: Robustness of interdependent networks under targeted attack Xuqing Huang, Jianxi Gao, Sergey Buldyrev, Shlomo Havlin, H. Eugene Stanley When an initial failure of nodes occurs in interdependent networks, a cascade of failure between the networks occurs. Earlier studies focused on random initial failures. Here we study the robustness of interdependent networks under targeted attack on high or low degree nodes. We introduce a general technique which maps the targeted-attack problem in interdependent networks to the random-attack problem in a transformed pair of interdependent networks. We find that when the highly connected nodes are protected and have lower probability to fail, in contrast to single scale-free (SF) networks where the percolation threshold $p_c=0$, coupled SF networks are significantly more vulnerable with $p_c$ significantly larger than zero. The result implies that interdependent networks are difficult to defend by strategies such as protecting the high degree nodes that have been found useful to significantly improve robustness of single networks. [Preview Abstract] |
Monday, February 27, 2012 9:36AM - 9:48AM |
A54.00009: Information Spreading in Context Dashun Wang, Zhen Wen, Hanghang Tong, Ching-Yung Lin, Chaoming Song, Albert-Laszlo Barabasi Information spreading processes are central to human interactions. Despite recent studies in online domains, little is known about factors that could affect the dissemination of a single piece of information. In this paper, we address this challenge by combining two related but distinct datasets, collected from a large scale privacy-preserving distributed social sensor system. We find that the social and organizational context significantly impacts to whom and how fast people forward information. Yet the structures within spreading processes can be well captured by a simple stochastic branching model, indicating surprising independence of context. Our results build the foundation of future predictive models of information flow and provide significant insights towards design of communication platforms. [Preview Abstract] |
Monday, February 27, 2012 9:48AM - 10:00AM |
A54.00010: Power-grid Network Partitioning and Cluster Optimization with Applications to Florida and Texas Per Arne Rikvold, Ibrahim Abou Hamad, Brett Israels, Svetlana V. Poroseva Cascading power-grid failures pose serious threats to lives and property, and it is desirable to contain them within a limited geographical area. One method to achieve this is Intelligent Intentional Islanding (I3): the purposeful partitioning of a grid into weakly connected ``islands'' of closely connected generators and loads. If such islands can be quickly isolated, the spread of faults can be limited. An additional constraint is that generating capacity and power demand within each island should be closely balanced to ensure self-sufficiency. I3 thus corresponds to constrained community detection in a network. After a matrix-based initial agglomeration of nearby loads and generators, we implement Monte Carlo simulated annealing to simultaneously optimize load-balance and internal connectivity of the resulting islands. The optimized network of islands is treated as a new network with the first-generation islands as the new nodes (``supergenerators'' and ``superloads''), and the same agglomeration and MC procedures are iteratively applied, reminiscent of real-space renormalization. Applications to the Floridian [1] and Texan high-voltage grids are demonstrated.\\[4pt] [1] I.\ Abou Hamad et al., Phys.\ Proc.\ {\bf 4}, 125-129 (2010); Phys.\ Proc.\ {\bf 15}, 2-6 (2011). [Preview Abstract] |
Monday, February 27, 2012 10:00AM - 10:12AM |
A54.00011: Computational Analysis of Topological Survivability of Large-Scale Engineering Networks with Heterogeneous Nodes Svetlana Poroseva The scale and complexity of modern networks, their integration, and the size of population and businesses they have impact on, make their massive damage catastrophic for the society and economy. Such damage is usually caused by adverse events and is not considered by traditional design practices. In the modern society, the likelihood of adverse events has substantially increased. Therefore, there is a need in evaluating the ability of a network to survive such damage. As the network topology is a key factor to consider, the goal of our research is to develop computational tools for quantifying its effect on the network survivability. ``Selfish'' algorithm will be presented that addresses exponential-time complexity associated with the problem of generation and analysis of all fault combinations possible in a given network. The reduction of computational complexity is achieved by mapping an initial network topology with multiple sources and sinks onto a set of simpler smaller topologies with multiple sources and a single sink. Application to the Texas power grid will be considered. [Preview Abstract] |
Monday, February 27, 2012 10:12AM - 10:24AM |
A54.00012: Cascading failures in interdependent lattice networks: from first order to second order phase transition Wei Li, Amir Bashan, Sergey Buldyrev, Eugene Stanley, Shlomo Havlin We study a system composed of two interdependent lattice networks A and B, where nodes in network A depend on a node within a certain shuffling distance $r$ of its corresponding counterpart in network B and vice versa. We find, using numerical simulation that percolation in the two interdependent lattice networks system shows that for small $r$ the phase transition is second order while for larger $r$ it is a first order. [Preview Abstract] |
Monday, February 27, 2012 10:24AM - 10:36AM |
A54.00013: Cascading behaviors in random directed dependency networks Yanqing Hu, Amir Bashan, Shlomo Havlin, Hernan Makse Cascading behaviors have been studied only for some specific dependency network systems. In this paper, we present a more general and realistic network system with both random connectivity and directed dependency links. Using percolation approach, we obtained the universal boundaries among first order transition, second order transition and unstable regimes, which depend only on less than fourth moment of degree distribution and the fractions of zero and one directed dependency link nodes. Moreover, besides the connectivity degree distribution, we also find the final state of dynamical cascading process is determined by out degree distribution of directed dependency links, and the in degree distribution only influence cascading speed. [Preview Abstract] |
Monday, February 27, 2012 10:36AM - 10:48AM |
A54.00014: Loops in hierarchical channel networks Eleni Katifori, Marcelo Magnasco Nature provides us with many examples of planar distribution and structural networks having dense sets of closed loops. An archetype of this form of network organization is the vasculature of dicotyledonous leaves, which showcases a hierarchically-nested architecture. Although a number of methods have been proposed to measure aspects of the structure of such networks, a robust metric to quantify their hierarchical organization is still lacking. We present an algorithmic framework that allows mapping loopy networks to binary trees, preserving in the connectivity of the trees the architecture of the original graph. We apply this framework to investigate computer generated and natural graphs extracted from digitized images of dicotyledonous leaves and animal vasculature. We calculate various metrics on the corresponding trees and discuss the relationship of these quantities to the architectural organization of the original graphs. This algorithmic framework decouples the geometric information from the metric topology (connectivity and edge weight) and it ultimately allows us to perform a quantitative statistical comparison between predictions of theoretical models and naturally occurring loopy graphs. [Preview Abstract] |
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