Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q0: Meet Your Future: Industrial Careers for Physicists
Sponsoring Units: FIAPRoom: 203
Wednesday, March 5, 2014 12:30PM - 2:00PM |
Q0.00001: Meet Your Future: Industrial Careers for Physicists In this special lunchtime session, representatives from industry will provide information about physics careers in the private sector. Topics will include research opportunities for physicists in industry, strategies for successfully pursuing industrial jobs, and advice on how to thrive in this exciting and challenging work environment. [Preview Abstract] |
Session Q1: Focus Session: Charge & Energy Transfer II
Sponsoring Units: DCPChair: Xiaoyang Zhu, Columbia University
Room: 103/105
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q1.00001: Optical Excitation and Energy Relaxation in Graphene: Layer by Layer Invited Speaker: Jiwoong Park Intralayer and stacking-dependent interlayer optical excitations, and the cooling of hot electrons are the critical processes underlying the operation of exciting new graphene-based optoelectronic and plasmonic devices. In this talk, I will discuss the optical excitations and subsequent energy relaxation in single layer graphene and coupled graphene layers. We will first discuss the hot electron cooling rate near the Fermi level by measuring the electron temperature as it cools dynamically. We found that a disorder-enhanced supercollision cooling mechanism provides a complete and unified picture of energy loss near the Fermi level over the wide range of electronic (15 to $\sim$ 3000 K) and lattice (10 to 295 K) temperatures, supported by measurements done both electrically (using photocurrent thermometry) and optically. In bilayer graphene, the misorientation (or twist) angle between the two layers creates new pathways for interlayer optical excitations resulting in an interlayer optical resonance with a strongly angle-dependent energy. Our Raman, broadband absorption/reflection, and transient absorption studies on bilayer graphene samples with known twist angles confirm that the dispersion of the interlayer resonance peaks follows closely the band structure of single layer graphene; however, its energy relaxation is significantly slower. These studies may lead to new methods for customizing fundamental optical characteristics of multilayer 2D materials, including the optical conductivity, (circular) polarization dependence, and the cooling dynamics of hot carriers. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q1.00002: Understanding the charge-transfer phenomena between prototypical electron-donors and acceptors: TTF-TCNQ as an example Changwon Park, Viktor Atalla, Sean Smith, Mina Yoon It is widely accepted that the charge transfer between the conventional electron donor and acceptor molecules is independent of their relative configurations and electrons are always transferred from the molecule with the lower ionization potential, the electron-donor, to the high electron affinity molecule, the electron-acceptor. Conventional first-principles density functional theory (DFT) supports this conclusion. However, the computational results are dominated by a term in the DFT exchange-correlation functional, which often results in qualitatively and quantitatively wrong conclusion due to an artifact. In our study of prototypical electron donor-acceptor molecules, TTF-TCNQ, we show that the conventional electronic picture is not valid and the relative orientation between TTF and TCNQ is equally important as the electronic structure of the individual molecules. Our results show that the current understanding of the donor-acceptor interaction and charge transfer mechanism has to be modified. 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] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q1.00003: Fast electron transfer at molecule-substrate interfaces Guido Fratesi, Carlo Motta, Mario Italo Trioni, Daniel Sanchez-Portal, Gian Paolo Brivio The development of efficient organic electronic devices depends substantially on the electronic coupling of the molecules at interfaces and on their arrangement at the nanometer length-scale. As an example, $\pi$-conjugated electronic systems maximize their coupling to a contact when they adsorb flat. An effective molecule-substrate interaction is mandatory for solar cells where excited electrons should be collected before recombination. Core electron spectroscopies are possibly the most suitable experimental technique to access fast electron transfer times, but introduce significant perturbation on the valence orbitals by the presence of core holes and bound excitons, further calling for theoretical analysis. This talk will focus on the investigation of elastic electron transfer processes at the molecule-substrate interface based on first-principles Green's function methods. For the electronic coupling of chromophores at semiconductor interfaces we concentrate on the linewidth for electrons excited in molecular LUMO states, as occur in photovoltaic devices or in resonant core spectroscopies, and discuss the effect of core-level excited atoms on the system properties. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q1.00004: Non-adiabatic vibrational-electronic resonance in a model dimer - Implications for Photosynthetic Energy Transfer Vivek Tiwari, David Jonas Recently Tiwari et al. [PNAS (2013)] have shown that for a model donor-acceptor system resonance between a vibrational quantum of energy of a weakly coupled Franck-Condon vibration and the excited state excitonic energy gap leads to an unavoidable nested energy funnel on the excited state of photosynthetic antennas. Anti-correlated nuclear motions on the two pigments are responsible for such non-adiabatic effects. Here we show that several vibrational modes lying close to an excitonic energy gap in the FMO antenna complex, and a finite width of vibrational-electronic resonance lead to an even stronger non-adiabatic vibrational-electronic mixing along a generalized energy tuning coordinate. Such a generalized tuning coordinate is similar to the ``tuning coordinate'' in a conical intersection. The 2D spectroscopic signatures of the resulting non-adiabatic effects are additive and lead to more than 2x enhancement of ground state anti-correlated vibrational wavepackets which are expected to dominate the long-lived 2D signatures. Thus, several near-resonant vibrations and a finite width of non-adiabatic coupling render the nested energy funnel in the FMO antenna as a robust and promising design principle for artificial energy and charge transport. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q1.00005: Dynamics of electron transfer and exciton formation at interfaces Invited Speaker: Martin Wolf The combination of inorganic semiconductors with organic molecules to hybrid systems promises superior functionality of the interface compared to optoelectronic properties of the single materials. We have investigated the electron dynamics of the ZnO(10-10) surface and the influence of hydrogen and several organic molecules on the electronic structure using time-resolved two-photon-photoemission (2PPE) spectroscopy. Hydrogen termination leads to the formation a metallic ZnO surface, whereas e.g. by pyridine adsorption a substantial work function reduction up to 2.9 eV is achieved, which can be useful controlling the energy level alignment at inorganic/organic interfaces. Furthermore, we directly monitor the hot electron relaxation in the ZnO conduction band and the formation of an excitonic state at the surface within a few ps, which decays mediated a thermal activated process on a 100 ps timescale. In a second set of experiments we have studied the dynamics of photoinduced electron transfer and solvation processes at the water ice-metal interface and the effect of co-adsorbed alkali ions (Na, K, Cs). Time-resolved 2PPE provides direct access to elementary processes like electron injection and the subsequent solvation dynamics which competes with the electron transfer back to the Cu(111) substrate. In particular, we study the electronic structure changes and ultrafast dynamics for the bulid-up of a solvation shell (up to about 6 water molecules) around individual alkali atoms at the metal surface. For ice mulitlayers doped with alkali ions we observe the formation of longlived electron alkali-water complexes. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q1.00006: Modeling recombination processes and predicting energy conversion efficiency of dye sensitized solar cells from first principles Wei Ma, Sheng Meng We present a set of algorithms based on solo first principles calculations, to accurately calculate key properties of a DSC device including sunlight harvest, electron injection, electron-hole recombination, and open circuit voltages. Two series of D-$\pi $-A dyes are adopted as sample dyes. The short circuit current can be predicted by calculating the dyes' photo absorption, and the electron injection and recombination lifetime using real-time time-dependent density functional theory (TDDFT) simulations. Open circuit voltage can be reproduced by calculating energy difference between the quasi-Fermi level of electrons in the semiconductor and the electrolyte redox potential, considering the influence of electron recombination. Based on timescales obtained from real time TDDFT dynamics for excited states, the estimated power conversion efficiency of DSC fits nicely with the experiment, with deviation below 1-2{\%}. Light harvesting efficiency, incident photon-to-electron conversion efficiency and the current-voltage characteristics can also be well reproduced. The predicted efficiency can serve as either an ideal limit for optimizing photovoltaic performance of a given dye, or a virtual device that closely mimicking the performance of a real device under different experimental settings. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q1.00007: Tunable Band Edges of TiO2 via Functionalization with Phosphonic Acid Adsorbates Guo Li, Jessy Rivest, Ian Sharp, Delia Milliron, Jeffrey Neaton The deliberate design of semiconductor surfaces with band edge energies optimal for electro- or photoelectrochemical applications is a grand challenge. We examine the extent that the band edges of anatase TiO2(101) can be effectively tuned via molecular adsorption on its surface. Using density functional theory, we compute TiO2 band edge energies for a series of phosphonic acid molecules, whose intrinsic dipole moments are significantly different in both magnitude and direction. The results reveal that the molecule-substrate binding leads to a large induced dipole moment, and the induced dipole upon adsorption varies with the binding nature and configuration. Repulsive dipole-dipole interactions between molecules lead to a striking coverage-dependence of the effective dipole moments. Interestingly, computed band edge shifts in TiO2 are in excellent agreement with the experimentally measured work-function changes. Implications for the role of such adsorbates on photoelectrochemical devices will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q1.00008: A novel theoretical probe of the SrTiO$_3$ surface under water-splitting conditions Kendra Letchworth-Weaver, Deniz Gunceler, Tom\'as Arias, Manuel Plaza, Xin Huang, Joel Brock, Joaquin Rodriguez-L\'opez, Hector Abru\~na Understanding the reaction mechanisms required to generate hydrogen fuel by photoelectrolysis of water is essential to energy conversion research. These reaction pathways are strongly influenced by the geometry and electronic structure of the electrode surface under water-splitting conditions. Electrochemical microscopy has demonstrated that biasing a SrTiO$_3$ (001) surface can lead to an increase in water-splitting activity. {\it In operando} X-ray reflectivity measurements at the Cornell High Energy Synchrotron Source (CHESS) correlate this increase in activity to a significant reorganization in the surface structure but are unable to determine the exact nature of this change. Joint Density-Functional Theory (JDFT), a rigorous yet computationally efficient alternative to molecular dynamics, provides a quantum-mechanical description of an electrode surface in contact with an aqueous environment, and a microscopically detailed description of the interfacial liquid structure. Our JDFT calculations determine the structure of the activated SrTiO$_3$ surface and explore why it is correlated with higher activity for water splitting. With no empirical parameters whatsoever, we predict the X-ray crystal truncation rods for SrTiO$_3$, finding excellent agreement with experiment. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:30PM |
Q1.00009: Efficient Plasmon-Induced Hot Electron Transfer and Photochemistry in Semiconductor-Au Nanorod Heterostructures Invited Speaker: Tianquan Lian In recent years, it has been shown that excitation of plasmons in metal nanostructures can lead to the injection of hot electrons into semiconductors and enhanced photochemistry. This novel plasmon-exciton interaction mechanism suggests that plasmonic nanostructures can potentially function as a new class of widely tunable and robust light harvesting materials for photo-detection or solar energy conversion. However, plasmon-induced hot electron injections from metal to semiconductor or molecules are still inefficient because of the competing ultrafast hot electron relaxation (via ultrafast electron-electron and electron-phonon scattering) processes within the metallic domain. In this paper we discuss a recent study on the plasmon-exciton interaction mechanisms in colloidal quantum-confined epitaxially-grown semiconductor-gold plexcitonic nanorod heterostructures. Using transient absorption spectroscopy, we show that optical excitation of plasmons in the Au tip leads to efficient hot electron injection into the semiconductor nanorod. In the presence of sacrificial electron donors, this plasmon induced hot electron transfer process can be utilized to drive photoreduction reactions under continuous illumination. Ongoing studies are examining how to further improve the plasmon induced hot electron injection efficiency through controlling the size and shape of the plasmonic and excitonic domains. [Preview Abstract] |
Session Q2: Focus Session: Surface Chemistry and Catalysis VI
Sponsoring Units: DCPChair: J.R. Schmidt, University of Wisconsin-Madison
Room: 102
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q2.00001: Ligand-Mediated Synthesis of Colloidal Nanoparticle Alloys Invited Speaker: Jill Millstone Small molecule ligand chemistry is used to mediate the incorporation and distribution of metals in and on discrete, colloidal nanoparticle substrates. Specifically, we examine the case of late d transition metals in Au and Pt hosts. The resulting structures are characterized by a variety of methods including X-ray absorption spectroscopy, electron microscopy, and photoelectron spectroscopy techniques. These multimetallic nanoparticles exhibit previously unobserved mixtures of metals such as continuously tunable Au-Co composition ratios, as well as unique physical properties including composition-tunable near-infrared photoluminescence. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q2.00002: The Double-edged Impact of Platinum Nano-Deposits on the Durability of Polymer Electrolyte Membranes --- A Theoretical Study Mohammad Javad Eslamibidgoli, Pierre-\'Eric Alix Melchy, Ata Roudgar, Michael H. Eikerling The attack of oxygen radicals is one of the main sources of chemical degradation in the polymer electrolyte membranes (PEM) of polymer electrolyte fuel cells. In this context, Pt in the membrane (PITM) that originates from Pt degradation in the cathode catalyst layer plays a double-edged role: surface reactions at PITM could facilitate the formation or quenching of radicals. The balance of these processes depends on the local electrochemical conditions, determined by thermodynamic parameters and local composition of the PEM. The objective of this work is to explore the equilibrium and kinetics of radical reactions at PITM as a function of local PEM conditions. We first determine the potential distribution of PITM based on a continuum model of crossover of reactant gases coupled with their local electrochemical reactions at Pt. Secondly, we determine the surface state of Pt for the given local potential using relevant experimental data and kinetic models of surface reactions at Pt [1, 2]. Lastly, we use this information as input for \textit{ab initio} calculations at the DFT level of specific processes involved in the radical balance at the Pt $|$ water interface.\\[4pt] [1] M. Wakisaka et al. Langmuir, 25, (2009)\\[0pt] [2] S. G. Rinaldo et al. submitted to Nat. Mater. (2013) [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q2.00003: Computational modeling of the surface properties of uranium dioxide using hybrid DFT Megan Hoover, Raymond Atta-Fynn Computational modeling of processes such as corrosion and the interaction of environmental impurities with the surfaces of actinide materials are important to the understanding of remediation processes for actinide-based nuclear waste from the biosphere. However, accurate modeling of bare surfaces of actinide materials is a necessary precursor to accurate modeling of surface interactions. This talk will be focused on atomistic modeling of uranium dioxide (UO$_{2})$ surfaces. The theoretical formalism is all-electron hybrid Density Functional Theory (DFT) based on the full-potential linearized augmented plane wave plus local basis method. Specifically, we computed the surface energies, work functions, incremental energies, and electronic band gaps for periodic slab structures for the (110) and (111) surfaces of UO$_{2}$. We observed that the anti-ferromagnetic semiconducting behavior in the bulk structure is retained in the surface structures. The convergence of surface properties with respect to slab thickness will be discussed. The trends in the surface electronic structures, particularly the localized behavior of the U 5$f$ electrons, in comparison with the bulk structure will also be elucidated. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q2.00004: Stability and metastability of clusters in a reactive atmosphere: theoretical evidence for unexpected stoichiometries of Mg$_M$O$_x$ Saswata Bhattacharya, Sergey Levchenko, Luca Ghiringhelli, Matthias Scheffler In heterogeneous catalysis, materials function at finite temperature and in an atmosphere of reactive molecules at finite pressure. As a first step towards understanding the catalytic behavior of metal-oxide clusters, we study the $(T,p)$ dependence of the composition, structure, and stability of the various isomers for each size $M$ of Mg$_M$O$_x$ clusters in an oxygen atmosphere. The calculations are performed via a massively parallel genetic algorithm in a cascade approach. With the term ``cascade'', we identify a multistep procedure in which successive steps employ higher levels of theory, with each next level using information obtained at the lower level. We find that small clusters ($M < 5$) are in thermodynamic equilibrium when $x>M$. The non-stoichiometric clusters exhibit peculiar magnetic behavior, suggesting the possibility of tuning magnetic properties by changing environmental pressure and temperature conditions. Furthermore, we show that density-functional theory (DFT) with a hybrid exchange-correlation (xc) functional is needed for predicting accurate phase diagrams of metal-oxide clusters. Neither a (sophisticated) force field nor DFT with (semi)local xc functionals are sufficient for even a qualitative prediction. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q2.00005: Density Functional Theory Studies of Sb(V) Oxyanion Adsorption on $\alpha$-Al$_2$O$_3$ 1$\overline{1}$02 surfaces Sai Kumar Ramadugu, Sara Mason We apply density functional theory and periodic slab models to understand the adsorption and reactivity of Sb(OH)$_6^{-1}$ on various surface terminations of $\alpha$-Al$_2$O$_3$ (1$\overline{1}$02). On all surfaces studied, Sb(V) adsorption is preferred when adsorbed in a bidentate, corner-sharing geometry. On the other hand, all theoretical bidentate edge-sharing modes are relatively high in energy. This is in good agreement with recent experiments that do not observe any bidentate edge-sharing adsorption of Sb(OH)$_6^{-1}$ on hydrous aluminum oxide. We identify factors that govern the preference for corner-sharing adsorption, and compare how well the surface reactivity can be predicted based on periodic DFT {\it vs.} quantum chemical cluster models. From the DFT projected density of states, we clearly show how strained ligands of Sb(V) cannot achieve good overlap with the electronic states of the surface, even at Sb-O distances predicted to be ideal by empirical bond-valence. The results thus provide a molecular-level understanding of Sb(V) specific adsorption and also allow us to assess other modeling approaches to computational geochemical surface science. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q2.00006: Artificial Photosystem I and II: Highly Selective solar fuels and tandem photocatalysis Yuchen Ding, Ignacio Castellanos, Logan Cerkovnik, Prashant Nagpal Artificial photosynthesis, or generation of solar fuels from CO$_{2}$/H$_{2}$O, can provide an important alternative for rising CO$_{2}$ emission and renewable energy generation. In our recent work, composite photocatalysts (CPCs) made from widebandgap nanotubes and different QDs were used to mimic Photosystem II (PS680) and I (PS700), respectively. By tuning the redox potentials using the size, composition and energy band alignment of QDs, we demonstrate highly selective (\textgreater 90{\%}) and efficient production of ethane, ethanol and acetaldehyde as solar fuels with different wavelengths of light. We also show that this selectivity is a result of precise energy band alignments (using cationic/anionic doping of nanotubes, QD size etc.), confirmed using measurements of electronic density of states, and alignment of higher redox potentials with hot-carriers can also lead to hot-carrier photocatalysis. This wavelength-selective CPCs can have important implications for inexpensive production of solar fuels including alkanes, alcohols, aldehydes and hydrogen, and making tandem structures (red, green, blue) with three CPCs, allowing almost full visible spectrum (410 $\sim$ 730nm) utilization with different fuels produced simultaneously. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q2.00007: K$^{+}$-hydration in a low-energy two-dimensional wetting layer on the basal surface of muscovite Peter J. Feibelman Density Functional Theory points to a key role of K$^{+}$ solvation in the low-energy two-dimensional arrangement of water molecules on the basal surface of muscovite. At a coverage of 9 water molecules per 2 surface potassium ions, there is room to accommodate the ions into wetting layers wherein half of them are hydrated by 3 and the other half by 4 water molecules, with no broken H-bonds, or wherein all are hydrated by 4. Relative to the ``fully connected network of H-bonded water molecules'' that M. Odelius \textit{et al.}. [Phys. Rev. Lett. \textbf{78}, 2855-- 2858 (1997)] found to form ``a cage around the potassium ions,'' the hydrating arrangements are several tens of meV/H$_{\mathrm{2}}$O better bound. Thus, low-temperature wetting on muscovite is not driven towards ``ice-like'' hexagonal coordination. Instead, solvation forces dominate.\\[4pt]Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q2.00008: Conversion of solar into chemical energy on plasmonic metal nanostructures Invited Speaker: Suljo Linic We will show that composite photo-catalysts combing plasmonic metallic nanoparticles of noble metals and semiconductor nanostructures exhibit improved photo-chemical activity compared to conventional photo-catalytic materials [1,2]. We will also show that plasmonic silver nanoparticles, optically excited with low intensity visible light, exhibit direct photo-catalytic activity in a number of oxidation reactions. We will discuss underlying mechanisms associated with these phenomena and predictive models that can capture the outcome of chemical transformations on these materials [2-4]. We propose that this new family of plasmonic metal photo-catalysts could prove useful for many heterogeneous catalytic processes that cannot be activated using conventional thermal processes on metals or photo-catalytic processes on semiconductors. I will show an example of such a process [5]. \\[4pt] [1] D. B. Ingram, S. Linic, \textbf{JACS,} 133, 5202, 2011.\\[0pt] [2] Suljo Linic, Phillip Christopher and David B., \textbf{Nature Materials,} \textbf{10}, 911, 2011.\\[0pt] [3] Ingram P. Christopher, H. Xin, S. Linic, \textbf{Nature Chemistry,} 3, 467, 2011.\\[0pt] [4] P. Christopher, H. Xin, M. Andiappan, S. Linic, \textbf{Nature Materials}, 11, 1044, 2012.\\[0pt] [5] M. Andiappan, J. Zhang, S. Linic, \textbf{Science,} 339, 1590, 2013 [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q2.00009: Impact of the vapour pressure of water on the equilibrium shape of ZnO nanoparticles: An ab-initio study Stephane Kenmoe, Mira Todorova, P. Ulrich Biedermann, Joerg Neugebauer ZnO powders and nanoparticles are used as catalysts and have potential applications in gas-sensing and solar energy conversion. A fundamental understanding of the exposed crystal facets, their surface chemistry and stability as function of environmental conditions is essential for rational design and improvement of synthesis and properties. Using density-functional theory calculations we study the adsorption of water on the non-polar low-index $(10\bar{1}0)$ and $(11\bar{2}0)$ surfaces of ZnO. Observing both molecular and dissociative H$_2$O adsorption, we analyse the contributions of water-surface and water-water interactions to the energies of the stable structure. Based on this insight we compute and analyse the impact of water adsorption on surface energies and the equilibrium shape of nanoparticles in a humid environment. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q2.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q2.00011: Asymmetric Wettability Directs Leidenfrost Droplets Rebecca Agapov, Jonathan Boreyko, Dayrl Briggs, Bernadeta Srijanto, Scott Retterer, C. Patrick Collier, Nickolay Lavrik Exploration of Leidenfrost droplets on nano- and microstructured surfaces are of great importance for increasing control over heat transfer in high power density systems using boiling phenomena. They also provide an elegant way to direct droplet motion in a variety of emerging fluidic systems. Here, we report the fabrication and characterization of tilted nanopillar arrays (TNPAs) that exhibit directional Leidenfrost water droplets under dynamic conditions. The batch fabrication of the TNPAs was achieved by glancing-angle anisotropic reactive ion etching of a thermally dewet platinum mask. In contrast to previously implemented macro- and microscopic Leidenfrost ratchets, our TNPAs induce \textit{no} preferential directional movement of Leidenfrost droplets under conditions approaching steady-state film boiling. This suggests that the observed droplet directionality is not a result of asymmetric vapor flow. Phase diagrams were constructed for the boiling behavior upon droplet impact onto TNPAs, straight nanopillar arrays, and smooth silicon surfaces. Asymmetric wettability and directional trajectory of droplets was exclusive to the TNPAs for impacts corresponding to the transition boiling regime, revealing this to be the mechanism for the droplet directionality. [Preview Abstract] |
Session Q3: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments II
Sponsoring Units: DCPChair: Francesco Paesani, University of California, San Diego
Room: 107
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q3.00001: Water in Room Temperature Ionic Liquids Invited Speaker: Michael Fayer Room temperature ionic liquids (or RTILs, salts with a melting point below 25 $^{\circ}$C) have become a subject of intense study over the last several decades. Currently, RTIL application research includes synthesis, batteries, solar cells, crystallization, drug delivery, and optics. RTILs are often composed of an inorganic anion paired with an asymmetric organic cation which contains one or more pendant alkyl chains. The asymmetry of the cation frustrates crystallization, causing the salt's melting point to drop significantly. In general, RTILs are very hygroscopic, and therefore, it is of interest to examine the influence of water on RTIL structure and dynamics. In addition, in contrast to normal aqueous salt solutions, which crystallize at low water concentration, in an RTIL it is possible to examine isolated water molecules interacting with ions but not with other water molecules. Here, optical heterodyne-detected optical Kerr effect (OHD-OKE) measurements of orientational relaxation on a series of 1-alkyl-3-methylimidazolium tetrafluoroborate RTILs as a function of chain length and water concentration are presented. The addition of water to the longer alkyl chain RTILs causes the emergence of a long time bi-exponential orientational anisotropy decay. Such decays have not been seen previously in OHD-OKE experiments on any type of liquid and are analyzed here using a wobbling-in-a-cone model. The orientational relaxation is not hydrodynamic, with the slowest relaxation component becoming \textit{slower} as the viscosity \textit{decreases} for the longest chain, highest water content samples. The dynamics of isolated D$_{2}$O molecules in 1-butyl-3-methylimidazolium hexafluorophosphate (BmImPF$_{6}$) were examined using two dimensional infrared (2D IR) vibrational echo spectroscopy. Spectral diffusion and incoherent and coherent transfer of excitation between the symmetric and antisymmetric modes are examined. The coherent transfer experiments are used to address the nature of inhomogeneous broadening by observing $\sim$ 100 fs time scale oscillations in the shape of the 2D IR spectra. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q3.00002: Comparative Study of the Intermolecular Dynamics of Benzene/Ionic Liquid Mixtures and Benzyl Functionalized Ionic Liquids: Femtosecond OKE Spectroscopic Measurements Edward Quitevis, Lianjie Xue, George Tamas Ionic liquids (ILs) are salts with melting points below 100 $^{\circ}$C that are comprised of an organic cation and an inorganic or organic anion. There is great interest in obtaining a molecular level understanding of their unique physical and chemical properties, of which one of them is their ability, despite being inherently polar liquids, to dissolve large quantities of nonpolar aromatic compounds. In order to understand further the solvation of aromatic molecules in ILs, we have performed optical effect (OKE) spectroscopic measurements on 1-benzyl-3-methylimidazolum bistriflate, 1,3-dibenzylimidazolum bistriflate and the corresponding 1:1 and 2:1 benzene/1,3-dimethylimazolium bistrifate (C$_{6}$H$_{6}$/C$_{1}$C$_{1})$ mixtures. In contrast to being free in benzene/IL mixtures, the benzene rings are tethered to the imidazolium ring via methylene linkages in the case of first two ILs. The intermolecular Kerr spectra indicate that the motion of the benzene rings becomes increasingly more restricted in going from neat benzene to benzene dissolved in 1,3-dimethylimazolium bistrifate to benzene rings tethered to the imidazolium ring. This restriction causes the Kerr spectra effectively to shift to higher frequency in going from neat liquid benzene to C$_{6}$H$_{6}$/C$_{1}$C$_{1}$ mixtures to benzylimidazolium ILs. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q3.00003: Solvation dynamics in room temperature ionic liquids studied by ultrafast vibrational spectroscopy Sean Garrett-Roe, Zhe Ren, Duane Couchot-Vore, Thomas Brinzer Room temperature ionic liquids are a challenging new area for understanding solvation dynamics. These solvent systems are liquids with a delicate balance of electrostatic, dispersion, and hydrogen-bonding forces which lead to complex structure and dynamics on many time- and length-scales. Here we probe the dynamics of thiocyanate ions in several imidazolium bis(trifluoromethylsulfonyl)amide ionic liquids from femtoseconds to 100 ps using ultrafast vibrational spectroscopy. Two-dimensional infrared (2D-IR) spectroscopy of thiocyanate ions detects both intertial motion (on the hundreds of femtosecond timescale) as well as slower, diffusive motions (on the tens of picosecond timescale). The 2D-IR experiments show that the rate of fluctuation of the electrostatic environment around the thiocyanate is sensitive to hydrogen bonding at the 2-position of the imidazolium ring, depends mildly on water concentration, changes with counter-ion, and is roughly independent of the thiocyanate concentration (up to 30 mM). The results are compared to ab initio simulations which predicted a 10 -- 15 picosecond hydrogen bond lifetime. The implications for topics such as the concept of ionicity, the effect of hydrogen bonding on viscosity, and structural and dynamical heterogeneity will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q3.00004: Comparative Study of the Intermolecular Dynamics and Physical Properties of Branched and Linear Alkyl Chain Imidazolium Ionic Liquids Lianjie Xue, Fehmi Bardak, George Tamas, Eshan Gurung, Edward Quitevis, Yung Koh, Sindee Simon The optical Kerr effect (OKE) spectra, densities, viscosities, and transition temperatures of 1-alkyl-3-methylimidazolium bis\textbraceleft (trifluoromethane)sulfonyl\textbraceright amide ionic liquids (ILs) with branched alkyl chains, -C$_{\mathrm{n-3}}$CH(CH$_{3})_{2}$ (branched ILs), were measured and compared to those with linear alkyl chains, -C$_{\mathrm{n-1}}$CH$_{3}$ (linear ILs), for $n$ $=$ 3, 4, 5, 6 and 7. The results show that a branched IL has a higher viscosity and transition temperature T$_{\mathrm{g}}$ than the corresponding linear IL with the same n, whereas the densities of each branched/linear IL pair are the same within experimental error. For short alkyl chains (n$=$3 and 4) the intermolecular part of the OKE spectrum of the branched ILs tends to be narrower and lower in frequency than that of the linear ILs. This suggests that branching softens the intermolecular modes. For long alkyl chains (n$=$5-7), the difference between the intermolecular spectrum of the branched IL and that of the linear IL with the same n decreases, which indicates that the branching effect becomes smaller when the alkyl chains get longer. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q3.00005: Structure and Dynamics of Nanoconfined Liquids Invited Speaker: Ward Thompson Dramatic changes in the molecular-level structure and dynamics of liquids occur upon nanoscale confinement in materials ranging from sol-gels to reverse micelles. A number of vibrational and electronic spectroscopic techniques are routinely used to probe the behavior of such nanoconfined liquids. However, in general, what information is contained in the spectra of confined liquids and how the complex molecular-level structural and dynamical properties can be extracted is still an open question. This issue will be discussed in the context of molecular dynamics simulations by examining the affected liquid properties and, when possible, the predicted spectroscopic signals for mesoporous amorphous silica systems. These confining frameworks have been relatively well characterized experimentally, present different surface chemistries, and are sufficiently transparent to permit the study of their contents by a variety of spectroscopies. A particular focus will be the molecular-level origins of the modified liquid behaviour, including those relevant to chemical processes, {\em e.g.}, reorientational and hydrogen-bond dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q3.00006: Confinement effects on collective water dynamics: Molecular dynamics study of optical Kerr response in silica nanopores Anatoli Milischuk, Branka Ladanyi We report the results of the study of the effects of confinement on collective dynamical properties of water in model nanopores at ambient conditions. The main focus is on approximately cylindrical pores composed of amorphous silica, with diameters ranging from 20 to 40 {\AA}, designed to represent MCM-41 materials. Results for hydrophilic and hydrophobic pores of similar dimensions, but with roughness reduced compared to silica nanopores, are also considered. The main quantity studied is the polarizability anisotropy time correlation function (TCF), which is related to the experimentally-observed optical Kerr effect (OKE) nuclear response. We investigate the effects on this TCF of the reduced molecular translational and rotational water mobility in the layers near the interface. We find that these effects lead to pore diameter dependent slowdown of polarizability anisotropy relaxation, in agreement with OKE experiments. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q3.00007: A Molecularly-Based Theory for Electron Transfer in Polar Solvents Bilin Zhuang, Zhen-Gang Wang Using field-theoretic methods, we develop a mean-field theory for charge solvation in equilibrium and nonequilibrium conditions, and apply it to study electron transfer reactions. The resulting dipolar mean-field theory (DMFT) considers the effects of solvent dipole moments and polarizabilities, and consists of simple sets of equations for the equilibrium and nonequilibrium conditions as well as an analytical expression for the free energy. With no adjustable parameters, the DMFT predicts the activation and the reorganization energies in good agreement with previous data. We have shown that, as DMFT is able to describe the solvent properties in the immediate vicinity of the charges, it is unnecessary to distinguish the inner-sphere and the outer-sphere solvent molecules in the calculation of reorganization energy. Furthermore, we examine the nonequilibrium free energy surfaces of electron transfer, and find that the parabolic approximation is not applicable for reactions with a large free energy change. In the case of solvent mixtures, we show that there are three classes of solvent structures around the ions, and the solvent structure is a key factor determining how the activation energy of electron transfer in a solvent mixture compares to those in the pure solvent components. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q3.00008: Polar nanoregions in water - a study of the dielectric properties of TIP4P/2005, TIP4P2005f and TTM3F Daniel Elton, Marivi Fernandez-Serra Using molecular dynamics simulation we present a critical comparison of the dielectric properties of three models of water - TIP4P/2005, TIP4P/2005f and TTM3F. Dipole spatial correlation is measured using the distance dependent Kirkwood function along with one dimensional and two dimensional dipole correlation functions. We find that the introduction of flexibility alone does not significantly affect dipole correlation and only affects $\varepsilon(\omega)$ at high frequencies. By contrast the introduction of polarizability increases dipole correlation and yields a more accurate $\varepsilon(\omega)$. Additionally the introduction of polarizability creates temperature dependence in the dipole moment even at fixed density, yielding a more accurate value for $d \varepsilon / d T$ compared to non-polarizable models. To understand the physical origin of the dielectric properties of water we make analogies to the physics of polar nanoregions (PNRs) in relaxor ferroelectric materials. We show that $\varepsilon(\omega,T)$ and $\tau_D(T)$ for water have striking similarities with relaxor ferroelectrics, a class of materials characterized by large frequency dispersion in $\varepsilon(\omega,T)$, VFT behaviour in $\tau_D(T)$, and the existence of PNRs. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q3.00009: Systematic investigation of the electronic and structural properties of chloride ion in aqueous solution by advanced density functional Arindam Bankura, Charles Swartz, Michael L. Klein, Xifan Wu In a recent photoelectron spectroscopy (PES) experiment,\footnote{B. Winter et al. J. Am. Chem. Soc., \textbf{128}, 3864 (2006)} electron binding energies have been measured in aqueous chloride ion solutions. The position of the highest occupied molecular orbital (HOMO) of the chloride ion was found to be 1.25-1.50 eV above with respect to the valence band maximum (VBM) of water. Theoretically, we have computed the PES for the aqueous chloride ion solutions, in which the molecular solvation structures are generated from the {\it ab initio} molecular dynamics using gradient-corrected (PBE) and hybrid density functional (PBE0).\footnote{X. Wu et al. Phys. Rev. B, \textbf {79}, 085102 (2009)} Using PBE level of theories we consistently found that HOMO level of the chloride ion on average below the VBM of the water. Whereas the HOMO of the anion was found above the VBM of water when the electronic structure calculations were carried out at the PBE0 level of theories. A substantial improvement in the result was found when the trajectory was generated using the corrections accounting for the effects of dispersion forces into the DFT-GGA scheme)\footnote{A. Tkatchenko et al. Phys. Rev. Lett., \textbf {102}, 073005 (2009)}. and hybrid density functional. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q3.00010: Relaxation Dynamics of the Solvated Electron in Water, Methanol and Ethanol Madeline H. Elkins, Holly L. Williams, Daniel M. Neumark The solvated electron, an isolated electron in solution, is of fundamental interest to the study of solvation. Lacking nuclear degrees of freedom, this highly reactive transient can act as a simple probe of solute-solvent interaction and condensed phase, non-adiabatic dynamics. We present time resolved photoelectron spectra of solvated electrons in water, methanol and ethanol microjets. Unlike prior results from transient absorption (TA) experiments in bulk\footnote{K. Yokoyamma et al., J. Phys. Chem. A, \textbf{102}, 6957 (1998)} or extrapolated from time resolved photoelectron spectroscopy (TRPES) of cluster anions\footnote{A. Bragg et al. Science, \textbf{306}, 669 (2004)}, our technique allows for state specific assignment of the individual features without relying on extrapolation methods. In water, our results reproduce the relaxation timescales found in the TA experiments and provide convincing support of the so called ``non-adiabatic'' mechanism. These results suggest that both binding energies and relaxation dynamics can be extrapolated from water cluster anion experiments; however, results in methanol and ethanol provide a qualitative agreement with TA experiments but a more complex story with regard to anionic cluster TRPES. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q3.00011: Quantum diffusion of atomic hydrogen isotopes in water David Bartels, Jonathan Walker Atomic hydrogen or deuterium radicals can readily be generated in water with a high energy electron beam, and studied using time-resolved electron paramagnetic resonance (EPR). The light isotope muonium, formed from an electron orbiting a positive muon (mass $=$ 0.11 amu), can also be studied by muonium spin resonance (MuSR) at facilities where muon beams are generated. It is to be expected that the diffusion of these isotopes would depend on their mass, and particularly in the case of muonium there should be effects of quantum zero point energy. The ring polymer molecular dynamics (RPMD) simulation method has already been applied to predict a large reduction of diffusion coefficient for muonium relative to hydrogen atoms$^{\mathrm{1}}$. In the present work, we present both EPR and MuSR measurements of spin exchange rate between the atomic hydrogen isotope and (Ni$^{\mathrm{2+}})_{\mathrm{aq}}$ ions. The spin exchange is a diffusion-limited process, and so should directly indicate the relative diffusion rates of the atomic isotopes. Surprisingly, the muonium diffusion appears to be more classical than quantum in character. New RPMD simulations with a quantized water model will be presented to model the experimental result. .(1) Markland, T. E.; Habershon, S.; Manolopoulos, D. E. \textit{J. Chem. Phys.} \textbf{2008}, \textit{128}, 194506. [Preview Abstract] |
Session Q4: Focus Session: Pyrochlore Magnets: Ordering and Freezing
Sponsoring Units: GMAGChair: Michel Gingras, University of Waterloo
Room: 112/110
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q4.00001: Effects of interaction strength on the ground states of magnetically frustrated pyrochlores Alannah Hallas, Angel Arevalo Lopez, Haidong Zhou, Graeme Luke, Christopher Wiebe The pyrochlore oxides are ubiquitous in frustrated magnetism because of their diverse array of novel magnetic ground states. The magnetism in these materials can be probed by the application of chemical pressure, that is, by varying the size of the non-magnetic ion. Germanium and lead are, respectively, the smallest and largest possible B-site cations for the pyrochlore lattice. We present thermodynamic and magnetization measurements on five new materials A$_2$Ge$_2$O$_7$ (A = Yb, Er, Tb) and A$_2$Pb$_2$O$_7$ (A = Pr, Nd). The strength of the magnetic interactions in these materials is strongly enhanced for the germanium pyrochlores and significantly reduced in the case of the lead pyrochlores. Comparison to the well-studied titanium and tin pyrochlores (B = Ti, Sn) provide context for our results. Our measurements reveal that, in some cases, the magnetic ground state for a given magnetic ion is stable against changes in chemical pressure. In other cases, varying the chemical pressure for a given magnetic ion gives rise to a distinctly different magnetic ground state. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q4.00002: Physical Properties of Ni$_{2}$GeO$_{4}$ Spinel Perturbed by Magnetic Dilution and Applied Pressure Jory Korobanik, Fereidoon Razavi Geometrically frustrated magnetic systems have yielded an interesting and rich playground for physicists. Recently, a new disordered low temperature state was discovered in the frustrated pyrochlore type Ho$_{2}$Ti$_{2}$O$_{7}$ which is termed spin ice [1]. This phase is the magnetic analog to water ice with local spin disorder replacing proton disorder. Geometric frustration arises when nearest neighbor exchange interactions cannot be simultaneously satisfied resulting in large macroscopic degeneracy. This has the effect of suppressing Neel ordering temperature [2]. This work seeks to understand the effects of applied pressure and magnetic dilution to the frustrated spinel Ni$_{2}$GeO$_{4}$. The parent material undergoes two closely spaced ordering events at T$_{1}=$12.1K and T$_{2} =$ 11.4K. [3] Upon dilution a downward shift in the ordering temperatures is observed with a destruction of the lower T2 transition. Heat capacity, AC and DC magnetometry are used to probe the changes in physical properties. \\[4pt] [1] Harris \textit{et al}, \textit{Phys. Rev. Lett. }\textbf{\textit{79,}}\textit{ 2554 (1997).}\\[0pt] [2] J. Greedan, \textit{J. Mater. Chem}., 11, 37-53 (2001).\\[0pt] [3] Lashley \textit{et al}, \textit{Phys. Rev. B} \textbf{78}, (2008). [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q4.00003: Half-Magnetization Plateau of a Dipolar Spin Ice in a [100] Field Ying-Jer Kao, Sheng-Ching Lin We report here numerical results of the low-temperature behavior of a dipolar spin ice in a magnetic field along the [100] direction. Tuning the magnetic field, the system exhibit a half-magnetization plateau at low temperature. This half-polarized phase should correspond to a quantum solid phase in an effective 2D quantum boson model, and the transition from the Coulomb phase with a power-law correlation to this state can be regarded as a superfluid to a quantum solid transition. We discuss possible experimental signatures of this half-polarized state. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q4.00004: Geometric Frustration with Disorder Nayoon Woo, Daniel M. Silevitch, Thomas F. Rosenbaum We study the effects of Nd doping on the geometrically-frustrated Heisenberg antiferromagnet Gadolinium Gallium Garnet (GGG), using linear and nonlinear ac magnetic susceptibility. Doping levels from 0.1 to 1 percent Nd alleviate the intrinsic frustration of pure GGG and elevate the ordering temperature compared to the pure material. We use nonlinear pump-probe magnetic susceptometry to examine cluster dynamics for both the pure and the doped series. At low frequency ($\sim$10 Hz), spectral hole burning is possible, indicating the presence of spin clusters with discrete energy levels largely decoupled from the overall spin bath. At kHz, we find a Fano resonance, revealing scattering pathways between spin cluster excitations and the bath. We trace the evolution of this resonance behavior as a function of dopant concentration. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q4.00005: ${\rm LiHo_xY_{1-x}F_4}$ in the highly-diluted limit Juan Carlos Andresen, Moshe Schechter, Vadim Oganesyan, Helmut G. Katzgraber The rare-earth material ${\rm LiHo_xY_{1-x}F_4}$ has attracted much attention recently, not only because it is well described by a long-range dipolar Ising model, but also because it has a rich phase diagram in the temperature--concentration plane that makes it especially interesting to explore exotic magnetic phenomena. The existence of a spin-glass phase in this material has been a long-standing controversy. In particular, it is unclear if the spin-glass phase extends to the low-concentration limit, or if an exotic anti-glass state emerges. Using large-scale Monte Carlo simulations we probe this difficult regime of the phase diagram. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q4.00006: Study on the magnetic properties of ${\mathrm{Y}_{2-x}\mathrm{Bi}_{x}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ by ${\mu}$SR and DC susceptibility T. Medina, T.J. Williams, T.J. Munsie, R.M. D'Ortenzio, L. Liu, B.J. Frandsen, Y.J. Uemura, M.C. Shapiro, S.C. Riggs, M.B. Stone, I.R. Fisher, C. Thompson, C. Marjerrison, H.A. Dabkowska, G.M. Luke Pyrochlore iridates have received considerable attention recently as they possess strong electron correlation and spin orbit coupling, giving rise to a finite temperature metal-insulator transition (MIT). The nature of this MIT transition is related to the magnetic order of the Ir atoms which also experience frustration. By doping ${\mathrm{Y}_{2}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ with Bi we are trying to elucidate the magnetic configuration of the iridium ions. Here we present a study on the magnetic properties of the ${\mathrm{Y}_{2-x}\mathrm{Bi}_{x}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ system using ${\mu}$SR and DC susceptibility. Our results show that pure ${\mathrm{Y}_{2}\mathrm{Ir}_{2}\mathrm{O}_{7}}$ has a magnetic transition to long-range order. Substituting the Bi by Y results in a lower temperature transition with a less homogeneous spin-glass like order with increasing {\it x}. We will show the resulting magnetic phase diagram for this system to understand how its magnetic properties behave near the metal insulator phase boundary. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q4.00007: Effective spin-1/2 exchange model for Tb$_2$Ti$_2$O$_7$: beyond the independent tetrahedra approximation Soumya Mukherjee, Stephanie Curnoe In the pyrochlore crystal terbium titanate (Tb$_2$Ti$_2$O$_7$) the magnetic Tb$^{3+}$ ions form a network of corner-sharing tetrahedra. The edges of the tetrahedra are nearest-neighbour exchange paths. The tetrahedra occur in two different orientations, therefore they can be divided into two sets. The independent tetrahedra approximation includes exchange interactions on only one set of tetrahedra and neglects interactions on the other. Although this approach can reproduce the main features of diffuse neutron scattering intensity patterns, it cannot describe any long range effects. In this work we look beyond the independent tetrahedra approximation by considering exchange paths on both sets of tetrahedra. Second order perturbation theory is used to find an effective spin-1/2 exchange model for Tb$_2$Ti$_2$O$_7$. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q4.00008: Thermal Order-by-Disorder at Criticality in XY Pyrochlore Magnets Michel Gingras, Behnam Javanparast, Alexandre Day, Zhihao Hao We investigate analytically and numerically the problem of long-range order selection via thermal fluctuations close to the critical region of the paramagnetic phase to long-range order transition in a system of interacting XY spins on the pyrochlore lattice and for which we consider the most general bilinear anisotropic nearest-neighbor spin Hamiltonian. At the standard mean-field theory (s-MFT) level, in a certain region of the parameter space of this Hamiltonian, the ordered state displays an accidental $U(1)$ degeneracy. This degeneracy is lifted by fluctuations beyond s-MFT and a certain form of order-by-disorder near criticality is thus fund to be at play. We analytically explore this selection at the microscopic level by using an extension of the method originally developed by Thouless, Anderson and Palmer (TAP) to study the effect of fluctuations in spin glasses. These TAP calculations provide an insight into the long-range order fluctuation-induced selection mechanism in terms of the spin-spin coupling constants of the microscopic Hamiltonian. We also employ a cluster mean field theory (c-MFT) to further explore numerically this problem. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q4.00009: RKKY interactions and anomalous Hall effect in metallic rare-earth pyrochlores SungBin Lee, Arun Paramekanti, Yong Baek Kim Motivated by experiments on Pr$_2$Ir$_2$O$_7$, we consider metallic pyrochlore systems A$_2$B$_2$O$_7$, where the A-sites are occupied by rare-earth local moments and the B-sites host 5d transition metal ions with itinerant strongly spin-orbit coupled electrons. Assuming non-Kramers doublets on the A-site, we derive the RKKY interaction between them mediated by the B-site itinerant electrons and find extended non-Heisenberg interactions. Analyzing a simplified model of the RKKY interaction, we uncover a local moment phase with coexisting spiral Ising-like magnetic dipolar and XY-like quadrupolar ordering. This state breaks time-reversal and lattice symmetries, and reconstructs the B-site electronic band structure, producing a Weyl Metallic phase with an intrinsic anomalous Hall effect and an undetectably small magnetization. We discuss implications of our results for Pr$_2$Ir$_2$O$_7$. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q4.00010: Competing order in spin ice Patrik Henelius, Taoran Lin, Zhihao Hao, Michel Gingras The spin ice family of materials is one of the foremost realizations of a frustrated system displaying a macroscopic ground state degeneracy down to very low temperatures. With the strongest interactions frustrated, small perturbations may eventually cause non-trivial and exotic ordering, which would be hidden in an unfrustrated system. In this study we find that Dy$_2$Ti$_2$O$_7$, a member of the spin ice family, is a prime example of this phenomenology. The value of the weak third neighbor interaction determines which ordered state is eventually selected. We calculate the phase diagram and explore the implications of recent zero-field specific heat measurements and previous neutron scattering data. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q4.00011: Chemical pressure effects on magnetism in the quantum spin liquid candidates Yb$_{2}$B$_{2}$O$_{7}$ (B = Sn, Ti, Ge) Zhiling Dun, Minsong Lee, Eunsang Choi, Alannah Hallas, Chris Wiebe, Alannah Hallas, Jason Gardner, Everton Arrighi, Ricardo Freitas, Angel Lopez, Haidong Zhou, Jinguang Cheng The linear and nonlinear AC susceptibility measurements of Yb-pyrochlores, Yb$_{2}$B$_{2}$O$_{7}$ (B = Sn, Ti and Ge), show a ferromagnetic ordering at 0.13 K but with short range ordering nature for Yb$_{2}$Sn$_{2}$O$_{7}$, a ferromagnetic ordering at 0.25 K for Yb$_{2}$Ti$_{2}$O$_{7}$, and an antiferromagnetic ordering at 0.62 K for Yb$_{2}$Ge$_{2}$O$_{7}$. These systematical results (i) clarified the nature of the controversial magnetic ground state in Yb$_{2}$Ti$_{2}$O$_{7}$; (ii) realized a distinct antiferromagnetic ordering state in Yb$_2$Ge$_2$O$_7$; and (iii) demonstrated that the application of chemical pressure through the series of Yb-pyrochlores can efficiently perturb the fragile quantum spin fluctuations of the Yb$^{3+}$ ions and lead to very different magnetic ground states. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q4.00012: Spin liquids and magnetic ordering in pyrochlores Ludovic Jaubert, Han Yan, Owen Benton, Nic Shannon By their diversity, rare earth pyrochlores have proven to be a very fertile testing ground for exotic phenomena in magnetism, ranging from monopoles in spin ice (Dy2Ti2O7), to textbook order-by-disorder transitions (Er2Ti2O7), Higgs mechanism in quantum spin ice (Yb2Ti2O7), potential spin liquid phases (Er2Sn2O7) mediated by lattice fluctuations (Tb2Ti2O7), and many more. In this talk, I will give a brief overview of this pyrochlore diversity, illustrated by direct comparison with experiments, both from the point of view of spin liquid stabilization and magnetic ordering processes. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q4.00013: Flucutation driven selection at crticality: the case of multi-k partial order on the pyrochlore lattice Zhihao Hao, Behnam Javanparast, Matthew Enjalran, Michel Gingras We study the problem of partially ordered phases with periodically arranged disordered sites on the pyrochlore lattice. The periodicity of the phases is characterized by one or more wave vectors $k = \{\frac{1}{2}\frac{1}{2}\frac{1}{2}\}$. Starting from a general microscopic Hamiltonian including anisotropic nearest-neighbor exchange, long-range dipolar interactions and second- and third-nearest neighbor exchange, we identify using standard mean-field theory (s-MFT) an extended range of interaction parameters that support partially ordered phases. We demonstrate that thermal fluctuations beyond s-MFT are responsible for the selection of one particular partially ordered phase, e.g. the ``4-$k$'' phase over the ``1-$k$'' phase. We suggest that the transition into the 4-$k$ phase is continuous with its critical properties controlled by the cubic fixed point of a Ginzburg-Landau theory with a 4-component vector order-parameter. By combining an extension of the Thouless-Anderson-Palmer method originally used to study fluctuations in spin glasses with parallel-tempering Monte-Carlo simulations, we establish the phase diagram for different types of partially ordered phases. Our result reveals the origin of 4-$k$ phase observed bellow 1K in Gd2Ti2O7. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q4.00014: High Magnetic Field Phase of the Pyrochlore Quantum Magnet Tb$_2$Ti$_2$O$_7$ Liang Yin, Jian-sheng Xia, Yasu Takano, Neil Sullivan, Eun-sung Choi, Qiu Ju Li, Xuefeng Sun By means of ac magnetic-susceptibility and vibrating-sample magnetization measurements in magnetic fields along [111], we find a new magnetic phase of Tb$_2$Ti$_2$O$_7$ between 15T and 16T and below 2.5K with the existing magnetic transition at 1~2T and below 0.6mK. This new magnetic transition implies that the long-range ordering, which has been discussed in other reports, exists between 1T and 15T at low temperatures. Above the critical field of this transition, Tb$_2$Ti$_2$O$_7$ tends to be fully-polarized up to 35T or form an unknown spin-configuration. A new temperature-field phase diagram is established below 4~K and in a field up to 35T. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q4.00015: Quantum critical dynamics of LiHoF4 Vadim Oganesyan, Haifu Ma We study spin-wave dynamics in the extended transverse Ising model appropriate for LiHoF4 in a field. The model includes electron and nuclear spins, as well as lattice deformations. Structure factors are computed and compared against neutron scattering data. [Preview Abstract] |
Session Q6: Focus Session: Emergent Properties in Bulk Complex Oxides: Fe-Oxide and Hexaferrite
Sponsoring Units: GMAG DMPChair: M. Khan, Miami University, Ohio
Room: 108
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q6.00001: Theory of magnetoelectric effects in field-induced canted affetiferromagnetic state in BiFeO$_{3}$ Nobuo Furukawa, Shin Miyahara We investigate static and dynamical magnetoelectric feffects in the field-induced antiferromagnetic state of the multiferroic compound BiFeO$_{3}$ with a distorted perovskite structure, based on calculations for a Heisenberg model with Dzyaloshinsky-Moriya interactions under external magnetic fields. Due to the distorted crystal structure, spins couple to electric polarizations via the spin-dependent metal-ligand hybridization mechanism. Due to the coupling, magnitude and direction of electric polarization depends on the external magnetic fields. Moreover, there exists a magnon which is both magneo- and electro-active due to magnetoelectric couplings, i.e., so-called a toroidalmagnon. As a result, the resonance of the toroidalmagnon shows non-reciprocal directional dichroism, i.e., the absorption intensity depends on the sign of the light propagating directions. We propose experimental conditions to clarify the existence of the couplings. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q6.00002: Microscopic Model for the Spectroscopic Modes of Multiferroic BiFeO$_{3}$ in a Magnetic Field Randy Fishman The zone-center modes measured by THz spectroscopy [1] provide the most detailed information available about the very small microscopic interactions responsible for the cycloid in multiferroic BiFeO$_{3}$. While a Dzaloshinskii-Moriya (DM) interaction perpendicular to the electric polarization \textbf{P} produces the cyloidal period, a DM interaction along \textbf{P} produces the small tilt in the cycloid, which leads to the weak ferromagnetic moment of the canted phase above a critical field of about 18 T. A microscopic model that includes both DM interactions as well as easy-axis anisotropy along \textbf{P} quantitatively predicts the field dependence of the spectroscopic frequencies [2]. Comparison with the measured frequencies indicates that only one of the three cycloidal domains survives above about 6 T.\\[4pt] [1] U. Nagel, R.S. Fishman, T. Katuwal, H. Engelkamp, D. Talbayev, H.T. Yi, S.-W. Cheong, and T. Room, \textit{Phys. Rev. Lett.} \textbf{110}, 257201 (2013)\\[0pt] [2] R.S. Fishman, \textit{Phys. Rev. B} \textbf{87}, 224419 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q6.00003: THz spectroscopy of spin waves in multiferroic BiFeO$_3$ in high magnetic fields Urmas Nagel, T. Katuwal, T. R\~o\~om, H. Engelkamp, D. Talbayev, H.T. Yi, S.-W. Cheong, Randy S. Fishman We have studied the magnetic field dependence of far-infrared active magnetic modes in a single ferroelectric domain BiFeO$_3$ crystal at low temperature. The modes soften close to the critical field of 18.8\,T along the [001] (pseudocubic) axis, where the cycloidal structure changes to the homogeneous canted antiferromagnetic state and a new strong mode with linear field dependence appears that persists at least up to 31\,T. A microscopic model that includes two Dzyaloshinskii-Moriya interactions and easy-axis anisotropy describes closely both the zero-field spectroscopic modes as well as their splitting and evolution in a magnetic field. The good agreement of theory with experiment suggests that the proposed model provides the foundation for future technological applications of this multiferroic material. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q6.00004: Static and Dynamic Magnetoelectric Effects in Multiferroic Hexaferrites Invited Speaker: Sae Hwan Chun Multiferroics, wherein magnetism and ferroelectricity coexist, are of great interest for the prospect of new multifunctional devices by utilizing cross-coupling between the electric and magnetic properties. In most multiferroics currently known, however, the magnetoelectric (ME) coupling does not reach the level enough for the practical applications and the cross control of electric polarization by magnetic field or magnetization by electric field has been realized only at low temperature. Hence, for use in the ME devices, it is essential to increase both the ME sensitivity and the operating temperature. From investigation of multiferroic hexaferrites, we discover a chemical route to effectively tailor the critical magnetic field inducing electric polarization in (Ba,Sr)$_{2}$Zn$_{2}$Fe$_{12}$O$_{22}$ (Zn$_{2}$Y-type) by Al-substitution, yielding a giant magnetoelectric susceptibility [1]. In (Ba,Sr)$_{3}$Co$_{2}$Fe$_{24}$O$_{41}$ (Co$_{2}$Z-type) hexaferrite single crystals, we realize the control of magnetization by electric field at room temperature [2]. In addition to those static ME properties, a dynamic ME effect, electric-dipole-active magnon resonance in THz frequency range, is also found in the Co$_{2}$Z-type hexaferrite, exhibiting the spectral weight even at room temperature [3]. The unprecedented supreme static and dynamic ME phenomena in the hexaferrites may provide a pathway to overcome the challenge in application of multiferroics for the real devices. \\[4pt] [1] S. H. Chun et al., Phys. Rev. Lett. 104, 037204 (2010). \\[0pt] [2] S. H. Chun et al., Phys. Rev. Lett. 108, 177201 (2012).\\[0pt] [3] S. H. Chun et al., in preparation. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q6.00005: A study for magnetic properties enhencement of strontium hexaferrite by first principles calculations Vivek Dikshit, Chandani Nandadasa, Seong-Gon Kim Owing to high magnetic anisotropy and saturation magnetization Strontium Hexaferrite is one of the most commonly used materials for hard magnets. In order to further improve the magnetic properties of the material we investigated the substitution different elements at Fe atom sites. Our calculation (using quantum mechanical DFT package, VASP) shows that both the properties: magnetization as well as magnetic anisotropy energy can be improved by a proper substitution. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q6.00006: Novel multi-Q spiral spin texture in SrFeO$_{3}$ S. Ishiwata, J. -H. Kim, D. S. Inosov, Y. Tokunaga, S. Seki, N. Kanazawa, R. Georgii, K. Seemann, G. Brandl, J. White, N. Egetenmeyer, J. Gavilano, Y.W. Long, Y. Kaneko, Y. Taguchi, T. Arima, B. Keimer, Y. Tokura A magnetic skyrmion discovered recently in chiral and cubic helimagnet such as MnSi [1] is of great interest for novel spintronic functions. SrFeO$_{3}$ has been known as a rare cubic perovskite showing both helimagnetic transition and metallic conduction. While the magnetic ground state has been believed to have a simple proper-screw-type helimagnetic order below, we found that SrFeO$_{3}$ hosts a rich variety of helimagnetic phases potentially containing novel skyrmion phases [2]. In the low magnetic field phases, a large topological Hall effect suggesting the formation of skyrmion lattice was observed. In this talk, based on the polarized and unpolarized neutron scattering studies, we will discuss the possible formation of novel type of three-dimensional skyrmion crystals, which can be characterized by quadruple-Q vectors along \textless 111\textgreater equivalents in a cubic lattice.\\[4pt] [1] S. M\"{u}hlbauer et al., Science 323, 915 (2009).\\[0pt] [2] S. Ishiwata et al., Phys. Rev. B 84, 054427 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q6.00007: Spin-Charge Coupling and Charge Order Phases in LuFe$_{2}$O$_{4}$ Chang-Jong Kang, B.I. Min Possible charge order phases in mixed-valent multiferroic LuFe$_{2}$O$_{4}$ are studied based on the first principles density functional theory. We have considered two different charge order phases of LuFe$_{2}$O$_{4}$ suggested by Angst et al. [1] and de Groot et al. [2], and investigated their electronic and magnetic properties systematically to determine the correct charge order phase that is consistent with the experiment. The systematic comparison of physical properties between two charge order phases will be discussed, and the corresponding spin-charge coupling effect will be examined. We have found that the spin-charge coupling effect is an essential ingredient in LuFe$_{2}$O$_{4}$.\\[4pt] [1] M. Angst et al., Phys. Rev. Lett. 101, 227601 (2008).\\[0pt] [2] de Groot et al., Phys. Rev. Lett. 108, 187601 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q6.00008: Optical properties of ferrites through their magnetic ordering and spin-reorientation temperatures Brian Holinsworth, Charles Brooks, Julia Mundy, Judy Cherian, Stephen McGill, Darrell Schlom, Janice Musfeldt Iron oxides have attracted a great deal of attention due to their high magnetic ordering temperatures and semiconducting band gaps, the combination of which is very attractive for applications. In this work, we investigate band gap behavior in charge-ordered LuFe$_{\mathrm{2}}$O$_{\mathrm{4}}$ and its cousin LuFeO$_{\mathrm{3}}$, showing that the latter is robust with temperature through the 130 K spin-reorientation and 440 K Neel transitions, and compare our findings with electronic structure calculations. Time permitting, we will discuss the spin polarized nature of the bands that form the leading edges of the gaps. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q6.00009: Electric field induced Verwey transition in single magnetite nanoparticles Qian Yu, Alireza Mottaghizadeh, Hongyue Wang, Christian Ulysse, Valentina Rebuttini, Nicola Pinna, Alexandre Zimmers, Herve Aubin In 1939, E.J.W. Verwey discovered that in magnetite (Fe3O4) electrons localize below a temperature T $\sim$ 120 K. He suggested that charge transport is due to electron exchange between ferric (Fe3$+)$ and ferrous (Fe2$+$) sites and the metal to insulator transition is due to the ordering of Fe cations into alternating layers of Fe3$+$ and Fe2$+$ ions. Using a method recently developed to fabricate single nanoparticle circuits, we trapped single nanoparticles of magnetite between nanometer-spaced electrodes that we used to study the electronic spectrum of the nanoparticles as function of temperature across the Verwey transition. In this tunnelling spectrum, we find the signature of polarons states. As function of temperature, one can observe that the density of states decreases to zero as the temperature approach the Verwey transition. Below the Verwey temperature, a clear gap is observed in the tunnelling spectrum. Above the Verwey temperature, no gap is observed. The absence of this gap indicates that electronic transport in the normal state of magnetite is due to polaron hopping, in contrast to the alternative scenario of activated band-like electronic transport. This work was supported by the French ANR grants 10-BLAN-0409-01 and 09-BLAN-0388-01. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q6.00010: Electronic structure and magnetic ground state properties of SrCoO$_{2.5}$ Chandrima Mitra, Randy S. Fishman, Satoshi Okamoto, Ho Nyung Lee, Fernando A. Reboredo ABO$_{3-\delta}$ type perovskite oxides are potential candidates for solid oxide fuel cells. The ones that crystallize in the orthorhombic brownmillerite-phase (ABO$_{2.5})$, such as SrCoO$_{2.5}$, are particularly interesting due to their crystal structure which contains ordered channels of oxygen vacancies. In this work we investigate theoretically the ground state electronic structure and magnetic properties of the brownmillerite phase of SrCoO$_{2.5}$. Strong correlations of the Co d electrons are treated within the local spin density approximations of Density Functional theory (DFT) with Hubbard U corrections (LSDA$+$U). The results are compared with the Heyd Scuzeria Ernzerhof (HSE) functional. The parameters computed with a U value of 7.5 eV are found to match closely to those computed within the HSE functional. Consistent with experimental observation a G-type antiferromagnetic structure is found to be the most stable one. From a Heisenberg Hamiltonian we compute the magnetic exchange interaction parameters, J, between the Co atoms which are then used to compute the spin-wave frequencies and inelastic neutron scattering intensities. The system has four spin-wave branches. The lowest energy mode was found to have the largest scattering intensity at the magnetic zone center. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q6.00011: Magnetically inhomogeneous ground-state below the first order valence transition in (Pr$_{1-y}$Y$_y$)$_{0.7}$Ca$_{0.3}$CoO$_{3-\delta}$ Daniel Phelan, Kanwal Preet Bhatti, Matthew Taylor, Shun Wang, Christopher Leighton Certain Pr-based cobaltites undergo an intriguing first-order metal-insulator transition on cooling, thought to be a manifestation of an abrupt valence transition (VT) where electron occupancy shifts from Pr to Co ions and thus depletes the hole concentration and inhibits ferromagnetic (FM) ordering. We discuss the inhomogenous nature of the magnetic ground-state (g.s.) of one such series of compounds, (Pr$_{1-y}$Y$_y$)$_{0.7}$Ca$_{0.3}$CoO$_{3-\delta}$. Although $y$=0 has a metallic, FM g.s., magnetization and transport studies indicate that the VT is stabilized upon Y substitution. Small angle neutron scattering measurements evidence short-range FM clusters of mean size $\sim$ 45 $\AA$, co-existing with low-density regions of long-range FM order in the g.s of $y$=0.075, which undergoes the VT at $\sim$ 80 K. These clusters are observed in magnetotransport, where a negative magnetoresistance and strong field-cooling effects occur as a result of inter-cluster tunneling. The results can be interpreted in the context of the system being driven into a phase-separated regime by the VT; however, the g.s. is significantly more inhomogeneous than in simpler cobaltites (e.g. La$_{1-x}$Sr$_x$CoO$_3$), likely due to inhomogeneities in the distributions of A-site ions and O vacancies. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q6.00012: Local Spin State Measurements in Critically Doped $La_{0.83} Sr_{0.17} CO_{3} $ Ahmet Gulec, Robert F. Klie Strontium doped $LaCO_{3} $ has fascinating magnetic phases which are believed to be directly related to Co spin states. Critically doped $La_{0.83} Sr_{0.17} CO_{3} $ undergoes a simultaneous Insulator to Metal Transition (IMT) and ferromagnetic (FM) order transition. In this work, we will utilized atomic-resolution Z-contrast imaging, annular bright field (ABF) imaging and electron energy-loss spectroscopy in the aberration-corrected JEOL JEM-ARM200CF in combination with cooling experiments to examine the local magnetic and spin-state transitions in critically doped $La_{0.83} Sr_{0.17} CO_{3} $ between 80 K and 300 K. Our energy-loss magnetic circular dichroism (EMCD) experiments confirm the non-localized increase in the dichromatic signal at low temperature, associated with a change in the co-ion spin state. On the other hand, by using he ABF imaging, a distortion of the $CO_{6} $ octahedral and the changes in the Co-O bond lengths within the same unit cell are observed. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q6.00013: Ferromagnetism and unconventional impurity effects in Rh- and Ga- doped LaCoO$_{3}$ Shinichiro Asai, Ryuji Okazaki, Ichiro Terasaki, Yukio Yasui The perovskite oxide LaCoO$_{3}$ has been long investigated because of a dramatic change of its spin state for Co$^{3+}$ ions with temperature variation. The Co$^{3+}$ ions (3$d^{6}$) in LaCoO$_{3}$ takes the non-magnetic low-spin state of \textit{t}$_{2g}^{6}$ at low temperature. The spin state of the Co$^{3+}$ ions in this system is sensitive to the chemical substitutions; we have found a weak ferromagnetism in a solid solution of LaCoO$_{3}$ and LaRhO$_{3}$ [S. Asai \textit{et al}., JPSJ 80, 104705 (2011).]. Since the two oxides are non-magnetic at low temperature, our finding is an example of ``order by disorder,'' where a non-magnetic impurity makes a non-magnetic oxide ferromagnetic. We have further investigated the magnetization and x-ray diffraction of LaCo$_{0.8-y}$Rh$_{0.2}$M$_{y}$O$_{3}$ (M = Rh, Ga) [S. Asai \textit{et al}., PRB 86, 014421 (2012).]. The magnetization decreases by the Ga$^{3+}$ substitution much more drastically than by the Rh$^{3+}$ substitution. It indicates that at least two kinds of Co$^{3+}$ ions exist in LaCo$_{0.8}$Rh$_{0.2}$O$_{3}$; one is nonmagnetic, and the other is magnetic. In this talk, we will also discuss the change of the lattice volume with the Rh$^{3+}$ and Ga$^{3+}$ substitution [S. Asai \textit{et al}., JPSJ 82, 114606 (2013).]. [Preview Abstract] |
Session Q7: Focus Session: Magnetic Domains and Domain Walls
Sponsoring Units: GMAGChair: Randy Dumas, Gothenburg University, Sweden
Room: 106
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q7.00001: Antiferromagnetic domain dynamics observed in Fe/CoO/MgO(001) system Qian Li, Jie Zhu, Gong Chen, Tian Ping Ma, Yan Huo, Yi Zheng Wu Antiferromagnetic (AFM) domain dynamics has rarely been researched in spite of its importance in the magnetic recording application and fundamental physics studies. Usually, the magnetic properties of the AFM layer can be studied by detecting the adjacent ferromagnetic (FM) layer due to the exchange coupling at the FM/AFM interface. In this contribution, we studied the AFM domain flipping process in a FM/AFM exchange-coupled system. Single-crystalline Fe/CoO/MgO(001) was prepared by molecular beam epitaxy and its magnetic properties were measured with magneto-optic Kerr effect. With magnetic field scanning along CoO[1-10] at 143K after field cooling along CoO[110], the hysteresis loop gradually changes from a double-split loop to an easy square loop, which indicates AFM spin rotating by 90 degrees during this process. Two mechanisms, AFM domain nucleation (DN) and domain wall motion (DWM), are clearly separated by analyzing remanence signal as a function of loop sequential number, and meanwhile the flipping rates are also obtained. For the first time, AFM DN and DWM energy barriers are quantitatively determined by temperature dependence measurement. Systematic results are got that energy barriers increase linearly with CoO thickness and decrease in larger magnetic field. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q7.00002: Order and topology in antiferromagnets with surfaces Michalis Charilaou, Frances Hellman We show using Monte Carlo simulations and mean-field theory that the antiferromagnetic (AFM) magnetization, arising from uncompensated spins, exhibits a unique thermodynamic behavior that differs from that of ferromagnets or of the N\'{e}el vector. More importantly, the net uncompensated magnetization is lower than that of the surface due to finite size effects. This phenomenon can be is manifested in thin films but it is in fact the same even in infinite systems with free surfaces, suggesting a topological order in uncompensated antiferromagnets. Moreover, we investigate the effects of defects and roughness on the magnetization of AFM and show that with increasing roughness the magnetization decreases non-monotonically and reaches values of only a few percent. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q7.00003: The surface magnetization study of Cr$_{2}$O$_{3}$ by spin polarized low energy electron microscopy Shi Cao, Ning Wu, Xin Zhang, Alpha N'Diaye, Gong Chen, Andreas Schmid, Will Echtenkamp, Valeria Lauter, Christian Binek, Peter Dowben The boundary magnetization at the surface of a Cr$_{2}$O$_{3}$ single crystal has been demonstrated by using spin-polarized low-energy electron microscopy (SPLEEM), indicating net surface spin polarization. This work shows that the placement of Cr$_{2}$O$_{3}$ single crystal in the single domain state, will result in net Cr$_{2}$O$_{3}$ spin polarization at the boundary, even in the presence of a gold overlayer. There are indications that the spin-polarized low-energy electron microscopy (SPLEEM) contrast for the two polarizations states is different. In addition, the boundary magnetization protected by the symmetry exists despite of the surface roughness/softness which was studied by the non-spin neutron reflectometry and low energy electron diffraction. Unoccupied surface oxygen sites and chromium sites are possible mechanism contributing to the surface ``softness,'' which will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q7.00004: Jamming Behavior of Domain Walls in an Antiferromagnetic Film Invited Speaker: Sunil Sinha Over the last few years, attempts have been made to unify many aspects of the freezing behavior of glasses, granular materials, gels, supercooled liquids, etc. into a general conceptual framework of what is called jamming behavior. This occurs when particles reach packing densities high enough that their motions become highly restricted. A general phase diagram has been proposed onto which various materials systems, e.g glasses or granular materials, can be mapped. We will discuss some recent applications of resonant and non-resonant soft X-ray Grazing Incidence Scattering to mesoscopic science, for example the study of magnetic domain wall fluctuations in thin films. For these studies, we use resonant magnetic x-ray scattering with a coherent photon beam and the technique of X-ray Photon Correlation Spectroscopy. find that at the ordering temperature the domains of an antiferromagnetic system, namely Dysprosium metal, behave very much also like a jammed system and their associated fluctuations exhibit behavior which exhibit some of the universal characteristics of jammed systems, such as non-exponential relaxation and Vogel-Fulcher type freezing.\\[4pt] Work done in collaboration with San-Wen Chen (UCSD), Hongyu Guo (UCSD), Keoki Seu (ALS/LBL), Karine Dumesnil (Institute Jean Lamour, Universite de Lorraine, Nancy, France) and Sujoy Roy (ALS/LBL). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q7.00005: Dependence of Exchange Bias on Interfacial and Bulk Antiferromagnetic Spins Ali C. Basaran, Thomas Saerbeck, Jose de la Venta, Henning Huckfeldt, Arno Ehresmann, Ivan K. Schuller One of the key issues in exchange bias is the dependence of pinned interfacial or bulk uncompensated antiferromagnetic spins. To address this important issue, we grew simultaneously several sets of ferromagnetic/antiferromagnetic (Ni/FeF$_{\mathrm{2}})$ bilayers capped with a nonmagnetic and inert gold layer of varying thickness. He-ion irradiation was employed to selectively create defects near the Ni/FeF$_{\mathrm{2\thinspace }}$interface or in the bulk of FeF$_{\mathrm{2}}$. The penetration depth of the ions with constant energy and dose was controlled by the gold capping layer thickness. Consequently, this leads to varying depth profiles of defects in the antiferromagnetic bulk. This was confirmed by numerical simulations of the ion damage. Detailed, quantitative, structural and magnetic characterizations were compared before and after the bombardment. These studies show that the creation of defects in the antiferromagnetic bulk is playing a crucial role in exchange bias [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q7.00006: Effect of morphology on exchange bias in NiMnSn and NiCoMnIn magnetic shape memory alloys Pavel N. Lapa, James A. Monroe, Brian E. Franco, Ibrahim Karaman, Igor V. Roshchin Exchange bias (EB) is one of puzzling magnetic properties of magnetic shape memory alloys (MSMA). Despite a few attempts to explain the mechanism, there is no comprehensive model describing it. The main obstacle is the lack of information about the magnetic structure of martensitic and austenite phases. In contrast to classical EB systems where the exchange coupling happens at the interface between ferromagnetic and antiferromagnetic layers, the EB in MSMA is attributed to coexistence of ferromagnetic and antiferromagnetic regions. We report the results of structural analysis obtained using wavelength-dispersive X-ray spectroscopy (WDS) and magnetic characterization of these samples. We observe a correlation of EB with the secondary heat treatment for NiCoMnIn alloys. Comparative first order reversal curve (FORC) analysis for NiMnSn samples with different heat treatment suggests a correlation between morphology and distribution of exchange bias values. Additionally, exchange bias in these alloys can be induced even after zero-field cooling by applying a constant field for 2 hours before measuring the magnetization curve. This behavior is consistent with magnetic glassiness observed in these alloys at low temperatures. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q7.00007: Element-selective investigation of domain structure in CoPd and FePd alloys using small-angle soft X-ray scattering C. Weier, R. Adam, R. Fr\"{o}mter, J. Bach, G. Winkler, A. Kobs, H.P. Oepen, P. Grychtol, H.C. Kapteyn, M.M. Murnane, C.M. Schneider Recent optical pump-probe experiments on magnetic multilayers and alloys identified $\it{perpendicular}$ spin superdiffusion as one of possible mechanisms responsible for femtosecond magnetization dynamics. On the other hand, no strong evidence for the ultrafast $\it{lateral}$ spin transport has been reported, so far. To address this question, we studied magnetic domain structure of CoPd and FePd thin films using small-angle scattering of soft X-rays. By tuning the synchrotron-generated X-rays to the absorption edges of Fe or Co we recorded Fourier images of the magnetic domain structure corresponding to a chosen element. Applying $\it{in-situ}$ magnetic fields resulted in pronounced rearrangement of domain structure that was clearly observed in scattering images. Our analysis of both the stand-alone, as well as magnetically coupled CoPd/FePd layers provides insight into the formation of domains under small magnetic field perturbations and pave the way to better understanding of transient changes expected in magneto-dynamic measurements. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q7.00008: Graphene mediated magnetic domain formation Iori Tanabe, Yi Wang, Lingmei Kong, Christian Binek, Frank Pasquale, Yuan Cao, Bin Dong, Jeffry Kelber, Peter Dowben Both graphene on Co and graphene on Co3O4/Co samples were investigate by the Raman spectroscopy and longitudinal magneto-optic Kerr effect (MOKE). While the graphene on Co (111) bilayer thin films exhibited high remnant magnetization in plane easy axis ferromagnetism, the graphene/Co3O4/Co trilayers exhibited little remnant magnetization. The latter is due to formation of a complex multidomain state at zero applied field. The role of graphene and Co3O4 will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q7.00009: Programmable manipulation of superparamagnetic microbeads at junctions using magnetic domain walls Elizabeth Rapoport, David Bono, Geoffrey Beach There has been a steady progression in the advancement of magnetic technologies for bead manipulation in chip-based devices. Recently, we demonstrated that with curvilinear magnetic tracks, both domain wall (DW)-driven transport and detection of superparamagnetic (SPM) beads can be achieved. Here, we demonstrate that the direction of bead motion at junctions in branched curvilinear structures can be precisely selected with a vertical field. Upon exiting a junction, a single DW is split into two of opposite configuration. A vertical field strengthens the bead-DW interaction for one DW configuration, while simultaneously weakening the interaction for the other. The result is preferential bead motion with one DW over the other, allowing for the design of complex bead routing networks. Numerical work is presented in support of the theoretical basis for selective motion, and experiment reveals a threshold vertical select field for a sample of nominally identical beads. This routing technique is also shown to be able to sort a mixed population of SPM beads by simple application of a vertical field. With this work, we add an essential capability to the set of DW-mediated SPM bead handling functions required for an integrated lab-on-a-chip platform. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q7.00010: Trajectory of dynamically propagating magnetic domain walls at nanowire vertices David M. Burn, Stephanie K. Walton, Megha Chadha, Katharina Zeissler, Lesley F. Cohen, Will R. Branford Nanoscale patterning techniques can be used to fabricate magnetic nanowire structures where the behavior of individual magnetic domain walls (DWs) can be investigated. In addition to the fundamental physical understanding of magnetism, research in this area is also driven by the potential to realize novel spintronic devices for technological applications. Magnetic DWs can support a wide variety of micromagnetic structures with different magnetization, chirality and topology based on their interaction with the nanostructure geometry. These interactions can govern the field dependent domain wall trajectory and subsequent magnetization reversal that takes place within nanowire vertex structures. In this work the additional factors affecting the trajectory due to the dynamic behavior of propagating DWs are investigated. This includes the time dependent periodic changes in the DW micromagnetic structure from Walker breakdown. These results have implications for future technological applications as well as suggesting processes that may govern magnetization reversal in artificial spin ice structures. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q7.00011: Simulation of Oscillatory Domain Wall Motion Driven by Spin Waves in Nanostrip with Perpendicular Magnetic Anisotropy Shang Fan Lee, Liang Juan Chang We numerically investigate the spin waves (SW) induced domain wall (DW) oscillatory motion in a nanostrip with perpendicular magnetic anisotropy by means of micromagnetic simulation. SW carries spin angular momentum and can interact with DWs via Spin Transfer Torque (STT). Propagating SW can drive a DW motion depending on the in-plane tilt angle $\varphi $ of the wall magnetization. We calculate the instantaneous velocity of DWs as a function of $\varphi $with different SW frequency $f$. We find that the DW motion under propagating SW depends not only on the frequencies $f$, but also on the in-plane tilt angle $\varphi $. The nanostrip considered is 50 nm wide and 4000 nm long. A DW at the center is subjected to a SW source 500 nm apart on the left with amplitude in the transverse direction and varying frequency $f$. The motions of the DW induced by the SW are accompanied by in-plane rotation of magnetization of DW. Once rotated by 90 degrees, the DW shows a backward motion towards the SW source. The oscillatory amplitude and frequency of the DW motion is analyzed. A phase diagram will be presented. This study provides new perspectives for the control and manipulation of DW in a nanostrip. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q7.00012: The creation of 360 degree domain walls in ferromagnetic nanorings by circular applied magnetic fields Jessica Bickel, Spencer Smith, Katherine Aidala 360$^{\circ}$ domain walls (DWs) are the proposed transition state of ferromagnetic nanorings which are candidate devices for magnetic memory. Using micromagnetic simulations [1], we examine the formation of 360$^{\circ}$ DWs created by the application of a circular Oersted field for the transition of a 5nm thick ring from a CCW to a CW vortex. The magnetic reversal begins by canting of the magnetization either inward or outward. As the spin continues to rotate, exchange interactions result in the rotation of adjacent spins. Finally, the rotate spin aligns with the applied magnetic field, creating a transition state made of two 180$^{\circ}$ DWs of opposite winding number. As the center of the rotated domain grows, the 180$^{\circ}$ walls of adjacent domains meet. Adjacent domains cant in opposite directions to lower the magnetostatic energy relative to canting in the same direction. Therefore 180$^{\circ}$ DWs at the boundaries have the same winding number and combine to form 360$^{\circ}$ DWs. Each pair of rotated domains results in a pair of two 360$^{\circ}$ DWs of opposite winding number. This work provides better understanding of the formation of 360$^{\circ}$ DWs and may lead to the ability to control the formation of DWs via geometry. [1] http://math.nist.gov/oommf [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q7.00013: Effect of curvature on domain wall motion in elliptical nanorings Fikriye Idil Kaya, Jessica Bickel, Katherine Aidala Understanding domain wall (DW) motion in ferromagnetic nanostructures is important to realize proposed magnetic data storage and logic devices. We investigate the effect of curvature on DW pinning and motion by studying elliptical rings using micromagnetic simulations [1]. Elliptical rings with constant width have varying curvature, with the lowest curvature at the minor axis, and the greatest curvature at the major axis. DWs can be created at any angular position within the ellipse by the application of an appropriate uniform magnetic field. However, only some of these positions are stable when the field is removed. We study the stability and depinning of the DWs by applying a slowly increasing elliptical magnetic field to determine the magnitude of the field at which the DWs begin to move. By varying the major to minor axis ratio, we examine the effect of curvature on DW pinning. A larger field is required to move DWs in regions of higher curvature (near the major axis) than lower curvature (near the minor axis). Overall, we see that increasing the major to minor axis ratio of elliptical nanorings requires increasing field strength to depin the DWs along the major axis. [1] Oommf software distributed by NIST at http://math.nist.gov/oommf [Preview Abstract] |
Session Q8: Focus Session: Spin Wave Phenomena and Spin Pumping and Damping
Sponsoring Units: GMAGChair: Carl Boone, National Institute of Standards and Technology, Boulder
Room: 104
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q8.00001: Spin wave dynamics and magnetization switching in exchange-coupled bilayers Invited Speaker: Takeshi Seki Magnetic moments under the application of electric current or rf magnetic field show uniform or non-uniform dynamical motions, which are keys to develop novel spintronic devices such as nanometer-sized auto-oscillators and logic circuits. Spin waves are representative of spatially non-uniform magnetization dynamics. We demonstrated that spin waves could be utilized also to reduce the switching field in exchange-coupled bilayers consisting of hard magnetic L10-FePt and soft magnetic Permalloy (Py; Ni81Fe19). The switching field of L10-FePt was drastically reduced when the spin waves were excited. This ``spin wave-assisted magnetization switching'' is a route to balance competing goals for high coercive field, which is essential to maintain a good thermal stability of magnetization in a nanometer region, and low switching field, leading to the device operation with low power consumption. Those are important to realize high-performance spintronic and magnetic storage devices.In this talk, the concept and mechanism of spin wave-assisted magnetization switching are introduced. By comparing the experiments and the numerical simulation, it is found that perpendicular standing spin wave modes are mainly excited in Py of the exchange-coupled bilayers and those spin waves affect the dynamics of L10-FePt through the exchange coupling mechanism at the interface. The significant reduction of switching field is achieved by exciting the spin waves with large oscillation amplitude. In addition, the spin wave-assisted magnetization switching shows the characteristic magnetic field angular dependence, which is totally different from that of uniform magnetization dynamics. We also show the spin wave dynamics in perpendicularly magnetized exchange-coupled bilayers. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q8.00002: Room-temperature extraction of spin lifetimes in metallic thin films via determination of the spin-pumping contribution to damping in ferromagnetic resonance experiments Carl Boone, Martin Schoen, Justin Shaw, Hans Nembach, Thomas Silva Recent room-temperature measurements yield spin diffusion lengths for Pt and Pd that are smaller than the bulk electron mean free path at room temperature. One proposed explanation is the thickness-dependence of conductivity that results in shorter momentum lifetimes at small Pt/Pd thicknesses. We measured spin transport properties in Pd and Pt thin films at room temperature via fitting of ferromagnetic resonance (FMR) damping vs. NM thickness with the spin pumping model for ferromagnet (FM)/normal metal (NM) multilayers. We use a broadband, perpendicular FMR system to obtain high-precision values for the damping. The fits are based upon spin diffusion equations that include both momentum and spin scattering processes. By measuring thickness-dependent conductivity of the same films, we correlate the charge and spin transport parameters, permitting us to test multiple models for spin scattering. We explicitly show that the spin scattering time $\tau_{sf}$ must be shorter than the momentum scattering time tau over some range of NM thicknesses to adequately fit the data. Invocation of a simple monotonic proportionality between $\tau_{sf}$ and $\tau$ fails to fit the data. However, an inverse proportionality $\tau \sim 1/\tau_{sf}$ can fit the data, and $\tau < \tau_{sf}$ for sufficiently thin NM layer. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q8.00003: Spin Pumping by Antiferromagnetic material Ran Cheng, Jiang Xiao, Arne Brataas, Qian Niu For a long time, spin-pumping is supposed to be impossible using antiferromagnetic materials (AFM) with compensated magnetization. We show that spin-pumping does not only exist in AFM with precessing staggered order, but is even stronger than its counterpart in ferromagnets. By calculating the scattering matrix of a normal metal/AFM interface based on a tight-binding model, we derive the pumped spin current in terms of the staggered order parameter and its time derivative. It is found that spin-pumping is of the same order of magnitude for both compensated and uncompensated interfaces. And the pumped spin current can be switched by changing the circular polarization of light exciting antiferromagnetic resonance, which introduces potential application. Besides spin current, we also define a new quantity--staggered spin current--and propose a novel pumping effect in AFM. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q8.00004: FMR spin pumping in YIG/ferromagnet bilayers (ferromagnet $=$ Fe, Co, Ni, Py) Fengyuan Yang, Hailong Wang, Chunhui Du, P. Chris Hammel Generation of pure spin currents from ferromagnets (FM) to normal metals (NM) has been extensively studied by thermal and ferromagnetic resonance (FMR) spin pumping. Recently, Miao et al. demonstrated thermal injection of spin currents from Y3Fe5O12 (YIG) into Py detected by inverse spin Hall effect (ISHE) in the FM [1]. The ISHE in FM is in fact the inverse anomalous Hall effect (SHE), but with all the signatures of ISHE in NMs. Here we report robust FMR spin pumping in YIG/FM bilayers with FM $=$ Fe, Co, Ni and Py using cavity FMR. The resonance fields of the FMs and YIG are clearly separated, which allows distinction of spin pumping induced ISHE voltages at the YIG resonance field and the voltage signals at the FM resonance fields. The ISHE voltages reaches 220 uV for YIG/Py(2nm) bilayer and tens of uV for all YIG/FM bilayers with 10-nm FM at an rf power of 200 mW. The sign of the ISHE voltages for Py and Ni are opposite to those for Fe and Co, which agrees with the opposite signs of AHE in Ni as compared to Fe and Co. \\[4pt] [1] B. F. Miao, S.Y. Huang, D. Qu, and C. L. Chien, ``Inverse Spin Hall Effect in a Ferromagnetic Metal,'' Phys. Rev. Lett. 111, 066602 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q8.00005: Spin current generation in graphene by dynamical spin pumping Simranjeet Singh, Daniel Mark\'o, Barbaros \"Ozyilmaz, Enrique del Barco Graphene is a promising material for spintronics applications given its unique properties. However, an efficient method to generate pure spin currents into this two-dimensional material is required to understand the spin dynamics and mechanisms associated to spin transport in graphene. Recently, we reported the first evidence of spin pumping in ferromagnet/graphene interfaces by studying the damping of the ferromagnet due to presence of graphene. We have extended the original studies towards different device configurations. Here we discuss the effect of the interface on the dynamical damping by studying different stacking orders of graphene and Permalloy layers. Our results confirm that the observed damping is indeed a signature of dynamical spin pumping wherein spin polarized currents are pumped into the graphene from the precessing magnetization of the ferromagnet. In addition, we performed comparative FMR studies of ferromagnet/Graphene strips buried underneath the central line of a coplanar waveguide. A larger FMR linewidth broadening is observed when the graphene layer protrudes away from the ferromagnet strip, indicating that the spin relaxation in graphene occurs away from the area directly underneath the ferromagnet being excited. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q8.00006: Enhancement of Pure Spin Currents in Spin Pumping Y$_3$Fe$_5$O$_{12}$/Cu/metal Trilayers Through Spin Impedance Matching P. Chris Hammel, Chunhui Du, Hailong Wang, Fengyuan Yang Spin pumping, driven thermally as well as by Ferromagnetic Resonance (FMR), is being widely used to generate pure spin currents from ferromagnets (FM) into normal metals (NM). Typically, the NM is chosen to be a spin-sink-Pt, W or Ta, while lighter metals such as Cu are rarely used, except to decouple the FM and spin sink. The efficiency of spin pumping is largely determined by the spin mixing conductance of the FM/NM interface. Here, we report a comparative study of spin pumping in $\rm Y_3 Fe_5 O_{12}$/Cu/Pt and $\rm Y_3 Fe_5 O_{12}$/Cu/W trilayers with varying Cu thicknesses. Remarkably, we find that insertion of a Cu interlayer between YIG and W substantially improves (over a factor of 4) the spin current injection into W while similar insertion between YIG and Pt degrades the spin current. This is a consequence of a much improved YIG/Cu spin mixing conductance relative to that for YIG/W. This result shows that high quality multilayer FM/NM heterostructures can enable spin mixing conductances to be engineered to enable optimal spin pumping efficiency. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q8.00007: Spin pumping and Gilbert damping in atomically flat nanometric thick YIG\textbar NM system H.M. Alyahyaei, Chi Tang, Bowen Yang, Jing Shi Epitaxial nanometric thick ytrrium iron garnet (YIG) films grown on (111) and (110) gadolliun gallium garnet (GGG) substrates via PLD exhibit an atomically flat surface. This extremely flat surface with a roughness $\sim$ 0.1 {\AA} offers a more controlled study of the physical mechanism behind the newly observed damping in YIG\textbar NM bilayer systems. Our bilayer systems consist of a 30 nm thick YIG film, either (111) or (110), and a non-magnetic layer, either beta-phase Ta or Pd, with thickness ranging from 0 to 20 nm. We have performed ferromagnetic resonance (FMR) experiments and observed systematic thickness dependence of the FMR linewidth. As the thickness of NM increases, the FMR linewidth increases rapidly and then slowly approaches saturation. The effect of the YIG surface on the Gilbert damping due to the magnetic proximity effect and on spin pumping in such bilayer systems will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q8.00008: Investigation of Spin Pumping in YIG/Cu/Py using Ferromagnetic Resonance Yu-ming Hung, Georg Wolf, Andrew D. Kent, Houchen Chang, Yiyan Sun, Mingzhong Wu Spin pumping in YIG/Au/Fe structures has been demonstrated where the YIG film serves as a spin battery, while the Fe film functions as a spin sink [1]. In principle, the insulating YIG film can also absorb spin currents through interfacial \emph{s-d} interactions and function as a spin sink for spin pumping. In this presentation we report on the coupling between the YIG and Permalloy (Py) films in YIG/Cu/Py systems from the viewpoint of the spin pumping effect, where both layers function as either a spin battery or a spin sink. We found an increased Gilbert damping for both the YIG and Py films by means of ferromagnetic resonance (FMR) measurements. We discuss the Gilbert damping constant ($\alpha$) of YIG(40nm), Cu(5nm)/Py(3nm), and YIG(40nm)/Cu(5, 20nm)/Py(3nm) and apply these values to spin diffusion model for the calculation of spin mixing conductance. These results show the spin pumping effect at both the ferrimagnetic/NM and ferromagnetic/NM interfaces in YIG/Cu/Py structures and the dual function of the YIG and Py films in terms of the generation and absorption of spin currents.\newline [1] B. Heinrich et. al., Phys. Rev. Lett. $\bf{107}$, 066604 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q8.00009: Modifying magnetic switching in permalloy film nanostructures using the native oxide A. Hojem, D. Bassett, D. Wesenberg, S.J. Mason, A.D. Avery, B.L. Zink Thin films of nickel-iron alloys of the nominal concentration near 80\% Ni, are very commonly used in applications and in fundamental studies of spin, charge and heat transport in nanomagnetic systems. These permalloy (Py) films are straightforward to grow by various techniques and typically produce predictable, controllable and repeatable magnetic properties, including small coercivity, low magnetocrystalline anisotropy, and low magnetostriction. We have found that greater complexity can be added to the switching behavior of thin films of permalloy by oxidation of thin ($\sim$4 nm) layers followed by subsequent growth of Py. Under some circumstances, this can cause apparent negative coercivity in the switching observed in anisotropic magnetoresistance (AMR) of micromachined strips with an expected shape anisotropy. Here we will present results on growth and AMR measurements of the effects in various oxidized Py-Py layered samples. It is not yet clear if the effects are reproducible enough to be used for intentional manipulation of switching behavior in Py nanostructures. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q8.00010: Control of perpendicular magnetic anisotropy and intrinsic Gilbert damping in $L1_{0} $ordered FePt(Pd) thin films Xin Ma, Pan He, Li Ma, Guangyu Guo, Haibin Zhao, Shiming Zhou, Gunter Luepke The dependence of perpendicular magnetic anisotropy (PMA) and intrinsic Gilbert damping $\alpha_{0} $ on some leading parameters, such as spin-orbital coupling strength $\xi $, are investigated in $L1_{0} $ ordered FePt(Pd) thin films by time-resolved magneto-optical Kerr effect measurements and spin dependent ab initio calculations. Continuous tuning of PMA and $\alpha_{0} $ over a wide range of magnitude is realized by modulating the chemical substitution and ordering. Spin orbital coupling strength can be effectively adjusted by replacing Pt with Pd atoms, which keeps other leading parameters with negligible changes. Measured PMA and $\alpha_{0} $ from experiment are proportional to $\xi^{1.6}$ and $\xi ^{2}$ at 200K, while first principle calculations predict for both a quadratic dependence on $\xi $. The degree of chemical order in real samples can also significantly affect PMA and $\alpha_{0} $through leading parameters other than spin orbital coupling strength. [Preview Abstract] |
Session Q10: Focus Session: Evolution, Co-evolution, and Game Theory
Sponsoring Units: DBIO GSNPChair: Alexandre Morozov, Rutgers University
Room: 201
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q10.00001: Fluctuation-induced patterns and rapid evolution in predator-prey ecosystems Invited Speaker: Nigel Goldenfeld Predator-prey ecosystems exhibit noisy, persistent cycles that cannot be described by intuitive population-level differential equations such as the Lotka-Volterra equations. Traditionally this paradox has been met by including additional nonlinearities such as predator satiation to force limit cycle behavior. Over the last few years, it has been realized that individual-level descriptions, combined with systematic perturbation techniques can reproduce the key features of such systems in a minimal way, without requiring many additional assumptions or fine tunings. Here I review work in this area that uses these techniques to treat spatial patterns and the phenomenon of rapidly evolving prey sub-populations. In the latter case, I show how stochastic individual-level models reproduce the key features observed in chemostats and in the wild, including anomalous phase shifts between predator and prey species, evolutionary cycles and cryptic cycles. This work shows that stochastic individual-level models naturally describe systems where evolutionary time scales surprisingly match ecosystem time scales. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q10.00002: Solutions to the public goods dilemma in bacterial biofilms Knut Drescher, Carey D. Nadell, Howard A. Stone, Ned S. Wingreen, Bonnie L. Bassler Bacteria frequently live in densely populated surface-bound communities, termed biofilms. Biofilm-dwelling cells rely on secretion of extracellular substances to construct their communities and to capture nutrients from the environment. Some secreted factors behave as cooperative public goods: they can be exploited by non-producing cells. The means by which public-good-producing bacteria avert exploitation in biofilm environments are largely unknown. Using experiments with \textit{Vibrio cholerae}, which secretes extracellular enzymes to digest its primary food source, the solid polymer chitin, we show that the public goods dilemma may be solved by two very different mechanisms: cells can produce thick biofilms that confine the goods to producers, or fluid flow can remove soluble products of chitin digestion, denying access to non-producers. Both processes are unified by limiting the distance over which enzyme-secreting cells provide benefits to neighbors, resulting in preferential benefit to nearby clonemates and allowing kin selection to favor public good production. Our results demonstrate new mechanisms by which the physical conditions of natural habitats can interact with bacterial physiology to promote the evolution of cooperation. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q10.00003: Beating Cheaters at Their Own Game Joseph Rauch, Jane Kondev, Alvaro Sanchez Public goods games occur over many different scales in nature, from microbial biofilms to the human commons. On each scale stable populations of cooperators (members who invest into producing some good shared by the entire population) and cheaters (members who make no investment yet still share the common goods) has been observed. This observation raises interesting questions, like how do cooperators maintain their presence in a game that seems to heavily favor cheaters, and what strategies for cooperation could populations employ to increase their success? We propose a model of a public goods game with two different player populations, S and D, which employ two different strategies: the D population always cheats and the S population makes a stochastic decision whether to cooperate or not. We find that stochastic cooperation improves the success of the S population over the competing D population, but at a price. As the probability of cheating by the S players increases they outcompete the D players but the total population becomes more ecologically unstable (i.e., the likelihood of its extinction grows). We investigate this trade off between evolutionary success and ecological stability and propose experiments using populations of yeast cells to test our predictions. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q10.00004: Stability of Zero-Sum Games in Evolutionary Game Theory Johannes Knebel, Torben Krueger, Markus F. Weber, Erwin Frey Evolutionary game theory has evolved into a successful theoretical concept to study mechanisms that govern the evolution of ecological communities. On a mathematical level, this theory was formalized in the framework of the celebrated replicator equations (REs) and its stochastic generalizations. In our work, we analyze the long-time behavior of the REs for zero-sum games with arbitrarily many strategies, which are generalized versions of the children's game Rock-Paper-Scissors.\footnote{J. Knebel, T. Krueger, M.F. Weber, E. Frey, Phys. Rev. Lett. 110, 168106 (2013).} We demonstrate how to determine the strategies that survive and those that become extinct in the long run. Our results show that extinction of strategies is exponentially fast in generic setups, and that conditions for the survival can be formulated in terms of the Pfaffian of the REs' antisymmetric payoff matrix. Consequences for the stochastic dynamics, which arise in finite populations, are reflected by a generalized scaling law for the extinction time in the vicinity of critical reaction rates. Our findings underline the relevance of zero-sum games as a reference for the analysis of other models in evolutionary game theory. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q10.00005: Dynamics of Coevolution and Branching in the Immune System Kimberly Schlesinger, Sean Stromberg, Jean Carlson We investigate the dynamics of coevolution between two coupled populations, in the context of the interaction between mutating pathogen and the adaptive immune response. Our model represents the binding affinities between antigen epitopes and lymphocyte receptors which mediate the interactions of the two populations, and which may change with pathogen mutation. We see diverse possible outcomes of infection, including early pathogen clearance, early pathogen escape from immune control, and an intermediate state of chronic infection, in which pathogen strains coexist with lymphocytes at relatively stable levels. The coevolutionary dynamics within this chronic infection state display emergent structure, including evolutionary branching that is fundamentally driven by the coevolutionary interaction and that results in the clustering of the pathogen population into distinct and independently evolving clusters. The increased fragility of the immune system as it distributes its resources to control a growing number of clusters can lead to the sudden out-of-control growth of the pathogen after months or years of chronic infection. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q10.00006: An obligatory bacterial mutualism in a multi-drug environment exhibits strong oscillatory population dynamics Arolyn Conwill, Eugene Yurtsev, Jeff Gore A common mechanism of antibiotic resistance in bacteria involves the production of an enzyme that inactivates the antibiotic. By inactivating the antibiotic, resistant cells can protect other cells in the population that would otherwise be sensitive to the drug. In a multidrug environment, an obligatory mutualism arises because populations of different strains rely on each other to breakdown antibiotics in the environment. Here, we experimentally track the population dynamics of two \textit{E. coli} strains in the presence of two different antibiotics: ampicillin and chloramphenicol. Together the strains are able to grow in antibiotic concentrations that inhibit growth of either one of the strains alone. Although mutualisms are often thought to stabilize population dynamics, we observe strong oscillatory dynamics even when there is long-term coexistence between the two strains. We expect that our results will provide insight into the evolution of antibiotic resistance and, more generally, the evolutionary origin of phenotypic diversity, cooperation, and ecological stability. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:42PM |
Q10.00007: Survivial Strategies in Bacterial Range Expansions Invited Speaker: Erwin Frey Bacterial communities represent complex and dynamic ecological~systems. Different environmental conditions as well~as bacterial~interactions determine the establishment and sustainability of bacterial diversity. In~this talk we discuss the~competition of three Escherichia coli~strains during range expansions on agar plates. In this bacterial model~system, a~colicin E2 producing strain C competes with a colicin~resistant strain R and with a colicin sensitive strain S for new~territory. Genetic engineering allows us to tune the growth rates of the~strains and to study distinct ecological scenarios.~These scenarios may~lead to either single-strain dominance, pairwise coexistence, or to the coexistence of~all three~strains. In order to elucidate the survival mechanisms of the~individual strains, we also developed a stochastic agent-based model to~capture the ecological scenarios in silico. In a combined theoretical and~experimental approach we are~able to show that the level of~biodiversity depends crucially on the composition of the inoculum, on~the relative growth~rates of the three strains, and on the effective reach~of colicin toxicity. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q10.00008: Widespread Attenuation of Antibiotics by Soil Bacteria Promotes Intermixed Microbial Diversity Kalin Vetsigin, Eric Kelsic, Jeffrey Zhao, Roy Kishony In natural soil environments, antibiotic sensitive bacteria coexist with antibiotic producers, even in close proximities. Efforts to understand diversity in microbial communities have focused on pairwise interactions between species, yet mathematical models of such interactions lead to distinct spatial domains of individual species, rather than to intermixed multi-species communities. In this work, we measured interactions between triplets of species and asked and how the presence of these higher-order interactions affects community structure and diversity. We developed a 3-species diffusion-based assay in which a modulator species either intensifies or attenuates the toxicity of compounds made by a producer species against a fluorescently labeled indicator species. We found that intensifying interactions were quite rare among soil bacteria, while attenuating interactions that protected nearby sensitive species from the antibiotic producer were abundant. Furthermore, many soil bacteria attenuated multiple classes of antibiotics with widely varying mechanisms of action. Computer simulations showed that such cross-species protection, when abundant, promoted the spontaneous formation and expansion of intermixed multi-species communities that overtook or assimilated single species domains. These findings suggest that drug attenuation is a widespread phenomenon that can be key to the coexistence of antibiotic producing and sensitive microbes in close proximity and thereby to the overall species diversity within soil microenvironments. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q10.00009: Population Genetics of Three Dimensional Range Expansions Maxim Lavrentovich, David Nelson We develop a simple model of genetic diversity in growing spherical cell clusters, where the growth is confined to the cluster surface. This kind of growth occurs in cells growing in soft agar, and can also serve as a simple model of avascular tumors. Mutation-selection balance in these radial expansions is strongly influenced by scaling near a neutral, voter model critical point and by the inflating frontier. We develop a scaling theory to describe how the dynamics of mutation-selection balance is cut off by inflation. Genetic drift, i.e., local fluctuations in the genetic diversity, also plays an important role, and can lead to the extinction even of selectively advantageous strains. We calculate this extinction probability, taking into account the effect of rough population frontiers. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q10.00010: Coevolution of CRISPR bacteria and phage in 2 dimensions Pu Han, Michael Deem CRISPR (cluster regularly interspaced short palindromic repeats) is a newly discovered adaptive, heritable immune system of prokaryotes. It can prevent infection of prokaryotes by phage. Most bacteria and almost all archae have CRISPR. The CRISPR system incorporates short nucleotide sequences from viruses. These incorporated sequences provide a historical record of the host and predator coevolution. We simulate the coevolution of bacteria and phage in 2 dimensions. Each phage has multiple proto-spacers that the bacteria can incorporate. Each bacterium can store multiple spacers in its CRISPR. Phages can escape recognition by the CRISPR system via point mutation or recombination. We will discuss the different evolutionary consequences of point mutation or recombination on the coevolution of bacteria and phage. We will also discuss an intriguing ``dynamic phase transition'' in the number of phage as a function of time and mutation rate. We will show that due to the arm race between phages and bacteria, the frequency of spacers and proto-spacers in a population can oscillate quite rapidly. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q10.00011: Genotype to Phenotype Mapping of the E. coli lac Promoter Jakub Otwinowski, Ilya Nemenman Genotype-to-phenotype maps and the related fitness landscapes that include epistatic interactions are difficult to measure because of their high dimensional structure. Here we construct such a map using the recently collected corpora of high-throughput sequence data from the 75 base pairs long mutagenized E. coli lac promoter region, where each sequence is associated with induced transcriptional activity measured by a fluorescent reporter. We find that the additive (non-epistatic) contributions of individual mutations account for about two-thirds of the explainable phenotype variance, while pairwise epistasis explains about 7\% of the variance for the full mutagenized sequence and about 15\% for the subsequence associated with protein binding sites. Surprisingly, there is no evidence for third order epistatic contributions, and our inferred fitness landscape is essentially single peaked, with a small amount of antagonistic epistasis. We identify transcription factor (CRP) and RNA polymerase binding sites in the promotor region and their interactions. We conclude with a cautionary note that inferred properties of fitness landscapes may be severely influenced by biases in the sequence data. [Preview Abstract] |
Session Q11: Nucleic Acids: Structure, Analysis, and Interaction with Proteins
Sponsoring Units: DBIORoom: 203
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q11.00001: The effect of long-range interactions in DNA melting Aaron Santos, William Klein` A theoretical understanding of the DNA melting transition may provide insight into the biological mechanisms of transcription and replication. If this process occurs via nucleation, it should exhibit several key features: metastability, rapid spontaneous growth, and droplet formation. In this talk, I describe the results of recent computational and theoretical studies on nearest-neighbor and long-range DNA models. While the models exhibit some characteristics of classical nucleation when the interaction range is short, they may undergo spinodal nucleation when the interaction range is long. In contrast to classical nucleation droplets, which are compact, spinodal critical droplets are diffuse, fractal-like, and similar to the metastable state. These results have clear implications for transcription and replication in biological DNA. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q11.00002: Correlated local bending of DNA and its effect on DNA flexibility Xinliang Xu, Jianshu Cao The flexibility of long DNA chains can be well described by the worm-like chain model (WLC) as a semi-flexible polymer with all local details coarse grained into one parameter, the persistence length l$_{\mathrm{p}}$ (approximately 150 base pairs). Recent experimental studies of DNA in the sub persistence length regime have shown a dramatic departure from WLC and suggested a length dependent DNA flexibility. Here we report an improved model of DNA flexibility with explicit considerations of a new length scale l$_{\mathrm{D}}$ (approximately 10 base pairs), over which DNA local bend angles are correlated (arxiv.org/abs/1309.7515). In this correlated worm-like chain (C-WLC) model, a finite length correction term is analytically derived and the persistence length is found to be contour length dependent. While our model reduces to the traditional worm-like chain model when treating long DNA at length scales much larger than l$_{\mathrm{p}}$, it predicts that DNA becomes much more flexible at shorter sizes, in good agreement with recent cyclization measurements of short DNA fragments around 100 base pairs. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q11.00003: Nucleosome phasing -- new insights Razvan Chereji Eukaryotic genomes are organized into arrays of nucleosomes, in which stretches of 147 base-pairs of DNA are wrapped around octameric histones. Recently, a new method of mapping nucleosome positions was developed, which gives a much higher accuracy than the typical MNase-seq method. I present a statistical mechanics model which is able to reproduce the high-resolution nucleosome positioning data. I show that the DNA sequence is not the main cause of the nucleosome phasing which is observed genome-wide, and I present the major nucleosome phasing elements. The statistical mechanics framework is general enough to be useful in explaining different experimental observations, and I present a few results of this model. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q11.00004: The DNA mismatch repair protein MutS forms a one-dimensional Tonks gas on DNA Ralf Bundschuh, Piotr Klajner, Jeungphill Hanne, Brooke M. Britton, Jianquan Liu, Jonghyun Park, Jong-Bong Lee, Richard Fishel MutS is a protein involved in DNA mismatch repair. It recognizes the mismatch, forms a sliding clamp around the DNA, and displaces other proteins bound to the DNA prior to the actual repair process. Here, we present a quantitative model of an ensemble of MutS molecules on a short strand of DNA with one mismatch. We model the ensemble as a Tonks gas of passively diffusing one-dimensional particles of finite extension and include clamp formation at the mismatch and random detachment. The distributions of MutS number bound to the DNA for different mismatch positions and different MutS concentrations in solution fit very well with distributions determined by single molecule experiments, thereby establishing the Tonks gas as an excellent model of MutS action on DNA. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q11.00005: Fluctuation and fidelity control of a non-proofreading polymerase Jin Yu Polymerases catalyze gene replication and transcription. They modulate activation barriers of nucleotide incorporation and amplify maximal free energy differentiation between the right and wrong nucleotides. It is essential for the polymerases to achieve sufficiently high fidelity at sufficiently high speed. We had noticed a small free energy bias in the translocation of T7 RNA polymerase (RNAP) that aids nucleotide selection. We investigated further how polymerases select against wrong nucleotides efficiently with given kinetics for the right, and with controlled differentiation capacities. We found that early selections on the reaction path outperform the late ones in error reduction. In particular, initial screening seems indispensable for lowering error rates without lowering much the speed. To see how exactly the nucleotide selection proceeds, we studied T7 RNAP also in atomistic simulations. We found that substantial nucleotide selection happens early, prior to full insertion of the nucleotide for complete Watson-Crick base pairing. A highly conserved residue brings up the small translocation energy bias by marginally blocking the active site. The residue senses the nucleotide species upon the nucleotide pre-insertion, and selectively `gates' the nucleotide during insertion. Our studies thus provide a kinetic survey of the nucleotide selection system along with underlying molecular mechanisms. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q11.00006: Loop cost in RNA secondary structures and the long-range cooperativity between RNA-binding proteins Yi-Hsuan Lin, Ralf Bundschuh The interactions between RNAs and RNA-binding proteins (RBPs) are significant in post-transcriptional regulation, and thus ensure that messenger RNAs can perform appropriate biological functions. Typically, in post-transcriptional regulation a single RNA is bound by multiple RBPs, which are likely to work together, resulting in ``cooperativity.'' This cooperativity can be a consequence of a mechanism mediated by RNA secondary structures, without assuming any direct interaction between the RBPs. The basic idea is that a bound RBP prohibits the nucleobases in its footprint from forming base pair bonds with other bases, thus changing the ensemble of RNA secondary structures, resulting in a shift on the binding probability of the other RBPs on the same RNA. We focus on the simplest RNA-protein complex: one RNA with two RBP binding sites. We study this effect analytically in the simplest model of RNA secondary structure formation, the molten RNA model. We measure the cooperativity as the correlation function between the RBPs and demonstrate that an algebraic correlation function occurs, implying that the cooperativity is long-range, and that a free energy cost for loop formation in the RNA secondary structures is the crucial ingredient that generates this cooperativity. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q11.00007: Single Molecule Observation of the Cyclization of Short DNA Duplex Teckla Akinyi, I-Ren Lee, Taekjip Ha In the presented work, a single molecule DNA cyclization assay was used to follow the looping kinetics of single DNA 83 bp molecules, utilizing single molecule fluorescence energy transfer (smFRET) technique. The assay was first prepared in a Na$^{+}$ free condition and the majority of the DNA was in its unlooped form. A sudden Na$^{+}$ jump was introduced at different concentrations (0.05-1.75M) and finally yielded DNA in its looping state by annealing the complementary single-strand overhangs of the assay. Looping and unlooping rates were obtained from the kinetic measurements. The result shows a positive and negative linear dependence of the Na$^{+}$ concentration to the looping and unlooping rate, respectively, until they reach a plateau at 500 mM. The plateau persists until about 1M. For concentrations beyond 1M, an immoderate increase in looping rate is noticed while the unlooping rate does so gradually. Above 1M Na$^{+}$ there is a preference of looping events that is attributed to the increase of the annealing rate of the overhangs rather than increased flexibility, consistent with earlier studies by Ibrahim Cisse \textit{et al. }(2012). A protein mediated cyclization assay was also used in experiments with HU protein in which a dramatic increase in the looping rate is noticeable. However in high HU concentration, looping is prohibited implying filament formation. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q11.00008: Structure and Dynamics of the tRNA-like Structure Domain of Brome Mosaic Virus Mario Vieweger, David Nesbitt Conformational switching is widely accepted as regulatory mechanism in gene expression in bacterial systems. More recently, similar regulation mechanisms are emerging for viral systems. One of the most abundant and best studied systems is the tRNA-like structure domain that is found in a number of plant viruses across eight genera. In this work, the folding dynamics of the tRNA-like structure domain of Brome Mosaic Virus are investigated using single-molecule Fluorescence Resonance Energy Transfer techniques. In particular, Burst fluorescence is applied to observe metal-ion induced folding in freely diffusing RNA constructs resembling the 3'-terminal 169nt of BMV RNA3. Histograms of E$_{\mathrm{FRET}}$ probabilities reveal a complex equilibrium of three distinct populations. A step-wise kinetic model for TLS folding is developed in accord with the evolution of conformational populations and structural information in the literature. In this mechanism, formation of functional TLS domains from unfolded RNAs requires two consecutive steps; 1) hybridization of a long-range stem interaction followed by 2) formation of a 3' pseudoknot. This three-state equilibrium is well described by step-wise dissociation constants $K_{1}$\textit{ (328(30) }$\mu M)$ and $K_{2}$\textit{ (1092(183) }$\mu M) $for [Mg$^{\mathrm{2+}}$] and $K_{1}$\textit{ (74(6) mM)} and $K_{2}$\textit{ (243(52) mM)} for [Na$^{\mathrm{+}}$]-induced folding. The kinetic model is validated by oligo competition with the STEM interaction. Implications of this conformational folding mechanism are discussed in regards to regulation of virus replication. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q11.00009: Re-sensitizing drug-resistant bacteria to antibiotics by designing Antisense Therapeutics Colleen Courtney, Anushree Chatterjee ``Super-bugs'' or ``multi-drug resistant organisms'' are a serious international health problem, with devastating consequences to patient health care. The Center for Disease Control has identified antibiotic resistance as one of the world's most pressing public health problems as a significant fraction of bacterial infections contracted are drug resistant. Typically, antibiotic resistance is encoded by ``resistance-genes'' which express proteins that carryout the resistance causing functions inside the bacterium. We present a RNA based therapeutic strategy for designing antimicrobials capable of re-sensitizing resistant bacteria to antibiotics by targeting labile regions of messenger RNAs encoding for resistance-causing proteins. We perform \textit{in silico} RNA secondary structure modeling to identify labile target regions in an mRNA of interest. A synthetic biology approach is then used to administer antisense nucleic acids to our model system of ampicillin resistant \textit{Escherichia coli}. Our results show a prolonged lag phase and decrease in viability of drug-resistant~\textit{E. coli~}treated with antisense molecules. The antisense strategy can be applied to alter expression of other genes in antibiotic resistance pathways or other pathways of interest. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q11.00010: RNA secondary structure critical exponents of random sequences near the glass transition William Baez, Ralf Bundschuh RNA forms elaborate secondary structures through intramolecular base pairing. These structures are important for the RNA's biological function but, due to the availability of a polynomial algorithm to calculate the partition function, they are also a model system for the study of statistical physics of disordered systems. In this context, it is known that below the denaturation temperature random RNA secondary structures can exist in one of two phases: a strongly disordered, low-temperature glass phase and a weakly disordered, high-temperature molten phase. The probability of two bases pairing in these phases have been shown to scale with the distance between the two bases as -3/2 and -1.33 in the molten and glass phases, respectively. In this study, we attempt to answer the question as to the value and behavior of this scaling exponent at and around the transition temperature. We present a precise determination of the location of the critical point and then use several methods to measure the exponent at this critical point including a comparison of different analytical models to describe finite-size effects developed within both phases. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q11.00011: Probe DNA-Cisplatin Interaction with Solid-State Nanopores Zhi Zhou, Ying Hu, Wei Li, Zhi Xu, Pengye Wang, Xuedong Bai, Xinyan Shan, Xinghua Lu Understanding the mechanism of DNA-cisplatin interaction is essential for clinical application and novel drug design. As an emerging single-molecule technology, solid-state nanopore has been employed in biomolecule detection and probing DNA-molecule interactions. Herein, we reported a real-time monitoring of DNA-cisplatin interaction by employing solid-state SiN nanopores. The DNA-cisplatin interacting process is clearly classified into three stages by measuring the capture rate of DNA-cisplatin adducts. In the first stage, the negative charged DNA molecules were partially discharged due to the bonding of positive charged cisplatin and forming of mono-adducts. In the second stage, forming of DNA-cisplatin di-adducts with the adjacent bases results in DNA bending and softening. The capture rate increases since the softened bi-adducts experience a lower barrier to thread into the nanopores. In the third stage, complex structures, such as micro-loop, are formed and the DNA-cisplatin adducts are aggregated. The capture rate decreases to zero as the aggregated adduct grows to the size of the pore. The characteristic time of this stage was found to be linear with the diameter of the nanopore and this dynamic process can be described with a second-order reaction model. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q11.00012: Entropic trapping of single DNA molecules emerging from a nanopore Xu Liu, Mirna Mihovilovic, Derek Stein We developed nanostructures with a cavity that receives and entropically traps a single DNA molecule after it translocates a nanopore in the cavity wall. The 1.5 $\mu $m-high, 2.2 $\mu $m-wide cavity has a 200 nm-wide opening across from the nanopore that is too large to affect the electrical resistance of the structure in solution, but small enough to confine $\lambda$ DNA. A voltage bias drew a DNA molecule through the nanopore, resulting in a blockage of the ionic current. 2 ms after the end of the translocation was detected, the bias was removed. A predetermined pause time, $t_p$, elapsed before a bias of the opposite polarity was applied. The current was monitored to detect the recapture of the same molecule. We found that the mean interval between the voltage reversal and the molecule's recapture, $t_r$, increased with $t_p$ until $t_p= 700$ ms, where it saturated at $t_r\approx 250$ ms. The molecules were recaptured with nearly unit efficiency for all $t_p$ tested, up to $t_p=50$ s. By contrast, when DNA emerged from a nanopore into an open reservoir with no cavity, $t_r$ increased continuously with $t_p$, and the probability of recapturing the molecule within 5 s of the voltage reversal dropped precipitously for $t_p>1$ s. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q11.00013: Mechanism of DNA Trapping in Nanoporous Structures during Asymmetric Pulsed-Field Electrophoresis Ya Zhou, D. Jed Harrison DNA molecules (\textgreater 100kbp) are trapped in separation sieves when high electric fields are applied in pulsed field electrophoresis, seriously limiting the speed of separation. Using crystalline particle arrays, to generate interstitial pores for molecular sieving, allows higher electric fields than in gels, (e.g 40 vs 5 V/cm), however trapping still limits the field strength. Using reverse pulses, which release DNA from being fully-stretched, allows higher fields (140 V/cm). We investigate the trapping mechanism of individual DNA molecules in ordered nanoporous structures. Two prerequisites for trapping are revealed by the dynamics of single trapped DNA, hernia formation and fully-stretched U/J shapes. Fully stretched DNA has longer unhooking times than expected by simple models. We propose a dielectrophoretic (DEP) force reduces the mobility of segments at the apex of the U or J, where field gradients are highest, based on simulations. A modified model for unhooking time is obtained after the DEP force is introduced. The new model explains the unhooking time data by predicting an infinite trapping time when the ratio of arm length differences (of the U or J) to molecule length $\Delta x/L<\beta $. $\beta $ is a DEP parameter that is found to strongly increase with electric field. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q11.00014: DNA transport and conformation in confined environments: novel separation mechanism using hydrodynamics and electrophoresis Hubert Ranchon, Qihao He, Joris Lacroix, Aur\'elien Bancaud Nanofluidics has gained popularity because it offered new solutions for single molecule manipulation and for the separation of biomolecules [1]. In addition to confinement, which enables to induce the elongation of DNA through steric repulsion, we recently showed that the degree of spreading of single molecules could be monitored by tuning the flow in nanochannels [2]. In this report we investigate the concomitant flow actuation with hydrodynamics and electrophoresis to transport DNA molecules in confined slit-like channels. We demonstrate that DNA size separation can be performed with no separation matrix, and we prove that our approach outperforms conventional separation methods, e.g. gel electrophoresis, in terms of separation performances, because we report power scaling dependence of up to -3 for the DNA mobility \textit{vs}. size response. We also describe the physics of DNA migration by single molecule microscopy and provide a mechanistic model of the separation.\\[4pt] [1] Dorfman, \textit{AIChE} 59, 346 (2013).\\[0pt] [2] He, \textit{Macromolecules} 46, 6195 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q11.00015: High-throughput DNA Stretching in Continuous Elongational Flow for Genome Sequence Scanning Robert Meltzer, Joshua Griffis, Mikhail Safranovitch, Gene Malkin, Douglas Cameron Genome Sequence Scanning (GSS) identifies and compares bacterial genomes by stretching long (60 -- 300 kb) genomic DNA restriction fragments and scanning for site-selective fluorescent probes. Practical application of GSS requires: 1) high throughput data acquisition, 2) efficient DNA stretching, 3) reproducible DNA elasticity in the presence of intercalating fluorescent dyes. GSS utilizes a pseudo-two-dimensional micron-scale funnel with convergent sheathing flows to stretch one molecule at a time in continuous elongational flow and center the DNA stream over diffraction-limited confocal laser excitation spots. Funnel geometry has been optimized to maximize throughput of DNA within the desired length range (\textgreater 10 million nucleobases per second). A constant-strain detection channel maximizes stretching efficiency by applying a constant parabolic tension profile to each molecule, minimizing relaxation and flow-induced tumbling. The effect of intercalator on DNA elasticity is experimentally controlled by reacting one molecule of DNA at a time in convergent sheathing flows of the dye. Derivations of accelerating flow and non-linear tension distribution permit alignment of detected fluorescence traces to theoretical templates derived from whole-genome sequence data. [Preview Abstract] |
Session Q12: Invited Session: Irreversibilty and Entropy Production in Biological Dynamics
Sponsoring Units: GSNP DBIOChair: Jeremy England, Massachusetts Institute of Technology
Room: 205
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q12.00001: Roles of Entropic Funnels and Irreversibility in RecA Mediated Homology Recognition Invited Speaker: Mara Prentiss The self-assembly complex systems requiring the correct pairing of more than approximately 3 distinct binding sites faces significant entropic barriers and can suffer from kinetic trapping in conformations containing some correct pairings. If the pairings have energies of the order of the thermal energy kT, then thermodynamic pairing cannot provide the stringencies required for biological systems. It is well known that kinetic proofreading systems can provide much better stringency by including an irreversible step; however, simple versions of such systems fundamentally tradeoff speed and stringency. RecA mediated homology recognition is an example of a system that can provide excellent rapid recognition that can last for days without irreversibility. The combination of speed and stability in the absence of irreversibility depends on the probability that accidental matches extend over of m contiguous binding sites. If the probability decreases sufficiently strongly with m, rapid and efficient homology recognition can occur via a system of checkpoints that limit the number of binding sites that can come in contact, which provides enthalpic and entropic advantages. Increasing the number of contacts requires passing sequence dependent energy barriers. The simplest version of such a system is an initial weakly bound state that is independent of site matching, which is separated from the next conformation by a sequence dependent barrier. The sequence dependent barrier for correct matches must be low enough for the correct match to progress to the next conformation before unbinding from the initial state, whereas the barrier for mismatches must be high enough that it is highly probable that the mismatch will unbind before they pass through the barrier. RecA employs a series of several sequence dependent barriers. The energy gap that reduces the need for irreversibility is the result of the correct pairing having orders of magnitude more contiguous matching sites than the nearest mismatch present in the sample. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q12.00002: Thermodynamics meets information in copolymerization processes Invited Speaker: Pierre Gaspard Copolymers are natural supports of information. This latter is contained in the sequence of monomeric units composing every copolymer. A well-known example is DNA in biology. At the molecular scale, the growth of a single copolymer is stochastic and proceeds by successive random attachments or detachments of monomers continuously supplied by the surrounding solution. The thermodynamics of copolymerization with or without a template shows that fundamental links exist between entropy production and the information content of the copolymer sequence [1,2]. During depolymerization, this information is erased in a way compatible with Landauer's principle [3]. These advances open new perspectives to understand information transmission during DNA replication and, more generally, information processing at the molecular scale in biology and polymer science. \\[4pt] [1] D. Andrieux and P. Gaspard, Nonequilibrium generation of information in copolymerization processes, Proc. Natl. Acad. Sci. USA 105, 9516 (2008). \\[0pt] [2] D. Andrieux and P. Gaspard, Molecular information processing in nonequilibrium copolymerizations, J. Chem. Phys. 130, 014901 (2009). \\[0pt] [3] D. Andrieux and P. Gaspard, Information erasure in copolymers, EPL 103, 30004 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q12.00003: Talk 4 Invited Speaker: Pankaj Mehta |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q12.00004: Reliable cell cycle commitment in budding yeast is ensured by signal integration Invited Speaker: Chao Tang Cells have to make reliable decisions in response to external and/or internal signals that can be noisy and varying. For budding yeast \textit{Saccharomyces cerevisiae}, cells decide whether and when to commit to cell division at the Start checkpoint. The decision is irreversible and has the physiological significance for coordinating cell growth with cell division. The trigger of the Start, the G1 cyclin Cln3 is a dynamic sensor of the nutrient and cellular conditions with low copy number and rapid turnover time. Here we quantitatively investigate how cells process the information from Cln3 to make the Start decision. By using an inducible Cln3 and monitoring the time cell waits before Start transition (G1 length), we find that G1 length is inversely proportional to Cln3 concentration, which implies that Start is triggered when the integration of Cln3 concentration over time exceeds certain threshold. We identify the Start repressor, Whi5 as the integrator. The instantaneous kinase activity of Cln3-Cdk1 is recorded over time on the phosphorylated Whi5, and the decision is made only when the phosphorylation level of Whi5 reaches a threshold. Furthermore, we find that Whi5 plays an important role for coordinating growth and division -- cells modulate Whi5 level in different nutrient conditions to adjust the Start threshold. The strategy of signal integration, which reduces noise and minimizes error and uncertainty, has been found in decision-making behaviors of animals. Our work shows that it is adopted at the cellular level, suggesting a general design principle that may be widely implemented in decision-making and signaling systems. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:30PM |
Q12.00005: Maximum entropy, Nonadditive entropies and Biology Invited Speaker: Steve Presse Gibbs once presciently noted that the elegance and simplicity of the principles of statistical physics were worthy of independent development outside of thermodynamics. Biophysical systems --from the single cell to the single protein level-- provide an ideal framework in which to test and apply far-from-equilibrium generalizations of statistical physics. Here we discuss two theoretical topics at the intersection of statistical physics and biology. First, we will describe a recipe for deriving, from first principles, probabilistic equations of motion from limited biophysical single particle tracking data. That is, we will show that maximum entropy principles can be used to determine the most likely statistical weights of trajectories from an ensemble of allowed system trajectories. For instance, using this reasoning, we can show under what circumstances Markov processes and chemical master equations rigorously follow from the data. Second, we will explore the logical implications of using a principle other than maximum entropy to select models (e.g. a model could be a trajectory ensemble in conformational space of a biomolecule) from non-equilibrium biophysical data. In particular, we will show that nonadditive entropy maximization can lead to biophysical models with features that go beyond what is warranted by the data. [Preview Abstract] |
Session Q13: Focus Session: Fe-based Superconductors-STM and Neutrons
Sponsoring Units: DMPChair: Andreas Kreyssig, Iowa State University
Room: 207
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q13.00001: The electronic phase diagram of NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As studied by scanning tunneling microscopy Invited Speaker: Yayu Wang Similar to the high $T_{\mathrm{C}}$ cuprates, the iron pnictide superconductors also lie in close proximity to a magnetically ordered phase. The interplay between magnetism and superconductivity (SC) is a central issue concerning the pairing mechanism. A key step for resolving this issue is to acquire a comprehensive picture regarding the nature of various phases and interactions in the iron-based compounds. In this talk we present doping, temperature, and spatial evolutions of the electronic structure of NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As studied by scanning tunneling microscopy. The spin density wave (SDW) gap in the parent state is directly observed, which shows a strongly asymmetric lineshape that is incompatible with conventional Fermi surface nesting. In the underdoped regime the SDW and SC phases are shown to microscopically coexist and compete with each other. The optimally doped sample exhibits a single SC gap, but in the overdoped regime another asymmetric gap-like feature emerges near the Fermi level. In contrast to the rich variations of the low energy electronic states, the high energy spectra of the NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As system remain nearly unchanged until the system enters the strongly overdoped non-SC regime. The implications of the local electronic structures on the pairing mechanism of the iron pnictides will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q13.00002: The upper critical field of NaFe$_{1-x}$Co$_x$As superconductors Saman Ghannadzadeh, Jack D. Wright, Francesca R. Foronda, Stephen J. Blundell, Simon J. Clarke, Paul A. Goddard Measurement of the upper critical field $H_{\rm c2}$ is a crucial part of the experimental effort to understand unconventional superconductivity. Studying the temperature dependence of $H_{\rm c2}$ and the interplay between the various pair-breaking mechanisms may hint towards the underlying pair-forming interaction. $H_{\rm c2}$ and its anisotropy are also sensitive to the cooper-pair symmetry, as well as the underlying dimensionality and electronic structure of the system. Here, we present the results of upper critical field measurements on the 111 system NaFe$_{1-x}$Co$_x$As in fields up to 45 T, across the phase diagram from the parent to the over-doped compound. We show that a multi-band model is required to describe $H_{\rm c2}$. We find the in-plane critical field to be strongly dominated by paramagnetic pair-breaking. In the parent compound the paramagnetic limit is shown to be equal to the BCS value; however addition of Co leads to a significant enhancement of the paramagnetic limit above the BCS value, most likely due to a combination of spin-orbit and strong-coupling effects. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q13.00003: Directly Visualizing Bogoliubov Quasiparticle Interference of LiFeAs: a Way Toward Understanding Superconductivity in Iron Pnictides Shun Chi, S. Johnston, G. Levy, S. Grothe, R. Szedlak, B. Ludbrook, Ruixing Liang, P. Dosanjh, S.A. Burke, A. Damascelli, D.A. Bonn, W.N. Hardy, Y. Pennec Imaging quasiparticle interference (QPI) is a way to probe the electronic states of a wide range of materials. In superconductors, QPI of Bogoliubov excitations is directly sensitive to the sign changes of the superconducting order parameter. In this talk, I present our investigation of QPI in superconducting LiFeAs by means of scanning tunneling microscopy/spectroscopy, angle resolved photoemission spectroscopy, and multi-orbital tight binding calculations. Using this combination we identify intra- and interband scattering vectors between the hole ($h$) and electron ($e$) bands in the QPI maps. Bogoliubov QPI, with a clear antisymmetric phase at positive and negative bias voltages near the superconducting gap, is revealed in the spatial modulations of the local density of states. The observation of both h-h and e-h scattering intensity variations is exploited using scattering selection rules for Bogoliubov quasiparticles. From this we infer an $s+-$ gap structure, where a sign change occurs in the superconducting order parameter between the $e$ and $h$ bands. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q13.00004: Doping dependence of nematicity in NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As imaged with scanning tunneling spectroscopy Ethan Rosenthal, Erick Andrade, Carlos Arguello, Rafael Fernandes, Li-Ying Xing, Xian-Cheng Wang, Chang-Qing Jin, Andrew Millis, Abhay Pasupathy Multiple experiments have found evidence for broken C$_{\mathrm{4}}$ rotational symmetry (nematicity) in the electronic structure of iron-based superconductors above the bulk magnetic and structural transition temperatures and across the doping phase diagram. Deducing the relationship between this broken symmetry state and the proximal superconducting state is essential to understanding the nature of the unconventional superconductivity. Many aspects of the nematic state still remain unknown given the bulk-probing nature of many measurements and their inability to probe beneath the superconducting dome. We use atomic-resolution, scanning tunneling spectroscopy (STS) to examine electronic nematicity across the phase diagram in NaFe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$As. We find that electronic anisotropy persists both above and below the superconducting dome from the parent compound to the overdoped regime. The strength of the nematicity decreases with increased doping and finally disappears in heavily doped, non-superconducting samples. With the spectral resolution of STS, we will discuss the energy dependence of the nematicity, as well as its interplay with magnetic and superconducting order. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q13.00005: Multiple superconducting gaps in hole-doped Ba$_{0.85}$K$_{0.15}$Fe$_{2}$As$_{2}$ observed by nano-scale Andreev reflection spectroscopy Guoxiong Su Recently, intensive attention has been paid to iron-based superconductors owing to their high transition temperature and intriguing physical properties, especially the mechanism for the superconductivity. Here we investigate the gap structure of Ba$_{0.85}$K$_{0.15}$Fe$_{2}$As$_{2}$ with Tc $\sim$ 25 K. By employing a novel experimental approach to point-contact Andreev reflection spectroscopy, experimental support has been found for the multiple superconducting gaps in hole-doped Ba$_{0.85}$K$_{0.15}$Fe$_{2}$As$_{2}$. The effects of temperature and magnetic field will be discussed as well. We also investigate the behavior of Andreev reflection spectrum at high biased voltage range. This work opens up new opportunity to understand the pairing mechanism and study gap structures in iron-based superconductors. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q13.00006: The low temperature magnetic structure of superconducting FeTeO$_{\mathrm{x}}$ films L.K. Narangammana, J.I. Budnick, W.A. Hines, C. Niedermayer, E.E. Alp, W. Bi, D.G. Hinks, D.E. Brown, B.O. Wells We compare the temperature dependent magnetic structure of superconducting FeTeO$_{\mathrm{x}}$ and non-superconducting FeTe films using neutron diffraction. Both show an antiferromagnetic transition below 70 K. The major difference between the two is that the superconducting FeTeO$_{\mathrm{x}}$ film shows a distinct reduction in magnetic order around the superconducting transition temperature (13 K) while the non-superconducting FeTe film shows a smoothly developing magnetic order as a function of temperature. Preliminary M\"{o}ssbauer spectroscopy studies done on the superconducting FeTeO$_{\mathrm{x}}$ indicate that film undergoes an antiferromagnetic transition below 50 K and magnetic order still exists in superconducting state. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q13.00007: Determination of a Magnetic Component to the Superconducting Condensation Energy for Fe$_{\mathrm{1+\delta }}$Se$_{\mathrm{x}}$Te$_{\mathrm{1-x}}$ Jonathan Leiner, Vivek Thampy, Mark Lumsden, Andrew Christianson, Douglas Abernathy, Brian Sales, Athena Sefat, Zhiqiang Mao, Jin Hu, Wei Bao, Collin Broholm A quantitative method to extract a magnetic component of the superconducting condensation energy from inelastic neutron scattering data is described and applied to Fe$_{\mathrm{1+\delta }}$Se$_{\mathrm{0.4}}$Te$_{\mathrm{0.6}}$. Based on the first moment sum-rule for the dynamic correlation function, the method is sensitive to changes in the inter-site magnetic correlation energy, $\Delta E_{\mathrm{ij}}$, associated with superconductivity. We find the length scale over which $\Delta E_{\mathrm{ij\thinspace }}$is appreciable coincides with the superconducting coherence length as determined by Scanning Tunneling Microscopy. The overall change in inter-site magnetic correlation energy is compared to the superconducting condensation energy determined through specific heat measurements. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q13.00008: Neutron Scattering Study of Low Energy Magnetic Excitation in FeTeSe System Zhijun Xu, Jinsheng Wen, John Schneeloch, Masaaki Matsuda, A.D. Christianson, Genda Gu, I.A. Zaliznyak, Guangyong Xu, J.M. Tranquada, R.J. Birgeneau We have performed neutron scattering and magnetization/transport measurements on a series of FeTe1-xSex system single crystals to study the interplay between magnetism and superconductivity. Comparing to pure FeTe1-xSex compounds, extra Fe and Ni/Cu doping on Fe-site can change physics properties of these samples, including resistivity, magnetization and superconducting properties. Our neutron scattering studies also show the Fe-site doping change low energy magnetic spectrum, including the magnetic excitations intensity, position and magnetic correlation length in these samples. On the other hand, the temperature dependence of the low energy magnetic fluctuations are also found to be different depending on the composition. This work is supported by the Office of Basic Energy Sciences, DOE. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q13.00009: Observation of acoustic-phonon-like mode driven by magnetic imbalance between neighboring Fe atoms in Fe$_{1+y}$Te ($y < 0.12$) David Fobes, Igor Zaliznyak, Zhijun Xu, Genda Gu, John M. Tranquada, Xu-Gang He, Wei Ku, Ovidiu Garlea We have studied the evolution with temperature of the low-energy inelastic spectra of Fe$_{1+y}$Te ($y < 0.12$), a parent compound of the iron-chalcogenide superconductor family, revealing an acoustic mode at an unexpected position. Recently, we found evidence for the formation of a bond-order wave leading to ferro-orbital order in the monoclinic phase, in part due to the observation of an elastic structural peak at (100) in the low-temperature monoclinic phase [D. Fobes, \textit{et al.}, arXiv:1307.7162]. In the inelastic spectra we observe a sharp acoustic-phonon-like mode dispersing out of the (100) position in the monoclinic phase. Surprisingly, the mode survives in the tetragonal phase, despite the absence of a Bragg peak at (100); such a peak is forbidden by symmetry. LDA calculations suggest this mode could involve significant magnetic scattering. By assuming in-phase virtual displacement of the Fe atoms from their equilibrium position in a frozen phonon calculation, we have found a small but significant imbalance in the magnetic moments between the two Fe atoms within the unit cell, suggesting magnetic contribution to the mode. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q13.00010: The non-magnetic collapsed tetragonal phase of CaFe$_{2}$As$_{2}$ and superconductivity in the iron pnictides J.H. Soh, G.S. Tucker, D.K. Pratt, D.L. Abernathy, M.B. Stone, S. Ran, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, R.J. McQueeney, A.I. Goldman The relationship between antiferromagnetic spin fluctuations and superconductivity has become a central topic of research in studies of superconductivity in the iron pnictides. We present unambiguous evidence of the absence of magnetic fluctuations in the non-superconducting collapsed tetragonal phase of CaFe$_{2}$As$_{2}$ via inelastic neutron scattering time-of-flight data, which is consistent with the view that spin fluctuations are a necessary ingredient for unconventional superconductivity in the iron pnictides. We demonstrate that the collapsed tetragonal phase of CaFe$_{2}$As$_{2}$ is non-magnetic, and discuss this result in light of recent reports of high-temperature superconductivity in the collapsed tetragonal phase of closely related compounds. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q13.00011: The structural and magnetic properties of Cs$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$ as determined by x ray and neutron scattering of powder and single crystal samples Keith Taddei, Omar Chmaissem, Mihai Sturza, Sevda Avci, Helmut Claus, Mercouri Kanatzidis, Stephan Rosenkranz, Ray Osborn The A$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_{2}$ family of iron selenides (A $=$ K, Rb and Cs) has proven an intricate system for the study of unconventional superconductivity, exhibiting high temperature superconductivity ($\sim$ 30 K) and a complex structural phase transition into a biphasic state coupled with a high temperature magnetic transition ($\sim$ 500 K). While isostructural to the 122 arsenides, significant structural differences are identified. In the selenides, iron vacancies in the tetrahedral FeSe layers become ordered below a high temperature structural transition defining a main phase $\surd 5 \times \surd 5$ superstructure. Coexistent with the main phase, a secondary phase of a previously contested structure is observed and it is in this biphasic state that superconductivity arises at $\sim$ 30 K. Both powder and single crystal samples show similar phase separation and coexistence. In this talk, I will discuss structural results and lattice parameter evolution obtained from neutron powder diffraction as well as single crystal x-ray diffraction with an emphasis on a novel magnetic structural model, the identification of the secondary phase, and the nature of coincidence of the magnetic, structural and secondary phase transitions. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q13.00012: Magnetic Excitations in Cr-Ru Superconducting Alloy Mehmet Ramazanoglu, Robert J. McQueeney, Thomas A. Lograsso, Deborah L. Schagel, Andreas Kreyssig, Alan I. Goldman, Daniel Pratt, Jeffrey W. Lynn, Garreth Granroth We have studied the spin fluctuations in the normal state of a cubic superconductor Cr$_{(1-x)}$Ru$_{x}$ for x$=$0.2. The electron doping created by Ru ions in Cr$_{(1-x)}$Ru$_{x}$ monotonically decreases the antiferromagnetic (AFM) spin density wave (SDW) transition temperature, T$_{N}$. As the Ru fraction increases through x$=$0.17 [1], the long-range SDW order is completely suppressed and the alloy becomes a superconductor. We have conducted a series of inelastic neutron scattering (INS) experiments with Ru concentration of x$=$0.2 with a superconducting transition temperature of Tc $\sim$1.6 K. The SDW fluctuations are found to be commensurate with the magnetic propagation vector of Q$_{AFM}=$(100). At high energy transfers, the strong spin fluctuations appearing near Q$_{AFM}$ reach beyond dE$=$120 meV, not unlike metallic Cr, where spin excitations are very energetic and can reach up to several hundreds of meV [2]. These excitations are also found to be down to energies of dE$=$2 meV. We discuss the possibility that superconductivity found in the Cr-Ru alloy system is unconventional. \\[4pt] [1] K. Chatani and Y. Endoh, J. Phys. Soc. Of Jpn, 72 , 17 , (2003)\\[0pt] [2] E. Fawcett, et. al. Rev. Mod. Phys. 66, 25 (1994) [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q13.00013: Physical properties of Rh substituted CaFe$_{2}$As $_{2}$ tuned by annealing/quenching Sheng Ran, Sergey Bud'ko, Paul Canfield Our previous work on CaFe$_{2}$As$_{2}$ single crystal grown out of FeAs flux has shown that a process of annealing and quenching can be used as an additional control parameter which can tune the ground state of CaFe$_{2}$As$_{2}$ systematically. We have also shown that CaFe$_{2}$As$_{2}$ is very pressure sensitive. Therefore, unlike the BaFe$_{2}$As$_{2}$ system, the effect of 4d transition metal substitution on CaFe$_{2}$As$_{2}$ is expected to be largely different from that of 3d transition metal substitution (e.g. cobalt or nickel substitution). In this talk we will present results of measurements on a Rh substituted CaFe$_{2}$As$_{2}$ system with different annealing/quenching temperatures. Phase diagrams with substitution level and annealing/quenching temperature as independent parameters are constructed and compared with that of other transition metal substitutions. [Preview Abstract] |
Session Q14: Invited Session: Physics of Proteins: New Insights on Hydrogen Bonding and Proton Transfer
Sponsoring Units: DBIO DCOMPChair: Wouter Hoff, Oklahoma State University
Room: 301-303
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q14.00001: Functional Dynamics and Proton Transfer in Proteins Invited Speaker: Steven Boxer Internal proton transfer between an enzyme and substrate is a common feature of many enzyme mechanisms. Likewise, internal proton transfer between the chromophore of green fluorescent protein (GFP) and amino acids on the inside of the beta barrel are important both in the ground and excited state. I will discuss an interesting connection between the proton transfer dynamics in GFP and those in an enzyme, ketosteroid isomerase (KSI), bound to substrate analogs. In both cases there is a tug of war between the protein and bound substrate analog or chromophore that depends on their affinities for a proton and which can be tuned either by changing the substrate/chromophore or the protein. This can be observed in the ground state by optical methods (absorption and IR) as well as by nmr, or in the excited state by time-resolved fluorescence or visible pump-IR probe measurements. In both cases the proton dynamics have important functional consequences. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q14.00002: Proton transfer and water exchange in the green fluorescent protein Invited Speaker: Noam Agmon The green fluorescent protein (GFP) is the only naturally occurring protein in which excited-state proton-transfer has been identified. Upon excitation, a proton is ejected from its chromophore, travelling through the ``privileged water molecule'' (PWM) and Ser205 to Glu222, on a 10 ps timescale or faster. However, time-resolved fluorescence from the chromophore exhibits a $t^{-\alpha }$ power-law decay extending into the ns regime. With increasing temperature, $\alpha $ switches from 1/2 (below 230 K) to 3/2 (above it). This has been interpreted as pseudo one-dimensional proton hopping along an internal ``proton wire,'' with an activated process that opens a ``doorway'' for proton escape to solution at the higher temperatures [1]. To identify such putative pathways, we have developed a computer code mapping all ``proton wires'' within a protein structure. Applying it to a X-ray GFP structure of 0.9 Angstrom resolution [2], a proton wire indeed continues from Glu222 along the axis of the GFP ``barrel,'' connecting to a negatively charged surface patch (a ``proton collecting antenna''?). This might explain the t$^{-1/2}$ behavior. However, a direct escape pathway opening from the chromophore to solution is not readily identified in the X-ray structure. Here we report molecular dynamics results showing that the PWM escapes to solution on the 100 ps timescale. This occurs by fluctuations of the beta-sheet, creating an opening through which water molecules can leave and enter the protein. The exact pathway of the PWM on its way in and out has been identified, as well as the water-exchange kinetics that follows a stretched-exponential time behavior.\\[4pt] [1] Agmon, N. \textit{J. Phys. Chem. B} 2007, \textbf{111}, 7870.\\[0pt] [2] Shinobu, A.; Palm, G. J.; Schierbeek, A. J.; Agmon, N. \textit{J. Am. Chem. Soc.} 2010, \textbf{132}, 11093. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q14.00003: Principles and Dynamics of Proton Transfer in Proteins Invited Speaker: Aihua Xie Proton transfer is broadly employed in protein functions, not only in energy transformation but also in biological signaling and enzymatic catalysis. Unlike electron transfer which has been well understood for nearly two decades, some key questions regarding the physical mechanism of proton transfer remains elusive after extensive studies. We will report a proof of concept study on principles and dynamics of proton transfer and its applications in proteins. In addition, we will discuss how to apply time-resolved infrared structural biology to probe and explore proton transfer during protein functions. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q14.00004: Proton transfer pathways in Photosystem II Invited Speaker: Hiroshi Ishikita Using quantum mechanics/molecular mechanics calculations and the 1.9-{\AA} crystal structure of Photosystem II (Umena, Y., Kawakami, K., Shen, J.-R., and Kamiya, N. (2011) Nature 473, 55-60), we investigated the H-bonding environment of the redox active tyrosine, TyrD and obtained insights that help explain its slow redox kinetics and the stability of TyrD radical. The water molecule distal to TyrD, 4 {\AA} away from the phenolic O of TyrD (O$_{\mathrm{TyrD}})$, corresponds to the presence of the tyrosyl radical state. The water molecule proximal to TyrD, in H-bonding distance to O$_{\mathrm{TyrD,}}$ corresponds to the presence of the unoxidised tyrosine. The H$^{+}$ released upon oxidation of TyrD is transferred to the proximal water, which shifts to the distal position, triggering a concerted proton transfer pathway involving D2-Arg180 and a series of waters, through which the proton reaches the aqueous phase at D2-His61. The water movement linked to the ejection of the proton from the hydrophobic environment near TyrD makes oxidation slow and quasi-irreversible, explaining the great stability of the TyrD radical. A symmetry-related proton pathway associated with TyrZ is pointed out and this is associated with one of the Cl$^{-}$ sites. This may represent a proton pathway functional in the water oxidation cycle. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:30PM |
Q14.00005: Contribution of Hydrogen Bonds to Protein Stability Invited Speaker: Nick Pace I will discuss the contribution of the burial of polar groups and their hydrogen bonds to the conformational stability of proteins. We measured the change in stability, $\Delta (\Delta $G), for a series of hydrogen bonding mutants in four proteins: villin head piece subdomain (VHP) containing 36 residues, a surface protein from \textit{Borrelia burgdorferi} (VlsE) containing 341 residues, and two proteins previously studied in our laboratory, ribonucleases Sa (RNase Sa) and T1 (RNase T1). Crystal structures were determined for three of the hydrogen bonding mutants of RNase Sa: S24A (1.1{\AA}), Y51F(1.5{\AA}), and T95A(1.3{\AA}). The structures are very similar to wild type RNase Sa and the hydrogen bonding partners always form intermolecular hydrogen bonds to water in the mutants. We compare our results with previous studies of similar mutants in other proteins and reach the following conclusions: 1) Hydrogen bonds contribute favorably to protein stability. 2) The contribution of hydrogen bonds to protein stability is strongly context dependent. 3) Hydrogen bonds by side chains and peptide groups make similar contributions to protein stability. 4) Polar group burial can make a favorable contribution to protein stability even if the polar groups are not hydrogen bonded. 5) The contribution of hydrogen bonds to protein stability is similar for VHP, a small protein, and VlsE, a large protein. [Preview Abstract] |
Session Q15: Pattern Formation & Nonlinear Dynamics
Sponsoring Units: DFDChair: Sandra Troian, California Institute of Technology
Room: 304
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q15.00001: Coherent structures for front propagation in fluids Kevin Mitchell, John Mahoney Our goal is to characterize the nature of reacting flows by identifying important ``coherent'' structures. We follow the recent work by Haller, Beron-Vera, and Farazmand which formalized the notion of lagrangian coherent structures (LCSs) in fluid flows. In this theory, LCSs were derived from the Cauchy-Green strain tensor. We adapt this perspective to analogously define coherent structures in \emph{reacting} flows. By this we mean a fluid flow with a reaction front propagating through it such that the propagation does not affect the underlying flow. A reaction front might be chemical (Belousov-Zhabotinsky, flame front, etc.) or some other type of front (electromagnetic, acoustic, etc.). While the recently developed theory of burning invariant manifolds (BIMs) describes barriers to front propagation in time-periodic flows, this current work provides an important complement by extending to the aperiodic setting. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q15.00002: Exact coherent structures: from fluid turbulence to cardiac arrhythmias Roman Grigoriev, Christopher Marcotte, Gregory Byrne Ventricular fibrillation, a life threatening cardiac arrhythmia, is an example of spatiotemporally chaotic state dominated by multiple interacting spiral waves. Recent studies of weak fluid turbulence suggest that spatiotemporal chaos in general can be understood as a walk among exact unstable regular solutions (exact coherent states, ECS) of nonlinear evolution equations. Several classes of ECS are believed to play a dominant role; most typically these are equilibria and periodic orbits or relative equilibria and relative periodic orbits for systems with global continuous symmetries. Numerical methods originally developed in the context of fluid turbulence can also be applied to models of cardiac dynamics which possess translational and rotational symmetries and, indeed, allowed us to identify relative equilibria and periodic orbits describing isolated spirals with, respectively, fixed and drifting cores. In order to find regular solutions featuring multiple interacting spirals a new approach is required that takes into consideration the dynamics of slowly drifting cores associated with local, rather than global, symmetries. We describe how local symmetries can be reduced and more general types of ECS computed that dominate spiral wave chaos in models of cardiac tissue. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q15.00003: Untying the Knot: Topological Vortex Dynamics Dustin Kleckner, Martin Scheeler, Davide Proment, William T.M. Irvine Knots and links are thought to be associated with topologically conserved quantities in many physical fields, including quantum and classical fluids, plasmas and electromagnetic fields. Observing topological dynamics in experiment, however, has proven difficult. Recent advances have made it possible to generate and measure vortex knots in classical fluids, revealing that they spontaneously untie themselves through a series of topology-changing reconnections. Similar behavior is found for simulations of superfluid knots. We will discuss these dynamics as well as their implications for the role of knots in fluids and other areas of physics. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q15.00004: Lie Group Reduction Analysis of the Moving Boundary Problem Governing Benard-Like Fluctuations in Nanofilms Zachary Nicolaou, Sandra Troian The underlying mechanism responsible for the spontaneous formation of nanopillar arrays in thin viscous films whose free surface is subject to a large transverse thermal gradient continues to be debated. Recent experimental measurements by our group strongly suggest that thermocapillary forces play a central role in the formation and growth of these 3D periodic structures, in a process somewhat akin to the conventional Benard problem. Here we present both analytic and numerical results of the governing thin film equation in the long wavelength approximation for films of constant viscosity subject to capillary and thermocapillary forces. We focus on exact reductions of the highly nonlinear, fourth order equation of motion which reveal steady state solutions, similarity solutions, and other reductions obtained through Lie group analysis. In particular, we predict the possibility of solutions describing isolated droplet formation well beyond the linearized regime. A linear stability analysis of these solutions has been carried out numerically and relevant bounds on droplet stability obtained within restricted parameter regimes. Experimental realization of such isolated droplet formations may find use in scientific and industrial applications such as nanolenses or other optical components. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q15.00005: Reanalysis of Incipient Wavelength Measurements in Free Surface Nanofilms Undergoing Benard Instability Kevin Fiedler, Sandra Troian Ultrathin liquid films whose free surface is subject to large thermal gradients are known to develop spontaneous periodic arrays of nanopillars. Theoretical predictions based on linear stability theory in the long wavelength approximation suggest that these formations arise either from fluctuations in the electrostatic forces between the fluid and opposing substrate, acoustic phonon radiation pressure within the film, or Benard instability due to surface thermocapillary forces. Experimental confirmation of the mechanism responsible for such emergent structures requires measurements of the pattern formation process at very early times, a difficult task given that incipient film distortions are of the order of a few nanometers. We reported last year that while our measurements of the dominant wavelength seem to rule out electrostatic effects and acoustic radiation pressure at the source of instability, there remained significant discrepancy between the predictions of the thermocapillary model and measurements of the dominant wavelength obtained from image Fourier analysis. We describe how earlier time analysis and more accurate modeling of the temperature field derived from substrate resistive heating leads to much closer agreement with predictions of the thermocapillary model than previously reported. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q15.00006: The Origin and Evolution of Icicle Ripples Antony Szu-Han Chen, Stephen W. Morris Natural icicles often exhibit ripples about their circumference which are due to a morphological instability. We present an experimental study that explores the origin of the instability, using laboratory-grown icicles. Contrary to theoretical expectations, icicles grown from pure water do not exhibit growing ripples. The addition of a non-ionic surfactant, which reduces the surface tension, does not produce ripples. Instead, ripples emerge on icicles grown from water with dissolved ionic impurities. We find that even very small levels of impurity are sufficient to trigger ripples, and that the growth speed of the ripples increases only approximately logarithmically with impurity concentration. With impurities present, the ripple wavelength remains constant under all other variations of the growth conditions. Ripples are observed to travel during their growth. For low impurity concentrations, they travel upward at speeds of mm/hr. For higher impurity concentrations, some ripples moved nonlinearly and different ripples on the same icicle sometimes traveled in opposite directions. Existing theories of ripple formation do not account for the effect of impurities and cannot be easily generalized to include them. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q15.00007: Sidebranch development in free dendritic crystal growth Andrew Dougherty We report measurements of the free dendritic crystal growth of NH$_4$Cl from supersaturated aqueous solution at small supersaturations, with a goal of understanding the origin and development of the sidebranching structure. The origin of sidebranches is not fully understood. The functional form and scaling of the sidebranches seem reasonably consistent with a noise-driven model, but the amplitude of the branches in this system appears larger than would be expected from simple thermal noise. An underlying dynamic oscillation can not be ruled out. We do sometimes observe short regions with highly regular branches, but such regularity is usually quite short-lived. In the context of directional solidification, Pocheau, Bodea, and Georgelin [Phys. Rev. E 80, 031601 (2009)] found randomly-distributed bursts of sidebranches that had strong coherence within each burst, but that were not well-correlated between bursts. In this talk, I will explore applying that idea to free dendritic growth. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q15.00008: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q15.00009: Characterizing Branched Flow Byron Drury, Anna Klales, Eric Heller Branched flow appears in a variety of physical systems spanning length scales from microns to thousands of kilometers. For instance, it plays an important role in both electron transport in two dimensional electron gases and the propagation of tsunamis in the ocean. Branches have typically been identified with caustics in the theoretical literature, but concentrations of flux recognizable as branches can arise from other mechanisms. We propose a generalized definition of branching based on a local measure of the stability of trajectories. We analytically and numerically study the characteristics of Hamiltonian flow in phase space and characterize the relationship between branch formation and trajectory stability. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q15.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q15.00011: Collisions of localized convection structures in binary fluid mixtures Manfred Luecke, Alexander Taraut, Boris Smorodin Collisions of a localized traveling wave structure with a localized stationary structure are investigated. In ethanol-water mixtures with appropriately chosen negative separation ratios both exist bistably in the unstable quiescent surrounding for a range of supercritical heating rates. Depending on the Rayleigh number we observe different evolution scenarios of the onvection structures that appear as a result of the collision. The incident localized traveling wave can be absorbed by the stationary structure and then the latter expands: either both of its fronts get unpinned and propagate into the quiescent fluid or only the one that is hit propagates while the opposing one remains pinned. For smaller Rayleigh numbers the stationary structure is destroyed while the incident localized traveling wave survives and a second one is created that moves ahead of the incoming one, both being coupled together. The mechanisms involved in these scenarios are analyzed and elucidated with the help of finite difference numerical simulations that are carried out subject to realistic boundary conditions. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q15.00012: Excitability dependent pattern formation Kaumudi Prabhakara, Azam Gholami, Eberhard Bodenschatz On starvation, the amoebae Dictyostelium discoideum emit the chemo-attractant cyclic adenosine monophosphate (cAMP) at specific frequencies. The neighboring amoebae sense cAMP through membrane receptors and produce their own cAMP. Soon the cells synchronize and move via chemotaxis along the gradient of cAMP. The response of the amoebae to the emission of cAMP is seen as spiral waves or target patterns under a dark field microscope. The causal reasons for the selection of one or the other patterns are still unclear. Here we present a possible explanation based on excitability. The excitability of the amoebae depends on the starvation time because the gene expression changes with starvation. Cells starved for longer times are more excitable. In this work, we mix cells of different excitabilities to study the dependence of the emergent patterns on the excitability. Preliminary results show a transition from spirals to target patterns for specific excitabilities. A phase map of the patterns for different combinations of excitability and number densities is obtained. We compare our findings with numerical simulations of existing theoretical models. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q15.00013: Noise-induced nucleation in a bistable tunnel diode circuit R.A. McGeehan, S.J. Jones, Yu. Bomze, S.W. Teitsworth We report the measurement of first-passage time distributions associated with electrical current switching in a bistable tunnel diode circuit driven by a noise generator with adjustable noise intensity $D$. In such a system, it is particularly interesting to study the behavior of the mean switching time $\tau $ near the boundary of the bistable regime where the metastable state approaches and collides with a saddle point in the underlying noise-free dynamical system. In the tunnel diode circuit and for sufficiently large noise intensity, we find a \textit{linear} scaling relationship $\ln \tau \propto \left| {V-V_{th} } \right|/D$ valid over several decades of time, where $V$ denotes the applied voltage and $V_{th}$ denotes the value corresponding to the end of the bistable regime. At smaller noise intensities, we typically find that the mean switching time versus $V-V_{th}$ possesses multiple scaling regimes. These experimental results are interpreted in light of theoretical work that shows how lateral charge transport dynamics can strongly affect the noise-induced nucleation events that lead to current switching [1]. [1] O. A. Tretiakov and K. A. Matveev, Phys. Rev. B \textbf{71}, 165326 (2005). [Preview Abstract] |
Session Q16: Extreme Mechanics: (more) Toys, Theory, and Fluid-Structure Interaction
Sponsoring Units: GSNPChair: Douglas Holmes, Virginia Polytechnic Institute and State University
Room: 401
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q16.00001: A bicycle with compliant training wheels, half way between a bicycle and a tricycle, is uncontrollable Andy Ruina We have built and tested a vehicle that can balance and steer like a bicycle, a tricycle, or anything in between. A \textit{bricycle} is essentially a bicycle with springy training wheels. The stiffness of the training wheel suspension can be varied from infinite, when the bricycle is a tricycle, to zero, when it is a bicycle. One might expect a smooth transition from tricycle to bicycle as the stiffness is varied, in terms of handling, balance and feel. But the situation is more complicated. Rather, the controllability of a bicycle depends on gravity. Without gravity, lean and direction cannot be controlled independently. Springy training wheels effectively reduce or even negate gravity. Indeed, experiments with the bricycle show problems when the total effective gravity is about zero. People can then still balance easily but can no longer turn the brike. The theory and experiment show a qualitative difference between bicycles and tricycles. A difference that cannot be met halfway. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q16.00002: A string falling onto a table James Hanna In the light of recent thought-provoking experiments on different types of chains, I consider the problem of an inextensible/incompressible string falling under gravity and undergoing a collision with a rigid table. The question of interest is whether the free upper end of the string experiences an acceleration greater than a free-falling string. I find that the answer is yes, subject to an assumption about the boundary condition at the colliding lower end. I propose some new experiments and simulations to assess the validity of the assumption and test the prediction. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q16.00003: Computational Design of Animated Mechanical Characters Stelian Coros, Bernhard Thomaszewski A factor key to the appeal of modern CG movies and video-games is that the virtual worlds they portray place no bounds on what can be imagined. Rapid manufacturing devices hold the promise of bringing this type of freedom to our own world, by enabling the fabrication of physical objects whose appearance, deformation behaviors and motions can be precisely specified. In order to unleash the full potential of this technology however, computational design methods that create digital content suitable for fabrication need to be developed. In recent work, we presented a computational design system that allows casual users to create animated mechanical characters. Given an articulated character as input, the user designs the animated character by sketching motion curves indicating how they should move. For each motion curve, our framework creates an optimized mechanism that reproduces it as closely as possible. The resulting mechanisms are attached to the character and then connected to each other using gear trains, which are created in a semi-automated fashion. The mechanical assemblies generated with our system can be driven with a single input driver, such as a hand-operated crank or an electric motor, and they can be fabricated using rapid prototyping devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q16.00004: Slinky Mechanics: Static Shapes and Unstable States Douglas Holmes, Andy Borum, Billy Moore, Raymond Plaut, David Dillard The floppy nature of a tumbling Slinky has captivated children and adults alike for over half a century. Highly flexible, the spring will walk down stairs, turn over in your hands, and--much to the chagrin of children everywhere--become easily entangled. The Slinky is an educational tool for demonstrating standing waves, and a structural inspiration due to its ability to extend to many times beyond its initial length without imparting plastic strain on the material. In this work, we provide a mechanical model that captures the static equilibrium configurations of the Slinky in terms of its geometric and material properties. We present both continuous and discrete models to capture a Slinky's static equilibria and unstable transitions. We compare these with experimental results obtained for the Slinky's static equilibrium shapes. We emphasize the importance of modeling coil contact, and determine the critical criteria for the Slinky to topple over in terms of a tilt angle, and the vertical displacement of one bale of coils. Finally, we provide a general description of highly flexible helical springs by considering the nondimensional potential energy of the spring, which characterizes the ``Slinkiness'' of a spring. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q16.00005: The Shape of a Developable M\"{o}bius Strip via Classical Elastic Cosserat Rod Theory Alexander Moore, Timothy Healey Recent efforts to find the equilibrium shape of an inextensible elastic M\"{o}bius strip have produced apparently conflicting approaches and results. While an earlier approach uses the traditional one dimensional Kirchhoff elastic rod, the latest effort claims that the strip must be modeled as an elastic two dimensional developable surface. This study explains the source of the discrepancy and demonstrates that a classical one dimensional Cosserat elastic rod can capture both types of behavior. Using numerical continuation methods, we show how to adapt traditional rod theory to approximate developable elastic strips and apply our method to the M\"{o}bius problem. We further analyze the stability of the equilibria obtained. The adapted rod theory holds promise for modeling the mechanics of other thin structures subject to curvature constraints. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q16.00006: Euler-Lagrange Elasticity: elasticity without stress or strain Humphrey Hardy A Euler-Lagrange (E-L) approach to elasticity is proposed that produces differential equations of elasticity without the need to define stress or strain tensors. The positions of the points within the body are the independent parameters instead of strain. Force replaces stress. The advantage of this approach is that the E-L differential equations are the same for both infinitesimal and finite deformations. Material properties are expressed in terms of the energy of deformation. The energy is expressed as a function of the principal invariants of the deformation gradient tensor. This scalar invariant representation of the energy of deformation enters directly into the E-L differential equations so that there is no need to define fourth order tensor material properties. By experimentally measuring the force and displacement of materials the functional form of the energy of deformation can be determined. The E-L differential equations can be input directly into finite element, finite difference, or other numerical models. If desired, stress and stain can be calculated as dependent parameters. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q16.00007: The ridge between two fracture tips Robert Schroll, Juan Francisco Fuentealba, Enrique Cerda The shape of a fracturing thin sheet is governed by Griffith's criterion, wherein both the system's energy and the applied force are minimized. For a thin sheet adhered to a substrate, the important energies are those of adhesion and bending of the sheet. Without adhesion, the ridge connecting the crack tips need not be developable, and in-plane stretching energy may become important. A reasonable assumption is that this ridge take the shape of a minimal ridge. We present experimental and numerical results that show the shape of this configuration does resemble the minimal ridge. However, an anomalous energy scaling is observed. We also show that the ridge shape, and therefore energy balance, depends on the length of the flap being pulled, which suggests a mechanism for controlling crack shapes. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q16.00008: Kinks in topological soft matter Bryan Chen, Nitin Upadhyaya, Vincenzo Vitelli Weakly connected mechanical systems near the isostatic threshold are fragile in the sense that they exhibit large deformations in response to tiny perturbations. Kane and Lubensky have recently defined a new topological invariant of isostatic mechanical lattices which leads within linear elasticity to zero energy modes at the boundary akin to the edge modes studied in topological quantum matter. What happens when such prototype topological soft materials are subject to an external mechanical perturbation? In our work, we demonstrate that the linear soft modes can often integrate to non-linear deformations described by topological solitons. These solitons that are moving kinks between distinct topological phases are the basic excitations of fragile mechanical systems. We illustrate the general soliton construction in the context of a 1D chain of rotors connected by springs that can be considered the archetype of a topological mechanical structure. In the continuum limit, this chain is described by a Lorentz invariant $\phi^4$ theory and the corresponding solitons exhibit a Lorentz contraction of the width, as their speed is raised. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q16.00009: The sedimentation of flexible filaments Saverio Spagnolie, Lei Li, Harishankar Manikantan, David Saintillan The dynamics of a flexible filament sedimenting in a viscous fluid are explored analytically and numerically. Compared with the well-studied case of sedimenting rigid rods, the introduction of filament compliance is shown to cause a significant alteration in the long-time sedimentation orientation and filament geometry. A model is developed by balancing viscous, elastic and gravitational forces in a slender-body theory for zero-Reynolds-number flows, and the filament dynamics are characterized by a dimensionless elasto-gravitation number. In the weakly flexible regime, a multiple-scale asymptotic expansion is used to obtain expressions for filament translations, rotations and shapes which match excellently with full numerical simulations. Furthermore, we show that trajectories of sedimenting flexible filaments, unlike their rigid counterparts, are restricted to a cloud whose envelope is determined by the elasto-gravitation number. In the highly flexible regime we show that a filament sedimenting along its long axis is susceptible to a buckling instability. A linear stability analysis provides a dispersion relation, illustrating clearly the competing effects of the compressive stress and the restoring elastic force in the buckling process. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q16.00010: A flexible fiber in a turbulent flow: a macroscopic polymer? Gautier Verhille, Christophe Brouzet, Patrice Le Gal We describe, for the first time, an experiment devoted to the study of the spatial conformation of a flexible fiber in a turbulent flow. We propose a model for the transition from rigid to flexible regimes as the intensity of turbulence is increased or the elastic energy of the fiber is decreased. This transition occurs for a fiber typical length which is observed experimentally and recovered by our analysis. We also demonstrate that the conformations of flexible fibers in a turbulent flow are analog to conformations of flexible polymers in a good solvent. This last result opens some new and creative ways to model flexible fiber distortions in turbulent flows while addressing fundamental problems in polymer dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q16.00011: Propulsion at low Reynolds number via beam extrusion Frederick Gosselin, Paul Neetzow We present experimental and theoretical results on the extrusion of a slender beam in a viscous fluid. We are particularly interested in the force necessary to extrude the beam as it buckles with large amplitude due to viscous friction. The problem is inspired by the propulsion of Paramecium via trichocyst extrusion. Self-propulsion in micro-organisms is mostly achieved through the beating of flagella or cilia. However, to avoid a severe aggression, unicellular Paramecium has been observed to extrude trichocysts in the direction of the aggression to burst away. These trichocysts are rod-like organelles which, upon activation, grow to about $40~\mathrm{\mu m}$ in length in 3 milliseconds before detaching from the animal. The drag force created by these extruding rods pushing against the viscous fluid generates thrust in the opposite direction. We developed an experimental setup to measure the force required to push a steel piano wire into an aquarium filled with corn syrup. This setup offers a near-zero Reynolds number, and allows studying deployments for a range of constant extrusion speeds. The experimental results are reproduced with a numerical model coupling a large amplitude Euler-Bernoulli beam theory with a fluid load model proportional to the local beam velocity. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q16.00012: The propulsion of filaments with natural curls Noor Khouri, Mohammad Jawed, Fang Da, Eitan Grinspun, Pedro Reis We consider a macroscopic analogue model for the locomotion of prokaryotic bacteria with a single flagellum and study the dynamics of a flexible helical filament that is rotated in a viscous fluid, at low Reynolds numbers. The scaling from the original micron-scale onto the desktop-scale is made possible by the prominence of geometry in the deformation process. Our filaments are custom fabricated with different geometric and material properties (with an emphasis on varying their intrinsic curvature), clamped at one end and rotated in a bath of glycerin. Geometrically nonlinear configurations of the filament can result from the coupling of the elastic forces of the filament and the viscous drag. Using digital imaging, we reconstruct the 3D deformed configurations of the rotating filament and quantify its dynamics. Our precision model experiments are combined with numerical tools ported from the computer graphics community. We couple the results from the experiments and simulations to quantify the effect of the control parameters on the propulsive force exerted by the rotating filament and rationalize the underlying mechanical instabilities. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q16.00013: Mechanics of fluid flow over compliant wrinkled polymeric surfaces Shabnam Raayai, Gareth McKinley, Mary Boyce Skin friction coefficients (based on frontal area) of sharks and dolphins are lower than birds, fish and swimming beetles. By either exploiting flow-induced changes in their flexible skin or microscale textures, dolphins and sharks can change the structure of the fluid flow around them and thus reduce viscous drag forces on their bodies. Inspired by this ability, investigators have tried using compliant walls and riblet-like textures as drag reduction methods in aircraft and marine industries and have been able to achieve reductions up to 19{\%} [1]. Here we investigate flow-structure interaction and wrinkling of soft polymer surfaces that can emulate shark riblets and dolphin's flexible skin. Wrinkling arises spontaneously as the result of mismatched deformation of a thin stiff coating bound to a thick soft elastic substrate. Wrinkles can be fabricated by controlling the ratio of the stiffness of the coating and substrate, the applied displacement and the thickness of the coating. In this work we will examine the evolution in the kinematic structures associated with steady viscous flow over the polymer wrinkled surfaces and in particular compare the skin friction with corresponding results for flow over non-textured and rigid surfaces. 1. K-S Choi et al.: Proc. R. Soc. Lond. A. 1997 [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q16.00014: Micro-mechanical lengthscales in soft elastic solids Edan Lerner, Eric DeGiuli, Gustavo D\"uring, Matthieu Wyart We provide numerical evidence and supporting scaling arguments that the response of soft elastic solids to a local force dipole is characterized by a lengthscale $\ell_c$ that diverges as unjamming is approached as $\ell_c \sim (z - 2d)^{-1/2}$, where $z \ge 2d$ is the mean coordination, and $d$ is the spatial dimension, at odds with previous claims based on numerics. We also show how the magnitude of the lengthscale $\ell_c$ is amplified by the presence of internal stresses in the disordered solid. Our data raise the possibility of a divergence of $\ell_c$ with proximity to a critical internal stress at which a buckling instability takes place. [Preview Abstract] |
Session Q17: Focus Session: Network of Networks
Chair: Antonio Scala, CNR-ISC Institute for Complex SystemsRoom: 402
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q17.00001: Multilevel Complex Networks and Systems Invited Speaker: Guido Caldarelli Network theory has been a powerful tool to model isolated complex systems. However, the classical approach does not take into account the interactions often present among different systems. Hence, the scientific community is nowadays concentrating the efforts on the foundations of new mathematical tools for understanding what happens when~multiple networks interact. The case of economic and financial networks represents a paramount example of multilevel networks. In the case of trade, trade among countries the different levels can be described by the different granularity of the trading relations. Indeed, we have now data from the scale of consumers to that of the country level. In the case of financial institutions, we have a variety of levels at the same scale. For example one bank can appear in the interbank networks, ownership network and cds networks in which the same institution can take place. In both cases the systemically important vertices need to be determined by different procedures of centrality definition and community detection. In this talk I will present some specific cases of study related to these topics and present the regularities found. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q17.00002: Epidemic Fault Propagation and Synchronization for Networks of Networks Gregorio D'Agostino, Huijuan Wang, Piet Van Mieghem, Shlomo Havlin, Eugene Stanley We have employed spectral methods to deal with both epidemics and diffusion processes on Networks of Networks (NoNs). Resorting to the largest eigenvalue of the adjacency matrix, we have estimated the threshold for epidemic processes. The algebraic connectivity has provided information on the slowest diffusion mode. The former quantities and their interplay are worth studying for their own sake. However, the motivation of the work arises in the context of Critical Infrastructure Protection. In fact, epidemics on NoNs provide a modeling of fault propagation on interdependent infrastructures and the diffusion processes are strictly related to their synchronization modes. Different theoretical approaches have been employed including exact bounds estimates, mean-field approximations and perturbative expansions. All these approaches provided tools to compare the resilience of NoNs with respect to both synchronization and fault propagation. We have modeled NoNs by different interdependent network models (BA, ER, RR etc) linked according to different strategies. Upon increasing the number of links among networks interesting emergent behaviors are observed. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q17.00003: Coexistence of critical regimes in interconnected networks Filippo Radicchi Networks in the real world do not exist as isolated entities, but they are often part of more complicated structures composed of many interconnected network layers. Recent studies have shown that such mutual dependence makes real networked systems exposed to potentially catastrophic failures. The theoretical approach to this problem is based on the study of the nature of the phase transitions associated to critical phenomena running on interconnected networks. In particular, it has been shown that many critical phenomena of continuous nature in isolated networks become instead discontinuous, and thus catastrophic, in interconnected networks when the strength of the connections between the various network layers is sufficiently large. We show that four main ingredients determine the critical features of a random interconnected network: the strength of the interconnections, the first two moments of the degree distribution of the entire network, and the correlation between intra- and inter-layer degrees. Different mixtures of these ingredients change the location of the critical points, and lead to the emergence a very rich scenario where phase transitions can be either discontinuous or continuous and different regimes can disappear or even coexist. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q17.00004: International and Domestic Business Cycles as Dynamics of a Network of Networks Yuichi Ikeda, Hiroshi Iyetomi, Hideaki Aoyama, Hiroshi Yoshikawa Synchronization in business cycles has attracted economists and physicists as self-organization in the time domain. From a different point of view, international and domestic business cycles are also interesting as dynamics of a network of networks or a multi-level network. In this paper, we analyze the Indices of Industrial Production monthly time-series in Japan from January 1988 to December 2007 to develop a deeper understanding of domestic business cycles. The frequency entrainment and the partial phase locking were observed for the 16 sectors to be direct evidence of synchronization. We also showed that the information of the economic shock is carried by the phase time-series. The common shock and individual shocks are separated using phase time-series. The former dominates the economic recession in all of 1992, 1998 and 2001. In addition to the above analysis, we analyze the quarterly GDP time series for Australia, Canada, France, Italy, the United Kingdom, and the United States from Q2 1960 to Q1 2010 in order to clarify its origin. We find frequency entrainment and partial phase locking. Furthermore, a coupled limit-cycle oscillator model is developed to explain the mechanism of synchronization. In this model, the interaction due to international trade is interpreted as the origin of the synchronization. The obtained results suggest that the business cycle may be described as a dynamics of the multi-level coupled oscillators exposed to random individual shocks. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q17.00005: Heterogeneous nodal responses in cascade dynamics on multiplex networks Kyu-Min Lee, Charles D. Brummitt, Kwang-Il Goh Structure and dynamics of multiplex network systems have been intensively studied recently, revealing nontrivial results such as facilitated cascading failures and new type of phase transitions unforeseen in the single-level systems. However, most studies about multi-layered, network of networks have mainly considered the case of single nodal response to multiple layers, that is, every node responds to the multiple layers in identical way. Most complex systems like human society, however, function not only through various kinds of relations but also through heterogeneous response behavior across agents, indicating a new level of complexity. To address it, here we formulate a threshold cascade model on multiplex networks with a mixture of two response functions: OR and AND rules. For the OR response, nodes are activated if enough neighbors in any layer are active, whereas for the AND response, the nodes activate only if enough neighbors in all layers are active. Coexistence of these two response rules is shown to control between facilitation and inhibition of cascading failures, and moreover, it can also control the type of transitions to global cascades between continuous and discontinuous ones. We will discuss the implication of the results in the context of social dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q17.00006: Mutual Percolation of Multiplex Networks with Link Overlaps Sangchul Lee, Byungjoon Min, Kyu-Min Lee, K.-I. Goh Many real-world complex systems operate through multiple layers of distinct interactions and the interplay between them. Most studies on multiplex networks, however, have largely ignored the effect of the link overlap across layers despite the strong empirical evidences for its significance. In this respect here we study the impact of link overlaps on mutual percolation of multiplex networks with two layers (duplex networks). We present the analytic solution based on the generating function approach that explicitly distinguishes the distinctive roles that the overlap- and non-overlap links play in establishing the mutual connectivity. The analytic solution is fully supported by extensive numerical simulations, thus successfully remedies the shortcoming of previously proposed theory by Cellai et al. [arXiv:1307.6359v1]. Our analytical results show that while the overlap links strongly facilitate mutual percolation by making components connected with overlap links yet it is unable to diminish the discontinuous nature of mutual percolation transition. Finally, we discuss the implication of our results to the robustness of duplex networks against link failures. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q17.00007: Coevolution model of multiplex networks Jin-hyeon Kim, Jung Yeol Kim, K.-I. Goh Many real-world complex systems can be represented as multiplex networks with multiple types of links. Each link type in the system defines network layers, which coexist and cooperate for the system's function. To understand such multiplex systems, we study a modeling framework based on coevolution of network layers. In our previous research, we introduced the coevolution of network layers as an evolutionary mechanism for the correlated multiplexity in growing networks [1]. We examined how the entangled growth of coevolving layers can shape the network structure and showed analytically and numerically that the coevolution can induce strong degree correlations across layers, as well as modulate degree distribution. In this research, we study several variants of the basic model with more realistic features such as the difference in the number of nodes and non-simultaneous arrivals of nodes in different layers, to characterize how these features also affect the correlation property of the multiplex structure. Further, we study the effect of negative coupling between layers in multiplex network evolution. \\[4pt] [1] J. Y. Kim and K. -I. Goh, Phys. Rev. Lett. 111, 058702 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q17.00008: Layer-crossing overhead and information spreading in multiplex social networks Byungjoon Min, K.-I. Goh Many real-world systems consist of multiple different layers of networks and interplay between them. Taking such multiplexity into account is important to a complete understanding of the structure and dynamics of complex systems. In this respect, we propose and study a model of information or disease spreading on multiplex social networks, in which agents interact or communicate through multiple channels (layers), and there exists a layer-switching overhead for transmission across the interaction layers. The model is characterized by the path-dependent transmissibility over a contact, which is dynamically determined, dependent on both incoming and outgoing transmission layers due to the switching overhead. We formulate a generalized theory with a mapping to deal with such a path-dependent transmissibility, and demonstrate dependency of epidemic threshold and epidemic outbreak size with respect to multiplexity characteristics such as the densities of network layers, layer-crossing costs, and type of seed infections. Our results suggest that explicit consideration of multiplexity can be crucial in realistic modeling of spreading processes on social networks. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q17.00009: Reliability theory for diffusion processes on interconnected networks Yasamin Khorramzadeh, Mina Youssef, Stephen Eubank We present the concept of network reliability as a framework to study diffusion dynamics in interdependent networks. We illustrate how different outcomes of diffusion processes, such as cascading failure, can be studied by estimating the reliability polynomial under different reliability rules. As an example, we investigate the effect of structural properties on diffusion dynamics for a few different topologies of two coupled networks. We evaluate the effect of varying the probability of failure propagating along the edges, both within a single network as well as between the networks. We exhibit the sensitivity of interdependent network reliability and connectivity to edge failures in each topology. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q17.00010: Synchronization in Networks of Coupled Chemical Oscillators Kenneth Showalter, Mark Tinsley, Simbarashe Nkomo, Hua Ke We have studied networks of coupled photosensitive chemical oscillators. Experiments and simulations are carried out on networks with different topologies and modes of coupling. We describe experimental and modeling studies of chimera and phase-cluster states and their relation to other synchronization states. Networks of integrate-and-fire oscillators are also studied in which sustained coordinated activity is exhibited. Individual nodes display incoherent firing events; however, a dominant frequency within the collective signal is exhibited. The introduction of spike-timing-dependent plasticity allows the network to evolve and leads to a stable unimodal link-weight distribution. M. R. Tinsley et al., Nature Physics 8, 662 (2012); S. Nkomo et al., Phys. Rev. Lett. 110, 244102 (2013); H. Ke et al., in preparation. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q17.00011: Percolation of localized attack on isolated and interdependent random networks Shuai Shao, Xuqing Huang, H. Eugene Stanley, Shlomo Havlin Percolation properties of isolated and interdependent random networks have been investigated extensively. The focus of these studies has been on random attacks where each node in network is attacked with the same probability or targeted attack where each node is attacked with a probability being a function of its centrality, such as degree. Here we discuss a new type of realistic attacks which we call a localized attack where a group of neighboring nodes in the networks are attacked. We attack a randomly chosen node, its neighbors, and its neighbor of neighbors and so on, until removing a fraction ($1-p$) of the network. This type of attack reflects damages due to localized disasters, such as earthquakes, floods and war zones in real-world networks. We study, both analytically and by simulations the impact of localized attack on percolation properties of random networks with arbitrary degree distributions and discuss in detail random regular (RR) networks, Erd\H{o}s-R\'{e}nyi (ER) networks and scale-free (SF) networks. We extend and generalize our theoretical and simulation results of single isolated networks to networks formed of interdependent networks. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q17.00012: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q17.00013: Discontinuous percolation transition at a finite threshold Byungnam Kahng, Young Sul Cho Recent interest of discontinuous percolation transitions (DPT) has been sparked by the explosive percolation model. Even though this model shows an abrupt percolation transition in finite-sized systems, it reveals that the jump of the order parameter shrinks to zero as the system size is increased. To disclose the mechanism of the DPT, a spanning-cluster-avoiding (SCA) model in the Euclidean space was introduced and analytically understood. However, the DPT in the SCA model is trivial because the percolation threshold is one. Thus, it is timely demanding to construct a general framework, under which a non-trivial DPT can take place at a finite threshold. Here, we propose the necessary conditions for the non-trivial DPT, and classify existing percolation models according to this criterion. Moreover, a model, satisfying those conditions and showing a non-trivial DPT, is introduced and discussed in the perspective of the network of networks. We anticipate this theoretical framework to be a platform for further researches on DPT in other disciplinary systems. [Preview Abstract] |
Session Q18: Disordered and Glassy Systems I
Sponsoring Units: DCMP GSNPChair: Alexei Sokolov, University of Tennessee/Knoxville
Room: 403
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q18.00001: Random matrix definition of the boson peak M. Lisa Manning, Andrea J. Liu The density of vibrational states for glasses and jammed solids exhibits universal features, including an excess of modes above the Debye prediction known as the boson peak, located at a frequency $\omega^*$. We show that the eigenvector statistics for modes in the boson peak are universal and emerge from the interplay of disorder and global translation invariance in the dynamical matrix. We demonstrate that a very large class of random matrices contains a band of modes with this same universal structure, and conjecture the existence of a new universality class. We characterize the eigenvector statistics as a function of coordination number, and find that one member of this new class reproduces the scaling of $\omega^{*}$ with coordination number that is observed near the jamming transition. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q18.00002: A New Method for Identifying Defects in Disordered Solids Sven Wijtmans, Lisa Manning Characterizing defects in solids is an important step to developing continuum equations for failure in materials. Defects in crystalline solids (i.e. dislocations) are easy to characterize, but in disordered solids the lack of crystalline order makes it difficult to identify where particle rearrangements are likely to occur. Recently, vibrational modes have been used to identify flow defects or ``soft spots'' in disordered solids. However, the algorithm contains several free parameters that are difficult to constrain and does not provide detailed information about the nature of the defects. Here we describe a new method for identifying defects. We add spring-like interactions between coarse-grained grid points, thereby suppressing long-wavelength sound modes. This allows us to identify the energy barriers and precise displacements corresponding to defects, and potentially avoids systematic effects generated by elastic interactions between defects. Plastic events do occur at defect locations, and are correlated with the defect energy barriers. We find that the energy barriers of defects are significantly lower that the energy barriers for the eigenvectors. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q18.00003: Topological Defects by Size Polydispersity Zhenwei Yao, Monica Olvera de la Cruz The engineering of defects in crystalline matter introduces entirely new physical properties of materials. The fascinating possible applications of defects, known as topological defects, provide great motivations to perform fundamental investigations to uncover their role on the physical properties of various systems. Here we investigate topological defects in size polydispersity on flat surfaces. Our simulations show that in polydispersed systems topological defects play the role of order-restoring. The perfect hexagonal lattice beyond a small defective region around the impurity particle is well protected. Moreover, size polydispersity is shown numerically here to be an essential ingredient to understand short-range attractions between like-charge disclinations. Our study suggests the promising potential of size polydispersity to engineer defects in real systems. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q18.00004: Origin of Rigidity in Dry Granular Solids Sumantra Sarkar, Bulbul Chakraborty In traditional solids, the resistance to shear is associated with broken translational symmetry as exhibited by a nonuniform density pattern. In this talk, we show that the emergence of shear rigidity in granular solids is a collective process, which is controlled solely by boundary forces, the constraints of force and torque balance, and the positivity of the contact forces, and not energetic or entropic considerations. We present a theoretical framework that connects rigidity to broken translational symmetry in a reciprocal space representing contact forces. We apply our theory to experimentally generated shear-jammed states and show that these states are indeed characterized by a persistent, non-uniform density modulation in force space, which emerges at the shear-jamming transition\footnote{ Sumantra Sarkar et al, Phys. Rev. Lett. 111, 068301}. Crucial to these analyses was an algorithm that was developed to obtain the reciprocal space structures for any real space configuration under mechanical equilibrium. Also, this algorithm help us identify the source of plastic failure which leads to avalanches in these systems. We argue that continuum theories of granular solidification and response should be based on the reciprocal space picture. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q18.00005: Role of Temperature on Self-organization of Networks Le Yan, Matthieu Wyart Both the thermodynamics and the dynamics of network glasses strongly depend on the network coordination. For instance, the fragility and the jump of specific heat at glass transition of covalent glass are minimal when the covalent network sits at the rigidity transition. We introduced a random network model with frozen topology to rationalize these observations (Yan, D\"urning, Wyart, PNAS(2013)). Here, we consider a novel model without frozen disorder, where the network topology can change dynamically. The model and the previous one shows nearly identical thermodynamic properties. In addition, this model allows us to compare rigidity percolation and jamming, and to study the presence of a rigidity window in covalent glasses. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q18.00006: Elastic Instability and Sound Dispersion in Amorphous Solids Eric DeGiuli, Adrien Laversanne-Finot, Gustavo During, Edan Lerner, Matthieu Wyart Connectedness and applied stress are key characteristics of amorphous solids at zero temperature. Rigidity can be lost either through unjamming as connectedness is decreased, or buckling as stress is increased. We present an effective medium theory which describes elastic behaviour in proximity to both unjamming and buckling. The theory successfully predicts (i) the dependence of the boson peak on pressure and coordination, (ii) negative sound dispersion and a kink in sound attenuation near the boson peak, as observed experimentally in molecular glasses, (iii) a characteristic frequency $\omega_0$ that vanishes at a critical pressure, and (iv) the previously derived stability diagram for $T=0$ amorphous solids. Our predictions for sound dispersion are similar to disorder-based approaches to the boson peak, however we resolve two inconsistencies of these approaches: (i) since disorder is secondary to connectedness and stress in our theory, we explain why some crystals and glasses have similar elasticity, and (ii) we explain the natural emergence of a mesoscopic length scale visible in response, which is absent in static structure. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q18.00007: Glassy Dynamics and Anomalous Diffusion in Self-Assembled Nanoparticle Monolayers Leandra Boucheron, Jacob Stanley, Yeling Dai, Sean You, Binhua Lin, Mati Meron, Suresh Narayanan, Alec Sandy, Zhang Jiang, Oleg Shpyrko We experimentally investigate the structure and dynamics of iron oxide nanoparticle thin films self-assembled at the liquid-air interface. Upon deposition on a water surface and subsequent lateral compression, iron oxide nanoparticles coated with oleic acid ligands self-assemble into a morphologically uniform quasi-2D monolayer. We examined the in-plane structure of these self-assembled films using Grazing-Incidence X-Ray Diffraction (GIXD) and investigated the interparticle dynamics using X-Ray Photon Correlation Spectroscopy (XPCS). The logarithmic relaxation of the surface pressure of the films post-compression suggests the presence of glassy dynamics in the system. Autocorrelation functions derived from XPCS measurements quantify the characteristic timescale of such dynamics and have been fit using the Kohlrausch-Williams-Watts (KWW) model to extract the degree of glassiness. Finally, the q-dependence of the interparticle dynamics in the films is supportive of an anomalous diffusion regime, $\langle x^{2}\rangle \propto t^{n}$, with $n>1$. I will discuss these results and their implications with regards to the nanoscale interactions involved in thin film self-assembly and rearrangement. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q18.00008: Melting Scenario for Coulomb-interacting Classical Particles in Two-dimensional Irregular Confinements Dyuti Bhattacharya, Amit Ghosal We report the thermal ``melting'' of self-formed rigid structures made of a small number of interacting classical particles in two-dimensional confined geometries. We will focus on the role of irregularities of the confinement on the melting of these Coulomb-interacting particles using Monte Carlo simulations. It will be shown that the interplay of long-range Coulomb repulsions between these particles and the irregular confinement yields a solid-like phase (termed as irregular Wigner molecules) at low temperatures that possesses a bond-orientation order. However, the positional order is depleted even at the lowest temperatures due to the disordered confinement. Upon including thermal fluctuations, this solid-like phase smoothly crosses over to a liquid-like phase by destroying the bond-orientation order. This cross-over will be demonstrated by the temperature dependence of several physical observables. The collapse of the solidity will be shown to be defect mediated, and aided primarily by the proliferation of free disclinations, initiated by intriguing tortuous path of correlated fluctuations. These results will help us quantifying the melting found in experiments on systems with confined geometries. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q18.00009: Efficient simulation scheme for studying spin-glass transitions Cheng-Wei Liu, Anatoli Polkovnikov, Anders W. Sandvik We propose and demonstrate an efficient simulation scheme for studying spin-glass transitions. This method is based on the idea of approaching the transition point through a non-equilibrium quench process, formally known as Kibble-Zurek mechanism. Recent studies have shown that there exists dual scaling behavior as a function of quench velocity. This dual scaling behavior allows us to extract transition point, static critical exponents as well as dynamic exponents to good numerical accuracy with an efficient computational effort. We have tested this approach with anti-ferromagnetic Ising model on 3-regular random graphs in terms of both classical and quantum phase transitions and we have obtained good agreement with known results. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q18.00010: Avalanches and hysteresis in frustrated superconductors and $XY$ spin-glasses Auditya Sharma, Alexei Andreanov, Markus Mueller We study avalanches along the hysteresis loop of long-range interacting spin-glasses with continuous XY symmetry - which serves as a toy model of granular superconductors with long-range and \ frustrated Josephson couplings. We identify sudden jumps in the $T=0$ configurations of the XY phases, as an external field is increased. They are initiated by the softest mode of the inverse\ susceptibility matrix becoming unstable, which induces an avalanche of phase updates (or spin alignments). We analyze the statistics of these events, and study the correlation between the no\ n-linear avalanches and the soft mode that initiates them. We find that the avalanches follow the directions of a small fraction of the softest modes of the inverse susceptibility matrix, sim\ ilarly as was found in avalanches in jammed systems. In contrast to the similar Ising spin-glass (Sherrington-Kirkpatrick) studied previously, we find that avalanches are not distributed with\ a scale-free power law, but rather have a typical size which scales with the system size. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q18.00011: Local activated dynamics in liquids Takuya Iwashita, Takeshi Egami Transport properties of glass-forming liquids, such as viscosity and diffusion, are an important and unsolved subject in condensed matter physics, and in particular the quantitative description of the dynamics of such liquids still remains incomplete. We studied the local activation process in a liquid in terms of the change in local coordination number Nc, which is the number of nearest neighbor atoms. We calculated the transition rate between Nc coordinated state and (Nc+1) or (Nc-1) coordinated state in 3D molecular dynamics simulation. The transition rate is dependent on Nc, and as temperature is lowered the transition rate exhibits a strong dependence on Nc, indicating the system becomes more heterogeneous at the atomic level. The analysis allows us to determine local activation energy as a function of temperature and Nc, and in Nc-configuration space a local energy landscape picture was constructed as a new conceptual view of liquid dynamics. This result provides an interesting and fundamental framework for describing the dynamics of liquids. We also discuss the principle of detailed balance for the transition rate in liquids. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q18.00012: A free-energy surface exploration algorithm for supercooled liquids and amorphous solids Kirk D. Lewis, Yongwoo Shin, Xi Lin Efficient exploration of the multidimensional free-energy surfaces (FES) of supercooled liquids and amorphous solids at low temperatures is extremely challenging. The recently developed autonomous basin-climbing (ABC) algorithm (JCP 130: 224504, 2009) allows the sluggish system to self-explore the multidimensional potential energy surface (PES) and climb out of deep energy basins through a series of collective activation and relaxation events. In this work, we present a new FES exploration algorithm that enforces an explicit temperature dependence on the ABC trajectories. The explicit temperature dependence is achieved by introducing an ensemble of walkers to collectively maintain the detailed balance criteria among all the relevant energy basins. Using this new algorithm, the metabasin correlation length of a binary Lennard-Jones supercooled liquid is identified at the glass transition temperature. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q18.00013: Microsecond molecular dynamics simulations of stress relaxation and slow quench in silica melts and glasses J. Matthew D. Lane Quench rates and stress relaxation in molecular dynamics simulations of glasses are usually studied on time-scales which are many orders of magnitude faster that those in experiment. We present results from relaxation of hydrostatic compressive stress in silica glass using classical molecular dynamics simulations. Structural variation will be discussed as a function of quench rate for glasses quenched 2 to 3 orders of magnitude slower than previously reported. Stress relaxation curves plotted in log t show time-temperature superposition holds over a wide-range of temperatures for 3\% initial volume compression. Silica melts and glasses were modeled with the BKS interatomic potential and were produced through a melt-quench process. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q18.00014: Modeling vitreous silica bilayers Avishek Kumar, Mark Wilson, David Sherrington, Michael Thorpe The recent synthesis and imaging of bilayers of vitreous silica has led to a wealth of new information [1-2]. We have modeled the experimentally-observed bilayer using a computer assembly procedure [3] to form a network of corner-sharing tetrahedra, which is then mirror-reflected to form a bilayer. We show that the vitreous silica bilayer has additional macroscopic degrees of freedom iff there is a symmetry plane through the center of the bilayer going through the central layer of oxygen ions that join the upper and lower monolayers. We have computer-refined the experimental coordinates to determine the density, and other structural characteristics such as the Si-Si pair distribution function, Si-O-Si bond angle distribution and the Aboav-Weaire law. [1] P. Y. Huang, S. Kurasch, A. Srivastava, V. Skakalova, J. Kotakoski,A. V. Krasheninnikov, R. Hovden, Q. Mao, J. C. Meyer, J. Smet,D. A. Muller, and U. Kaiser, Nano. Lett. 12, 1081 (2012). [2] M. Heyde, S. Shaikhutdinov, and J. J. Freund, Chem. Phys. Lett.550, 1 (2012). [3] M. Wilson, A. Kumar, D. Sherrington, M.F. Thorpe, Phys. Rev. B (87) 214108 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q18.00015: Plasticity, defect dynamics and superfluid phenomena in deformed solid $^4$He Debabrata Sinha, Surajit Sengupta, Chandan Dasgupta, Oriol Valls We present a numerical study of a continuum plasticity field coupled to a Ginzburg-Landau model for superfluidity. The results suggest that a supersolid fraction may appear as a long-lived transient during the time evolution of the plasticity field at higher temperatures where both dislocation climb and glide are allowed. Supersolidity, however, vanishes with annealing. As the temperature is decreased, dislocation climb is arrested and any residual supersolidity due to incomplete annealing remains frozen. We show that superfluid response will be experimentally observable only if certain mechanical and structural conditions are satisfied. Study of the superfluid phenomena in solid $^4$He in the presence of a dynamic defect density gives rise to many interesting observations and may provide a resolution to some of the perplexing issues concerning a variety of experiments on bulk solid $^4$He. [Preview Abstract] |
Session Q19: Focus Session: Theory and Simulations of Macromolecules VII - Chain Conformation
Sponsoring Units: DPOLYChair: Ting Ge, The University of North Carolina at Chapel Hill
Room: 404
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q19.00001: Evolution of chain conformation and entanglements as related to the origin of stress overshoot during startup shear of entangled polymer melts Zhen-Gang Wang, Yuyuan Lu, Lijia An, Shi-Qing Wang Using Brownian Dynamics simulation, we determine the chain orientation and stretching and their connection to stress overshoot in an entangled polymer melt undergoing startup shear at rates lower than the reciprocal of the Rouse time yet higher than the reciprocal reptation time. In this regime, the prevailing tube theory envisions little chain stretching and monotonic increase of the radius of gyration to a saturated value, and attributes the stress overshoot to excessive chain orientation. In contrast, our results reveal that there is significant chain stretching which persists well beyond the Rouse time and contributes substantially to the initial stress growth. In particular, stress overshoot is found to be primarily due to chain retraction after considerable stretching rather than chain over-orientation. The coil size shows non-monotonic dependence on the strain. Furthermore, up to many Rouse times, the relaxation of the initial entanglements is slower than that under the quiescent condition. These results point to fundamental deficiencies in the molecular picture of the tube model for startup shear. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q19.00002: Effects of Bond Stiffness on Structural Transitions of Flexible Polymers Tomas Koci, Michael Bachmann Utilizing advanced parallel Monte Carlo simulation methods we examine the structural transitions of a coarse-grained flexible polymer model. In this model, the bond elasticity or effective bond stiffness is considered to be a parameter. Pseudophase diagrams in temperature-stiffness space are constructed by using energy dependent canonical quantities to demonstrate the effects of the changes in the bond flexibility on the liquid and solid structural phases. With increasing bond flexibility we observe the disappearance of the liquid phase and the fusion of the collapse and the freezing transitions. The notoriously difficult sampling of entropically suppressed energetic regions near strong first-order transitions is improved by employing generalized ensemble methods. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q19.00003: Ordering transitions in confined melts of semiflexible polymers: A Monte Carlo simulation Wolfgang Paul, Viktor Ivanov, Marcus Mueller, Kurt Binder Using grand-canonical Monte Carlo simulations of the bond-fluctuation model confined between two hard walls we study the effect of confinement on the isotropic-nematic transition of a melt of semi-flexible chains. The walls have a stiffening effect on the chains in their vicinity leading to an ordering transition at the walls preempting the one in the bulk (surface-induced ordering). For a semi-infinite system the thickness of the ordered nematic layer increases with a complete wetting transition upon approaching bulk coexistence. For a finite extension, D, between the walls, the ordered surfaces induce a shift of the first order isotropic-nematic transition in the bulk of the film (capillary nematization). When D becomes comparable to the extension of a chain, the first order isotropic-nematic transition line ends in a critical point. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q19.00004: Stress-strain relation of K4 phenolic resins by classical MD simulation Katsumi Hagita From viewpoint of collaboration among mathematics, material sciences, and physics, K4 lattice has been much interested. K4 lattice is confirmed to be backbone of Schwarz G surface, which is one of triply periodic minimal surface called Gyroid. Double Gyroid is nested lattice of two single Gyroid. As exploration of new materials inspired from pure mathematics, we proposed K4 Phenolic resins. The resins consist of Phenol and hydrocarbon originated from formaldehyde. The hydrocarbon has a role to connect two phenols. Maximum number of connection from one phenol is 3. Then, as an ideal case, backbone of Phenol resins can be form K4 lattice structure. In the present study, for studying elastic modulus, we performed classical MD simulation with deformation by using LAMMPS packages to estimate stress-strain relation. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q19.00005: Zip-DNA: A Novel DNA Structure Formed Under Mechanical Stress Alexander Balaeff, Ivan Mikhailov, Malakhat Turabekova, Stephen Craig, David Beratan Zip-DNA is a novel DNA structure predicted by molecular dynamics simulations of forced DNA extension. In the zip-DNA form, the Watson-Crick hydrogen bonds are broken and nucleobases from the opposite DNA strands interdigitate with each other, forming a continuous single-base aromatic stack. The B-Zip DNA structural transition is proposed to be responsible for the famous overstretching plateau on the force-extension curve of DNA. The simulations show that zip-DNA may either self-assemble from force-melted DNA strands or evolve from B-DNA through an earlier recognized S-DNA. Zip-DNA is shown to be consistent with multiple experimental observations; notably, the S-DNA transition state is shown to be a highly disordered state consistent with experimentally measured thermodynamic characteristics of DNA extension. We predict that zip-DNA possesses increased molecular conductivity compared to the B-DNA form and, therefore, may find applications in molecular electronics. A conductive state of a stretched non-complementary double-stranded DNA would, if detected, become a ``smoking gun'' experiment validating the existence of zip-DNA. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q19.00006: Excluded-volume interaction induced stiffness of comb polymer with densely grafted side-chains Feng Qiu Excluded-volume interaction has been widely recognized to cause expansion of polymer chain at large length scale. However, its effect on chain conformations at small length scale has been studied to less extent. Here we consider a comb polymer with its backbone densely grafted by side-chains as a model system. The method analogue to solving the electrostatic persistence length problem for either rigid or flexible polyelectrolytes is employed. For comb polymers with rigid backbone near the rod limit, the excluded-volume interaction induced persistence length scales linearly with the volume of the side-chain. While for flexible backbone, the persistence length depends on the side-chain volume more weakly. Field theoretic method that is relevant to address this problem is also explored and discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q19.00007: Effect of bending stiffness and confinement on a polymer chain under tension Peter Poier, Christos N. Likos, Richard Matthews Type II topoisomerase are enzymes that (un)knot DNA. There is experimental evidence that a certain type II topoisomerase preferentially cleaves adenine (A) and thymine (T) rich regions of the DNA. It is believed that AT-rich sequences are more flexible than random ones. This raises the question of whether the flexibility of the preferred cleavage sites of topoisomerase II could play an important role in the regulation of knotting. With this motivation we study the effect of the bending stiffness and confinement on the free-energy cost of a knot in a polymer chain under tension. For the polymer chain we use a coarse-grained model. Via thermodynamic-integration we calculate the change of the free-energy cost of a knot due to modifications of the bending stiffness. The free-energy cost exhibits a minimum at a non-zero value for the bending stiffness. Our simulations suggest that this minimum is related to a suppression of the bending at the points where the strands of the polymer cross in the knotted region. We study how the minimum of the free-energy cost is affected by changing the knot type and introducing a two dimensional confinement for the polymer chain. The results of this work might be of importance for the localization of knots in DNA. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q19.00008: Molecular mechanics modeling of compressible polymer solutions based on an isobaric-isothermal ensemble. Moeed Shahamat, Alejandro Rey Understanding the thermodynamics and physical properties of polyethylene (PE) in hydrocarbons is significant for its industrial production. Of recent interest are atmostic simulations which provide a detailed molecular level insight without experimental efforts. This paper reports on the density of solutions of PE in hexane using molecular dynamics (MD) simulations at high pressures. The computed densities increase monotonically with raising external pressure and compare quite favorably with experimental and theoretical data. Furthemore, the effect of cut-off distance to density is investigated and it is shown that the density increases with increasing the cut-off radius. It has been revealed that for pressures below 100 bar the mixture density displays a large dependece on cut-offs and for higher pressures solution density and non-bonded interactions demostrate weak sensitivity to cut-off distance. Analysis of the pair distribution function versus pressure shows that the amplitude of the first peak increases and the radial distribution function shifts to shorter separations reflecting structural change of the condensed phase. The implemented MD-NPT approach in this research provides a good insight into the polymer-polymer, polymer-solvent, and solvent-solvent interactions. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q19.00009: Frank elastic constants in LC mesophases of polymeric semiconductors Patrick Gemuenden, Kurt Kremer, Kostas Ch. Daoulas Liquid crystalline (LC) mesophases of polymeric semiconductors [1], e.g. poly(alkylthiophenes), can facilitate processing to obtain morphologies with improved properties. We develop a particle-based modeling approach to study nematic mesophases of such systems. The method uses soft, directional interactions [2] and is inspired by field theoretical approaches to LCs [3]. It enables us to generate large morphologies and calculate Frank elastic constants (FC). Besides interesting theoretical questions related to the behavior of FCs in polymer nematics, they are important when linking particle-based with continuum media descriptions of LCs. We calculate FC related to bend, splay and twist deformations from the fluctuation spectra of the local nematic director. The magnitudes of FC measured in the simulations agree with those reported in experiments on polymer nematics. We discuss their dependence on system parameters, e.g. chain length, and we compare with predictions by analytical field theory [4].\\[4pt] [1] Ho et al., Macromolecules 43, 7895 (2010)\\[0pt] [2] Gem\"{u}nden et al. Macromolecules 46, 5762 (2013)\\[0pt] [3] Pryamitsyn \& Ganesan, J. Chem. Phys. 120, 5824 (2004)\\[0pt] [4] Le Doussal \& Nelson, Europhys. Lett. 15, 161 (1991) [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q19.00010: Stability of Polymeric Crystalline Polymorphs Daniel W. Sinkovits, Sanat K. Kumar In the search for polymeric materials with novel properties, such as high dielectric constant and low loss, an important attribute of a material is its crystal structure. Most polymers can crystallize into multiple polymorphs whose properties vary. Therefore, the question of which polymorphs are thermodynamically preferred under what conditions is of great importance. We generate polymorphs using atomistic molecular dynamics simulations and tackle the question of stability using a combination of molecular dynamics and Monte Carlo techniques. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q19.00011: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q19.00012: \textit{A priori }Determination of the Rheological Properties and Slip Phenomena of Polymer Melts John Dorgan, Nicholas Rorrer This work builds on previous studies...$^{\mathrm{1,\thinspace 2}}$ done utilizing a coarse grained Dynamic Monte Carlo algorithm. The algorithm is able to capture the molecular scale details of flowing polymer melts in nanoscopic geometries. In shear flow all of the known observable viscoelastic behavior associated with polymer melts is predicted in a completely \textit{a priori }manner. For example, the correct scaling of the zero shear viscosity with molecular weight is observed. When polydispersity is introduced, the viscosity curves demonstrate a broadening behavior, exhibiting the same zero shear viscosity but lower viscosities at higher shear rates. Slip phenomena has been investigated under both shear (Couette) and parabolic (Poiseulle) flow. These two different flow cases are demonstrated to be profoundly different in their molecular scale details. All of the predicted results are in \textit{post facto} agreement with many experiments and help shed fundamental insight into the molecular scale behavior of polymer fluid dynamics. .1. Dorgan, J. R.; Rorrer, N. A.; Maupin, C. M., \textit{Macromolecules }\textbf{2012,} \textit{45} (21), 8833-8840. 2. Dorgan, J. R.; Rorrer, N. A., \textit{Physical Review Letters }\textbf{2013,} \textit{110} (17) [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q19.00013: Autocorrelation study for a coarse-grained polymer model Kai Qi, Michael Bachmann By means of Metropolis Monte Carlo simulations of a coarse-grained model for flexible polymers, we investigate how the integrated autocorrelation times of different energetic and structural quantities depend on the temperature. We show that, due to critical slowing down, an extremal autocorrelation time can also be considered as an indicator for the collapse transition that helps to locate the transition point. This is particularly useful for finite systems, where response quantities do not necessarily exhibit clear indications for pronounced thermal activity. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q19.00014: The Configuration and Dynamics of Self-Attractive Flexible and Semi-Flexible Polymers Ronald Larson, Indranil Saha Dalal, Miqiu Kong We study ``bead-rod'' chains containing stiff Fraenkel springs with nearly fixed Kuhn length, but with varying numbers of rods representing each Kuhn length, N$_{\mathrm{r,K}}$, modeled by incorporating a bending potential between consecutive rods. We find converged results as we increase the number of rods per Kuhn step. We find that at high $\varepsilon ^{\mathrm{\ast }}$N$_{\mathrm{r,K}}$, where $\varepsilon^{\mathrm{\ast }}$ is the attractive interaction strength per bead normalized by kT, collapsed globules are produced at moderate dimensionless chain diameter $\sigma^{\mathrm{\ast }}=$1/4, while for $\sigma^{\mathrm{\ast }}=$1, helices are formed, and for $\sigma^{\mathrm{\ast }}=$1/16, tori, folded bundles, and finally globules, are formed as $\varepsilon ^{\mathrm{\ast }}$N$_{\mathrm{r,K}}$ increases. Under shear, a universal tumbling state is found where chain width in the shear gradient direction is independent of chain length and proportion to shear rate to the fourth power. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q19.00015: Floquet-Bloch theory for polymers in a periodic Ricardo Pablo Pedro, David Tempel, Alfredo Alexander-Katz Anderson localization in disordered systems predicts the localization of electronic wave functions and the resulting absence of diffusion. The phenomenon is much more general and has been observed in a variety of systems. In the case of the polymer, the behavior of it in a periodic potential is equivalent to the behavior of a quantum-machanicial particle in a periodic potential. According to this mapping our results for polymers in a periodic potential ara valid for localization of a quantum-mechanical particle in a periodic potential. Besides, one of our motivations for studying polymers in a periodic potential is because it reveals interesting aspects of a self-organization of the adsorbed polymers onto a surface with periodic potential. In order to describe the properties of time-periodic polymer system, we consider the potential time dependent which is periodic in time and space and we evaluate the solutions using the powerful nonperturbative Floquet-Bloch theory which is formulated for linear systems. Finally, we also consider a more interesting problem of when disorder is included in the time-periodic system, where localization of the wave function can occur. [Preview Abstract] |
Session Q20: Focus Session: Organic Electronics and Photonics - Charge Transport
Sponsoring Units: DMP DPOLYChair: Brian Collins, National Institute of Standards and Technology
Room: 405
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q20.00001: Role of fluctuations on electron transport in soft materials Enrique Gomez Soft materials are characterized by weak intermolecular interactions and disorder. Although the extent of spatial fluctuations depends on the molecular structure, lattice fluctuations can be on the order of the unit cell dimensions in molecular crystals and soft materials. For example, our results from Quasi-elastic Neutron Scattering demonstrates that longer side chains in poly(3-alkylthiophene)s leads to an enhancement in motion of the thiophene rings. To ascertain the effect of dynamics on charge transport, we have developed a simple model to describe the roles of longitudinal and transverse modes of the intermolecular spacing between molecules on intrinsic electron mobilities. We demonstrate that the intrinsic mobility of soft materials appears thermally-activated by assuming fluctuations in a harmonic potential and an exponential decay in the charge transfer rate with intermolecular distance. For example, for poly(3-hexylthiophene), we can extract the characteristic decay as a function of separation distance from Density Functional Theory calculations and the extent of fluctuations from Molecular Dynamics simulations to predict the temperature dependence of the charge mobility. We find that the temperature dependence appears Arrhenius with activation energy of approximately 50 meV for a wide temperature range, in good agreement with experiments. This model suggests that fluctuations in the lattice spacing of soft materials lead to a significant intrinsic dependence of electron transport on temperature, regardless of the presence of band-tail states or traps. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q20.00002: Temperature activated transport tuned by libration in the charge-transfer salt trans-stilbene -- 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (STB-F$_{4}$TCNQ) Katelyn P. Goetz, Derek Vermeulen, Margaret E. Payne, Jiang Hui, Hu Peng, Cynthia S. Day, Christian Kloc, Veaceslav Coropceanu, Laurie E. McNeil, Oana D. Jurchescu A common route to solubility in organic semiconductors is chemical functionalization. This adds librational modes to the molecules, which was theoretically predicted to impact charge transport. We discuss the effect of libration on charge transport in the charge-transfer complex trans-stilbene--2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (STB-F$_{4}$TCNQ). This material has a 300 K mobility of 0.3 cm$^{2}$V$^{-1}$s$^{-1}$ that decreases with an activation energy of 170 meV to 235 K, where it transitions to temperature independence. X-ray diffraction indicates that the cause of this is the freezing of the libration of the ethylene moiety within STB below 235 K. Above 235 K, it increases in amplitude with increasing temperature. Fourier difference maps suggest that the charge density unaccounted for by the STB and F$_{4}$TCNQ molecules is localized on the molecules at low temperature and more delocalized after the transition. This agrees with XRD and Raman spectroscopy estimates for the degree of the ground state donor to acceptor charge transfer, indicating zero transfer below and 0.1 electrons above the transition temperature, highlighting the strong coupling between molecular motion and charge transport. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q20.00003: Incorporating Decoherence in the Dynamic Disorder Model of Organic Semiconductors Wei Si, Yao Yao, Chang-Qin Wu The transport phenomena in crystalline organic semiconductors, such as pentacene, have drawn much attention recently, where the electron-phonon interaction plays a crucial role. An important advance is the dynamic disorder model proposed by Troisi \textit{et. al.}, which is successful in determining the carrier mobility and explaining the optical conductivity measurements. In this work, we aim to incorporate the decoherence effects in the dynamic disorder model, which is essential for the self-consistent description of the carrier dynamics. The method is based on the energy-based decoherence correction widely used in the surface hopping algorithm. The resulting dynamics shows a diffusion process of wave packets with finite localization length, which scales with the decoherence time. In addition, the calculated mobility decreases with increasing temperature. Thus the method could describe a band-like transport based on localized states, which is the type of transport anticipated in these materials. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q20.00004: Two extreme limits of carrier injection in organic semiconductor FETs Thangavel Kanagasekaran, Hidekazu Shimotani, Yoichi Tanabe, Satoshi Heguri, Katsumi Tanigaki The metal-semiconductor (MS) contact between a metal electrode and an organic semiconductor is generally in the Schottky limit, and the barrier height against carrier injection from the electrode is greatly dependent on the work function (?m) of the electrode. Consequently, air-unstable metals with low ?m's such as Ca are necessary for electron injection. Here, we report that the Schottky limit can be converted to the Bardeen limit and the carrier injection barrier height can become independent of the electrode work function. This is exemplified using tetratetracontane as a surface modification layer on an SiO2 dielectric gate insulator and the unambiguous evidences are given. Based on this finding we demonstrate an air-stable light-emitting organic field-effect transistor using Au electrodes for both hole and electron injection. A light emmitting FET stable in air is demonstrated using Au-Au electrodes. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q20.00005: Gate Voltage Dependent Resistance across Interspherulite Boundaries in Solution-Processed Organic Semiconductor Thin Films Anna Hailey, Marcia Payne, John Anthony, Yueh-Lin Loo Grain boundaries formed by impinging spherulites in solution-processed organic semiconductor thin films limit charge transport in organic field-effect transistors that comprise these polycrystalline active layers. Thin films of triethylsilylethynyl anthradithiophene (TES ADT) exhibit limited order upon spin-coating; subsequent exposure to 1,2-dichloroethane vapor induces growth of TES ADT spherulites, with a mixture of low-angle (LA) and high-angle (HA) interspherulite boundaries (ISBs) defining neighboring spherulites. Our ability to control the directionality of TES ADT growth allows us to prescribe the formation of ISBs, forming exclusively LA [0$\pm$20$^{\circ}$] and HA ISBs [90$\pm$20$^{\circ}$] over macroscopic distances. Gated four-point probe transistor measurements allow us to quantify differences in resistance within spherulites and across these LA and HA ISBs as the devices are switched from ``off'' to ``on'' states. Surprisingly, for devices in the ``on'' state, the gate-independent resistances across LA [6$\pm$3M$\Omega$] and HA ISBs [16$\pm$6M$\Omega$] remain quantitatively different from that within an individual spherulite [1.9$\pm$0.7M$\Omega$], suggesting that the angle of mismatch of the ISB continues to affect charge transport, even after all traps are filled. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q20.00006: Charge Energy Transport in Hopping Systems with Rapidly Decreasing Density of States Dan Mendels An accurate description of the carrier hopping topology in the energy domain of hopping systems incorporating a rapidly decreasing density of states and the subsequent energetic position of these systems' so called effective conduction band is crucial for rationalizing and quantifying these systems' thermo-electric properties, doping related phenomena and carrier gradient effects such as the emergence of the General Einstein Relation under degenerate conditions. Additionally, as will be shown, the 'mobile' carriers propagating through the system can have excess energies reaching 0.3eV above the system quasi-Fermi energy. Hence, since these mobile carriers are most prone to reach systems interfaces and interact with oppositely charged carriers, their excess energy should be considered in determining the efficiencies of energy dependent processes such as carrier recombination and exciton dissociation. In light of the stated motivations, a comprehensive numerical and analytical study of the topology of hopping in the energetic density of such systems (i.e. the statistics regarding which energy values carriers visit most and in what manner) was implemented and the main statistical features of the hopping process that determine the position in energy of the system's effective conduction band were distilled. The obtained results also help shed light on yet to be elucidated discrepancies between predictions given by the widely employed transport energy concept and Monte Carlo simulations. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q20.00007: Field-induced low temperature transport in polythiophene thin films Evan Kang, Eunseong Kim Low temperature charge transport in poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) field-effect transistors (FETs) was systematically investigated. The temperature dependent transport behavior was studied by varying drain-source electric field and gate bias. Thermally-assisted hopping is dominant at high temperatures. At low temperatures, the temperature dependence becomes weaker and tunneling becomes the prevailing transport mechanism. Under high drain-source electric field, the additional field-driven current leads to the non-ohmic current-voltage relations. The results will be discussed with previously suggested models, such as Poole-Frenkel-type hopping [1], Efros-Shklovskii hopping [2], multistep tunneling [3], and field emission [1, 4]. [1] J. H. Worne, J. E. Anthony, and D. Natelson, Appl. Phys. Lett. 96, 053308 (2010) [2] A. S. Dhoot, G. M. Wang, D. Moses, and A. J. Heeger, Phys. Lev. Lett. 96, 246403 (2006) [3] J. H. Wei, Y. L. Gao, and X. R. Wang, Appl. Phys. Lett. 94, 073301 (2009) [4] J. M. Beebe, B. Kim, J. W. Gadzuk, C. D. Frisbie, and J. G. Kushmerick, Phys. Rev. Lett. 97, 026801 (2006) [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q20.00008: Low trap density of states in solution-deposited organic semiconductors by Vibration Assisted Crystallization Peter Diemer, Christopher Lyle, Yaochuan Mei, Christopher Sutton, Marcia Payne, John Anthony, Veaceslav Coropceanu, Jean-Luc Bredas, Oana Jurchescu Solution-deposited organic thin-film transistors suffer from defects at the semiconductor/dielectric interface due to disorder. These defects act as trapping sites and lead to inferior performance compared to single crystals. During the evaporation of a solvent, the solute molecules are driven to minimize their configuration energy: molecules may settle into local energy minimum configurations, characterized by molecular displacements with respect to the global minimum. We demonstrate that applying gentle vibrations of 100Hz or less to the solution during film crystallization perturbs and partially re-dissolves the dislocated molecules, allowing them to escape the local energy minimum and crystallize into the global energy minimum. This results in markedly improved performance in devices based on several solution-cast organic semiconductors due to a decrease in trap density at the organic/dielectric interface. The performance of our Vibration Assisted Crystallization transistors approach that of the corresponding single-crystal devices, as shown in transistors made from 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene on SiO$_{\mathrm{2}}$ dielectric, with mobility of 3 cm$^{\mathrm{2}}$/Vs, subthreshold slope of 0.43 V/dec, and threshold voltage of 0.7 V. The low interfacial trap density of 6.76x10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$ eV$^{\mathrm{-1}}$ agrees well with the results of quantum mechanical calculations. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q20.00009: Dynamical- and static-disorder effects on charge transport property of organic semiconductors Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose In comparison with inorganic materials, electron transfer energy of typical organic semiconductors is small in the range of 10 - 100meV, which is comparable to the magnitude of dynamical disorder of transfer energy originating from the thermal fluctuations of molecular motions. Furthermore, the static disorder inevitably exists in realistic organic devices and disturbs the transport of charge carrier. To clarify the influence of the dynamical and static disorders on the mobility, we employ a realistic static-disorder potential, which is deduced from the data obtained by electron-spin-resonance spectroscopy. We evaluate the carrier mobilities of pentacene and rubrene semiconductors under the realistic situation, using our time-dependent wave-packet diffusion method. [1] In this methodology, we carry out the quantum-mechanical time-evolution calculations of wave packets and the classical molecular dynamics simulations simultaneously. We clarify the relation between the charge transport property and these disorders. We will talk about these results in my presentation. [1] H. Ishii, K. Honma, N. Kobayashi, K. Hirose, Phys. Rev. B, 85 (2012) 245206. H. Ishii, N. Kobayashi, K. Hirose, Phys. Rev. B (to be published). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q20.00010: A general relationship between disorder, aggregation and charge transport in conjugated polymers Invited Speaker: Rodrigo Noriega The large variety of semiconducting polymers is a result of the synthetic capabilities to tune the optoelectronic properties of materials. Such variability also leads to a wide range of microstructures when solid films of conjugated polymers are cast from solution. Indeed, the large number of degrees of conformational freedom of these macromolecules and their weak intermolecular interactions result in complex microstructures - displaying a coexistence of amorphous and ordered phases with varying degrees of order. Understanding the limitations of charge transport in conjugated polymers is difficult owing to the unusual range of disorder they exhibit. These microstructures highlight the necessity to study the contributions of electronic processes at various length scales. In this talk, I will describe X-ray diffraction studies that show a large amount of disorder in semiconducting polymers while optoelectronic measurements show charges stay in the semi-ordered regions. Considering a particular system, P3HT with variable molecular weight, very similar transitions are observed in the behavior of the lattice disorder parameter (paracrystallinity) and charge mobility. Additionally, a comprehensive comparison of structure-property data across a wide range of materials uncovers strong similarities between seemingly diverse families of conjugated polymers. These insights allow the grouping of materials and identification of the importance of short-range ordered aggregates in transport. This generalization explains the seemingly contradicting high performance of recently reported, poorly ordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials. Specifically, the key to designing high mobility polymers is not in increasing crystallinity but rather in increasing their tolerance to an inevitably large amount of disorder within the aggregates by allowing more efficient intra- and intermolecular charge transport/transfer at the segmental level. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q20.00011: Low-Temperature Structural Phase Transition in a Soluble Oligoacene and Its Effect on Charge Transport Jeremy W. Ward, Abdulmalik Obaid, Cynthia S. Day, John E. Anthony, Oana D. Jurchescu Small-molecule organic semiconductors are of great interest to understanding fundamental properties of charge transport in organic semiconductors as they offer relatively structurally simple model systems. The crystal packing plays a crucial role in determining the electronic performance of a material, as we demonstrate for the case of fluorinated 5,11-bis(triethylsilylethynyl)anthradithiophene. Increased interest in this compound is driven by the recent demonstrations of its high stability and high performance in organic field-effect transistors. This material exhibits a structural phase transition around $T =$ 294 K, however properties below $T$ $=$ 230 K have not been investigated in detail. We identify an additional polymorph that forms below $T =$ 200 K and shows distinct properties compared to the previously reported polymorphs. We identity the phase transition generating the new polymorph using grazing incidence X-Ray diffraction, field-effect transistor electrical characterization and differential scanning calorimetry. The evolution of the field-effect mobility with temperature shows a one order of magnitude increase in value as the films transition from a pure phase to a co-existence of two phases. The structural changes in the film modify the injection picture in these devices, and irreversibly increase the contact resistance two orders of magnitude. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q20.00012: Characterization of charge motion in Poly(3-hexylthiophene) field effect transistors with Scanning Probe Microscopy Jason P. Moscatello, Morgen Patterson, Andrew R. Davis, Kenneth R. Carter, Katherine E. Aidala Poly(3-hexylthiophene) (P3HT) is a promising conductive organic polymer for applications such as organic FETs and photovoltaics. Key to proper utilization of P3HT is the understanding of how charges move and are trapped in the polymer, which directly affects the mobility of the charges as well as device efficiency. Scanning probe techniques, such as Kelvin Probe Force Microscopy, offer the advantage of being able to observe charges and local potentials down to the nano-scale. We present our work using scanning probe techniques to study charge injection and flow through P3HT FETs. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q20.00013: Photo-Patterned Ion Gel Electrolyte-Gated Thin Film Transistors Jae-Hong Choi, Yuanyan Gu, Kihyun Hong, C. Daniel Frisbie, Timothy P. Lodge We have developed a novel fabrication route to pattern electrolyte thin films in electrolyte-gated transistors (EGTs) using a chemically crosslinkable ABA-triblock copolymer ion gel. In the self-assembly of poly[(styrene-r-vinylbenzylazide)-b-ethylene oxide-b-(styrene-r-vinylbenzylazide)] (SOS-N$_{\mathrm{3}})$ triblock copolymer and the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]), the azide groups of poly(styrene-r-vinylbenzylazide) (PS-N$_{\mathrm{3}})$ end-blocks in the cores can be chemically cross-linked via UV irradiation ($\lambda =$ 254 nm). Impedance spectroscopy and small-angle X-ray scattering confirmed that ion transport and microstructure of the ion gel are not affected by UV cross-linking. Using this chemical cross-linking strategy, we demonstrate a photo-patterning of ion gels through a patterned mask and the fabricated electrolyte-gated thin film transistors with photo-patterned ion gels as high-capacitance gate insulators exhibited high device performance (low operation voltages and high on/off current ratios). [Preview Abstract] |
Session Q21: Focus Session: Polymers for Energy Storage and Conversion III - Ion Transport
Sponsoring Units: DPOLY GERAChair: Dan Hallinan, Florida State University
Room: 406
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q21.00001: Probing Structural Changes in Poly(3-hexylthiophene) (P3HT) During Electrochemical Oxidation with In Situ X-ray Scattering Jacob L. Thelen, Shrayesh N. Patel, Anna E. Javier, Nitash P. Balsara Mixtures of poly(3-hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO) block copolymer and lithium bis(trifluromethanesulfonyl) imide (LiTFSI) salt can microphase separate into electron (P3HT) and ion (PEO/LiTFSI) conducting domains. P3HT is a semicrystalline polymer with intrinsically semiconducting electronic properties. Electrochemical oxidation (doping) of the P3HT block provides the P3HT-b-PEO/LiTFSI mixtures with electronic conductivity suitable for lithium battery operation[1][2]. Due to the presence of the solid-state electrolyte (PEO/LiTFSI) in intimate contact with the microphase separated P3HT domains, electrochemical oxidation of P3HT can be performed entirely in the solid state; therefore, P3HT-b-PEO/LiTFSI provides a unique opportunity to study the structural changes in P3HT induced by oxidation. We use in situ x-ray scattering techniques to probe structural changes in P3HT during electrochemical oxidation and correlate these changes with previously observed enhancements in electron mobility[2]. [1] Javier, A. E., Patel, S. N., Hallinan, D. T., Srinivasan, V., Balsara, N. P., Angew. Chem. Int. Ed. Engl., 50, 9848-51 (2011). [2] Patel, S. N., Javier, A. E., Balsara, N. P. ACS Nano, 7, 6056-6068 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q21.00002: Design of superionic polymers for energy storage applications Yangyang Wang, Fei Fan, Alexander Agapov, Kunlun Hong, Tomonori Saito, Jimmy Mays, Alexei Sokolov Replacing traditional liquid electrolytes by polymers will significantly improve electrical energy storage technologies. Despite the significant advantages for applications in electrochemical devices, the use of solid polymer electrolytes has been impeded by their poor ionic conductivity. By analyzing the relationship between ionic conductivity and segmental relaxation, we demonstrate that polyether-based solid electrolytes have intrinsic limitations for ionic transport at ambient and low temperatures, due to strongly coupled segmental and ion dynamics. On the other hand, the ionic conductivity in rigid polymer can be strongly decoupled from segmental relaxation, in terms of both temperature dependence and relative transport rate, thus providing a significant advantage over traditional polyether-based electrolytes. Our analysis emphasizes that decoupling of ionic transport from segmental dynamics is the key for macromolecular design of superionic polymers. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q21.00003: Effects of Confinement and Interface on Ion Transport Properties of Block Copolymer Electrolytes Moon Jeong Park, Onnuri Kim, Gyuha Jo There is growing interest in blending polymers and ionic liquids (ILs) as a simple route to obtain solid-state ion conductors for a wide variety of electrochemical devices such as lithium batteries and fuel cells. Since the ion transport and mechanical properties of these materials are generally coupled, the development of IL-impregnated polymer electrolytes with improved ionic conductivity and optimized mechanical stability has lately been the subject of extensive studies employing diverse combinations of polymers and ILs. In this work, we present fascinating experimental insights into confinement- and interface-driven modulation of ion transport properties for block copolymer electrolytes. By varying the type of ILs, qualitatively similar lamellar morphology was identified, however, the highest conductivity was only achieved when ILs were confined within ionic domains with a sharp interface. In contrast, a high degree of intermixing of ionic and non-ionic domains at the interface resulted in a reduction by one order of magnitude in the conductivity owing to the creation of tortuous ion conduction pathways. This work suggests the future prospects for designing desired nanostructures as efficient ion conductors. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q21.00004: Using Tapered Block Copolymers to Create Conducting Nanomaterials Invited Speaker: Thomas Epps, III Soft materials, such as polymers, colloids, surfactants, and liquid crystals, are a technologically important class of matter employed in a variety of applications. One sub-class of soft material, \textbf{block copolymers}, provides the opportunity to design materials with attractive chemical and mechanical properties based on the ability to assemble into periodic structures with nanoscale domain spacings. Several applications for block copolymers currently under investigation in my group include battery and fuel cell membranes, analytical separations membranes, nano-tool templates, precursors to electronic arrays, and drug delivery vehicles. One area of recent progress in the group focuses on the behavior of conventional block copolymer and tapered block copolymer systems for lithium battery membrane applications. We find that we can tune poly(styrene-$b$-ethylene oxide) diblock copolymer nanostructures by adjusting the lithium counterion and lithium salt concentration, as well as the taper volume fraction and composition. Additionally, we can estimate the effective interaction parameters ($\chi_{\mathrm{eff}})$ for the salt-doped copolymers to determine the overall influence of tapering on the energetics of copolymer assembly. These tapered materials allow us to design nanostructured membrane systems with increased conductivity and improved mechanical properties in ion transport devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q21.00005: Mechanisms Underlying Ionic Mobilities in Nanocomposite Polymer Electrolytes Venkat Ganesan, Benjamin Hanson, Victor Pryamitsyn Recently, a number of experiments have demonstrated that addition of ceramics with nanoscale dimensions can lead to substantial improvements in the low temperature conductivity of the polymeric materials. However, the origin of such behaviors, and more generally, the manner by which nanoscale fillers impact the ion mobilities remain unresolved. In this communication, we report the results of atomistic molecular dynamics simulations which used multibody polarizable force-fields to study lithium ion diffusivities in an amorphous poly(ethylene-oxide) (PEO) melt containing well-dispersed TiO$_{2} $ nanoparticles. We observed that the lithium ion diffusivities decrease with increased particle loading. Our analysis suggests that the ion mobilities are correlated to the nanoparticle-induced changes in the polymer segmental dynamics. Interestingly, the changes in polymer segmental dynamics were seen to be related to the nanoparticle's influence on the polymer conformational features. Overall, our results indicate that addition of nanoparticle fillers modify polymer conformations and the polymer segmental dynamics, and thereby influence the ion mobilities of polymer electrolytes. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q21.00006: Characterization of PEO-b-P(STFSILi) as a Single-Ion Block Copolymer Electrolyte for Lithium Batteries Adriana Rojas, Sebnem Inceoglu, Nitash Balsara Block copolymers containing a poly(ethylene oxide) (PEO) ion-conducting block and a polystyrene (PS) structural block mixed with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt have been studied in the past as solid electrolytes for lithium batteries. However, transport of these ionic species result in concentration gradients during battery operation, and the energy expended by this process is inefficient. In other words, these electrolyte systems have low cation transference numbers. A single-ion block copolymer electrolyte has been synthesized where the TFSI anion of LiTFSI is covalently bound to the PS backbone. Li can dissociate from the immobilized anion, enabling the conduction of Li ions with a theoretical transference number of unity. AC impedance spectroscopy and small angle X-ray scattering are used to determine charge transport and morphological properties of these PEO-b-P(STFSILi) block copolymers over a range of molecular weights. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q21.00007: Morphology and Ionic Associations in Polyphosphazene Ionomers Joshua Bartels, Andrew Hess, Harry Allcock, Ralph Colby, James Runt Polyphosphazene ionomers with short chain poly(ethylene oxide) (PEO) moieties, bound ammonium cations, and free iodide anions were previously synthesized and their conductivity was studied through dielectric relaxation spectroscopy (DRS). Polyphosphazenes provide interesting conductive materials to study because of their low glass transition temperature and unique inorganic backbone. Non-ionic (poly[bis(methoxyethoxy)phosphazene] (MEEP) and two high ion content phosphazene ionomers were studied by multiple angle X-ray scattering (MAXS). Room temperature scattering shows the polymers are completely amorphous. Two peaks are observed in non-ionic MEEP and correspond to the phosphazene backbone-backbone spacing and to the amorphous halo of PEO. When longer ion-containing pendants are incorporated, an increase in the spacing is observed. A third peak is observed in the ionic systems and is interpreted as the average spacing between ions. The average ion separation closely matches the spacing of monomers and suggests the ions are not aggregated but stay in isolated pairs even at high ion content. Although there is no observable ionic aggregation, the conducting ion concentration remains low due to strong cation-anion associations. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q21.00008: Dynamics and morphologies of course-grained ionomer melts under an external electric field Christina Ting, Mark Stevens, Amalie Frischknecht Ionomers have been identified as potential solid electrolytes in battery applications. However, these systems are hindered by strong electrostatic interactions that can lead to ionic aggregation, making ion diffusion very slow. To develop a molecular understanding of how the ion transport depends on the system morphology and ultimately the ionomer chemistry, we perform molecular dynamics simulations. We apply an external electric field to a melt of course-grained polymers with charged groups along or pendant to the backbone, and explicit counterions. We observe ionic aggregate morphologies that merge and whose shape anisotropy, depending on the strength of the field, may decrease or increase (along the field direction). We also quantify the dependence of the drift mobility of the ions on the aggregate morphologies and the field strength. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q21.00009: Ion distributions in electrolyte confined by multiple dielectric interfaces Yufei Jing, Jos W. Zwanikken, Vikram Jadhao, Monica de la Cruz The distribution of ions at dielectric interfaces between liquids characterized by different dielectric permittivities is crucial to nanoscale assembly processes in many biological and synthetic materials such as cell membranes, colloids and oil-water emulsions. The knowledge of ionic structure of these systems is also exploited in energy storage devices such as double-layer super-capacitors. The presence of multiple dielectric interfaces often complicates computing the desired ionic distributions via simulations or theory. Here, we use coarse-grained models to compute the ionic distributions in a system of electrolyte confined by two planar dielectric interfaces using Car-Parrinello molecular dynamics simulations and liquid state theory. We compute the density profiles for various electrolyte concentrations, stoichiometric ratios and dielectric contrasts. The explanations for the trends in these profiles and discuss their effects on the behavior of the confined charged fluid are also presented. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q21.00010: How does crystalline structure affect ionic conductivity in solid polymer electrolyte? Shan Cheng, Derrick Smith, Christopher Li Solid polymer electrolytes (SPEs) have drawn intensive attention due to their potential applications in all-solid-state lithium batteries. Ion conduction in this system is generally considered to be confined in the amorphous polymer/ion phase and through segmental motion assisted hopping. In poly(ethylene oxide) (PEO) based SPEs, the crystalline nature of the polymer complicates the ion transport behavior. Herein we quantitatively show that the effect of polymer crystallinity on ion transport is two-fold: one is structural (tortuosity) and the other is dynamic (tethered chain confinement). We decouple these two effects by designing, and fabricating a model polymer single crystal electrolyte system with controlled crystal structure, size, crystallinity and orientation. The tortuosity effect results in a high conductivity anisotropy (10$^{2}$ -10$^{3})$ from directions parallel and transverse to PEO crystal lamellae. On the other hand, the dynamic effect is negligible at relatively high ion content, suggesting that semicrystalline polymer is a valid system for practical polymer electrolytes design. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q21.00011: Decoupling of ionic conductivity from structural dynamics in polymerized ionic liquids Ciprian Iacob, Joshua Sangoro, James Runt, Friedrich Kremer Charge transport and structural dynamics in low molecular weight and polymerized 1-vinyl-3-pentylimidazolium bis(trifluoromethylsulfonyl)imide ionic liquids (ILs) are investigated by a combination of broadband dielectric spectroscopy, dynamic mechanical spectroscopy and differential scanning calorimetry. While the dc conductivity and fluidity exhibit practically identical temperature dependence for the non-polymerized IL, a significant decoupling of ionic conduction from structural dynamics is observed for the polymerized IL. In addition, the dc conductivity of the polymerized IL is found exceed that of its molecular counterpart by four orders of magnitude at their respective calorimetric glass transition temperatures. This is attributed to the unusually high mobility of the anions even at lower temperatures when the structural dynamics is significantly slowed down. A simple physical explanation of the possible origin of the remarkable decoupling of ionic conductivity from structural dynamics is proposed. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q21.00012: Conductivity and Stability of Photopolymerized Polymer Electrolyte Network Thein Kyu, Ruixuan He, Yu-Ming Chen, Jialin Mao, Yu Zhu A melt-processing window has been identified within the wide isotropic region of the phase diagram of ternary blends consisting of poly (ethylene glycol diacrylate) (PEGDA), tetraethylene glycol dimethyl ether (TEGDME) and lithium bis(trifluoromethane) sulfonamide (LiTFSI). Upon UV-crosslinking of PEGDA in the isotropic window, the polymer electrolyte membrane (PEM) network thus formed is completely transparent and remains in the single phase without undergoing polymerization-induced phase separation or polymerization-induced crystallization. These PEM networks are solid albeit flexible and light-weight with safety and space saving attributes. The ionic conductivity as determined by AC impedance spectroscopy exhibited very high room-temperature ionic conductivity on the order of $\sim$10$^{-3}$ S/cm in several compositions, viz., 10/45/45, 20/40/40 and 30/35/35 PEGDA/TEGDME/LiTFSI networks. Cyclic voltammetry measurement of these solid-state PEM networks revealed excellent electrochemical stability against lithium reference electrode. The above study has been extended to the anode (graphite) and cathode (LiFePO$_{4}$) half-cell configurations with lithium as counter electrode. Charge/discharge cycling behavior of these half cells will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q21.00013: Holographic polymerization for highly conductive robust electrolyte membranes Derrick Smith, Wenda Wang, Timothy Bunning, Christopher Li The roles of nanostructure and confinement for ion mass transport in polymer electrochemical applications are key for improving the diffusion characteristics and mechanical robustness of solid electrolyte membranes. The challenges in fabricating highly controlled model systems are largely responsible for the interdependent ambiguities between nanostructures and the corresponding ion transport behavior. In this work, holographic polymer electrolyte membrane volume gratings comprised of alternating layers of robust cross-linked polymer resin and electrolyte, with an average d-spacing of 180 nm, were fabricated using holographic polymerization. These one-dimensional confinement structures were used to quantitatively study the anisotropic ionic conductivity properties and correlate this behavior to nano-confinement and phase mixing. Anisotropies greater than 5000 have been observed, and conductivities approaching 10$^{\mathrm{-4}}$ S/cm in robust freestanding films. In this case, the cross-linked resin serves as both load-bearing scaffold layers and as an electrolyte blending agent. These membranes serve as a platform in next generation nanostructured blend systems with enhanced mechanical properties for electrochemical applications. [Preview Abstract] |
Session Q22: Focus Session: Directed Assembly of Hybrid Materials II
Sponsoring Units: DPOLYChair: Sanat Kumar, Columbia University
Room: 407
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q22.00001: Role of Polymer-Graft Architecture on the Cohesive Interactions, Assembly and Thermo-Mechanical Properties of Particle Brush Materials Invited Speaker: Michael R. Bockstaller Recent progress in the area of surface-initiated controlled radical polymerization (SI-CRP) has enabled the synthesis of polymer-grafted particulates with precise control over the architecture of grafted chains. The resulting ``particle brush materials'' are of interest both from a fundamental as well as applied perspective because structural frustrations (that are associated with the tethering of chains to a curved surface) imply a sensitive dependence of the conformation of surface-grafted chains on the architecture of the particle brush. The opportunity to control chain conformation in hierarchically organized hybrid materials with precisely controlled microstructure renders particle brush materials intriguing building blocks for innovative material systems that could have a transformative impact on a range of ``soft material'' technologies. In the first part, this presentation will discuss experimental results that illustrate the role of polymer graft modification on the interaction between brush particles in solution and the solid state as well as the assembly characteristics of particle brushes in the solid state. The opportunities provided by ``merging'' of the physical properties of ordered particle superlattice structures with the mechanical properties and processibility of polymer materials will be demonstrated for the example of ``plastic colloidal crystal'' structures. In the second part, this presentation will showcase results of ongoing experimental studies that aim to harness the distinctively different property characteristics associated with ``stretched'' and ``relaxed'' polymer segments to facilitate novel property combinations in particle brush materials that are absent in binary particle/polymer blend systems. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q22.00002: Ordering Nanoparticles with Polymer Brushes Shengfeng Cheng, Mark Stevens, Gary Grest Ordering nanoparticles into a desired super-structure is crucial for their technological applications. We use molecular dynamics simulations to study the assembly of nanoparticles with polymer brushes randomly grafted to a plane surface and with varying densities. In the starting state, the nanoparticles are mostly dispersed in the solvent that wets the polymer brush. After the solvent is evaporated, the nanoparticles either enter the brush or straddle the top of the brush, depending on the strength of the nanoparticle/brush interaction. In the case of engulfed nanoparticles, a 2-dimensional array is only formed when the brush density is finely tuned to accommodate just a single layer of nanoparticles. When the brush density is higher or lower than this optimal value, the packing of nanoparticles shows large fluctuations in space and its quality diminishes. In the case of weak nanoparticle/brush interactions, a hexagonal packing with almost no defects is always found as long as the brush density is higher than some critical value. We also report an interesting healing effect of nanoparticles weakly interacting with the brush that can make a low-density brush more uniform. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q22.00003: Nonisotropic Assembly of Single-Component Hairy Nanoparticles R. Vaia, H. Koerner, L. Drummy, B. Benicewicz, Y. Li Solvent-free assemblies of hairy nanoparticles (HNPs) are providing avenues to avoid issues of mixing, agglomeration and limited inorganic content that plague traditional nanocomposites that are based on polymer-nanoparticle blending. We demonstrate that for a range of graft densities, depletion forces acting on high molecular weight poly(styrene) (120kDa) grafted to SiO$_{2}$ (r0 $=$ 8nm) lead to non-isotropic organization of the nanoparticle center of mass. The order within the neat HNP assembly (aHNP) and its elongational characteristics evolve as the architecture of the polymeric corona in solution transitions from concentrated (CBP) to semidilute (SDPB) polymer brush regimes. Specifically, local HNP packing adopts a non-isotropic arrangement at intermediate graft densities ($\sigma \quad =$ 0.01 -- 0.1 chains/nm$^{2})$ where the CPB-to-SDPB transition in solution is approximately r0. In concert, the neat HNP assembly responds to elongational deformation in a manner analogous to semi-crystalline elastomers. The correlation between the corona architecture of the HNP and the physical characteristics of the solvent free aHNP point toward a possible approach to tune mechanical, optical and electrical properties of single component hybrids in a manner analogous to block-copolymer mesoscale morphology. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q22.00004: Study of Rigid Polymers Grafted on Silica Nanoparticle as a Function of Coverage using Molecular Dynamics Simulations Sabina Maskey, J. Matthew D. Lane, Gary S. Grest, Dvora Perahia Nanoparticles (NPs) hybrids consisting of para dialkyl phenyleneethynylenes (PPEs) grafted to a silica NPs were studied in solution using molecular dynamic simulations. PPEs are rigid polymers whose conformation determines their degree of conjugation and assembly which in turn affects the electro-optical response of the NPs-polymer hybrids. Here we report the effect of coverage of PPE chains on their conformation, correlated with their interaction with the solvent. We have previously shown that at low coverage in good solvent, a star like hybrid is formed with the PPEs chained assuming a fully extended configuration in the corona, and associated with each other to form clusters with reduced solvent quality. Here we show that similar to the low concentration regime, increasing the coverage of PPEs in good solvents results in a homogenous corona with starched out PPE molecules. However, at higher coverage, clustering of chains becomes distinctive and their number increases with increase in the coverage of PPEs. We find that the clusters are temperature responsive and dissociate with increasing temperature. This control over clustering of the corona chains offers a mechanism to tune the electro-optical behavior of the hybrid as well as direct their assembly. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q22.00005: Polymer Conformation and Topological Defects in Systems of Hairy and DNA hybridized Nanoparticles Chris Knorowski, Alex Travesset Systems of hairy and DNA hybridized Nanoparticles are able to self-assemble into an array of superlattices. Understanding the role the polymer plays is critical to predicting the superlattice structure. In this talk, we use Molecular Dynamics to study hairy nanoparticles where the grafted polymer is modeled explicitly. We study self-assembly starting from a liquid and following the nucleation and growth of large nanoparticle superlattices (2000NP). We explore the role of polymer stretching as well as the geometric frustration of the polymer for both spherical and cubic nanoparticles. We also provide a characterization of the dynamics, including topological defects. Further, we will discuss the difficulties and methods for simulating large lattices in molecular dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q22.00006: Polymer-Grafted Nanoparticles in Polymer Melts: Modeling using Combined SCFT-DFT Approach Valeriy Ginzburg Nanoparticles (silica, carbon black, etc.) are often used as fillers to improve physical (thermal conductivity, coefficient of thermal expansion) and mechanical (modulus, strength) properties of polymer materials. In many cases, however, lack of nanoparticle dispersion in the polymer limits the utility of a resulting nanocomposite material. To improve dispersion, one often grafts organic chains (``ligands'') onto the surface of the particles; if the ligands are chemically miscible with the matrix polymer, it helps the particles to disperse more uniformly. In recent years, many new morphologies (``wires'', ``sheets'', ``networks'', etc.) were observed in such nanocomposites. Here, we adapt our earlier formalism combining Self-Consistent Field Theory (SCFT) for polymers with Density Functional Theory (DFT) for the particles; the modified formalism explicitly incorporates the grafted chains into SCFT. We then perform several simulations to study the dependence of morphology on the length and density of grafted chains, as well as the nanoparticle loading. The results are in qualitative agreement with predictions of earlier theories in the limit of lower particle loadings, and predict new morphologies (``bundles of wires'') for the case of larger particle loadings. The method can be easily extended to more complex cases (for example, where the matrix and/or ligand itself is a blend or block copolymer). [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q22.00007: Self-Assembly of Supramolecular Composites under Cylindrical Confinement Peter Bai, Kari Thorkelsson, Peter Ercius, Ting Xu Block copolymer (BCP) or BCP-based supramolecules are useful platforms to direct nanoparticle (NP) assemblies. However, the variety of NP assemblies is rather limited in comparison to those shown by DNA-guided approach. By subjecting supramolecular nanocomposites to 2-D cylindrical confinement afforded by anodic aluminum oxide membranes, a range of new NP assemblies such as stacked rings, and single and double helices can be readily obtained, as confirmed by TEM and TEM tomography. At low NP loadings (3 v{\%}), the nanostructure conforms to the supramolecule morphology. However, at higher NP loadings (6-9 v{\%}), the nanostructure deviates significantly from the morphology of supramolecular nanocomposites in bulk or in thin film, suggesting that frustrated NP packing, in addition to simple supramolecule templating, may play a significant role in the self-assembly process. The present studies demonstrate that 2-D confinement can be an effective means to tailor self-assembled NP structures and may open further opportunities to manipulate the macroscopic properties of NP assemblies. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q22.00008: Polymer Structure and Dynamics in Polymer / Layered-Silicate Nanocomposites Invited Speaker: Spiros H. Anastasiadis Polymer/layered silicate nanocomposites are of particular interest among different nanohybrids because of their anticipated superior properties. Mixing polymers with layered inorganic materials can lead to three different types of structure, depending on the interactions between the constituents: phase separated, intercalated and exfoliated. Intercalated hybrids, where the polymer is confined within the inorganic galleries, can serve as model systems for the study of the static and dynamic properties of macromolecules in nano-confinement. We describe our recent efforts to elucidate the effects of severe confinement utilizing hydrophilic nanohybrids of PEO or hyperbranched polymers mixed with Na$^{+}$-MMT. Intercalated hybrids with mono-, bi- and tri-layers of chains are obtained for all compositions covering the complete range from pure polymer to pure clay. Severe confinement influences significantly the structure of the polymer: the PEO chains intercalated within the inorganic galleries as well as those in close proximity to the outside walls are purely amorphous; it is only when there is significant excess polymer outside the completely filled galleries that the bulk polymer crystallinity is abruptly recovered. In contrast, when the inorganic is incorporated as silica nanoparticles, the crystallinity varies smoothly with composition whereas a population with a lower melting temperature near the inorganic surfaces is observed under strong confinement. The dynamics of the polymers confined within the galleries is probed by quasi-elastic neutron scattering and dielectric spectroscopy. The very local dynamics of the confined chains show similarities with those in bulk, whereas the segmental dynamics depend very strongly on the polymer/inorganic interactions varying from much faster to much slower or even frozen dynamics as the strength of the interactions increases. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q22.00009: Sulfonated Poly(styrene) Chains Grafted on Magnetic Nanoparticles Yang Jiao, Anton Yevelev, Javier Parra, Pinar Akcora Iron oxide nanoparticles functionalized with poly(styrene) (PS) chains at various grafting densities and loadings present stable and ordered nanostructures for tuning the mechanical and conductive properties in polymer composites. Strings, spherical and anisotropic clusters and well-dispersed particles are achieved with PS-grafted Fe$_{3}$O$_{4}$ nanoparticles in PS matrices upon varying the system parameters. In this work, we report the effect of sulfonic group locations on the aggregation state of polymer-grafted nanoparticles. Structures formed by the random and diblock copolymers of PS-poly(styrene sulfonate) (PSS) grafted particles will be discussed with small-angle x-ray scattering (SAXS) measurements in solution and melts. The conformational changes in PS-grafted chains and ion-containing grafts will be also presented in small-angle neutron-scattering (SANS) results to understand the role of polymer on the assembly of particles at the low grafting density. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q22.00010: Assembly of diblock copolymer grafted nanoparticles in a homopolymer blend matrix Cara Estridge, Arthi Jayaraman Hybrid materials comprised of nanoscale fillers embedded in a polymer matrix, also terms polymer nanocomposites, are used in many applications, such as photovoltaics, photonics, automobile parts, where their macroscopic properties are governed by the nanocomposite morphology. The structure and composite morphology is controlled by the interactions of the nanoscale fillers and the polymer matrix. In this talk we show using molecular simulations that functionalization of the nanoparticle surface with AB diblock copolymer grafts is a way to tune the interactions between the grafted particle and the A and B homopolymer blend matrix. Specifically, our work demonstrates that by tailoring the copolymer composition and the copolymer grafting density one can tune the location of the copolymer grafted particles in the matrix, (e.g. within a domain versus interface of two domains). Additionally, in the case where the grafted particles locate themselves at the interface between the two domains, the interfacial tension is reduced below that possible with bare ungrafted particles at the interface. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q22.00011: Additive-Driven Self-Assembly of Well Ordered Mesoporous Carbon/Iron Oxide Nanoparticle Composites for Supercapacitors Ying Lin, Xinyu Wang, Gang Qian, James Watkins Supercapacitors have attracted significant attention as energy storage devices for applications to meet the requirements of fast charge and discharge, high power density, and long cycle life. Recent research efforts demonstrate that the metal oxide- mesoporous carbon nanocomposite materials are indeed a class of promising electrode materials for high performance supercapacitors. However several major drawbacks for metal oxide-carbon nanocomposite materials remain, such as relatively low loadings of the metal oxide, aggregation of nanoparticles, and the lack of an ordered mesoporous structure. Here we demonstrate that well ordered mesoporous carbon/iron oxide composites can be prepared through simple carbonization of blends of block copolymers serving as the source of carbon and a porogen, e.g., poly(t-butyl acrylate)-block-polyacrylonitrile (PtBA-b-PAN), and iron oxide nanoparticles (NPs). Strong interactions between phenol-functionalized iron oxide NPs and polyacrylonitrile result in a preferential dispersion of the nanoparticles within the PAN domains and leads to ordered nanostructured mesoporous carbon framework containing upto 30 wt% iron oxide nanoparticles after pyrolysis. The specific capacitance of composites with 30 wt.% Fe2O3 NPs reaches 235 F/g. [Preview Abstract] |
Session Q23: Invited Session: Dynamics of Fluids at Interfaces
Sponsoring Units: DFD DPOLYChair: Joshua McGraw, Saarland University
Room: 505-507
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q23.00001: Fluid transport at the nanoscale: application to osmotic energy harvesting Invited Speaker: Lyd\'eric Bocquet |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q23.00002: Directed Assembly at Interfaces of Isotropic and Anisotropic Fluids Invited Speaker: Kathleen Stebe We exploit fields inherent to soft materials that contain colloidal particles to induce interactions and to direct particles to assemble into well-defined structures at given locations. The unifying concepts are that colloidal particles create deformations that store energy in soft matter, and that soft matter can be molded to create energy fields with which the particle sourced deformations can interact. Furthermore, since soft matter can be readily reconfigured, these approached pave the way to reconfigurable structures. Two examples are presented. In one, we exploit curvature fields at fluid interfaces to generate capillary interactions that steer particles along curvature gradients to given locations. Anisotropic particles adopt preferred orientations and migrate to sites of high curvature. The role of different aspects of the particle-sourced deformation and the imposed curvature field in driving these orientations and migrations is discussed. In the second example, we exploit elastic energies that arise in confined liquid crystals. By confining a nematic liquid crystal in a structure with well-defined anchoring conditions, the nematic director and its associated defect field can be molded to store elastic energy. This energy steers particles within the bulk or particles trapped at the nematic-air interface. We demonstrate this concept by creating defect rings around immersed microposts in a nematic liquid crystal. Particles trapped at the nematic air interface interact with this energy field, forming assemblies mimicking the defect texture. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q23.00003: Coalescence of drops on a substrate Invited Speaker: Jacco Snoeijer When two drops come into contact they will rapidly merge and form a single drop. Here we address the coalescence of drops on a substrate -- an elementary process encountered for example during condensation and inkjet printing. We focus on the dynamics just after contact, by characterizing the growth of the thin bridge connecting the two drops. For very viscous drops we present similarity solutions for the bridge, and find that the bridge size grows linearly with time $t$. Both the dynamics and the self-similar bridge profiles are verified quantitatively by experiments. We then consider the coalescence of water drops, for which viscosity can be neglected and liquid inertia becomes rate-limiting for the merging process. Once again, we find that experiments display self-similarity, but now the bridge size grows with as $t^{2/3}$ or $t^{1/2}$, depending on whether the contact angle is above or below $90^\circ$. A geometry-based scaling theory is able to capture these observations. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q23.00004: Correlation between surface topography and slippage Invited Speaker: Marcus Mueller Using Molecular Dynamics simulations of a polymer liquid flowing past flat and patterned surfaces, we investigate the influence of corrugation, wettability and pressure on slippage and friction at the solid-liquid interface. For one-dimensional, shallow, rectangular grooves, we observe a gradual crossover between the Wenzel state, where the liquid fills the grooves, and the Cassie state, where the corrugation supports the liquid and the grooves are filled with vapor. Using two independent flow set-ups, we characterize the near-surface flow by the slip length and the position, at which viscous and frictional stresses are balanced according to Navier's partial slip boundary condition. This hydrodynamic boundary position depends on the pressure inside the channel and may be located above the corrugated surface. In the Cassie state, we observe that the edges of the corrugation contribute to the friction. These simulation data illustrate the gradual crossover between the macroscopic behavior, where the friction is reduced in the Cassie state, and molecular scale corrugation, where the substrate roughness increases friction. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:30PM |
Q23.00005: Contact-line dynamics of colloidal interfaces Invited Speaker: Dirk Aarts The ongoing miniaturization in science and technology raises new questions about the behavior of liquids in confinement. One particularly suitable way to study strongly confined liquids is by combining colloid science with soft-lithography techniques. Here, we will focus on contact-line dynamics: as our model system we use a mixture of spherical colloids and non-adsorbing polymers, which allows us to directly study contact-line motion and the accompanying entrainment and pinch-off instabilities at the scale of the thermal interface fluctuations. We interpret our findings within a mesoscopic theoretical framework, where the small separation between fluid and system length scales can be matched to that of the experiments. It turns out that in this regime of length-scale overlap thermal fluctuations, wettability and gravity all play a crucial role in describing the dynamics. [Preview Abstract] |
Session Q24: Novel Instrumentation and Measurements for Biomedical Research
Sponsoring Units: GIMSChair: Larry Nagahara, Office of Physical Scinences - Oncology, National Cancer Institute
Room: 504
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q24.00001: Numerical dosimetry of transcranial magnetic stimulation coils Lawrence Crowther, Ravi Hadimani, David Jiles Transcranial magnetic stimulation (TMS) is a non-invasive neuromodulation technique capable of stimulating neurons by means of electromagnetic induction. TMS can be used to map brain function and shows promise for the diagnosis and treatment of neurological and psychiatric disorders. Calculation of fields induced in the brain are necessary to accurately identify stimulated neural tissue during TMS. This allows the development of novel TMS coil designs capable of stimulating deeper brain regions and increasing the localization of stimulation that can be achieved. We have performed numerical calculations of magnetic and electric field with high-resolution anatomically realistic human head models to find these stimulated brain regions for a variety of proposed TMS coil designs. The realistic head models contain heterogeneous tissue structures and electrical conductivities, yielding superior results to those obtained from the simplified homogeneous head models that are commonly employed. The attenuation of electric field as a function of depth in the brain and the localization of stimulating field have been methodically investigated. In addition to providing a quantitative comparison of different TMS coil designs the variation of induced field between subjects has been investigated. We also show the differences in induced fields between adult, adolescent and child head models to preemptively identify potential safety issues in the application of pediatric TMS. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q24.00002: Novel transcranial magnetic stimulation coil for mice Stephen March, Spencer Stark, Lawrence Crowther, Ravi Hadimani, David Jiles Transcranial magnetic stimulation (TMS) shows potential for non-invasive treatment of various neurological disorders. Significant work has been performed on the design of coils used for TMS on human subjects but few reports have been made on the design of coils for use on the brains of animals such as mice. This work is needed as TMS studies utilizing mice can allow rapid preclinical development of TMS for human disorders but the coil designs developed for use on humans are inadequate for optimal stimulation of the much smaller mouse brain. A novel TMS coil has been developed with the goal of inducing strong and focused electric fields for the stimulation of small animals such as mice. Calculations of induced electric fields were performed utilizing an MRI derived inhomogeneous model of an adult male mouse. Mechanical and thermal analysis of this new TMS helmet-coil design have also been performed at anticipated TMS operating conditions to ensure mechanical stability of the new coil and establish expected linear attraction and rotational force values. Calculated temperature increases for typical stimulation periods indicate the helmet-coil system is capable of operating within established medical standards. A prototype of the coil has been fabricated and characterization results are presented. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q24.00003: A Label-Free, Redox Biosensor for Detection of Disease Biomarkers Michelle M. Archibald, Binod Rizal, Timothy Connolly, Michael J. Burns, Michael J. Naughton, Thomas C. Chiles Technologies to detect early stage cancer would provide significant benefit to cancer disease patients. Clinical measurement of biomarkers offers the promise of a noninvasive and cost effective screening for early stage detection. We have developed a novel 3-dimensional ``nanocavity'' array for the detection of human cancer biomarkers in serum and other fluids. This all-electronic diagnostic sensor is based on a nanoscale coaxial array architecture that we have modified to enable molecular-level detection and identification. Each individual sensor in the array is a vertically-oriented coaxial capacitor, whose dielectric impedance is measurably changed when target molecules enter the coax annulus. We are designing a nanocoaxial biosensor based on electronic response to antibody recognition of a specific disease biomarker ($e.g.$ CA-125 for early-stage ovarian cancer) on biofunctionalized metal surfaces within the nanocoax structure, thereby providing an all-electronic, ambient temperature, rapid-response, label-free redox biosensor. Our results demonstrate the feasibility of using this nanocoaxial array as an ultrasensitive device to detect a wide range of target proteins, including disease biomarkers. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q24.00004: Nanocoax neurointerface array recordings of \textit{Hirudo medicinalis} neurons Jeffrey R. Naughton, Binod Rizal, Margaret H. Aasen, Michael J. Burns, Thomas C. Chiles, Michael J. Naughton We report results for a nanocoax-based neuroelectronic array. The device was used in real time to noninvasively couple to a ganglion sac located along the main nerve cord of the leech \textit{Hirudo medicinalis}. This allowed for extracellular recording of synaptic activity in the form of spontaneous synapse firing in pre- and post-synaptic somata. In addition, we show the ability to actuate localized stimulation (Faradaic regime) which, in some circumstances, appears to facilitate electroporation, which itself enables intracellular measurements. In conjunction with this latter recording with one subarray, we measured changes in the local field potential (extracellular) with another array at a second site, allowing us to calculate the action potential propagation or conduction speed. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q24.00005: Unlocking the Full Potential of MR Imaging of Multi-Spin Solids Jared Rovny, Sean Barrett, Merideth Frey Advances in magnetic resonance pulse sequences have allowed dramatic increases in imaging resolution of single-spin solids by exploiting the internal dynamics of radio-frequency pulses. The ``Quadratic Echo" pulse sequence [1] used to accomplish this provides results about 1000-fold worse when applied to multi-spin solids primarily due to heteronuclear dipolar interactions. Our preliminary goal is to discover an effective decoupling scheme for Phosphorus and Hydrogen in wet bone samples and integrate it with the complicated Quadratic Echo pulse sequence by advancing our understanding of the Hamiltonian dynamics of these systems. Initial trials will focus on simple systems and naive decoupling schemes, the results of which will serve to improve our understanding of the internal spin dynamics and guide further trials. Results will be presented from a benchmark study of Ammonium Dihydrogen Phosphate crystal as a simple multi-spin system under continuous-wave decoupling. Implications of these results and further possibilities for more complicated decoupling schemes will be discussed. [1] Proc. Natl. Acad. Sci. USA \textbf{109}, 5190 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q24.00006: Accelerating multidimensional NMR and MRI experiments using iterated maps Sean Barrett, Merideth Frey, Zachary Sethna, Gregory Manley, Suvrajit Sengupta, Kurt Zilm, J. Patrick Loria Techniques that accelerate data acquisition without sacrificing the advantages of fast Fourier transform (FFT) reconstruction could benefit a wide variety of magnetic resonance experiments. Here we discuss an approach for reconstructing multidimensional nuclear magnetic resonance (NMR) spectra and MR images from sparsely-sampled time domain data, by way of iterated maps [1]. This method exploits the computational speed of the FFT algorithm and is done in a deterministic way, by reformulating any \textit{a priori} knowledge or constraints into projections, and then iterating. In this paper we explain the motivation behind this approach, the formulation of the specific projections, the benefits of using a `QUasi-Even Sampling, plus jiTter' (QUEST) sampling schedule, and various methods for handling noise. Applying the iterated maps method to real 2D NMR and 3D MRI of solids data, we show that it is flexible and robust enough to handle large data sets with significant noise and artifacts. [1] M. A. Frey, Z. Sethna et al., Journal of Magnetic Resonance v237, 100 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q24.00007: TBD Invited Speaker: Thomas Thundat |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q24.00008: Feedback-driven tracking and trapping of a single fluorescent nanoparticle in a confocal microscope Lloyd Davis, James Germann, Jason King, Brian Canfield Improved techniques for recording the three-dimensional motion and spectroscopic dynamics of single fluorescent emitters with ever higher temporal and spatial resolution and for longer periods of observation will benefit future studies of molecular behavior and cellular mechanisms in biomedical research. Feedback-driven tracking and trapping, which relies on rapid determination of particle position followed by low latency application of motion to counteract Brownian diffusion, has been demonstrated by a number of techniques, each with their advantages and shortcommings. We have recently demonstrated a new method for tracking the motion of single emitters with diffusivities up to $\sim$ 12 square-microns/second by use of a confocal fluorescence microscope with four slightly spatially offset temporally modulated laser foci for position determination and a 3D-piezo stage to counteract diffusion. Here, the instrument achieves single-molecule sensitivity but the update rate of the piezo stage limits the response of the tracking. Also, we have recently shown trapping of a single nanoparticle by use of astigmatic imaging for position determination and a simple four-electrode microfluidic device for applying electrokinetic motion in three dimensions. Here, the frame rate of the imaging limits the response of the trap. We discuss combining the advantages of each these methods and the projected capabilities. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q24.00009: Laser speckle visibility acoustic spectroscopy in soft turbid media Fr\'{e}d\'{e}ric Wintzenrieth, Sylvie Cohen-Addad, Marie Le Merrer, Reinhard H\"{o}hler We image the evolution in space and time of an acoustic wave propagating along the surface of turbid soft matter by shining coherent light on the sample. The wave locally modulates the speckle interference pattern of the backscattered light and the speckle visibility\footnote{P. K. Dixon et D. J. Durian, ``Speckle Visibility Spectroscopy and Variable Granular Fluidization,'' Phys. Rev. Lett., vol. 90, no 18, p. 184302, 2003.} is recorded using a camera. We show both experimentally and theoretically how the temporal and spatial correlations in this pattern can be analyzed to obtain the acoustic wavelength and attenuation length. The technique is validated using shear waves propagating in aqueous foam.\footnote{F. Wintzenrieth, S. Cohen-Addad, M. Le Merrer, et R. H\"{o}hler, ``Laser speckle visibility acoustic spectroscopy in soft turbid media,'' Phys. Rev. E, 2013. (Submitted)} It may be applied to other kinds of acoustic wave in different forms of turbid soft matter, such as biological tissues, pastes or concentrated emulsions. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q24.00010: Graphene-based platform for nano-scale infrared near-field spectroscopy of biological materials Omar Khatib, Joshua D. Wood, Gregory P. Doidge, Gregory L. Damhorst, Aniruddh Rangarajan, Rashid Bashir, Eric Pop, Joseph W. Lyding, Dimitri N. Basov In biological and life sciences, Fourier Transform Infrared (FTIR) spectroscopy serves as a noninvasive probe of vibrational fingerprints used to identify chemical and molecular species. Near-field spectroscopy, based on the illumination of an atomic force microscope (AFM) tip with an infrared laser, allows for determination of IR properties of a material at nanometer length scales. However, application of near-field IR spectroscopy to most biological systems has thus far been elusive. Physiological conditions required for experimentation are incompatible with typical implementations of nano-FTIR. Recently it became possible to trap water and small biomolecules underneath large-area graphene sheets grown by chemical vapor deposition (CVD). The graphene layer serves as an IR-transparent cover that allows for a near-field interrogation of the underlying layers. We discuss the applicability of near-field IR nano-imaging and spectroscopy to trapped biomolecules in aqueous environments. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q24.00011: Biological Applications of Extraordinary Electroconductance (EEC) L.C. Tran, F.M. Werner, S.A. Solin Rapid detection of biomolecular concentration is a fundamental goal for lab on a chip diagnostic systems. The Extraordinary Electroconductance (EEC) sensor, a stacked, AuTi-GaAs metal semiconductor hybrid structure (MSH), has been previously demonstrated to have an electric field sensitivity of 3.05V/cm[1] in a mesoscopic-scale structure fabricated at the center of a parallel plate capacitor. In this work, we demonstrate the first successful application of EEC sensors as electrochemical detectors of molecular binding to the sensor surface. The negatively charged avidin derivative, captavidin, was applied with varying captavidin concentrations in phosphate buffered saline (PBS). The four-point measured resistance of bare EEC sensors was shown to increase by a factor of four due to captavidin binding at the sensor surface, as compared to a baseline binding assay in which the captavidin binding sites were blocked. Calculations for approximate electric field strengths introduced by a bound captavidin molecule will also presented. EEC sensors' four point measurements showed robustness and stability in spite of variations in the functional, linking layer. Ref [1] A.K.M. Newaz, et al, Phys Rev B. 79, 195308 (2009). [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q24.00012: Ultra-low field SQUID magnetic resonance for biomedical research P. Bhupathi, I. Hahn We are developing a SQUID (Superconducting QUantum Interference Device)-magnetometer system operating at 4K, for electron paramagnetic resonance (EPR) detection from room temperature samples in magnetic fields of the order of a Gauss. The magnetometer consists of a home-built, a second order gradiometer pick-up coil inductively coupled to the input of a commercially available two-stage dc SQUID amplifier with high bandwidth suitable for EPR, as well as NMR detection at wide range of frequencies up to a few MHz. Preliminary tests were done on samples of Pt powder at 4K and NMR signals have been detected in fields of few tens of gauss, with a minimum system sensitivity for spin concentration of $\sim $10$^{\mathrm{17}}$. We are currently developing an optimal SQUID gradiometer and a low temperature dewar for the EPR measurements. We plan to operate at low EPR excitation frequencies of a few MHz with the advantages of negligible sample heating and high penetration depth in biological systems. We discuss the prospects for \textit{in vivo} biomedical EPR imaging. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q24.00013: Single Cell Magnetic Measurements with a Superconducting Quantum Interference Device Johanna C. Palmstrom, Jennifer Arps, Bo Dwyer, Beena Kalisky, John R. Kirtley, Kathryn A. Moler, Lisa C. Qian, Aaron J. Rosenberg, Brian Rutt, Sui Seng Tee, Eric Theis, Elana Urbach, Yihua Wang Magnetic nanoparticles play an important role in numerous biomedical applications such as magnetic resonance imaging and targeted drug delivery. There is a need for tools to characterize individual magnetic nanoparticles and the magnetic properties of individual cells. We use a scanning superconducting quantum interference device (SQUID) to observe the magnetic fields from single mammalian cells loaded with superparamagnetic iron oxide nanoparticles. We show that the SQUID is a useful tool for imaging biological magnetism and is capable of resolving cell to cell variations in magnetic dipole moments. We hope to correlate these magnetic images with real space imaging techniques such as optical and scanning electron microscopy. The visualization of single cell magnetism can be used to optimize biological magnetic imaging techniques, such as MRI, by quantifying the strength of magnetic dipole moments of in vitro magnetic labeling. [Preview Abstract] |
Session Q25: Focus Session: Thermoelectrics: Controlling Spin
Sponsoring Units: GERA DMP FIAPChair: Joseph Heremans, Ohio State University
Room: 503
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q25.00001: Electronic Correlations and Thermoelectric Performance Invited Speaker: Brian Sales Most of the recent progress in improving thermoelectric performance has been due to a reduction of the lattice thermal conductivity. In current state of the art thermoelectric materials the lattice thermal conductivity is near its minimum value, and further improvement in ZT is likely to come from improving the power factor, which depends on the electronic structure. This presentation will discuss the possibility of using electronic correlations to enhance ZT. This work is supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q25.00002: Negative magnetoresistance and s-f scattering in Pb$_{1-x}$Eu$_{x}$Se Sunphil Kim, Yibin Gao, Bin He, Gloria Lehr, Yeseul Lee, Mercouri Kanatzidis, Donald Morelli, Joseph P. Heremans PbSe is cost effective over PbTe and has been a good potential p-type thermoelectric material, showing zT $>$1.\footnote{Yeseul Lee et al., J. Am. Chem. Soc. 135, 2013, 5152-5160.} Also, addition of Eu increases the band gap.\footnote{Arnim Lambrecht et al., J. Cryst. Growth 108, 1991, 301-308.} The valence band of Pb$_{1-x}$Eu$_{x}$Se appears theoretically favorable generating degeneracy between L-points and $\Sigma$-points. Here, we synthesize and characterize p-type doped Pb$_{1-x}$Eu$_{x}$Se:Na using combination of co-melting and Spark Plasma Sintering methods. Thermoelectric, thermomagnetic, and galvanomagnetic properties (electrical resistivity, Seebeck, and Hall) are measured and reported, along with Magnetization and Magnetoresistance. We found out that magnetic scattering is dominant in Pb$_{1-x}$Eu$_{x}$Se, showing negative magnetoresistance. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q25.00003: Intermediate Valence Tuning and Seebeck Coefficient Optimization in Yb-based Low-Temperature Thermoelectric Materials Gloria Lehr, Donald Morelli, Hyungyu Jin, Joseph Heremans Several Yb-based intermediate valence compounds have unique thermoelectric properties at low temperatures. These materials are interesting to study for niche applications such as cryogenic Peltier cooling of infrared sensors on satellites. Elements of different sizes, which form isostructural compounds, are used to form solid solutions creating a chemical pressure (smaller atoms -- Sc) or relaxation (larger atoms -- La) to alter the volume of the unit cell and thereby manipulate the average Yb valence. Magnetic susceptibility measurements show a strong correlation between the Seebeck coefficient and the ratio of trivalent to divalent Yb in these compounds. Two different Yb-based solid solution systems, Yb$_{\mathrm{1-x}}$Sc$_{\mathrm{x}}$Al$_{2}$ and Yb$_{\mathrm{1-x}}$La$_{\mathrm{x}}$Cu$_{\mathrm{2}}$Si$_{2}$, demonstrate that the concentration of Yb can be used to tune both the magnitude of the Seebeck coefficient as well as the temperature at which its absolute maximum occurs. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q25.00004: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q25.00005: Thermomagnetic properties of single-crystal Holmium Sarah J. Watzman, Yibin Gao, Stephen R. Boona, Joseph P. Heremans This talk will present results of experiments intended to experimentally map out the complete thermomagnetic transport tensor of elemental holmium. An emphasis has been placed on examining the evolution of these properties as the material crosses between its various magnetic phases, including the unusual helical anti-ferromagnetic state. This state is particularly interesting due to the gradual rotation of the local magnetic moments that leads to their net global cancellation, as the impact of this type of ordering on the thermomagnetic transport properties has yet to be fully explored. Specifically, we will report results of the magneto-thermopower, magneto-thermal conductivity, and the Nernst coefficient on single crystal samples. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q25.00006: Phonon induced magnetism in ionic materials Oscar D. Restrepo, Nikolas Antolin, Hyungyu Jin, Joseph P. Heremans, Wolfgang Windl Thermoelectric phenomena in magnetic materials create exciting possibilities in future spin caloritronic devices by manipulating spin information using heat. An accurate understanding of the spin-lattice interactions, i.e. the coupling between magnetic excitations (magnons) and lattice vibrations (phonons), holds the key to unraveling their underlying physics. We report ab initio frozen-phonon calculations of CsI that result in non-zero magnetization when the degeneracy between spin-up and spin-down electronic density of states is lifted for certain phonon displacement patterns. For those, the magnetization as a function of atomic displacement shows a sharp resonance due to the electronic states on the displaced Cs atoms, while the electrons on indium form a continuous background magnetization. We relate this resonance to the generation of a two-level system in the spin-polarized Cs partial density of states as a function of displacement, which we propose to be described by a simple resonant-susceptibility model. Current work extends these investigations to semiconductors such as InSb. ODR and WW are supported by the Center for Emergent Materials, an NSF MRSEC at OSU (Grant DMR-0820414).HJ and JPH are supported by AFOSR MURI Cryogenic Peltier Cooling, Contract \#FA9550-10-1-0533. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q25.00007: Observation of a magnetic field dependence of the lattice thermal conductivity Hyungyu Jin, Oscar Restrepo, Nikolas Antolin, Wolfgang Windl, Stewart Barnes, Joseph Heremans Can phonons respond to magnetic fields? From the simple point of view of the classical lattice vibrations, there is no clue that phonons possess any magnetic characteristics. Here, we report for the first time that the lattice thermal conductivity can show a response to an external magnetic field in a non-magnetic semiconductor crystal. We observe a magnetic field dependence of the lattice thermal conductivity in a high quality 2x10$^{15}$ Te doped single crystal of InSb. The electronic contribution is over 10$^{6}$ times smaller than the lattice. The effect is observed in the temperature regime where the Umklapp processes start appearing, and still mainly involve phonons with long mean free paths. A special thermal design is employed to obtain a high accuracy heat flux measurement. Detailed experimental procedures and results are presented along with a brief discussion about possible origins of the effect. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q25.00008: A possible origin for the colossally large Seebeck coefficient in FeSb$_{2}$ Hidefumi Takahashi, Ryuji Okazaki, Ichiro Terasaki, Yukio Yasui Narrow-gap semiconductor FeSb$_{2}$ has attracted interest because of the recent observation of a colossal Seebeck coefficient $S\simeq -45$ mV/K at 10 K.[A. Bentien $et$ $al$., EPL 80, 17008 (2007).] This compound has a small energy gap $\Delta \sim 5$ meV and $\mid S\mid $ rapidly increases below 40 K, suggesting that $\Delta$ is formed by an unusual mechanism such as a strong electron correlation. However, the reported maximum values of $S$ are remarkably different from sample to sample, ranging from $-500$ $\mu$V/K to $-45$ mV/K. We report a systematic study of ppm-level impurity effects of magnetic and transport properties with single crystals.[H. Takahashi $et$ $al$., JPSJ. 80, 054708 (2011).] A purest sample has a small carrier concentration ($<10^{16}$ cm$^{-3}$ below 30 K) and a large $S$ ($-1400$ $\mu$V/K at 20 K), indicating that the large $S$ predominantly comes from the small carrier density. Moreover, we have measured the magnetic field dependence of transport properties of the purest crystal to investigate the relation between the electronic states and transport properties. We successfully explain the results in terms of an extrinsic semiconductor with ppm-level impurities, suggesting that the large $S$ arises from the low carrier concentration with a phonon-drag [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q25.00009: On the thermal Hall effect in the electrically insulating ferrimagnet yttrium iron garnet Stephen R. Boona, Joseph P. Heremans This talk will present results from our recent experiments aimed at measuring the elements of the magneto-thermal conductivity tensor $\kappa _{\mathrm{ijk}}$ of the electrically insulating ferrimagnet yttrium iron garnet (YIG). We will report evidence of a non-zero contribution from $\kappa_{\mathrm{xyz}}$, which suggests the existence of a thermal Hall effect in this material. We will discuss the ramifications of these results in relation to spin caloritronic experiments, as well as some possible concepts for thermal-to-electrical energy conversion applications based on this phenomenon. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q25.00010: Studies on magnetoresistance and magneto-thermopower of single cobalt nanowire D. Kim, J. Kally, M.H.W Chan, N. Samarth, D. Tadigadapa We have studied the magnetoresistance and magneto-thermopower of suspended individual cobalt nanowire using microfabricated thermoelectric workbench. The workbench has embedded heater and thermocouple to provide a temperature gradient along a nanowire and measure temperature of both ends of nanowire. The cobalt nanowire was synthesized by direct electrodeposition and dispersed in solution. It was confirmed that the nanowire is single crystal with hexagonal close-packed structure by TEM analysis. Cobalt nanowires with 70-nm-diameter nanowires were drop-cast on the device and focused-ion-beam-induced deposition of platinum was used to provide mechanical anchors and good electrical and thermal contact between nanowire and the workbench. The magnetic field was applied perpendicular and parallel to wire axis. The absolute value of thermopower increased with perpendicular magnetic field and the value in the saturation state was 0.9 {\%} higher A negative magnetoresistance was observed with 1.3 {\%} smaller resistance in the saturation state. Because of increasing thermopower and decreasing resistance thermoelectric power factor was improved with magnetic field [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q25.00011: Spin-Seebeck effect due to thermally driven spin-polarized electron transport on the surface of a three-dimensional topological insulator Po-Hao Chang, Farzad Mahfouzi, Naoto Nagaosa, Branislav Nikolic We study the spin Seebeck effect on the surface of a three-dimensional topological insulator (TI), such as Bi$_2$Se$_3$, in a geometry in which temperature bias is applied parallel to the surface. This generates spin-polarized charge current with polarization component $P_x \simeq 60\%$ along the direction of transport due to surface spin-orbit coupling. The spin current injected from the surface into a third nonmagnetic voltage probe, covering portion of the TI surface across its width, is converted via the inverse spin Hall effect (ISHE) into the voltage signal yielding the spin-Seebeck coefficient \mbox{$|S_{xy}|^{\mathrm{max}} \simeq 30$ nV/K} (assuming the SH angle of Pt voltage probe). Our prediction relies crucially on specific orientation of quintuple layers of Bi$_2$Se$_3$ with respect to the TI surface and direction of transport, as well as on the corresponding proper coupling of electronic spin states with support on the Bi and Se sublattice to spins emitted or absorbed by the three attached normal metal leads. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q25.00012: Thermoelectric corrections to quantum voltage measurement Charles Stafford, Justin Bergfield The voltage measured by a floating probe of a nonequilibrium quantum system is shown to exhibit nontrivial thermoelectric corrections at finite temperature. The voltage probe is modelled as a scanning potentiometer/thermometer that is allowed to equilibrate with a quantum system via local tunnel coupling. Once equilibrated, the net electrical and heat currents flowing into the probe are zero. This generalizes Buettiker's theory of voltage measurement [1] at zero temperature to finite-temperature systems. In a quantum conductor with electrical bias, it is shown that the probe temperature generally differs from ambient temperature due to Peltier cooling/heating within the system, and that the temperature difference can be sizeable for modest bias voltages. Conversely, if the probe is held at ambient temperature, its voltage is shifted from the equilibrated value, leading to a significant error in voltage measurement. However, if there is a large thermal coupling of the probe to the ambient environment, thermal coupling between the probe and system becomes unimportant, and the voltage measurement becomes similar to the process at zero temperature, with negligible thermoelectric corrections. [1] M. Buttiker, Phys. Rev. B 40, 3409 (1989). [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q25.00013: Galvanomagnetic and Thermoelectric Properties of Bi$_{2}$Se$_{3-x}$S$_{x}$ Prepared by Spark Plasma Sintering and Annealing Bin He, Yi-Bin Gao, Joseph P. Heremans Bi$_{2}$Se$_{3}$ belongs to the tetradymite class of semiconductors, many of which are known thermoelectric materials. Bi$_{2}$Se$_{3}$ has intrinsic Se vacancies that tend to make it n-type, and the conduction band density of states (DOS) is too low to give a high ZT. Here sulfur is added to pure Bi$_{2}$Se$_{3}$ in order to increase the DOS and reduce vacancy concentration. A group of Bi$_{2}$Se$_{3-x}$S$_{x}$ samples are prepared by SPS, with x varying from 0\% to 10\%. The samples are cut into halves, with one half measured directly and the other annealed before measurement. Thermoelectric properties are measured from 80K to 420K. The Seebeck coefficient increases after annealing while the Hall measurements show the carrier concentration to drop from about 10$^{19}$/cm$^{3}$ to about 10$^{18}$/cm$^{3}$. The DOS increases a little with x, as expected, and the best power factor reached is about 16 $\mu$W/cmK$^{2}$. While the electron concentration can thus be controlled, further work is needed to increase the conduction band DOS more. [Preview Abstract] |
Session Q26: Focus Session: Materials in Extremes: Reactive Chemistry
Sponsoring Units: GSCCM DCOMP DMPChair: Timothy Germann, Los Alamos National Laboratory
Room: 502
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q26.00001: Fast Quantum Molecular Dynamics Simulations of Shock-induced Chemistry in Organic Liquids Invited Speaker: Marc Cawkwell The responses of liquid formic acid and phenylacetylene to shock compression have been investigated via quantum-based molecular dynamics simulations with the self-consistent tight-binding code LATTE. Microcanonical Born-Oppenheimer trajectories with precise conservation of the total energy were computed without relying on an iterative self-consistent field optimization of the electronic degrees of freedom at each time step via the Fast Quantum Mechanical Molecular Dynamics formalism [A. M. N. Niklasson and M. J. Cawkwell, Phys. Rev. B, \textbf{86}, 174308 (2012)]. The conservation of the total energy in our trajectories was pivotal for the capture of adiabatic shock heating as well as temperature changes arising from endo- or exothermic chemistry. Our self-consistent tight-binding parameterizations yielded very good predictions for the gas-phase geometries of formic acid and phenylacetylene molecules and the principal Hugoniots of the liquids. In accord with recent flyer-plate impact experiments, our simulations revealed i) that formic acid reacts at relatively low impact pressures but with no change in volume between products and reactants, and ii) a two-step polymerization process for phenylacetylene. Furthermore, the evolution of the HOMO-LUMO gap tracked on-the-fly during our simulations could be correlated with changes transient absorption measured during laser-driven shock compression experiments on these liquids. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q26.00002: Slow N-O chemistry in detonating oxygen balanced mixtures Nir Goldman, Sorin Bastea The chemical evolution and states of matter of energetic materials under detonation conditions remains an open question despite decades of research. Reaction zones for many energetic materials are inferred from hydrodynamic measurements to be anywhere between nanosecond to microsecond time scales. However, the molecular level processes that govern these reaction zone lengths are poorly understood for many organic materials and composites. To this end, we have conducted quantum molecular dynamics simulations of zero and positive oxygen balance mixtures of hydrogen peroxide/nitromethane under detonation conditions to close to equilibrium time scales. We observe the formation of metastable nitrogen oxide intermediates that effectively act as an oxygen ``trap'' by directly slowing the formation of the equilibrium products CO$_{2}$ and N$_{2}$. This is in sharp contrast to the decomposition mechanism of carbon-rich, negative oxygen balanced energetic materials, where N-O chemistry equilibrates extremely rapidly, and carbon condensation and carbon-oxygen bond chemistry are the rate limiting steps to achieving chemical equilibrium. Further work is underway to fully determine the kinetic parameters for N-O chemistry under these conditions for possible use in hydrocode models. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q26.00003: Shock compression of glow discharge polymer (GDP): density functional theory (DFT) simulations and experiments on Sandia's Z machine Kyle R. Cochrane, T. Ao, R.W. Lemke, S. Hamel, M.E. Schoff, B.E. Blue, M.C. Herrmann, T.R. Mattsson Glow discharge polymer (GDP) is used extensively as capsule/ablation material in inertial confinement fusion (ICF) capsules. Accurate knowledge of the equation of state (EOS) under shock and release is particularly important for high-fidelity design, analysis, and optimization of ICF experiments since the capsule material is subject to several converging shocks as well as release towards the cryogenic fuel. We performed Density Functional Theory (DFT) based quantum molecular dynamics (QMD) simulations, to gain knowledge of the behavior of GDP - for example regarding the role of chemical dissociation during shock compression, we find that the dissociation regime along the Hugoniot extends from 50 GPa to 250 GPa. The shock pressures calculated from DFT are compared experimental data taken at Sandia's Z-machine. The GDP samples were grown in a planar geometry to improve the sample quality and maintained in a nitrogen atmosphere following manufacturing, thus allowing for a direct comparison to the DFT/QMD simulations. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q26.00004: Modeling of amorphous poly-CO structure with N and He I.G. Batyrev, W.D. Mattson Density functional theory simulations of amorphous poly-CO structure were performed with addition of N or He atoms to crystalline delta phase of CO. For the CO-N mixtures the concentration of N was varied in the range from 6.25 {\%} to 50{\%} with different distribution of N atoms in the unit cell. For all studied concentrations and initial configurations, isotropic compression led to polymerization beginning at a pressure of 11 GPa. For the CO-He mixtures, the concentration of He atoms in delta phase of CO was 6.25{\%}. Formation of random networks begins at 9 GPa and at 11 GPa all CO molecules have formed a combination of closed rings and chain type structures without isolated CO molecules with a density of 2.40 g/cm3. He atoms facilitate complete formation of the random structure at lower pressure than that for pure poly-CO, which isn't completely polymerized until compressed to a pressure of 18 GPa. He atoms also help stabilize the structure while lowering the pressure down to 100 Bar with only few CO molecules detaching in the process. Without He atoms at the same pressure there are approximately ten times the number CO molecules occupying voids in the random network. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q26.00005: Vibrational Energy Relaxation in Common High Energy Density Materials Through Reactive Molecular Dynamics Simulations Mitchell Wood, Alejandro Strachan We use molecular dynamics with the reactive force field ReaxFF to study the decomposition and subsequent reactions of the nitramine HMX under induced by electric fields and temperature. We find that electric fields of appropriately chosen frequencies can trigger chemical decomposition for total energy input significantly smaller than thermally excited systems. In addition, the energy barriers associated with exothermic chemical reactions are also dependent on the character of the excitation for electric field driven samples. We are able to characterize the frequency-dependent energy input and subsequent equilibration using the power spectra of atomic velocities and we find that the non-equilibrium nature of the energy distribution obtained via electric field excitations is responsible for the dependence of energy threshold for decomposition on type of perturbation. Timescales and decay pathways for vibron energy are discussed for HMX and other energetic materials. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q26.00006: Shock response of Ni/Al reactive inter-metallic composites Mathew Cherukara, Timothy Germann, Edward Kober, Alejandro Strachan Intermolecular reactive composites find diverse applications in defense, microelectronics and medicine, where strong, localized sources of heat are required. Motivated by experimental work which has shown that high-energy ball milling can significantly improve the reactivity as well as the ease of ignition of Ni/Al inter-metallic composites, we present large scale ($\sim$41 million atom) molecular dynamics simulations of shock-induced chemistry in porous, polycrystalline, lamellar Ni/Al nano-composites, which are designed to capture the microstructure that is obtained post milling. Shock propagation in these porous, lamellar materials is observed to be extremely diffuse, leading to substantial inhomogeneity in the local stress states of the material. We describe the importance of pores as sites of initiation, where local temperatures can rise to several thousands of degrees, and chemical mixing is accelerated by vortex formation and jetting in the pore. We also follow the evolution of the chemistry after the shock passage by allowing the sample to ``cook'' under the shock induced pressures and temperatures for up to 0.5 ns. Multiple ``tendril-like'' reaction fronts, born in the cauldron of the pores, propagate rapidly through the sample, consuming it within a nanosecond. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q26.00007: The influence of mesostructural properties in concentric Ni-Al laminates on the accommodation of large plastic strain during high strain rate dynamic loading Karl Olney, Po-Hsun Chiu, Andrew Higgins, Matthew Serge, David Benson, Vitali Nesterenko Ni-Al laminates have been shown to be good candidates for use in reactive material systems due to their ability to release chemical energy via an intermetallic reaction initiated by thermal or mechanical stimuli. The Thick Walled Cylinder (TWC) technique allows for the testing and complete recovery of samples during a tunable high strain/strain rate plastic deformation process under plane strain conditions. Ni-Al laminates constructed from alternating concentric Ni (25.4 micron thickness) and Al (38.1 micron thickness) layers demonstrated that the cooperative buckling of layers was the dominant mode of plastic strain accommodation. Intermetallic reaction spots were observed in many of the apexes of these buckles. Alterations to mesoscale properties in these laminates using the TWC method help to understand the role of the mesostructural properties on the accommodation of plastic strain and possible development of intermetallic reaction. Finite element simulations show good agreement with the TWC experiments and provided insights into the evolution of the mesostructure during the collapse. These insights may be used to tailor the mesostructure to enhance the reactivity in the Ni-Al laminates. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q26.00008: Atomistic Calculations of Nanosecond Timescale Kinetics of Shock-Compressed SiO2 Invited Speaker: Evan Reed Unraveling the behavior of SiO2 under high dynamic pressures is important for understanding meteor impacts, laser-induced damage of optics, and interpretation of shock compression experiments in a geophysical context. This behavior is made complicated by the presence of several high pressure phases and complex kinetic processes that yield long-lived metastable states. Quantitative understanding of the kinetics is nearly as important as the thermodynamics for this material. Here we make the first atomistic calculations of kinetic processes in SiO2 shocked to pressures near the stishovite and melt boundary. We perform atomistic calculations with up to 1 million atoms for timescales of 10 nanoseconds to elucidate the nucleation and growth and subsequent grain size evolution of the stishovite phase. These calculations are enabled by the multi-scale shock technique (MSST) implemented in LAMMPS. We further study the role of the quantum nature of the nuclei at 1 million atom scales using a fast semiclassical variation of MSST called QBMSST. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q26.00009: A Gibbs Formulation for Reactive Condensed Phase Materials with Phase Change D. Scott Stewart A large class of applications have pure, condensed phase constituents that come into contact, chemically react and simultaneously undergo phase change. Phase change in a given molecular material has often been considered to be separate from chemical reaction. Continuum modelers of phase change often use a phase field model whereby an indicator function is allowed to change from one value to another in regions of phase change, governed by evolutionary (Ginzburg-Landau) equations. Whereas classic chemical kinetics literally count species concentrations and derive evolution equations based on species mass transport. We argue that the latter is fundamental and is the same as the former, if all species, phase or chemical are treated as distinct chemical species. A self consistent continuum-thermomechanical model, must account for all significant energetic quantities and have correct molecular and continuum limits in the mixture. The use of Gibbs potentials for all relevant species, chemical and phase does this neatly and allows the use of classical potentials, while allowing for modeling of phase interaction terms and reaction rate and phase change kinetics. The phase field quantities are the mass fractions. One has a single stress tensor, and a single temperature. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q26.00010: Physical Properties of Supercritical Silica Carl Greeff, Daniel Sheppard Supercritical states of silica -- states lying above the vapor dome -- are produced on release from strong shocks applied to solid quartz, as may have occurred in a planetary impact that formed the moon. These states are also reached in shocks applied to highly porous aerogels. In the supercritical region, ion-ion correlations and thermal excitation of electrons are significant and non-trivial. Models used in wide-ranging equations of state such as Sesame generally have simplified physics in this region. We present results from extensive quantum molecular dynamics simulations for the equation of state and optical properties of supercritical silica. The simulations are found to reproduce the aerogel Hugoniot very well. We examine the nature of the ion correlations and electronic spectrum with an eye toward improving equation of state models in this difficult region. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q26.00011: Developing a Reactive Potential for Hydrocarbons Under Extreme Deformation Thomas O'Connor, Lars Pastewka, Jan Andzelm, Mark Robbins In traditional molecular dynamics, interaction potentials define and maintain a fixed topology of chemical bonds. The development of reactive cluster potentials, such as the AIREBO and the REAXFF, have allowed modelers to explore dynamic bonding processes at the expense of more complex many-particle interactions. The complexity of these many-particle interactions makes parameterization of such models difficult; consequentially, current cluster potentials quickly lose fidelity outside a limited range of ambient pressures and temperatures, corresponding to near equilibrium states of the bonds described. In order to accurately describe the dynamics of extreme loading and failure, existing reactive cluster potentials must be modified to improve the description of highly strained bond configurations. Here we present a modified AIREBO potential for hydrocarbons with accuracy extended to highly strained intra (and inter) molecular configurations. Implementing recently developed techniques of bond-screening and a reparameterization of van der Waal's interactions based on second-order Moller-Plesset perturbation theory, we explore the equilibrium states of condensed phase hydrocarbons under high pressure and yield under extreme loads. [Preview Abstract] |
Session Q27: Focus Session: Electron-hole Interaction in Nanoparticles
Sponsoring Units: DCOMPChair: Ari Chakraborty, Syracuse University
Room: 501
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q27.00001: Excitation gaps of finite-sized systems from Optimally-Tuned Range-Separated Hybrid Functionals Invited Speaker: Leeor Kronik Excitation gaps are of much significance in electronic structure theory. Within many-body perturbation theory, the fundamental gap is the difference between the lowest quasi-hole and quasi-electron excitation energies and the optical gap is addressed by including the quasi-electron - quasi-hole interaction. A long-standing challenge has been the attainment of a similar description within density functional theory (DFT), with much debate on whether this is achievable even in principle. Here, I describe a new DFT approach to this problem. Anchored in the generalized Kohn-Sham framework, our method is based on a range-split hybrid functional with exact long-range exchange. Its novel feature is that the range-splitting parameter is determined from first principles, per-system, based on satisfaction of physical constraints. For finite objects, this approach mimics successfully the quasi-particle excitation picture. It allows the extraction of the fundamental and optical gap from one underlying functional, based on the ground-state HOMO-LUMO gap and the lowest excitation of linear-response time-dependent DFT, respectively. It is equally accurate for the difficult case of charge-transfer excitations and it produces accurate outer-valence simulated photoelectron spectra. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q27.00002: First-Principles Investigation of Hot Carrier Relaxation in Quantum-Dots Kyle Reeves, Lesheng Li, Yosuke Kanai A combination of fewest-switches surface hopping simulations and Kohn-Sham (KS) density functional theory allows us to obtain key insights into hot carrier relaxation processes in materials. In particular, a first-principles approach allows us to investigate the influence of atomistic details such as the surface chemistry on the relaxation process in quantum dots. At the same time, the accuracy of the KS single-particle energies impacts the simulated non-adiabatic relaxation rate. Using a small gold nanocluster as a model system, we investigate how relaxation rates are influenced by the single-particle energies by considering a hybrid functional in DFT and the GW approximation. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q27.00003: Investigation of effect of particle size and heterojunction on electron-hole interaction in CdSe and CdSe/ZnS quantum dots Jennifer Elward, Jeremy Scher, Arindam Chakraborty The focus of this work is to explore both the effect of dot size and the effect of heterojunction on the electron-hole (e-h) interaction in nanoparticles. The exciton binding energy (EB), electron-hole recombination probability (P-eh) and average electron-hole separation (R-eh) are important metrics used to quantify the e-h interaction. The form of the wavefunction is critical for accurate representation of these properties. In this work, the explicitly correlated Hartree-Fock (XCHF) and explicitly correlated configuration interaction (XCCI) methods are used. The effect of dot size on e-h interaction was investigated by studying a CdSe quantum dot (QD) system, with diameters ranging from 1-20 nm. The EB and P-eh were found to be strongly dependent on dot size, however, the scaling of each property was distinctly different. The effect of heterojunction was explored by studying a CdSe/ZnS core/shell QD system. The effects of increasing shell thickness, core size and effect of volume versus effect of heterojunction were assessed by computing the EB, P-eh and R-eh. It was found that these properties are also dependent on size, however, the dependence of the CdSe/ZnS core/shell system was markedly different from the CdSe core-only system. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q27.00004: Electronic Properties of Defects at the Surface of Embedded Silicon Nanoparticles Nicholas Brawand, Marton Voros, Giulia Galli Using density functional theory calculations we predicted the single particle energies and lifetimes of dangling bond defects at the surface of various Si nanoparticles (NPs) embedded in amorphous SiO$_2$ matrices [1]. We found that both the lifetimes and the single particle excitation energies of these defects depend on the size of the NP. However, the energy positions of the dangling bond defect levels are always within the NP gap, irrespective of size, between 1 and 2 nm. Our findings suggest that the presence of silicon NPs within amorphous SiO$_2$ may impact the functionality of silicon-on-insulator nanophotonic devices operating near 1.5 $\mu$m [2]. [1] Li, Tianshu et al. Phys. Rev. Lett., 107, 206805 (2011) [2] Bogaerts, W. et al. Journal of Lightwave Technology, 23, 401-412 (2005) [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q27.00005: Quasiparticle band structures and thermoelectric transport properties of Mg$_{2}$Si, Mg$_{2}$Ge, and Mg$_{2}$Sn Guangsha Shi, Emmanouil Kioupakis Mg$_{2}$Si, Mg$_{2}$Ge, and Mg$_{2}$Sn are narrow-gap semiconductors with large Seebeck coefficients and favorable thermoelectric properties. We calculated the quasiparticle band structures of Mg$_{2}$Si, Mg$_{2}$Ge, and Mg$_{2}$Sn using density functional and many-body perturbation theory in the GW approximation. The calculated band gaps are in good agreement with experiment. The inclusion of semicore states in the valence is necessary to obtain accurate band gaps for Mg$_{2}$Ge and Mg$_{2}$Sn. We used the maximally localized Wannier function method and the Boltzmann transport equation in the constant relaxation-time approximation to determine the Seebeck coefficient and the electrical and carrier thermal conductivities. We discuss the importance of quasiparticle corrections to accurately determine the Seebeck coefficients at high temperatures. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q27.00006: Optical, electronic and transport properties of tetrahedrites Simon Kohl, Jason Vielma, David Foster, Guenter Schneider Doped Tetrahedrites Cu$_{12-x}$TM$_{x}$Sb$_{4}$S$_{13}$ (TM={Fe,Mn,Zn}) have recently attracted interest as thermoelectric materials. We present an ab-initio study based on density functional theory of the optical, electronic and transport properties of these materials. We find in Cu$_{12-x}$Zn$_{x}$Sb$_{4}$S$_{13}$: 1. the band-gap can be tuned through chalcogenide substitution and the optical absorption is very large making tetrahedrites attractive also as solar absorber materials. A point defect study of the Zn rich tetrahedrite (x=2) based on supercell calculations indicates p-type conductivity and Cu-Zn antisite defects are the dominant acceptor defect with Cu-vacancies also contributing. The calculated hole concentration is much larger than what is expected from conductivity measurements. We discuss these results in the context of the observed unusual, variable range hoping like electronic transport properties. Finally we present results of thermopower calculations based on semiclassical Boltzmann theory and discuss the applicability of these approach for tetrahedrites. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q27.00007: Strong core hole in resonant inelastic x-ray scattering (RIXS) Robert Markiewicz, John Rehr, Arun Bansil We apply a lattice version of Mahan, Nozi{\`e}res, and de Dominicis theory$^1$ to RIXS calculations to understand the role of the core hole. The model reproduces the decomposition of the RIXS spectrum into well- and poorly-screened components. While the calculation can reproduce the full multiband spectrum, single pair excitations contribute the dominant part to the RIXS spectrum, and they can be described as the dynamic structure function $S(q,\omega )$ dressed by matrix element effects. We find evidence for an edge singularity at the RIXS threshold, similar to that found in x-ray absorption. We will discuss comparisons with long core hole lifetime calculations, and extensions to a system with antiferromagnetic order. 1. G.D. Mahan, Phys. Rev. {\bf 163}, 612 (1967); P. Nozi\`eres and C.T. De Dominicis, {\it ibid}. {\bf 178}, 1097 (1969). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q27.00008: Thermoelectric properties of titanium dioxide polymorphs from first principles Dylan Bayerl, Emmanouil Kioupakis Titanium oxides are promising materials for high-temperature thermoelectrics because of their high Seebeck coefficients, thermal stability, and natural abundance. We use first-principles calculations to investigate the thermoelectric transport properties of several titanium dioxide polymorphs. Our methodology is based on density functional and many-body perturbation theory within the GW approximation. The maximally localized Wannier function method is employed to interpolate the GW bands in the Brillouin zone. We use the Boltzmann transport formalism within the constant relaxation time approximation to determine the temperature and carrier-density dependence of the Seebeck coefficient, electron mobility, and electron thermal conductivity from the calculated electronic band structures. We demonstrate agreement with experimentally measured transport parameters and enhanced power factor at high temperature in certain heavily doped phases. This research was supported as part of CSTEC, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science. Computational resources were provided by the DOE NERSC facility. [Preview Abstract] |
Session Q28: Focus Session: Superconducting Qubits: Trajectories & Measurement
Sponsoring Units: GQIChair: Andrew Cross, IBM
Room: 601
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q28.00001: Observing interferences between past and future quantum states in resonance fluorescence Invited Speaker: Landry Bretheau In quantum physics, measurement results are random but their statistics can be predicted at any time assuming some knowledge about the system in the past. Additional knowledge from a future measurement deeply changes these statistics in the present and leads to purely quantum features. In particular conditioned average outcomes of a weak measurement, so-called weak values, were shown to go beyond the conventional range, give a way to directly measure complex values, and can be used to enhance the sensitivity of quantum meters. Recently, these concepts have been considered in the general case of open quantum systems where decoherence occurs. Then, what are the properties of weak values for the unavoidable measurement associated to decoherence, the one performed by the environment? Here, we answer this question in the simplest open quantum system: a quantum bit in presence of a relaxation channel. We continuously monitor the fluorescence emitted by a superconducting qubit driven at resonance. Conditioned on initial preparation and final single shot measurement outcome of the qubit state, we probe weak values displaying all the above properties. The fluorescence signal exhibits interferences between oscillations associated to past or future quantum states. The measured data are in excellent agreement with a recently developed formalism. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q28.00002: Observing single quantum trajectories of a superconducting qubit: introduction Kater Murch, Steven Weber, Chris Macklin, Irfan Siddiqi The length of time that a quantum system can exist in a coherent superposition is determined by how strongly it interacts with its environment. Unmonitored environmental fluctuations can be viewed as a source of noise, causing random evolution of the quantum system from an initially pure state into a statistical mixture. However, by accurately measuring the environment in real time, the quantum system can be maintained in a pure state and its time evolution described by a ``quantum trajectory'' determined by the measurement outcome. We use weak measurements to monitor a microwave cavity embedding a superconducting qubit and track the individual quantum trajectories of the system. We perform quantum state tomography at discrete times along the trajectory to verify that we have faithfully tracked the state of the quantum system as it diffuses on the surface of the Bloch sphere. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q28.00003: Observing single quantum trajectories of a superconducting qubit: ensemble properties and driven dynamics Steven Weber, K.W. Murch, A. Chantasri, J. Dressel, A.N. Jordan, I. Siddiqi We use weak measurements to track individual quantum trajectories of a superconducting qubit embedded in a microwave cavity. Using a near-quantum-limited parametric amplifier, we selectively measure either the phase or amplitude of the cavity field, and thereby confine trajectories to either the equator or a meridian of the Bloch sphere. We analyze ensembles of trajectories to determine statistical properties such as the most likely path and most likely time connecting pre and post-selected quantum states. We compare our results with theoretical predictions derived from an action principle for continuous quantum measurement. Furthermore, by introducing a qubit drive, we investigate the interplay between unitary state evolution and non-unitary measurement dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q28.00004: Backtracking quantum trajectories with analog feedback G. de Lange$^{*}$, D. Rist\`e$^{*}$, M.J. Tiggelman, C. Eichler, L. Tornberg, G. Johansson, A. Wallraff, R.N. Schouten, L. DiCarlo Circuit quantum electrodynamics offers a nearly ideal platform for the fundamental study of continuous quantum measurement. A nondemolition measurement of a superconducting qubit can be performed via homodyne detection of microwave transmission through a dispersively coupled cavity. By boosting the homodyne signal with a nearly noiseless phase-sensitive parametric amplifier, we experimentally show that a form of measurement backaction, consisting of stochastic quantum phase kicks on the measured qubit, is highly correlated with the fluctuations in the continuous homodyne record. We demonstrate a real-time analog feedback scheme that counteracts these phase kicks and thereby reduces measurement-induced dephasing. We develop a numerical optimization technique to overcome the bandwidth limitations of the amplification chain and provide a theoretical model for the optimization result. A quantum efficiency of 50\% is extracted for the complete analog feedback loop. Finally, we discuss the integration of this analog feedback technique to improve performance in our recent demonstration [1] of entanglement by dispersive parity measurement. $^{*}$equal contribution. [1] D. Rist\`e {\it et al.}, Nature 502, 350 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q28.00005: Waveguide implementation of a fluxonium qubit W.C. Smith, A. Kou, I.M. Pop, R.J. Schoelkopf, M.H. Devoret Enclosing a fluxonium qubit in a 3D cavity has recently been shown to provide remarkable qubit relaxation times of order 1 ms. This is realized by inductively coupling the qubit to an on-chip antenna, itself electromagnetically coupled to a microwave readout cavity. In order to improve measurement contrast, we propose reading out the on-chip antenna directly by embedding the substrate in a rectangular waveguide impedance-matched to open coaxial lines. This scheme allows for greater flexibility in engineering the dispersive shift of the readout resonator. Independently, we can optimize the readout resonator external quality factor to maximize measurement contrast. Results of dispersive shift calculations, electromagnetic simulations, and experimental design will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q28.00006: Measurement of a fluxonium qubit embedded in a waveguide A. Kou, W.C. Smith, I.M. Pop, R.J. Schoelkopf, M.H. Devoret The fluxonium qubit in a 3D cavity has been shown to have energy relaxation times of order 1 ms. The qubit readout contrast, however, was only approximately 10 degrees. High qubit readout contrast and fast operations on a qubit state are necessary ingredients for any implementation of a quantum computer. We have designed and measured a fluxonium qubit embedded in a waveguide in order to achieve these goals. We present measurements of the dispersive shift on a Josephson junction resonator inductively coupled to the fluxonium qubit as well as the relaxation times of the fluxonium qubit in this new architecture. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q28.00007: Quantum jumps of a fluxonium qubit U. Vool, I.M. Pop, K. Sliwa, B. Abdo, T. Brecht, S. Shankar, M. Hatridge, R.J. Schoelkopf, M. Mirrahimi, L. Glazman, M.H. Devoret The fluxonium qubit has recently been shown to have energy relaxation time (T1) of the order of 1 ms, limited by quasiparticle dissipation. With the addition of a Josephson Parametric Converter (JPC) to the experiment, trajectories corresponding to quantum jumps between the ground and 1st excited state can be measured, thus allowing the observation of the qubit decay in real time instead of that of an ensemble average. Our measurement fidelity with the JPC is in excess of 98{\%} for an acquisition time of 5 us and we can thus continuously monitor the quantum jumps of the qubit in equilibrium with its environment in a time much shorter than its average relaxation time. We observe in our sample a jump statistics that varies from being completely Poissonian with a long (500 us) mean time in the ground state to being highly non-Poissonian with short (100 us) mean time in the ground state. The changes between these regimes occur on time scales of seconds, minutes and even hours. We have studied this effect and its relation to quasiparticle dynamics by injecting quasiparticles with a short intense microwave pulse and by seeding quasiparticle-trapping vortices with magnetic field. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q28.00008: Quasi-particle population of superconducting islands probed by quantum jumps of a fluxonium qubit I.M. Pop, U. Vool, K. Sliwa, B. Abdo, T. Brecht, R.J. Schoelkopf, M. Mirrahimi, L. Glazman, M.H. Devoret The origin and the dynamics of nonequlibrium quasiparticles in superconducting circuits remain an open problem. One of the most sensitive systems that could be used to measure quasiparticles is the fluxonium qubit. Recently, this artificial atom has demonstrated relaxation times on the order of 1 ms, limited by quasiparticle dissipation. Moreover, the sensitivity to quasiparticle loss can be tuned in situ by applying a magnetic flux. By using a quantum limited amplifier (a Josephson Parametric Converter) we can observe quantum jumps between the 0 and 1 states of a fluxonium qubit in thermal equilibrium with the environment. The distribution of the times in-between the quantum jumps reveals quantitative information about the population and dynamics of quasiparticles. Our data is entirely consistent with the hypothesis that our system is sensitive to single quasiparticle excitations. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q28.00009: Fluxon readout for superconducting qubits Kirill Fedorov, Anastasia Shcherbakova, Alexey Ustinov We demonstrate an experiment on coupling of a single Josephson vortex (fluxon) in a long annular Josephson junction (AJJ) with a flux qubit. Using a possibility to measure the microwave radiation induced by a fluxon oscillations in the AJJ, the interaction of a current dipole generated by the flux qubit and the propagating fluxon was studied. We discuss relativistic dynamics of the Josephson vortex scattering on the current dipole. We detected specific periodic variations of the fluxon oscillation frequency versus magnetic flux through the qubit. We found that quantum states of the flux qubit can be distinguished by measuring small frequency shifts of the coupled fluxon oscillations. The fluxon readout for the superconducting flux qubit was experimentally performed by measuring of a characteristic energy spectrum of the latter. The demonstrated approach is compatible with the existing low-temperature digital RSFQ (Rapid Single Flux Quantum) electronics and may be useful as a scalable interface between classical computers and respective quantum counterparts. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q28.00010: Generating entanglement via measurement between two remote superconducting qubits N. Roch, M.E. Schwarzt, F. Motzoi, C. Macklin, R. Vijay, A.W. Eddins, A.N. Korotkov, B. Whaley, M. Sarovar, I. Siddiqi Measurement has traditionally been viewed as a mechanism for restoring classical behavior: a quantum superposition, once observed, transforms into a single classical state. However, for some quantum systems it is possible to design a measurement that probabilistically projects onto an entangled state, thereby purifying, rather than destroying, quantum correlations. We use a joint dispersive readout to entangle two superconducting qubits, in individual cavities, separated by more than a meter of coaxial cable. We obtain a concurrence of 0.35, which is consistent with transmission losses and detector efficiency. The intensity of the readout pulse can be continuously varied, enabling us to monitor the dynamics of entanglement generation. The data agree with both a Bayesian model and a full master equation treatment. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q28.00011: Continuous measurement of two spatially separated superconducting qubits: Quantum trajectories and feedback M.E. Schwartz, N. Roch, F. Motzoi, B. Whaley, A.N. Korotkov, M. Sarovar, I. Siddiqi Measurement can be harnessed to probabilistically generate entanglement in the absence of local interactions, for example between spatially separated quantum objects. Continuous weak measurement allows us to observe the dynamics associated with this process. In particular, we perform joint dispersive readout of two superconducting transmon qubits separated by one meter of coaxial cable. We track the evolution of a joint quantum state under the influence of measurement, both as an ensemble and as a set of individual quantum trajectories. We analyze the statistics of such quantum trajectories and find good agreement with a Bayesian formalism for a two-body quantum system. Such tracking opens the door to continuous feedback-stabilized remote entanglement. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q28.00012: Quantum Zeno effect in a strongly measured superconducting qubit D.H. Slichter, R. Vijay, S.J. Weber, C. Mueller, A. Blais, I. Siddiqi A qubit undergoing strong measurement is continuously projected into an eigenstate of the measured observable. A simultaneous resonant qubit drive will give rise to transitions between qubit states, but the presence of measurement slows the rate of these transitions -- a phenomenon known as the quantum Zeno effect. We observe this effect in a transmon qubit using linear circuit QED readout and a near-quantum-limited following amplifier. The experimental measurement record, consisting of a series of quantum jumps between states, is analyzed to extract qubit transition rates. We study the dependence of these rates on measurement strength and qubit drive amplitude and compare with theoretical predictions. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q28.00013: Local comb generation in nonlinear TiN superconducting resonators David Pappas, Michael R. Vissers, Robert Erickson, Martin Sandberg, Jiansong Gao Low loss superconducting nonlinear resonators are extensively used for qubit readout as well as photon detectors. These devices are typically capacitively coupled to a launch line. When driven at high power, a shift in resonant frequency is observed due to the kinetic inductance of the TiN superconductor. At higher power, the resonant frequency mixes with the drive tone to produce a series of peaks that are observed to be equally spaced at the detuning frequency, i.e. a ``local comb.'' The full circuit analysis of this system is derived. The renormalized resonant frequency is obtained and the local comb is derived from a first order successive approximation. [Preview Abstract] |
Session Q29: Graphene Defects and Functionalization
Sponsoring Units: DCMPChair: Carl Ventrice, College of Nanoscale Science and Engineering, SUNY Albany
Room: 603
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q29.00001: Direct observation and simulations of atomically resolved low loss images in graphene Myron Kapetanakis, Mark Oxley, Juan-Carlos Idrobo, Wu Zhou, Stephen Pennycook, Sokrates Pantelides Aberration-corrected scanning transmission electron microscopy (STEM) at low voltages provides atomic-resolution imaging of many two-dimensional materials, such as pristine graphene, using core-loss and low-loss spectra. Traditionally, EELS-STEM imaging and density functional theory (DFT) simulations were carried out by two different communities with minimal overlap. One community includes diffraction but ignores solid-state effects in the spectra, while the other includes solid-state effects but leaves out diffraction and interference. Recent work has combined DFT calculations and dynamical scattering to allow the simulation of probe position dependent core-loss spectra. In this talk we describe extension of this work to calculations of STEM images based on low-loss spectroscopy. It is usually assumed that such signals are highly delocalized, since plasmons represent a collective excitation. Considering that not all low-loss excitations are plasmonic in nature, we examine the role of interband transitions in the formation of atomic resolution low-loss images. We compare experimental results that show atomic resolution lattice images of graphene based on low-loss signals with simulations of images based on low-loss scattering potentials. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q29.00002: Simulated non-contact atomic force microscopy for polycyclic aromatic hydrocarbons James Chelikowsky, Minjung Kim Theoretical simulations of non-contact atomic force microscopy (AFM) play an important role in analyzing measured images. Recent non-contact AFM studies on polycyclic aromatic hydrocarbons have achieved atomic resolution that is not observed in other imaging techniques such as scanning tunneling microscopy. In particular, AFM images for these hydrocarbons have resolved bond orders of individual carbon-carbon bonds. Here we present simulated AFM images for several polycyclic aromatic hydrocarbons and explain the role of molecular functionalized AFM tips. Our work is based on solving the electronic structure problem using real space pseudopotentials constructed within density functional theory. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q29.00003: A viable geometry for graphene and single-atom crystals from atoms alone Hamed Pour Imani, Alejandro Pacheco Sanjuan, Zhengfei Wang, Mihajlo Vanevic, Salvador Barraza-Lopez The geometry of a single layer crystals is determined by four invariant parameters from the metric and curvature tensors. We directly study these invariant parameters using a novel framework from atomic positions in terms of angles, areas, vertex and normal vectors from atoms on the lattice for arbitrary elastic regime and atomic conformation, without resorting to differential geometry and continuum elasticity. The results can enable the study the electrical and mechanical properties of atom-thick crystals in a framework complementary to differential geometry and continuum elasticity. [1,2,3] References: 1. ``Graphene's morphology and electronic properties from discrete differential geometry'', Alejandro A. Pacheco Sanjuan, Zhengfei Wang, Hamed Pour Imani, Mihajlo Vanevic and Salvador Barraza-Lopez, Submitted on July 11, 2013. 2. J. V. Sloan et al., Phys. Rev. B 87, 155436 (2013). 3. Barraza-Lopez et al., Solid State Comm 166, 70 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q29.00004: Spatial fluctuations in barrier height at the graphene-silicon carbide Schottky junction Shivani Rajput, Mingxing Chen, Ying Liu, Yaoyi Li, Michael Weinert, Lian Li When graphene is interfaced with a semiconductor, a Schottky contact forms with rectifying properties. Graphene, however, is also susceptible to the formation of ripples upon making contact with another material. In this work, we report intrinsic ripple- and electric field-induced effects at the graphene-semiconductor Schottky junction, by comparing chemical vapor deposited graphene transferred onto semiconductor surfaces of opposite polarization: the hydrogen-terminated Si and C- faces of $\alpha $-SiC. Using scanning tunneling microscopy/spectroscopy and first-principles calculations, we show the formation of a narrow Schottky dipole barrier approximately 10 {\AA} wide, which facilitates the observed effective electric field control of the Schottky barrier height. We further find atomic-scale spatial fluctuations in the Schottky barrier that directly follow the undulation of ripples on both graphene-SiC junctions. These findings reveal fundamental properties of the graphene/semiconductor Schottky junction. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q29.00005: Collective properties of two-dimensional Dirac electron system with a superconducting pairing interaction Daisuke Inotani, Yoji Ohashi, Susumu Okada Recently, the possibility of the superconductivity in graphene are attracting a lot of attention because of its novel properties associated with the pure two-dimensionality, as well as the Dirac fermion nature of the electrons. In this work, we investigate the collective properties of the superconducting graphene. Including the attractive s-wave pairing interaction, as well as the long range Coulomb interaction between the electrons in the tight-binding model for the honeycomb lattice, we calculate the generalized density-density correlation function within the random phase approximation in both normal and superconducting state at T=0. In normal state, we find that a stable collective excitation associated with the superconducting pairing fluctuations appears due to the linear dispersion relation of the electrons. On the other hand, in superconducting state, the phase mode remains stable even at T=0, although the dispersion relation of the phase mode is strongly modified by the Coulomb interaction in the long wave-length region. This result is in contrast to the conventional superconductors in which the phase mode disappears at T=0 by the so-called Anderson-Higgs mechanism. We show that this novel property of the phase mode arises from the pure two-dimensionality of the system. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q29.00006: Interaction phenomena at topological transitions in strongly anisotropic Dirac materials Valeri Kotov It is known that a topological (Lifshitz) transition can take place in graphene, strained uniaxially in the zig-zag direction. At such a transition the spectrum becomes semi-Dirac like, with linear, ultrarelativistic dispersion in one direction, and quadratic momentum dependence in the other. This type of transition also occurs in other materials [1] as well as in artificial graphene lattices [2]. We have found that long-range Coulomb interactions can lead to profound effects at such topological transitions. In particular, an unusually strong log squared renormalization behavior was found in the effective fermion mass, ultimately leading to very strong changes in the shape of the critical fermion spectrum. We also study the stability of such exotic spectrum towards spontaneous gap formation (excitonic transition). Ultimately we find that the interaction effects are much stronger at topological transitions in strongly anisotropic Dirac materials, compared to ``conventional'' isotropic graphene. \\[4pt] [1] G. Montambaux et al, A universal Hamiltonian for the motion and the merging of Dirac cones in a two-dimensional crystal, Eur. Phys. J. B 72, 509 (2009).\\[0pt] [2] M. Bellec et al, Topological transition of Dirac points in a microwave experiment, Phys. Rev. Lett. 110, 033902 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q29.00007: Simulations of noble gases adsorbed on graphene Sidi Maiga, Silvina Gatica We present results of Grand Canonical Monte Carlo simulations of adsorption of Kr, Ar and Xe on a suspended graphene sheet. We compute the adsorbate-adsorbate interaction by a Lennard-Jones potential. We adopt a hybrid model for the graphene-adsorbate force; in the hybrid model, the potential interaction with the nearest carbon atoms (within a distance $r_{\mathrm{nn}})$ is computed with an atomistic pair potential $U_{\mathrm{a}}$; for the atoms at r\textgreater r$_{\mathrm{nn}}$, we compute the interaction energy as a continuous integration over a carbon uniform sheet with the density of graphene. For the atomistic potential $U_{\mathrm{a}}$, we assume the anisotropic LJ potential adapted from the graphite-He interaction proposed by Cole et.al. This interaction includes the anisotropy of the C atoms on graphene, which originates in the anisotropic $\pi $-bonds. The adsorption isotherms, energy and structure of the layer are obtained and compared with experimental results. We also compare with the adsorption on graphite and carbon nanotubes. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q29.00008: Electron supercollimation in graphene using one-dimensional disorder potentials Sangkook Choi, Cheol-Hwan Park, Steven G. Louie Due to its unique electronic structure, electrons in graphene interact with external potential in a counter-intuitive way, manifesting various different interesting characteristics Here we present another surprising, counter-intuitive electron transport phenomenon in graphene. We discovered that electron supercollimation can be induced by 1D disorder potentials. An electron wave packet is guided to propagate undistorted along the fluctuating direction of the external disorder potential, independent of its initial motion. The more disorder, the better is the supercollimation. This robust novel phenomenon is expected to have significant implications in the fundamental understanding of transport in graphene, as well as other materials with Dirac cone physics, and the potential to be exploited in the design of devices based on these materials. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q29.00009: Interaction graphene with metallic and semiconductor surfaces. Ab initio approach to the lattice dynamics Alejandro Molina Sanchez, Ludger Wirtz The interaction of graphene with substrates can alter its electronic and vibrational properties and is relevant for the practical use of graphene. In this work, we describe the graphene-substrate interaction through the theoretical study of the vibrational properties. We focus on three paradigmatic cases where the interaction strength changes gradually: graphene@BN, graphene@Ir(111), and graphene@SiC. We use \textit{ab initio} methods to obtain the phonon band structure, the density of states, and the strength of the electron-phonon coupling. Graphene on boron nitride exhibits a weak interaction but a non-negligible shift of the 2D Raman band. We explain this observation by connecting the increase of the dielectric screening with the softening of the electron-phonon interaction. Graphene on iridium, also displays weak interaction but the substrate is a metal. In this case the electron-electron interaction in graphene is screened by a metal electron gas. In the last case, we study the buffer layer of graphene on silicon carbide. The strong hybridization of graphene with silicon carbide changes substantially the electronic structure of graphene. All the calculations are compared to experimental data of Raman spectroscopy and angle-resolved inelastic electron scattering. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q29.00010: The role of SiO2/graphene interface on morphology of pentacene overlayer Gvido Bratina, Manisha Chhikara We have examined by atomic force microscopy submonolayer of vacuum-evaporated pentacene on exfoliated graphene on SiO$_2$. Two-dimensional (2D) growth is observed on as-transferred graphene. When the samples were heated to 300$^\circ$C for three hours, pentacene formed elongated, three-dimensional (3D) 20-nm-high islands. Strikingly similar pentacene morphology was observed on graphene that was transferred onto SiO$_2$, which was treated by hexamethyldisilazane (HMDS). We have also examined pentacene morphology on many-layer graphene (MLG) that was transferred onto untreated, and HMDS-treated SiO$_2$. In both cases we observed 2D growth. The observed differences in pentacene morphology can be attributed to the changes in graphene surface energy due to different interface layers. Interfacial water layer in the as-transfered graphene samples reduces the surface energy of graphene through the dipole field[1]. This results in 2D pentacene morphology. As the water layer is removed (via HMDS treatment or high-temperature annealing) the graphene surface energy favors 3D growth. As the graphene surface is moved away from the interface on MLG, the effect of the interface is reduced and pentacene grows as 2D layer. [1]J. Sabio, et al., Phys. Rev. B \textbf{77}, 195409 (2008). [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q29.00011: Phase diagram in two-dimensional Hubbard model: variational cluster approximation Armen Kocharian, Kun Fang, Gayanath Fernando, Alexander Balatsky, Kalum Palandage The Variational Cluster Approximation (VCA) is used to rigorously calculate the intrinsic phase diagram in bipartite two-dimensional (2d) Hubbard structures such as square and honeycomb lattice geometries with attraction and repulsion of electrons. The Mott-Hubbard gap, manifested as a smooth metal-insulator transition at finite $U>0$ in both square and honeycomb lattices at half filling ($n=1$), is in agreement with the generic 2d phase diagram. However, a density variation with the chemical potential displays their distinct structural differences away from half filling. Near $n=1$ at equilibrium we found discontinuous transition in square lattices signaling a phase separation instability into an inhomogeneous state with hole rich (metallic) and hole poor ($n=1$-insulating) regions. In contrast, a smooth density transition in honeycomb geometry describes a continuous evolution of homogenous (metallic) state. Incorporation of long-range input in VCA using U$>$0 and U$<$0 models displays antiferromagnetic and superconducting ground states respectively. The implication of VCA results to HTSCs, topological insulators as well as comparison to other studies is discussed. The VCA provides strong support for spontaneous phase separation instability found in our quantum cluster calculations. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q29.00012: Modification of graphene chemistry for metal nanoparticle growth: the effect of substrate selection Anna Zaniewski, Robert Nemanich Graphene and metal nanoparticle composites are a promising class of materials with unique electronic, optical, and chemical properties. In this work, graphene is used as a reducing surface to grow metal nanoparticles out of solution-based metal precursors. The nanoparticle formation is found to strongly depend upon the graphene substrate selection. The studied substrates include silicon oxide, silicon, lithium niobate, and copper. Our results indicate that the chemical properties of graphene depend upon this selection. For example, for the same reaction times and concentration, the reduction of gold chloride to gold nanoparticles on graphene/lithium niobate results in 3{\%} nanoparticle coverage compared to 20{\%} coverage on graphene/silicon and 60{\%} on graphene/copper. This work is supported through the National Science Foundation under Grant {\#} DMR-1206935 . [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q29.00013: Self-Organized Platinum Nanoparticles Elevated on Freestanding Graphene Matthew Ackerman, Peng Xu, Steven Barber, James Schoelz, Dejun Qi, Paul Thibado, Lifeng Dong, Jianhua Yu, Fangfang Xu, Mehdi Neek-Amal, Francois Peeters Freestanding graphene membranes were successfully functionalized with platinum nanoparticles (Pt NPs) using a single-step sputtering deposition process. The membranes were imaged using high-resolution transmission electron microscopy, revealing a homogeneous distribution of uniformly sized, single-crystal Pt NPs that exhibit a preferred orientation and nearest-neighbor distance. The NPs were also found to be partially elevated by the graphene substrate, as deduced from atomic-resolution scanning tunneling microscopy (STM) images. Furthermore, the electrostatic force between the STM tip and sample was utilized to estimate the binding energy of the NPs to the suspended graphene. Local strain accumulation due to elevation during the growth process is thought to be the origin of the NP self-organization. Such detailed insight into the atomic nature of this functionalized system was only possible through the cooperation of dual microscopic techniques combined with molecular dynamics simulations. The findings are expected to shape future approaches to develop high-performance electronics based on nanoparticle-functionalized graphene as well as fuel cells using Pt NP catalysts. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q29.00014: Fracture size effects in defected graphene Alessandro Luigi Sellerio, Stefano Zapperi We investigate fracture in a monolayer graphene with a small concentration of vacancies by molecular dynamics simulations. We simulate monolayers of varying size encompassing more than three decades, and we systematically study the mechanical failure following simulated ``constant engineering strain'' conditions. We compute the fracture strength distribution as a function of system size and defect concentration and compare the results with extreme value statistics. We highlight similarities and differences between the size effects expected in the fracture of macroscopic disordered solids with those observed at the nanoscale. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q29.00015: Effective Properties of Graphene with Large Periodic Anti-dots Bing Zhang, Ping Sheng Antidot graphene is an interesting material system which can exhibit a bandgap. In this work we present the effective properties of graphene with large periodic anti-dots, obtained by solving the Weyl equation numerically, as well as through the k dot p theory calculation. We find the dispersion relation to be hyperbolic in character, leading to an effective mass m* with an altered Fermi velocity v$_{\mathrm{eff}}$ as compared to the pristine case. The gap is exactly given by 2m*c$^{\mathrm{2}}$, where c is the speed of light. The dependence of m* and v$_{\mathrm{eff}}$ on geometric parameters is investigated. A remarkable enhancement of the Coulomb interaction parameter is seen in the antidot graphene, in the region close to the bottom of the band. This can explain the appearance of a Coulomb quasi-gap discovered recently in this material system. [Preview Abstract] |
Session Q30: The Physics of Climate
Sponsoring Units: GPCRoom: 605
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q30.00001: Stochastic Stommel box models for the thermohaline structure of the oceans Dibyendu Mandal, Jeffrey B. Weiss, Baylor Fox-Kemper, Royce K.P. Zia Bistability of the thermohaline circulation of the oceans has been implicated in various climate shifts in the past. The origin of the bistability lies in ocean-atmosphere interactions, as can be understood from a simple, deterministic two-box model proposed by H. Stommel (1961). Because of the rapidly varying nature of the atmosphere relative to the ocean it is more appropriate to treat the interactions stochastically, but, studies of stochastic Stommel models have been limited. Stochastic Stommel models have the further potential of explaining the features of the global temperature-salinity distribution in the oceans. We propose several such models, of varying complexity, which provide the blueprints to understand both empirical data and general circulation models. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q30.00002: Nonequilibrium life-cycles in Ocean Heat Content Jeffrey B. Weiss, Baylor Fox-Kemper, Dibyendu Mandal, Royce K.P. Zia Natural climate variability can be considered as fluctuations in a nonequilibrium steady state. A fundamental property of nonequilibrium steady states is the phase space current which provides a preferred direction for fluctuations, and is manifested as preferred life-cycles for climate fluctuations. We propose a new quantity, the phase space angular momentum, to quantify the phase space rotation. In analogy with traditional angular momentum, which quantifies the rotation of mass in physical space, the phase space angular momentum quantifies the rotation of probability in phase space. It has the additional advantage that it is straightforward to calculate from a time series. We investigate the phase space angular momentum for fluctuations in ocean heat content in both observations and ocean general circulation models. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q30.00003: Sensitivity of deep tropical convection to changes in the thermodynamic environment Sharon Sessions, David Raymond, Saska Gjorgjievska Accurately modeling the effects of climate change using global models relies heavily on the representation of unresolved convection. This is because a major uncertainty in models is due to the effect of clouds and water vapor. Reducing this uncertainty requires a better understanding of convective processes. Deep tropical convection is especially important since it simultaneously drives global circulation and evolves as a result in changes to the convective environment induced by the general circulation. We investigate how changes to the thermodynamic environment--specifically changes in temperature and moisture--modify tropical convection. Unsurprisingly, increases in environmental moisture result in convection with higher precipitation rates. However, a counter-intuitive result is that increases atmospheric stability associated with a cooling in the lower troposphere and a warming aloft also produce higher precipitation rates. Understanding this result has provided significant insight to tropical cyclogenesis, and may be important for understanding other types of large scale organization of tropical convection. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q30.00004: Uncertainties and complexities in small-scale ocean surface mixing processes Invited Speaker: Hezi Gildor Ocean mixing and dispersion processes are intermittent in time, nonlinear, and inhomogeneous in space. Much is known about processes with a spatial scale of a few tens of km (that can be studied using satellite data) and about very fine-scale processes (turbulent motions of milimeters to meters that can be studied using microstructure turbulence profilers). However, there is a lack of both observations and understanding of the so-called ``submesoscale'' processes, composed of motions on a scale of a few kilometers. It is well recognized that submesoscale processes play a critical role in modulating large-scale circulation, ecological functioning, and the dispersion of pollutants. Due to limited computer power, present day ocean and climate models resolve processes on scales down to a few tens of km, so submesoscale processes have to be parameterized. Accurate parameterizations of these processes are critical in simulating and predicting ocean circulation and changes in the climate. In this talk, I will focus on submesoscale horizontal mixing. Recent advances in ocean observation systems enable us to reconstruct quasi-synoptic maps of the ocean surface velocity field, over large areas and at high spatial (100s of meter) and temporal (30 min) resolutions. These surface current observations allow the computation of Lagrangian trajectories of many virtual particles. Based on these trajectories, one can compute various measures for mixing and identify Lagrangian Coherent Structures (LCS) using various methods (such as Finite Time and Finite Size Lyapunov Exponents). I will demonstrate, using surface current measurements by High Frequency radar, the existence of temporary submesoscale barriers to mixing. This has important implications for a wide range of predictions. We were also able to verify the existence of these barriers using aerial-photographs. Using a non-stationary Lagrangian stochastic model, I will present a method for estimating the upper bound of the horizontal eddy diffusivity based on the existence of such barriers. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q30.00005: Atomic-scale mechanism of incorporation of carbon dioxide in coal Yingdi Liu, Hongli Dang, Pongtorn Charoensuppanimit, Sayeed Mohammad, Khaled Gasem, Sanwu Wang Global warming is attributed to the rise of CO$_{2}$ concentration in the atmosphere. Sequestration of CO$_{2}$ into geological formations has been suggested for mitigating this phenomenon. Coalbeds are investigated as potential storage sites. Numerous experimental studies have demonstrated that coal swelling occurs after the injection of CO$_{2}$ into coal seams. However, the atomic-scale mechanism of such a phenomenon has not been well established. We report first-principles density-functional-theory calculations for the interaction between CO$_{2}$ and the coal network. The calculations show that the activation energies for incorporation of CO$_{2}$ into the coal bonding network are low at $\sim$ 0.9-1.3 eV depending on the bonding sites. We have found that the incorporated configurations are stable at low temperatures. However, high temperatures could stimulate the dissociation of CO$_{2}$ from such configurations as the activation energies are low at $\sim$ 0.5-0.9 eV, suggesting that coal swelling is reversible at high temperatures. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q30.00006: Calibration of cavity ring-down spectrometry, integrating nephelometery, and condensation particle counting for distinguishing aerosol scattering/absorption properties Solomon Bililign, Sujeeta Singh, Damon Smith, Marc Fiddler Aerosol optical depth, Angstrom exponent, size distribution are critical for radiative forcing models and are necessary to adequately parameterize biomass aerosols and dust in RCMs to improve our understanding of the impacts of aerosols on regional climate. Recent studies on the impact of atmospheric heating by dust and black carbon in the Tibetan Plateau (TP) showed that the heating will lead to enhanced pre-summer monsoon surface warming and early snow melts the TP region.. As a first step to characterize biomass aerosols we use completely scattering particles (polystyrene latex (PSL) spheres) and absorbing spheres to compare three techniques: cavity ring-down spectrometry (CRDS), integrating nephelometery, and Mie and T-Matrix theory; along with ancillary techniques, including condensation particle counting (CPC) and differential mobility analysis (DMA). In this work we compare the values and uncertainties of the scattering and backscattering cross section at 589 nm, scattering efficiency, and scattering {\AA}ngstrom coefficient determined from CRDS, nephelometery, and Mie theory and T-matrix theory. This is applied to PSL spheres 100-300 $\mu $m diameter and extended to 400 $\mu $m absorbing PSL spheres. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q30.00007: Variability of Aerosol Optical Properties Based on Particle Size, Concentration and Origin Rudra Aryal, Seth Malhotra This work provides time series of size segregated aerosol optical depth (AOD), absorption angstrom exponent, single scattering albedo, aerosol size distribution observed over Tudor Hill, Bermuda. Aerosol optical properties (absorption and scattering) are compared with corresponding chemical compositions. It is observed that coarse particle light scattering is dominated by sea salt particles and fine aerosol light scattering is dominated by non-sea salt sulfate. The concentration of coarse sea salt aerosols shows a strong correlation with the wind speed however chemical composition observed in fine particles did not show any connection with the wind speed. The possibility of different origins of aerosol particles such as from continental, oceanic, industrial etc. will be presented based on the back trajectory analyses and the chemical composition. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q30.00008: Multiscale Atmospheric Physics Modeled by Cumulant Expansions Brad Marston, Greg Chini We investigate a systematic and physically based approach to modeling subgrid physics statistically with the use of an expansion in equal-time cumulants. To accomplish this we replace the zonal average employed in previous work\footnote{S. M. Tobias and J. B. Marston, Phys. Rev. Lett. {\bf 101}, 104502 (2013).} with a low-pass filter that separates small and large scales in the zonal direction. The statistics are non-local, inhomogeneous, and anisotropic; the sole approximation is the neglect of 3-point and higher correlation functions. The closure respects the conservation of energy, enstrophy, and angular momentum. An advantage of the formulation is that correlations between large and small scale processes are treated explicitly without the introduction of phenomenological parameterizations. The approach is tested against full numerical simulation of idealized 1- and 2-layer models of the atmospheric general circulation\footnote{J. B. Marston, Ann. Rev. Cond. Matt. Phys. {\bf 3}, 285 (2012).} and shown to yield accurate low-order statistics. (The computer model used to perform these tests runs on OS X and is publicly available.\footnote{URL http://appstore.com/mac/gcm}) We identify important multiscale interactions and discuss the computational cost of the new scheme. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q30.00009: Can We Eliminate the Major Tornado Threats in Tornado Alley? R. Tao The recent devastating tornado attacks in Oklahoma, Iowa, Nebraska, and South Dakota raise an important question: can we do something to eliminate the major tornado threats in Tornado Alley? Violent tornado attacks in Tornado Alley are starting from intensive encounters between the northbound warm air flow and southbound cold air flow. As there is no mountain in Tornado Alley ranging from west to east to weaken or block such air flows, some encounters are violent, creating instability: The strong wind changes direction and increases in speed and height. As a result, it creates a supercell, violent vortex, an invisible horizontal spinning motion in the lower atmosphere. When the rising air tilts the spinning air from horizontal to vertical, tornadoes with radii of miles are formed and cause tremendous damage. Here we show that if we build three east-west great walls in the American Midwest, 300m high and 50m wide, one in North Dakota, one along the border between Kansas and Oklahoma to east, and the third one in the south Texas and Louisiana, we will diminish the tornado threats in the Tornado Alley forever. We may also build such great walls at some area with frequent devastating tornado attacks first, then gradually extend it. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q30.00010: How can you tell whether Earth is warming Up? Juan Restrepo, Shankar Venkataramani, Darin Comeau, Hermann Flaschka How does one determine whether the high summer temperatures in Moscow of a few years ago was an extreme climatic fluctuation or the result of a systematic global warming trend? How does one perform an analysis of the causes of this summer's high temperatures in the US, if climate variability is poorly constrained? It is only under exceptional circumstances that one can determine whether a climate signal belongs to a particular statistical distribution. In fact, climate signals are rarely ``statistical.'' It is thus often the case that one relies on statistical assumptions in order to compute a trend. There are other challenges in obtaining a trend: inherent multi-scale manifestations, and nonlinearities/non-Gaussianity, incomplete knowledge of climate variability. We propose a non-parametric notion of a trend, we call the tendency, that can handle multi-scale time series and that does not rely on statistical assumptions. Its primary utility lies in the analysis of time series with the aim of discerning structure from processes that could be modeled as noise. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q30.00011: Iceberg capsize hydrodynamics and the source of glacial earthquakes Lynn Kaluzienski, Justin Burton, Mac Cathles Accelerated warming in the past few decades has led to an increase in dramatic, singular mass loss events from the Greenland and Antarctic ice sheets, such as the catastrophic collapse of ice shelves on the western antarctic peninsula, and the calving and subsequent capsize of cubic-kilometer scale icebergs in Greenland's outlet glaciers. The latter has been identified as the source of long-period seismic events classified as glacial earthquakes, which occur most frequently in Greenland's summer months. The ability to partially monitor polar mass loss through the Global Seismographic Network is quite attractive, yet this goal necessitates an accurate model of a source mechanism for glacial earthquakes. In addition, the detailed relationship between iceberg mass, geometry, and the measured seismic signal is complicated by inherent difficulties in collecting field data from remote, ice-choked fjords. To address this, we use a laboratory scale model to measure aspects of the post-fracture calving process not observable in nature. Our results show that the combination of mechanical contact forces and hydrodynamic pressure forces generated by the capsize of an iceberg adjacent to a glacier's terminus produces a dipolar strain which is reminiscent of a single couple seismic source. [Preview Abstract] |
Session Q31: Graphene: Raman Spectroscopy and Phonons
Sponsoring Units: DMP DCMPChair: Tony Heinz, Columbia University
Room: 607
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q31.00001: How strain affect graphene's optical contrast on SiO2/Si gratings Xuanye Wang, Khwanchai Tantiwanichapan, Roberto Paiella, Anna Swan Optical contrast interference provides a fast and easy method for identifying graphene layer number the widely used silicon dioxide/silicon substrates. Uniaxial strain deforms the band structure and consequently it has been suggested that optical absorption in visible range varies with strain.\footnote{Many-Electron Effects on Optical Absorption Spectra of Strained Graphene, Liang et al, arXiv:1110.0212 [cond-mat.mtrl-sci]} Here we explore how uniaxial strain affects the optical contrast.~ Mechanically cleaved single layer graphene is deposited onto sinusoidal shaped SiO2 optical diffraction grating. Graphene strains as it conforms to the corrugated surface. We observe a dramatic optical contrast change for the graphene on the grating under white light and different color channels. To quantitatively analyze this optical response, we map graphene's strain distribution by analyzing redshift of G and 2D peaks positions in Raman line scan measurement and~ compare with~ AFM measurements~ to compare optical and Raman results with how well~ graphene conforms to the corrugated surface.~ We explore different surface treatments that vary the friction between the graphene and the corrugated oxide in order to control the strain and the conformation. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q31.00002: Th Q1 e thermal stability of graphene in air investigated by Raman spectroscopy Haiyan Nan, Zhenhua Ni, Jun Wang, Zainab Zafar, Zhixiang Shi, Yingying Wang The thermal stability in air of graphene synthesized by either chemical vapor deposition or mechanical cleavage is studied. It is found that single layer graphene prepared by both methods starts to show defects at $\sim$500 $^{\circ}$C, indicated by the appearance of a disorder-induced Raman D peak. The defects are initially sp3 type and become vacancy like at higher temperature. On the other hand, bilayer graphene shows better thermal stability, and the D peak appears at $\sim$600 $^{\circ}$C. These results are quite different from those annealing in vacuum and controlled atmosphere. Raman images show that the defects in chemical vapor deposition graphene are not homogeneous, whereas those in mechanical cleavage graphene are uniformly distributed across the whole sample. The factors that affect the thermal stability of graphene are discussed. Our results could be important for guiding the future electronics process and chemical decoration of graphene. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q31.00003: Relationship between Transport Properties and Raman Spectra in Electron Beam Irradiated Graphene Hikari Tomori, Rineka Hiraide, Hirokazu Tanaka, Youiti Ootuka, Akinobu Kanda Raman spectroscopy is commonly used to characterize disorder in graphene. Increase of structural defects leads to raise in intensity of the Raman D band for low defect densities. Defects also cause degradation of graphene transport properties. Thus, a certain relationship is expected between the Raman spectra and transport properties in graphene. Here, we investigate Raman spectra and transport properties of graphene as a function of the amount of electron beam irradiation. The electron beam irradiation leads to generation of the Raman D band and decrease of carrier mobility. We find that the intensity ratio of Raman D to G peaks is inversely proportional to square of the carrier mean free path. The proportionality coefficient is proportional to the carrier density. This kind of relationship has not been reported so far. Our result may pave the way for evaluating graphene transport properties with Raman spectroscopy. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q31.00004: Theory of topological Raman band in graphene Ken-ichi Sasaki, Kouta Tateno, Hideki Gotoh In the Raman spectrum measured at the edge of graphene, the $D$ band appears as a prominent peak. The edge $D$ band is useful in the characterization of the armchair edge because it is a result of an intervalley scattering of the electron by the armchair edge. The intervalley scattering is not unique to the armchair edge, but common to topological defects, such as pentagons and heptagons. Since topological defects are found throughout the honeycomb network of carbon, the characterization is an important issue. Based on our recent work on the activation mechanism of the edge $D$ band [1], we found a novel type of the $D$ band induced by a topological defect, which we call a topological $D$ band [2]. A photo-excited carrier with a non-zero winding number is the key to activating a topological $D$ band. A topological $D$ band can be distinguished from the conventional edge $D$ band by its peak position and non-dispersive nature, because the selection rules for electron-phonon matrix elements are altered in an essential way by the presence of the topological defect. [1] Sasaki et al., Crystals 3(1) 120 (2013). [2] Sasaki et al., Phys. Rev. Lett. 111, 116801 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q31.00005: Electronic structure modification of graphene on d-band metal surfaces and its Raman signature Sinisa Coh, Qin Zhou, Alex Zettl, Marvin L. Cohen, Steven G. Louie We find strong modifications of the graphene electronic structure when it is placed on a platinum surface. Additionally, these modifications strongly depend on the relative orientation of the graphene and platinum lattices. We expect that the same will occur whenever graphene is brought in contact with a surface of a material that has d-orbital close to the Fermi level. We demonstrate experimentally and theoretically that these modifications leave a distinct signature in the Raman spectrum of graphene. Out of two prominent graphene Raman peaks, one is unaffected (the G peak) while the other (the 2D peak) is severely affected, in proportion with the modification of the graphene electronic structure. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q31.00006: Influence of dopants on carrier dynamics and low-frequency phonon modes in bilayer graphene Ramakrishna Podila, Benoy Anand, Ajay Sood, Reji Philip, Apparao Rao Controlling the electronic structure of graphene with substitutional doping is central to many fascinating applications. For example, graphene's unique band structure coupled with its nonlinear optical properties (NLO) allows it to serve as a saturable absorber in an all-carbon optical diode . Although dopant effects are often correlated to the dopant concentration in the graphene lattice, the role of dopant's local bonding environment has not been explored in sufficient detail (R. Podila et al., Appl. Phys. Lett., 101, 123108 (2012)). Here, we present the effect of substitutional nitrogen (N) doping on the saturable absorption characteristics and carrier dynamics of chemical vapor deposited bi-layer graphene. We find that the saturation depth and carrier relaxation times are greatly influenced by the dopant density, and the N bonding configurations. The latter is inferred from the low-frequency phonon modes which are derived from the Fourier transform pump-probe spectra of N-doped bilayer graphene. Understanding the role of dopants on the NLO properties of graphene offers the possibility of tailoring graphene for opto-electronic applications \textit{via} defect engineering. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q31.00007: Raman Spectroscopy as an Accurate Probe of Defects in Graphene Joaquin Rodriguez-Nieva, Eduardo Barros, Riichiro Saito, Mildred Dresselhaus Raman Spectroscopy has proved to be an invaluable non-destructive technique that allows us to obtain intrinsic information about graphene. Furthermore, defect-induced Raman features, namely the $D$ and $D^{\prime}$ bands, have previously been used to assess the purity of graphitic samples. However, quantitative studies of the signatures of the different types of defects on the Raman spectra is still an open problem. Experimental results [1] already suggest that the Raman intensity ratio $I_{D}/I_{D^{\prime}}$ may allow us to identify the nature of the defects. We study from a theoretical point of view the power and limitations of Raman spectroscopy in the study of defects in graphene. We derive an analytic model that describes the Double Resonance Raman process of disordered graphene samples, and which explicitly shows the role played by both the defect-dependent parameters as well as the experimentally-controlled variables. We compare our model with previous Raman experiments, and use it to guide new ways in which defects in graphene can be accurately probed with Raman spectroscopy. [1] A. Eckmann, \textit {et al}., Nano Lett 12,3925 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q31.00008: Investigation of 2D materials by wide-field Raman imaging Jae-Ung Lee, Hyeonsik Cheong Raman spectroscopy is a very useful tool to investigate 2D materials such as graphene, hBN, and MoS2. Due to the uniqueness of the Raman spectrum of each material, we can use various Raman features to distinguish the number of layers, and other external effects (strain, doping, and temperature) on the sample. To study the spatial variations of the Raman features, confocal Raman imaging technique have been used conventionally. But due to limited beam size ($\sim$ 1 $\mu$m) of confocal Raman systems, investigating a large area of the samples would consume a lot of time. In contrast to conventional confocal Raman systems, the wide-field Raman system has advantages for fast and large area investigation. A shaped laser beam with the size of few-hundred microns is shone onto a sample, and only a specific wavelength is transmitted through a tunable band pass filter and directly imaged onto an EMCCD. We exfoliated three most common 2D materials (graphene, hBN, and MoS2) on the same SiO2/Si substrates. The optical contrast images of these materials are difficult to distinguish. But in wide-field Raman system, we can characterize the samples within a few seconds. This demonstrates that the wide-field Raman system provides a useful platform to characterize 2D materials. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q31.00009: The imprint of transition metal d-orbitals on a graphene Dirac cone: A Raman investigation Qin Zhou, Sinisa Coh, Marvin Cohen, Steven Louie, A. Zettl We investigate the influence of SiO2, Au, Ag, Cu, and Pt substrates on the Raman spectrum of graphene. Experiments reveal particularly strong modifications to the intensity, position, width, and shape of the Raman signal of graphene on platinum, compared to that of suspended graphene. The modifications also strongly depend on the relative orientation of the graphene and platinum lattices. Surprisingly, the interaction between graphene and platinum is often considered as weak Van der Waals interaction. We theoretically investigates the observations from electromagnetic shielding, charge transferring and from hybridization of electronic states in graphene and platinum. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q31.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q31.00011: Tunable Infrared Phonon Anomalies in Trilayer Graphene Zhiqiang Li, Chun Hung Lui, Emmanuele Cappelluti, Tony F. Heinz Trilayer graphene in both ABA (Bernal) and ABC (rhombohedral) stacking sequences is shown to exhibit intense infrared absorption from in-plane optical phonons. The phonon feature, lying at 1580 cm$^{\mathrm{-1}}$, changes strongly with electrostatic gating. For ABC-stacked graphene trilayers, we observed a large enhancement in phonon absorption amplitude, as well as softening of the phonon mode, as the Fermi level is tuned away from charge neutrality. A similar, but substantially weaker, effect is seen in samples with the more common ABA stacking order. The strong infrared response of the optical phonons and the pronounced variation with electrostatic gating and stacking order reflect the interactions of the phonons and electronic excitations in the two systems. The key experimental findings can be reproduced within a simplified charged-phonon model that considers the influence of charging through Pauli blocking of the electronic transitions. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q31.00012: Unusual ultra-low frequency fluctuations in freestanding graphene Steven Barber, Peng Xu, Mehdi Neek-Amal, Matthew Ackerman, James Schoelz, Paul Thibado, Ali Sadeghi, Francois Peeters Intrinsic ripples in freestanding graphene have been difficult to study with common experimental methods. In notable breakthroughs, ripple geometry was recently imaged using scanning electron microscopy as well as scanning tunneling microscopy (STM), but these measurements are thus far limited to static configurations. Thermally-activated flexural phonon modes could generate dynamic changes in curvature which would be of great interest to observe. Here, we present how to track the vertical movement of a one-square-angstrom region of suspended graphene using STM. This allows a direct measurement of the out-of-plane trajectory at one point in space over long periods of time. Based on these data, we present a model from elasticity theory to explain the very-low frequency oscillations that are observed. Unexpectedly, we sometimes detect a sudden colossal jump, which we interpret as due to mirror buckling. This innovative technique provides a much needed atomic-scale probe for the time-dependent behaviors of intrinsic ripples in freestanding graphene, and it represents a fundamental advance in the use of STM. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q31.00013: Strong enhancement of electron-phonon coupling in doped-graphene Choongyu Hwang, Duck Young Kim, D.A. Siegel, Kevin T. Chan, J. Noffsinger, A.V. Fedorov, Marvin L. Cohen, J.B. Neaton, B. Johansson, A. Lanzara Fundamental physical properties of a material are affected by many-body interactions. Among them, the interactions of electrons to phonon modes not only govern transport properties of the material, but also play an important role in realizing novel phenomena, when such an electron-phonon coupling is strongly enhanced. By using angle-resolved photoemission spectroscopy, we study strong enhancement of electron-phonon coupling of doped graphene. Our finding provides a viable route to realize strongly correlated electron phases in graphene. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q31.00014: Scattering of flexural acoustic phonons at graphene grain boundaries Edit Helgee, Andreas Isacsson We have studied the scattering of long-wavelength flexural phonons against grain boundaries in graphene using molecular dynamics. The grain boundaries consist of arrays of dislocations, where the size of each dislocation is of the order of magnitude of the lattice constant. The small size of the dislocations suggests that long-wavelength phonons should be unaffected by the boundary. However, dislocations cause out-of-plane buckling of the graphene sheet. The width of the buckles can be on the order of nanometers, large enough to interact with long-wavelength vibrations. Of the two grain boundaries considered here, one shows no buckling while the other displays an out-of-plane buckling 0.5 nm high and approximately 1.5 nm wide. For the flat grain boundary, the phonon transmission approaches unity at long wavelengths. The buckled grain boundary, on the other hand, yields transmission coefficients between 0.4 and 0.6 for wavelengths exceeding 1 nm. Also, the flexural vibrations couple to longitudinal modes at the buckled grain boundary. This indicates that grain boundaries scatter long-wavelength flexural phonons, provided that the boundary causes out of plane buckling of the graphene sheet. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q31.00015: Interplay between electron-phonon and Coulomb interactions in the honeycomb lattice Laura Classen, Michael M. Scherer, Carsten Honerkamp We study the impact of electron-phonon interactions on the many-body instabilities of electrons in the honeycomb lattice and their interplay with local and non-local short-ranged Coulomb interactions at charge neutrality. Therefore, we consider the in-plane optical phonon branches giving the most important contribution to the electron-phonon coupling and calculate the effective phonon-mediated electron-electron interaction by integrating out the phonon modes. The ordering tendencies are studied by means of a momentum-resolved functional renormalization group approach allowing for an unbiased investigation of the appearing instabilities. In the case of an exclusive and supercritical phonon-mediated interaction, we find a nematic ground state being favored over the s-wave superconducting state conjectured from a simple mean-field treatment. We further discuss the influence of phonon-mediated interactions on the instabilities induced by onsite, nearest neighbor and next-nearest neighbor density-density interactions. We find an extension of the parameter regime of the spin density wave order going along with an increase of the critical scales where ordering occurs. [Preview Abstract] |
Session Q32: Invited Session: Quantum Computing Architectures
Sponsoring Units: GQIChair: Andrew Landahl, Sandia National Laboratories
Room: 708-712
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q32.00001: Quantum Compiling for Topological Quantum Computing Invited Speaker: Krysta Svore In a topological quantum computer, universality is achieved by braiding and quantum information is natively protected from small local errors. We address the problem of compiling single-qubit quantum operations into braid representations for non-abelian quasiparticles described by the Fibonacci anyon model. We develop a probabilistically polynomial algorithm that outputs a braid pattern to approximate a given single-qubit unitary to a desired precision. We also classify the single-qubit unitaries that can be implemented exactly by a Fibonacci anyon braid pattern and present an efficient algorithm to produce their braid patterns. Our techniques produce braid patterns that meet the uniform asymptotic lower bound on the compiled circuit depth and thus are depth-optimal asymptotically. Our compiled circuits are significantly shorter than those output by prior state-of-the-art methods, resulting in improvements in depth by factors ranging from 20 to 1000 for precisions ranging between $10^{-10}$ and $10^{-30}$. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q32.00002: Error correction for adiabatic quantum computing Invited Speaker: Kevin Young Adiabatic quantum computing (AQC) is an alternative to the standard circuit model of quantum computation, wherein a quasistatic Hamiltonian, whose ground state at time T = 0 is simple and easily prepared, evolves slowly so that by the final time T = 1 the ground state encodes the answer to a problem. This procedure admits a novel set of natural algorithms for optimization problems, and is computationally equivalent to the circuit model in the absence of noise. But noise, and its effect on computational power, cannot be ignored. In light of this, AQC is particularly intriguing, possessing an intrinsic resilience to certain kinds of errors, including flawed time-dependent control Hamiltonians, dephasing in the energy basis, and energy relaxation. But these are far from the only errors afflicting quantum information processors, and any practical model of computation must be fault-tolerant to all expected forms of noise and error. With respect to this broader class of errors, true fault tolerance for AQC has remained elusive. In this talk we will discuss our progress towards this goal: (1) we identified and solved a variety of challenges on the road to error correction and fault tolerance in AQC; and (2) we identified a couple of major roadblocks, which appear insurmountable, and make us ultimately pessimistic that fault-tolerant AQC will ever be achieved. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q32.00003: Architectures for measurement-based quantum computation Invited Speaker: Robert Raussendorf As our experience so far shows, building a quantum computer is not going to be easy. There are fundamental difficulties to overcome, such as decoherence, and suitable technologies and materials need to be identified. In between those two extremes lies the challenge of quantum computer architecture. Shall or shall we not envision a quantum computer as a von-Neumann type device, with CPU here and memory there? How are the qubits supposed to be wired? How do realistic physical constraints such translation invariance, planarity or bounded degree of the qubit connectivity graph affect quantum computer architecture? I will discuss these questions from the angle of measurement-based quantum computation. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q32.00004: Synthesizing Logic in Fault-Tolerant Quantum Computers Invited Speaker: Cody Jones Quantum computers hold the promise of solving problems believed to be intractable using conventional computation, but this potential is impeded by the apparent difficulty in engineering reliable quantum hardware. One solution is quantum error correction (QEC), which enables fault-tolerant computation at the expense of a sizable overhead in qubits and gates. In this talk, I discuss several recent advancements in QEC to reduce the resource overhead in contemporary error-correction schemes like the surface code. Quantum logic can be encoded into so-called ``magic states,'' and the burden of error correction is shifted to verifying a well-characterized state, instead of protecting an arbitrary quantum process from errors. I discuss some of the recent work in magic-state distillation and its extensions to multi-qubit gates like Toffoli, which are ubiquitous in quantum algorithms. For operations in the surface code, resource overheads are improved by as much as two orders of magnitude. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:30PM |
Q32.00005: The quest for self-correcting quantum memory Invited Speaker: Olivier Landon-Cardinal A self-correcting quantum memory is a physical system whose quantum state can be preserved over a long period of time without the need for any external intervention. The most promising candidates are topological quantum systems which would protect information encoded in their degenerate groundspace while interacting with a thermal environment. Many models have been suggested but several approaches have been shown to fail due to no-go results of increasingly general scope. In this presentation, I will explain the desiderata for self-correction, review the recent advances and no-go results, and describe the current endeavours to define a self-correcting system in 2D and 3D. [Preview Abstract] |
Session Q33: Quantum Entanglement II
Sponsoring Units: GQIRoom: 706
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q33.00001: Universal Renyi mutual information in classical systems: the case of kagome ice Armin Rahmani, Gia-Wei Chern We study the Renyi mutual information of classical systems characterized by a transfer matrix. We first establish a general relationship between the Renyi mutual information of such classical mixtures of configuration states, and the Renyi entropy of a corresponding Rokhsar-Kivelson--type quantum superposition. We then focus on chiral and nonchiral kagome-ice systems, classical spin liquids on the kagome lattice, which respectively have critical and short-range correlations. Through a mapping of the chiral kagome ice to the quantum Liftshitz critical field theory, we predict a universal subleading term in the Renyi mutual information of this classical spin liquid, which can be realized in the pyrochlore spin ice in a magnetic field. We verify our prediction with direct numerical transfer-matrix computations, and further demonstrate that the nonchiral kagome ice (and the corresponding quantum Rokhsar-Kivelson superposition) is a topologically trivial phase. Finally, we argue that the universal term in the mutual information of the chiral kagome ice is fragile against the presence of defects. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q33.00002: Eigenstate Thermalization and the Sign-Structure of Quantum Many-Body Eigenstates Matthew Fisher, Tarun Grover Eigenstate Thermalization Hypothesis (ETH) posits that a generic finite-energy density eigenstate of an ergodic quantum system satisfies the ``volume law'' of entanglement entropy -- the bipartite Renyi entanglement entropies associated with a subregion scale in proportion to the subregion's volume. Here we argue that the volume law for Renyi entropies originates from the intricate ``sign structure'' of the many body eigenstates. Specifically, we show that the amplitude fluctuations in a many body eigenstate carry very little entanglement compared to the fluctuations in the sign of the wavefunction, and it is the latter which are essential for the aforementioned volume law. We present analytical and numerical results that support these conclusions. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q33.00003: Entanglement Temperature and Entanglement Entropy of Excited States Gabriel Wong, Israel Klich, Leopaldo A. Pando Zayas, Diana Vaman We derive a general relation between the ground state entanglement Hamiltonian and the physical stress tensor within the path integral formalism. For spherical entangling surfaces in a CFT, we reproduce the \emph{local} ground state entanglement Hamiltonian derived by Casini, Huerta and Myers. The resulting reduced density matrix can be characterized by a spatially varying ``entanglement temperature.'' Using the entanglement Hamiltonian, we calculate the first order change in the entanglement entropy due to changes in conserved charges of the ground state, and find a local first law-like relation for the entanglement entropy. Our approach provides a field theory derivation and generalization of recent results obtained by holographic techniques. However, we note a discrepancy between our field theoretically derived results for the entanglement entropy of excited states with a non-uniform energy density and current holographic results in the literature. Finally, we give a CFT derivation of a set of constraint equations obeyed by the entanglement entropy of excited states in any dimension. Previously, these equations were derived in the context of holography. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q33.00004: How universal is the entanglement spectrum? Anushya Chandran, Vedika Khemani, Shivaji Sondhi It is now commonly believed that the ground state entanglement spectrum (ES) exhibits universal features characteristic of a given phase. In this letter, we show that this belief is false in general. Most significantly, we show that the entanglement Hamiltonian can undergo quantum phase transitions in which its ground state and low energy spectrum exhibit singular changes, even when the physical system remains in the same phase. For broken symmetry problems, this implies that the ES and the Renyi entropies can mislead entirely, while for quantum Hall systems the ES has much less universal content than assumed to date. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q33.00005: Entanglement at an O(3) Critical Point with a Numerical Linked-Cluster Expansion Ann B. Kallin, Rajiv Singh, Miles Stoudenmire, A. John Berlinsky, Roger Melko Using the Numerical Linked-Cluster Expansion technique on rectangular clusters, we study the scaling of Renyi entanglement entropies at an O(3) quantum critical point, realized through the spin-1/2 Heisenberg bi-layer. There is a subleading logarithmic contribution to the entanglement due to the presence of a vertex in the entanglement boundary, with a coefficient that is known to be universal. We compute this ``corner coefficient'' and compare our value to that from both a non-interacting field theory, and the Ising fixed point in 2+1 dimensions. The corner coefficient has the potential to distinguish between these and other universality classes, through a variety of numerical calculations of strongly interacting quantum critical points. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q33.00006: Entanglement entropy in mesoscopic conductors Konrad Thomas, Christian Flindt The degree of entanglement in a many-body quantum system can be characterized by the entanglement entropy. We consider the entanglement entropy generated between two electronic reservoirs connected by a quantum point contact (QPC) [1,2]. The entanglement entropy is obtained from the fluctuations of the electric current which we evaluate numerically exact using a tight-binding model of the system [3]. Within our approach we can investigate the influence of time-dependent modulations, including the opening and closing of the QPC [4]. We focus on electronic conductors, but our ideas may also be realized in cold atomic gases. \newline\newline [1] I.~Klich and L.~S.~Levitov, Phys. Rev. Lett. 102, 100502 (2009)\newline [2] H.~F.~Song, C.~Flindt, S.~Rachel, I.~Klich, and K.~Le~Hur, Phys. Rev. B 83, 161408(R) (2011) \& Phys. Rev. B 85, 035409 (2012)\newline [3] K.~Sch\"{o}nhammer, Phys. Rev. B 75, 205329 (2007)\newline [4] K.~H.~Thomas and C.~Flindt, in.~prep. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q33.00007: Entanglement entropy of fermionic quadratic band touching model Xiao Chen, Gil Young Cho, Eduardo Fradkin The entanglement entropy has been proven to be a useful tool to diagnose and characterize strongly correlated systems such as topologically ordered phases and some critical points. Motivated by the successes, we study the entanglement entropy (EE) of a fermionic quadratic band touching model in $(2+1)$ dimension. This is a fermionic ``spinor'' model with a finite DOS at k=0 and infinitesimal instabilities. The calculation on two-point correlation functions shows that a Dirac fermion model and the quadratic band touching model both have the asymptotically identical behavior in the long distance limit. This implies that EE for the quadratic band touching model also has an area law as the Dirac fermion. This is in contradiction with the expectation that dense fermi systems with a finite DOS should exhibit $L\log L$ violations to the area law of entanglement entropy (L is the length of the boundary of the sub-region) by analogy with the Fermi surface. We performed numerical calculations of entanglement entropies on a torus of the lattice models for the quadratic band touching point and the Dirac fermion to confirm this. The numerical calculation shows that EE for both cases satisfy the area law. We further verify this result by the analytic calculation on the torus geometry. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q33.00008: Particle entanglement in continuum many-body systems via quantum Monte Carlo C.M. Herdman, P.-N. Roy, R.G. Melko, A. Del Maestro Entanglement of spatial bipartitions, used to explore lattice models in condensed matter physics, may be insufficient to fully describe itinerant quantum many-body systems in the continuum. We introduce a procedure to measure the R\'{e}nyi entanglement entropies on a particle bipartition, with general applicability to continuum Hamiltonians via Path Integral Monte Carlo methods. Via direct simulations of interacting bosons in one spatial dimension, we confirm a logarithmic scaling of the single-particle entanglement entropy with the number of particles in the system. The coefficient of this logarithmic scaling increases with interaction strength, saturating to unity in the strongly interacting limit. Additionally, we show that the single-particle entanglement entropy is bounded by the condensate fraction, suggesting a practical route towards its measurement in future experiments. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q33.00009: Entanglement scaling in the quantum Heisenberg bilayer model Stefan Wessel, Johannes Helmes We employ quantum Monte Carlo simulations to quantify the bipartite entanglement in the spin-1/2 quantum Heisenberg model on the square lattice bilayer in terms of the second R\'enyi entropy. The dependence on the interlayer coupling of the dominant area law contribution to the entanglement is analyzed, in particular its enhancement across the quantum phase transition. In addition, we study the various sub-leading logarithmic correction terms due to Goldstone excitations and corner contributions, related to the subsystem's geometry. We compare our numerical findings to previous analytical predictions and discuss limitations in extracting universal contributions due to finite size restrictions for numerical studies. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q33.00010: Entanglement properties of the antiferromagnetic-singlet transition in the two dimensional Hubbard model Richard T. Scalettar, Chia-Chen Chang, Rajiv R.P. Singh Entanglement entropy is a manifestation of quantum coherence. In a many body system, it can provide distinct signatures of quantum criticality and topological order. Measurements of entanglement entropy typically require knowledge of the many body wave functions. Due to their non-local nature, it is difficult to evaluate entanglement properties of correlated systems using numerical methods that rely on local operators. Here we present a study of Renyi entanglement entropy (EE) for fermonic bilayer Hubbard model at half-filling using a recently proposed formalism [1] within the determinantal quantum Monte Carlo framework. We obtain temperature dependence of the Renyi EE. At low temperatures, a sharp signal in the EE is observed as the system undergoes the singlet-antiferromagnetism transition. Scaling properties of the Renyi EE resulting from different bipartite divisions of the bilayer are explored. In the non-interacting limit, the results of the simulations are compared with those obtained with the correlation matrix method. \\[4pt] [1] T. Grover, Phys. Rev. Lett. {\bf 111}, 130402 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q33.00011: Collapsing Schrodinger Cats in the Density Matrix Renormalization Group Hongchen Jiang, Leon Balents In this paper, we propose a modified Density Matrix Renormalization Group (DMRG) algorithm to preferentially select minimum entropy states (minimally entangled states) in finite systems with asymptotic ground state degeneracy. The algorithm adds a ``quench'' process to the conventional DMRG method, which mimics the decoherence of physical systems, and collapses non-locally entangled states such as Schrodinger cats. We show that the method works for representative models with ground state degeneracy arising from either topological order or spontaneous discrete symmetry breaking. In the minimal entropy states thus obtained, properties associated with thermodynamic limit, such as topological entanglement entropy and magnetic order parameters, can be obtained directly and efficiently. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q33.00012: Emergent irreversibility and entanglement spectrum statistics Eduardo Mucciolo, Claudio Chamon, Alioscia Hamma We study the problem of irreversibility when the dynamical evolution of a many-body system is described by a stochastic quantum circuit. Such evolution is more general than Hamitonian, and since energy levels are not well defined, the well-established connection between the statistical fluctuations of the energy spectrum and irreversibility cannot be made. We show that the entanglement spectrum provides a more general connection. Irreversibility is marked by a failure of a disentangling algorithm and is preceded by the appearance of Wigner-Dyson statistical fluctuations in the entanglement spectrum. This analysis can be done at the wavefunction level and offers a new route to study quantum chaos and quantum integrability. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q33.00013: Nonsymmetrized Correlations in Mesoscopic Current Measurements Wolfgang Belzig, Adam Bednorz, Christoph Bruder, Bertrand Reulet A long-standing problem in quantum mesoscopic physics is which operator order corresponds to noise expressions like $\langle I(\omega)I(-\omega)\rangle$, where $I(\omega)$ is the measured current at frequency $\omega$. Symmetrized order describes a classical measurement while nonsymmetrized order corresponds to a quantum detector, e.g., one sensitive to either emission or absorption of photons. We show that both order schemes can be embedded in quantum weak-measurement theory taking into account measurements with memory, characterized by a memory function which is independent of a particular experimental detection scheme [A. Bednorz, C. Bruder, B. Reulet, and W. Belzig, Phys. Rev. Lett. \textbf{110}, 250404 (2013)]. We discuss the resulting quasiprobabilities for different detector temperatures and how their negativity can be tested on the level of second-order correlation functions already. Experimentally, this negativity can be related to the squeezing of the many-body state of the transported electrons in an ac-driven tunnel junction. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q33.00014: Four and Five-body non-local correlations in pure and mixed states Santosh Shelly Sharma, Naresh Kumar Sharma In our earlier works [1], quantifiers of four and three-body correlations based on four qubit invariants had been constructed for pure states. The principal construction tools, local unitary invariance and notion of negativity fonts, make it possible to outline the process of selective construction of meaningful invariants that quanify $N$ and $ N-1 $ qubit correlations. It is found that, in general, starting from degree $ k $ invariants relevant to detection and quantifcation of specific type of non-local quantum correlations in $(N-1)$ $(N>2)$ qubit system, one can construct degree $k $ coefficients of an $N$-qubit bilinear form. When $k=2^{N-2}$ ($N>2$), one of the invariants of degree $2^{N-1}$ quantifies N-body non-local correlations The process is recursive. While for few body systems it yields analytical expressions in terms of functions of state coefficients, for larger systems it can be the guiding principle to numerical caculations of invariants. To illustrate the process, an expression for a five qubit correlation quantifier for pure states is constructed. In addition, the extension to specific rank two mixed states through convex-roof extension is investigated. [1] S. Shelly Sharma and N. K. Sharma, Phys. Rev. A 87, 022335 (2013); Phys. Rev. A 82, 052340 (2010). [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q33.00015: Rotational Covariance and GHZ Contradictions for three or more particles of any dimension Jay Lawrence Greenberger-Horne-Zeilinger (GHZ) states are characterized by a special symmetry under independent uniaxial rotations of particles. Observables representing particular detector arrangements transform covariantly and exhibit a wealth of GHZ contradictions. Hidden variables cannot reproduce this covariance for any number of particles (N ? 3) of any spin S (or dimension d = 2S + 1). However, finding specific and experimentally verifiable contradictions covering all cases requires increasingly more complex arguments, utilizing more observables, for more difficult cases [1]. We illustrate a new method that utilizes explicit reference to hidden variable failure, which succeeds for the most difficult. The method is applied to the case of three particles of any prime dimension. 1. J. Lawrence, eprint arXiv:1308.3808 [quant/ph]. [Preview Abstract] |
Session Q34: Bose Gases
Sponsoring Units: DAMOPChair: Michael Foss-Feig, JQI
Room: 704
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q34.00001: Damping of excitations in a dipolar Bose gas Ryan Wilson, Stefan Natu In a Bose-condensed gas, quasiparticle excitations can undergo damping via effective condensate-mediated interactions in the collisionless regime. Motivated by recent experimental advances with condensates of highly magnetic atoms, we consider quasiparticle damping in Bose gases with dipolar interactions, where the dispersion exhibits a roton-maxon character in the appropriate trapping geometry. Following standard perturbative arguments, we derive the rates for quasiparticle damping in a collisionless Bose gas interacting with long range interactions. We find that in the experimentally relevant temperature regime, phonons and rotons are effectively undamped in a dipolar gas owing to the nature of the low energy dispersion. Furthermore, by tuning the external magnetic field, the dipolar interaction can be made strongly anisotropic, which leads to a non-trivial dependence of the damping processes on the direction of the applied magnetic field. We discuss the implications of this work for recent experiments with highly magnetic atoms. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q34.00002: Breakdown of the scale invariance in a near-Tonks-Girardeau gas: some exact results and beyond Zhedong Zhang, Gregory Astrakharchik, Steven Choi, H\'{e}l\`{e}ne Perrin, Thomas Bergeman, Maxim Olshanii In this presentation, we consider {\it elementary} monopole excitations of harmonically trapped one-dimensional Bose gas in the vicinity of a Tonks-Girardeau limit. Using the Girardeau Fermi-Bose mapping we obtain the first dominant correction to the excitation frequency, beyond the scale-invariance-protected value of $2\omega$. In limit of a large number of atoms, our result coincides with the upper bound predicted by Menotti and Stringari [Phys. Rev. A \textbf{66}, 043610 (2002)]. We find further that, surprisingly, the frequency of the {\it collective} excitations, obtained using the perturbation theory [Phys. Rev. Lett. \textbf{81}, 4541 (1998)], is found to be substantially below the Menotti-Stringari bound. In the latter case, the value of the frequency correction is $9/4$ times higher than in the former. Finally, an ab initio numerical calculation of the collective excitation frequency returns to the value predicted for the elementary excitation. We conjecture that the sharp boundary of the TG cloud, characterized by an infinite density gradient, renders the perturbation theory for the collective excitation frequencies unapplicable. We also discuss an extension of our results to the case of spin-polarized $p$-wave-interacting fermions in a cold waveguide. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q34.00003: Cavity-mediated near-critical dissipative dynamics of a driven condensate Baris Oztop, Manas Kulkarni, Hakan Tureci We [1] investigate the near-critical dynamics of atomic density fluctuations in the non-equilibrium self-organization transition of an optically driven quantum gas coupled to a single mode of a cavity. In this system cavity-mediated long-range interactions between atoms, tunable by the drive strength, lead to softening of an excitation mode recently observed in experiments. This phenomenon has previously been studied within a two-mode approximation for the collective motional degrees of freedom of the atomic condensate which results in an effective open-system Dicke model. Here, including the full spectrum of atomic modes we find a finite lifetime for a roton-like mode in the Bogoliubov excitation spectrum that is strongly pump-dependent. The corresponding decay rate and critical exponents for the phase-transition are calculated explaining the non-monotonic pump-dependent atomic damping rate observed in recent experiments. We compute the near-critical behavior of the intra-cavity field fluctuations. We highlight the role of the finite size of the system in the suppression of it below the expectations of the open Dicke model.\\[4pt] [1] M. Kulkarni, B. Oztop, H. E. Tureci, arXiv:1306.3889 (PRL 2013, in press) [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q34.00004: Nonlinear novel oscillation of polaritons in the optical microcavity Yongchang Zhang, Xiangfa Zhou, Guangcan Guo, Xingxiang Zhou, Han Pu, Zhengwei Zhou As a kind of new state of matter, Bose-Einstein condensation (BEC) in a dilute gas of trapped atoms is able to exhibit quantum phenomena on macroscopic scales. Recently, BEC of microcavity polaritons had been experimentally demonstrated. As a kind of bosonic quasi-particle which generates from the strong light-matter coupling, the polariton can be manipulated by the external laser field, and it provides a platform to simulate strongly correlated many-body models in the photon-coupled microcavity array. In this talk we present a scheme for simulating the nonlinear tunneling between two bosonic condensations in the microcavity system. Due to the controllability of the polariton, the effective nonlinear tunneling between two condensates of polaritons can be easily induced by the external controlling fields. In our work, a kind of two modes polariton model is derived, in which nonlinear tunneling strength depends on the difference of the particles in such two kinds of modes. We investigate the mean-field behaviors for such kind of double-mode polariton model, and we find that it is analogous to the model of the pendulum with variable pendulum length. Furthermore, some novel oscillation modes are revealed. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q34.00005: Transport Theory for Dilute Bose-Einstein Condensates Linda Reichl, Erich Gust We obtain microscopic expressions for the six hydrodynamic modes of a dilute Bose-Einstein condensate: two transverse (shear) modes and four longitudinal modes corresponding to first and second sound [1]. Our microscopic expressions include both the speed of the two types of sound and the rate of relaxation of the sound waves. We obtain numerical values for the shear viscosity of a dilute BEC composed of bosons that interact via a contact potential. Our values for the shear viscosity are obtained using the eigenvalues and eigenvectors of the three types of collision operators that govern the relaxation of the condensate [2]. 1. L.E. Reichl and Erich D. Gust, Phys. Rev. A 88 053603 (2013). 2. E. D. Gust and L.E. Reichl, J. Low Temp. Phys. 170 43 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q34.00006: Three-body loss rate of unitary Bose gas Weiran Li, Johannes Hofmann, Stefan Natu Quantum gases at unitarity can exhibit interesting features, for instance their universal thermodynamics. In the past, unitary Fermi gases in degenerate limit have been studied extensively. As recent experiments [1, 2] show unitary Bose gases can be stabilized at relatively high temperatures, we would like to ask an important question whether a Bose gas can persist in a well defined thermodynamic state at lower temperatures, even to the degenerate limit [3] where the medium affects the three-body loss rate crucially. By calculating the three-body recombination rate while taking into account the scattering with the medium, we have an estimate of the temperature (scale) above which thermodynamic quantities of a metastable branch can be studied in a unitary Bose gas. [1] Rem, B. S., et al. ``Lifetime of the Bose Gas with Resonant Interactions." Physical review letters 110.16 (2013): 163202. [2] Fletcher, Richard J., et al. ``Stability of a unitary Bose gas." Physical review letters 111.12 (2013): 125303. [3] Makotyn, Philip, et al. ``Universal dynamics of a degenerate unitary Bose gas." arXiv preprint arXiv:1308.3696 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q34.00007: Many-body Physics of Rydberg Polaritons Alexander Edelman, Peter Littlewood Electromagnetically induced transparency (EIT) in cold dense atomic gasses with Rydberg states in has attracted considerable interest as a means of realizing strong nonlinear photon-photon interactions. The coherent light-matter coupling provided by the EIT medium combined with interactions between Rydberg states whose strength and shape can be engineered provide a parameter space with rich phenomenology including photon bound states and spatially ordered structures. Past theoretical treatments of these systems have relied on equations of motion to predict dynamics in particular limits. Here we present progress towards a full many-body path integral description that considers interaction effects beyond the Dicke model as well as the intrinsically non-equilibrium nature of the system, and explore a tentative phase diagram. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q34.00008: Excitation of ultra-long-range nd Rydberg molecules David Anderson, Stephanie Miller, Georg Raithel A recently discovered class of ultra-long-range Rydberg molecules has generated a significant amount of theoretical and experimental interest [1,2]. The binding mechanism of these molecules arises from a scattering-induced, attractive interaction between the low-energy electron of a highly-excited Rydberg atom and a nearby neutral perturber [3]. The bond formed between a Rydberg atom and a ground-state atom via this interaction, and the nature of the resulting molecule, is largely dependent on the Rydberg electron wave function. We present here the experimental observation of ultra-long-range $^{87}Rb_2$ Rydberg molecules formed by a $Rb(nd_j)$ Rydberg atom and a $Rb(5s_{1/2})$ ground-state atom, for principal quantum numbers 34$\le n \le$40. The molecular ground states are isolated spectroscopically and their measured binding energies scale as $\sim n^{-6}$, in good agreement with theory [1]. The molecular binding energies are found to be the same for angular momentum $j=5/2$ and $3/2$ of the $nd_{j}$ Rydberg state over a selected $n$ range, within the measurement precision. [1] C. H. Greene, A. S. Dickinson, and H. R. Sadeghpour, PRL, 85, 2458-2461 (2000). [2] V. Bendkowsky et al., Nature, 458, 1005-1008 (2009). [3] E. Fermi, Il Nuovo Cimento, 11, 1934. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q34.00009: Field-theoretical Study of the Bose Polaron - Challenges for Quantum Simulation with ultracold Atoms Richard Schmidt, Steffen Patrick Rath We study the properties of the Bose polaron, an impurity strongly interacting with a Bose-Einstein condensate, using a field-theoretic approach and make predictions for the spectral function and various quasiparticle properties that can be tested in experiment. We find that most of the spectral weight is contained in a coherent attractive and a metastable repulsive polaron branch. We show that the qualitative behavior of the Bose polaron is well described by a non-selfconsistent T-matrix approximation by comparing analytical results to numerical data obtained from a fully selfconsistent T-matrix approach. The latter takes into account an infinite number of bosons excited from the condensate. Finally we discuss the implications of our results for the attempted quantum simulation of the Froehlich Hamiltonian using ultracold atoms. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q34.00010: Fermionized photons in one-dimensional coupled cavities David L. Feder, Adam G. D'Souza, Barry C. Sanders We consider the properties of a one-dimensional array of evanescently coupled high-finesse cavities each containing a single neutral atom, in the limit of low photon densities. The ground state of the corresponding Jaynes-Cummings-Hubbard (JCH) model is obtained numerically using the Density Matrix Renormalization Group algorithm. We find strong evidence for the existence of a Tonks-Girardeau phase, in which the photons are strongly fermionized, between the Mott-insulating and superfluid phases as a function of the inter-cavity coupling. Results for photon and spin excitation densities, one- and two-body correlation functions, and superfluid and condensate fractions are all found to be consistent with this conclusion. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q34.00011: Shortcuts to adiabaticity in quantum many-body systems: a quantum dynamical microscope Adolfo del Campo The evolution of a quantum system induced by a shortcut to adiabaticity mimics the adiabatic dynamics without the requirement of slow driving. Engineering it involves diagonalizing the instantaneous Hamiltonian of the system and results in the need of auxiliary non-local interactions for matter-waves [1,2]. Here experimentally realizable driving protocols are found for a large class of single-particle, many-body, and non-linear systems without demanding the spectral properties as an input. The method is applied to the expansion of a trapped ultracold gas which spatially scales up the size of the cloud while conserving the quantum correlations of the initial many-body state. This shortcut to adiabatic expansions acts as a quantum dynamical microscope [3]. [1] Adolfo del Campo, Shortcuts to adiabaticity by counter-diabatic driving, Phys. Rev. Lett. 111, 100502 (2013). [2] Adolfo del Campo, Marek M. Rams, Wojciech H. Zurek, Assisted finite-rate adiabatic passage across a quantum critical point: Exact solution for the quantum Ising model, Phys. Rev. Lett. 109, 115703 (2012) [3] Adolfo del Campo, Frictionless quantum quenches in ultracold gases: a quantum dynamical microscope, Phys. Rev. A 84, 031606(R) (2011). [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q34.00012: Quantum Shock waves and Population Inversion in Collisions of Ultracold Atomic Clouds Sebastiano Peotta, Massimiliano Di Ventra Using Time-Dependent Density Matrix Renormalization Group (TDMRG) we study the collision of one-dimensional atomic clouds confined in a harmonic trap and evolving with the Lieb-Liniger Hamiltonian [1]. It is observed that the motion is essentially periodic with the clouds bouncing elastically in agreement with the results of the ``quantum Newton cradle'' experiment of Kinoshita et al. [Nature 440, 900 (2006)]. We compare the results for the density profile against a hydrodynamic description with the pressure term taken from the Bethe Ansatz solution of the Lieb-Liniger model. We find that hydrodynamics can describe the breathing mode of a harmonically trapped cloud for arbitrary long times while it breaks down almost immediately for the collision of two clouds due to the formation of shock waves (gradient catastrophe). Concomitantly with the shock waves formation we observe a local energy distribution typical of population inversion, i.e., an effective negative temperature. Our results are an important step towards understanding the hydrodynamics of quantum many-body systems out of equilibrium and the role of integrability in their dynamics. \\[4pt] [1] S.. Peotta and M. Di Ventra, arXiv:1303.6916 [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q34.00013: Thermometry of ultracold atoms based on momentum-distribution noise Tommaso Roscilde Ultracold atoms have the puzzling feature of representing, within a good approximation, a microcanonical system, whose temperature cannot be controlled in a direct manner. Yet thermometry is essential for the use of cold atoms as quantum simulators reconstructing, e.g., equilibrium phase diagrams of strongly correlated models. Here I propose a very general thermometry scheme based on the fluctuations of the momentum distribution - a primary observable of cold-atom experiments. Relying on model-independent fluctuation-dissipation relations, the temperature can be estimated from a combined measurement of 1) the gradient of the momentum distribution, or its response to the application of a gauge field; 2) the fluctuations of the momentum distribution. This estimator provides the exact temperature in a translationally invariant system, or in a lattice system without interactions and further confining potentials - in these cases the fluctuations of the (quasi-)momentum distribution are purely thermal. When quantum fluctuations are also present, their effect does not jeopardize the thermometry down to temperatures well below the onset of quantum degeneracy; in the case of bosons, the proposed thermometry becomes exact in the thermodynamic limit in the presence of Bose condensation. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q34.00014: Probing the optical conductivity of harmonically-confined charge neutral quantum gases Eugene Zaremba, Zhigang Wu, Edward Taylor Using a linear response formulation, we study the centre-of-mass response of a harmonically trapped gas to a small amplitude, time-dependent displacement of the trap. We show that the response to this kind of excitation is directly related to the bulk optical conductivity. Thus, a measurement of the time-dependent centre-of-mass dynamics of the cloud provides information about the complex bulk conductivity tensor of the many-body system. For systems with pure harmonic confinement, the response is prescribed by the generalized Kohn theorem and is independent of interactions and quantum statistics. However, non-trivial responses and optical conductivities arise when the harmonicity of the system is compromised by the presence of an additional external potential, such as an optical lattice or impurity. We demonstrate the usefulness of this scheme by calculating the optical conductivity of a one-dimensional Mott insulator of Bose or Fermi atoms confined in a harmonic trap, as well as the optical Hall conductivity of an ideal rotating trapped gas interacting with a Gaussian impurity. Our calculations provide a proof-of-principle demonstration that our proposal should be able to give considerable information about the optical conductivity of strongly-correlated quantum gases. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q34.00015: Rapid coherent control of population transfer in lattice systems Shumpei Masuda, Stuart Rice During the last three decades there have been dramatic advances in understanding of the requirements for control of quantum dynamics. Lattice models are widely used to describe quantum systems, examples of which are a BEC in an optical lattice, a network of nonlinear waveguides and optical fibers, etc. The existing studies clearly reveal the value of the ability to manipulate BECs in optical lattices for the purpose of preparing well-defined quantum states. We have been stimulated by this observation to extend the theory of accelerated adiabatic transfer to lattice systems so as to determine the potential that drives specified state-to-state population transfer without excitation of unwanted quantum states. In this talk we provide a derivation of that driving potential, and we apply the theory to site-to-site population transfer of a BEC in a quasi-one-dimensional optical lattice. We show that modulation of the lattice potential can transfer the population of the BEC between sites of the lattice without unwanted excitations. The theory is applicable to any lattice in which the on-site potential is tunable. We also demonstrate the robustness of the accelerated population transfer to approximation of the driving potential. [Preview Abstract] |
Session Q35: Focus Session: Hybrid Quantum Systems
Sponsoring Units: DAMOPChair: Charles Clark, JQI/NIST
Room: 702
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q35.00001: Cavity optomechanics - Manipulating mechanical motion at the quantum level Invited Speaker: Andreas Nunnenkamp Cavity optomechanics is a rapidly-growing field in which mechanical degrees of freedom are coupled to modes of the electromagnetic field inside optical or microwave resonators. These devices may lead to ultra-sensitive mass and force sensors, provide long-range interaction between distant qubits, and serve as probes of quantum mechanics at increasingly large mass and length scales [for a review see e.g. Physics Today 65, 29 (2012)]. Adapting laser-cooling techniques from atomic physics several experiments have recently observed mechanical motion close to the quantum ground-state. This paves the way for exploiting mechanical systems in the quantum regime. In this talk I will address three problems. First, I will demonstrate that signatures of the intrinsically nonlinear interaction between light and mechanical motion in cavity optomechanical systems can be observed even when the cavity line width exceeds the optomechanical coupling [PRL 111, 053603 (2013)]. Second, I will discuss optomechanical systems in which the position of a mechanical oscillator modulates the line width of the cavity [NJP 15, 045017 (2013) and PRA 88, 023850 (2013)]. Finally, I will present a recent study on synchronization in a self-sustained oscillator coupled to an external harmonic drive [arXiv:1307.7044]. Work done in collaboration with Kjetil B{\o}rkje, Christoph Bruder, Steven M. Girvin, John D. Teufel, Stefan Walter, and Talitha Weiss. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q35.00002: Design and construction of a cavity electro-opto-mechanical system Robert Peterson, Reed Andrews, Thomas Purdy, Katarina Cicak, Raymond Simmonds, Cindy Regal, Konrad Lehnert The parallel advances in the fields of electromechanics and optomechanics have raised the prospect of coupling mechanical motion to both electrical and optical fields. Such a hybrid device has many applications, including transduction of quantum information between microwave and optical frequencies. We demonstrate a cavity electro-opto-mechanical device with a mechanical resonator formed by a thin Si${}_3$N${}_4$ membrane. Partial metallization of the membrane with niobium completes a superconducting electrical circuit fabricated using a ``flip-chip'' technique. This package is integrated into a free-space high-finesse Fabry-Perot cavity, whose spatial mode interacts with the non-metallized portion of the membrane. We report on device performance and discuss future directions for design of hybrid electro-opto-mechanical devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q35.00003: Microwave to optical state transfer with a cavity electro-opto-mechanical system Reed Andrews, Robert Peterson, Thomas Purdy, Katarina Cicak, Raymond Simmonds, Cindy Regal, Konrad Lehnert Quantum-coherent conversion between gigahertz-frequency ``microwave light'' and terahertz-frequency ``optical light'' would combine the processing power and scalability of superconducting qubits with the low-loss and long-distance distribution of optical fibers. Here we use an electro-opto-mechanical device to reversibly convert classical signals between microwave and optical light with an efficiency of ten percent. The frequency conversion is coherent and occurs over a 10 kHz bandwidth. This new type of converter opens new possibilities for generating and distributing entanglement, and is potentially capable of quantum-coherent frequency conversion. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q35.00004: Fiber-Cavity Optomechanics with Superfluid Helium Nathan E. Flowers-Jacobs, Anna D. Kashkanova, Alexey B. Shkarin, Scott W. Hoch, Christian Deutsch, Jakob Reichel, Jack G.E. Harris In a typical optomechanical device, the resonance frequency of a cavity is coupled to mechanical motion through the radiation pressure force. To date, experimental cavities have predominately coupled to a resonant mechanical mode of a solid structure, often a lithographically-defined beam or membrane. We will describe our progress towards realizing an optomechanical device in which an optical fiber-cavity couples to the acoustic modes of superfluid helium. In this system, the optical modes and the acoustic modes of the superfluid are co-located between the mirrored ends of two fiber optic cables. Changes in the density of the superfluid change the effective length of the cavity which results in a standard, linear optomechanical coupling between the 300 MHz acoustic resonances and the 200 THz optical resonances. This type of device is motivated by the self-aligning nature of the acoustic and optical modes (which eases the difficulties of operating at cryogenic temperatures) and by the low optical and mechanical losses of superfluid helium. Although we expect the mechanical quality factor to be limited by acoustic radiation into the glass fiber, we will describe a proposal to realize a dual-band Bragg mirror to confine the optical and acoustic modes more efficiently. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q35.00005: Optomechanical Squeezing of Light Thomas Purdy, Pen-Li Yu, Robert Peterson, Nir Kampel, Cindy Regal Cavity optomechanical systems in the radiation-pressure-shot-noise dominated regime display a variety of quantum effects including measurement backaction heating and quantum correlations between light and mechanical motion. One consequence of latter effect is the creation of squeezed light at the output of such a system. We have generated optomechanically squeezed light using a membrane mechanical resonator inside an optical cavity. The quantum noise of the output light is measured to be reduced by 1.7 dB compared to the shot noise level. Additionally, since the mechanical motion is correlated with quantum fluctuations of the light, readout of the motion provides a non-destructive measurement of the light. We use this type of measurement along with active feedback to produce optical squeezing under conditions where passive optomechanical squeezing is absent. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q35.00006: A Qubit-Coupled Nanomechanical Resonator Integrated with a Superconducting CPW Cavity Yu Hao, Francisco Rouxinol, Seung-Bo Shim, Matt LaHaye In this work we discuss some of our first results integrating a qubit-coupled nanomechanical resonator with a superconducting transmission line resonator. This hybrid circuit QED system is composed of a capacitively-coupled superconducting charge-type qubit and UHF-range flexural nanoresonator, which are both embedded within a superconducting niobium coplanar waveguide (CPW) cavity. Phase-sensitive transmission measurements of the CPW cavity are used to spectroscopically probe the qubit-coupled nanoresonator via the qubit-state-dependent dispersive shift of the cavity frequency. We will discuss the design and measurement of the latest generation of these devices and the prospects for using this system to read-out the number-states statistics of a nanomechanical resonator at low thermal occupancy. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q35.00007: Optomechanical Metamaterials: Dirac polaritons, Gauge fields, and Instabilities Vittorio Peano, Michael Schmidt, Florian Marquardt Freestanding photonic crystals can be used to trap both light and mechanical vibrations. These ``optomechanical crystal'' structures have already been experimentally demonstrated to yield strong coupling between a photon mode and a phonon mode, co-localized at a single defect site. Future devices may feature a regular superlattice of such defects, turning them into ``optomechanical arrays.'' We predict that tailoring the optomechanical band structure of such arrays can be used to implement Dirac physics of photons and phonons, to create a photonic gauge field via mechanical vibrations, and to observe a novel optomechanical instability. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q35.00008: Entangled photon pairs from three coupled optomechanical cells Z.J. Deng, S.J.M. Habraken, F. Marquardt Optomechanics, which couples light to the mechanical motion of an object, is a very important research field. To show features different or superior to the classical counterparts, one major goal in the field of optomechanics is to generate nonclassical states such as squeezed states, entangled states, or states with negative Wigner functions for either or both the optical and mechanical degrees of freedom. In this work, we will discuss on how to generate entangled photon pairs from three coupled optomechanical cells, where each cell consists of a standard optomechanical system and different cells are coupled by photon tunneling. Due to the symmetry of the setup and with the help of mechanical motion, the photons in the driven optical normal mode will be scattered into the other two optical normal modes, where the entangled photon pairs correlated by frequency can be collected. We have investigated the squeezing and entanglement properties of the output light beams, and how these properties would be changed under the influence of the mechanical thermal noise and intrinsic optical losses. Moreover, we find that a suitable choice of parameters can lead to large steady-state entanglement in this proposed setup. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q35.00009: Optical squeezing via dissipation in optomechanics Andreas Kronwald, Florian Marquardt, Aashish A. Clerk The generation of quantum squeezed light is of interest from both fundamental and practical points of view. For example, squeezed light can be used to improve the measurement sensitivity in gravitational wave detectors or even in biophysical applications. In this talk, we discuss a simple yet surprisingly effective mechanism which allows the generation of squeezed output light from an optomechanical cavity, where mechanical motion is coupled to cavity photons via radiation pressure. In contrast to the well known mechanism of ``ponderomotive squeezing'' (realized recently in experiments [1-3]), our scheme generates squeezed output light by explicitly using the dissipative nature of the mechanical resonator. We show that our scheme has many advantages over ponderomotive squeezing; in particular, it is far more effective in the good cavity limit commonly used in experiments. Furthermore, the squeezing generated in our approach can be directly used to enhance the intrinsic measurement sensitivity of the optomechanical cavity; one does not have to feed the squeezed light into a separate measurement device. \\[4pt] [1] D. W. C. Brooks et al., Nature 488, 476 (2012).\\[0pt] [2] A. H. Safavi-Naeini et al., Nature 500, 185 (2013).\\[0pt] [3] T. P. Purdy et al., Phys. Rev. X 3, 031012 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q35.00010: Towards a high quality three-dimensional superconducting electromechanical cavity Adam Reed, Reed Andrews, Brad Mitchell, Konrad Lehnert Macroscopic mechanical resonators coupled to microwave circuits enable quantum control of mechanical motion. These experiments are often limited by undesired loss in the electrical resonator. Current devices are typically fabricated on planar structures with materials that limit the electrical quality. Motivated by the high electrical quality obtainable in three-dimensional superconducting resonators, we explore such non-planar architectures. We present preliminary results of an electromechanical device that moves away from a planar geometry by combining a high quality mechanical resonator with a three-dimensional microwave resonator. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q35.00011: Enhancing optomechanical coupling via the Josephson effect Jani Tuorila, Tero Heikkil\"a, Fransesco Massel, Rapha\"el Khan, Mika Sillanp\"a\"a Cavity optomechanics offers one of the most promising prospects for studying large systems in the quantum limit. The key element within this approach is to employ strong radiation-pressure coupling between mechanical motion and electromagnetic field. However, challenges arise because such a coupling is far too weak in typical systems. We show that the charge tuning of the non-linear Josephson inductance in a single-Cooper-pair transistor can be exploited to create a radiation pressure -type coupling between mechanical and microwave resonators. With experimentally achievable parameters, we find that the usually measured bare coupling can be amplified by a large factor, up to a strength required of the quantum limit. Instead of the non-linearity arising from the strong radiation pressure, we show that the main non-linearity in this setup originates rather from a cross-Kerr type of coupling between the resonators, allowing the access to individual phonon numbers via the measurement of the cavity. Our predictions can be readily tested in the state of the art circuit optomechanical devices. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q35.00012: Engineering Phononic Bandgap Shield for High-$Q$ Silicon Nitride Membrane Resonators K. Cicak, P.-L. Yu, N.S. Kampel, Y. Tsaturyan, T.P. Purdy, R.W. Simmonds, C.A. Regal High-stress $\rm Si_3 N_4$ membrane mechanical resonators exhibit ultrahigh $Q$-frequency products. These millimeter-sized, macroscopic objects should exhibit quantum properties and can be integrated into opto-mechanical, electro-mechanical, and even hybrid electro-opto-mechanical systems. They represent an enabling technology for mediating quantum information transfer between vastly different frequency domains. Experimentally achieving high $Q$-factors with these membranes is hindered by coupling to support structures providing a path for energy loss to the environment (i.e. clamping or support losses). We have microfabricated membranes embedded into phononic crystals etched into the silicon support structure. In order to realize acoustic isolation and shielding from the environment, these structures are engineered to have phononic bandgaps $\sim$1 MHz wide centered around membrane mode frequencies in the MHz range. In this talk, we will discuss device design (aided by finite-element simulation) and fabrication. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q35.00013: Demonstration of the Phononic Bandgap Isolation for Silicon Nitride Membrane Resonators Pen-Li Yu, Katarina Cicak, Nir Kampel, Yeghishe Tsaturyan, Thomas Purdy, Raymond Simmonds, Cindy Regal Silicon nitride membranes offer great potential for sensing weak forces at the standard quantum limit, realizing a mesoscopic quantum harmonic oscillator, and converting quantum information between different quantum systems. An important current limitation to these applications comes from the acoustic coupling between the membrane and its support structure. Such coupling can be controlled by micromachining the support structure to create a phononic crystal. With such a structure, we demonstrate the phononic bandgap isolation for MHz $\rm Si_3 N_4$ membrane resonators. We probe the membrane modes and the non-membrane modes by measuring the displacement spectra of the membrane and different components of the support structure. We find that inside the observed bandgaps, the density and amplitude of the non-membrane modes are greatly suppressed, and the membrane modes are shielded from an external mechanical drive by a factor up to 30 dB. [Preview Abstract] |
Wednesday, March 5, 2014 5:30PM - 5:42PM |
Q35.00014: Single polariton optomechanics Juan Restrepo, Cristiano Ciuti, Ivan Favero We explore theoretically a hybrid quantum system combining cavity quantum electrodynamics and optomechanics[1]. We solve analytically the Hamiltonian of the system showing the nature of the dressed atom-cavity-mechanics polarons and study its dynamical behavior in presence of dissipation and finite temperature. In particular we study cooling to the ground state in the single-polariton regime and peculiar statistics for the mechanical resonator in this hybrid configuration. {[1]} J. Restrepo, C. Ciuti and I. Favero, (arXiv:1307.4282) [Preview Abstract] |
Session Q36: Topological Quantum Information
Sponsoring Units: GQIRoom: 703
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q36.00001: Matrix Product States for Chiral Topological Phases B. Andrei Bernevig, Benoit Estienne, Nicolas Regnault, Yangle Wu I show how, using interacting conformal field theory, an MPS representation can be obtained for both the ground-state and the quasihole excitations of chiral topological states of matter. I show that the advance allows for the accurate calculation of quantities such as topological entanglement entropy and non-abelian braiding. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q36.00002: Entanglement spectrum of Levin-Wen model for topological phases in two dimensions Yuting Hu, Yong-Shi Wu We obtain explicitly the entanglement spectrum of ground states and excited states of the doubled Fibonacci Levin-Wen model. The entanglement spectrum has the topological degeneracy. We show that they exhibit the fractional exclusion statistics of chiral anyons. Moreover, we show that the entanglement spectrum can be mapped to a grand canonical ensemble of 1d system of the chiral Fibonacci anyons on the boundary, at a finite temperature determined by the quantum dimension of Fibonacci anyons. Finally, we show how that the topological quantum numbers of the bulk states can be detected by the entanglement spectrum [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q36.00003: Bulk Entanglement Spectrum Reveals Quantum Criticality within a Topological State Timothy Hsieh, Liang Fu A quantum phase transition is usually achieved by tuning physical parameters in a Hamiltonian at zero temperature. Here, we demonstrate that the ground state of a topological phase itself encodes critical properties of its transition to a trivial phase. To extract this information, we introduce a partition of the system into two subsystems both of which extend throughout the bulk in all directions. The resulting bulk entanglement spectrum has a low-lying part that resembles the excitation spectrum of a bulk Hamiltonian, which allows us to access a topological phase transition from a single wavefunction by tuning either the geometry of the partition or the entanglement temperature. As an example, this remarkable correspondence between topological phase transition and entanglement criticality is rigorously established for integer quantum Hall states. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q36.00004: Exotic circuit elements from hybrid superconductor/quantum Hall systems David Clarke, Jason Alicea, Kirill Shtengel Heterostructures formed by quantum Hall systems and superconductors have recently been shown to support widely coveted Majorana fermion zero-modes and still more exotic `parafermionic' generalizations [1-3]. Here we establish that probing such zero-modes using quantum Hall edge states yields \emph{non-local} transport signatures that pave the way towards a variety of novel circuit elements. In particular, we demonstrate quite generally that at low energies the zero-modes convert chirally moving quasiparticles into oppositely charged quasiholes propagating in the same direction---that is, they swap the sign of the chiral edge currents [4]. One may then construct new and potentially useful circuit elements using this `perfect Andreev conversion' process, including superconducting current and voltage mirrors as well as transistors for fractional charge currents. Characterization of these circuit elements should provide striking evidence of the zero mode physics. \\ \\ 1. Clarke, Alicea, Shtengel, Nat. Commun. 4, 1348 (2013) \\ 2. Lindner, Berg, Refael, Stern, Phys. Rev. X 2, 041002 (2012) \\ 3. Cheng Phys. Rev. B 86, 195126 (2012) \\ 4. Barkeshli, Qi, arXiv:1302.2673 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q36.00005: Exact zero modes and decoherence in systems of interacting Majorana fermions Guang Yang, Dmitri Feldman Majorana fermions often coexist with other low-energy fermionic degrees of freedom. In such situation, topological quantum computation requires the use of fermionic zero modes of a many-body system. We classify all such modes for interacting fermions and show how to select the mode that maximizes the decoherence time. We find that in a typical interacting system the maximal decoherence time is within one order of magnitude from the decoherence time of a qbit, based on the local part of the fermion parity operator. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q36.00006: Braiding Majorana states in helical magnetic atom chains Ching-Kai Chiu, Mohammad Vazifeh, Marcel Franz A helical magnetic atom chain deposited on the top of a superconductor can be realized as a 1D topological superconductor. We propose an innovative braiding protocol for Majorana zero modes at the ends of the magnetic chains for topological quantum computing. Braiding of exchanging particles can be implemented by moving only a \emph{single} Majorana mode from one end to the other end. During the movement, the other Majorana mode teleports to the beginning position of the moving Majorana mode due to the finite size coupling of Majorana modes. Furthermore, the operation of changing the signs of the two Majorana modes can be achieved by rotating the direction of the magnetic moments by pi without moving Majorana modes at the ends. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q36.00007: Robustness of zero-modes in parafermion chains Adam Jermyn, Roger Mong, Jason Alicea Several models for 1D topological phases are known to host zero-modes that enable high-fidelity quantum information storage and manipulation. The Majorana fermion chain provides a classic example. Here the system supports Majorana zero-modes that guarantee two-fold degeneracy in the ground state and excited states to within exponential accuracy. Chains of ``parafermions''--which represent generalized Majorana fermions--also support zero-modes, but, curiously, only under much more restricted circumstances as shown recently by Fendley. We shed light on this interesting finding by exploring the properties of ground-states and excited states in parafermion chains using analytic methods as well as DMRG and exact diagonalization of a truncated Hilbert space model. We show that the absence of exact zero-modes admits a simple physical picture in terms of domain-wall dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q36.00008: Topological Insulating Phases of Non-Abelian Anyonic Chains Wade DeGottardi I will present work on the topological insulating phases of non-abelian anyonic chains, focusing on antiferromagnetically coupled spin-1/2 su(2)$_k$ chains at any level $k$. The topological phases of these systems are characterized by anyonic end modes. A detailed discussion of the two most prominent cases is given: Majorana fermions (at $k = 2$) and Fibonacci anyons ($k = 3$). A renormalization group approach which allows for a straightforward determination of the topological phases of these systems will be discussed. This work reveals a deep connection between topological order and spontaneous symmetry breaking in these systems. It will be argued that the emergent anyons may be more easily manipulated than the physical quasiparticles and could be used to perform quantum computation. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q36.00009: Entanglement Inequalities for Majorana Fermions in Semiconductor Nanowires David Drummond, Alexey Kovalev, Chang-Yu Hou, Leonid P. Pryadko, Kirill Shtengel Recent work has provided evidence that unpaired Majorana fermions may exist at the ends of a semiconductor nanowire in the presence of s-wave superconductivity, a magnetic field, and strong spin-orbit coupling. While Majorana fermions are interesting in their own right as self-conjugate quasiparticles, they are also sought after because they could serve as the stable building blocks of topological quantum computing. We propose an experiment that would establish the entanglement of these Majorana fermions by testing an analog of the Bell and CHSH inequalities in nanowire systems. Our proposal is viable with realistic system parameters, simple ``keyboard'' gating, and projective measurement. Simulation results indicate entanglement can be demonstrated with a relatively small amount of accuracy in the gate operations. Our proposal for testing entanglement inequalities can also be adapted to other systems where Majorana fermions may be present, such as topological insulators. In addition to providing further evidence for the existence of the unpaired Majorana fermions, our proposal could be used as an experimental stepping stone to more complicated braiding experiments. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q36.00010: Detection of zero-modes induced by defect in the Kitaev quantum wire model Sheng-Wen Li, Zeng-Zhao Li, Chang-Pu Sun The Kitaev quantum wire model has two Majorana edge states for open boundary condition. The existence of a defect on a homogenous quantum wire would effectively cut off the wire at this position and generate new boundaries. In this case, another pair of low-energy modes would emerge, localized on both sides of this site, whose energies also approach zero for strong defect. We build up an exactly solvable quantum Langevin equation to describe the electrical current of the quantum wire contacted with two normal leads. If the lead is put besides different sites of the quantum wire, we obtain different transportation profile. When the lead is contacted with the site beside the defect, we would observe a splitting in the differential conductance spectrum, which is determined by the defect strength. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q36.00011: Teleportation-induced entanglement of two nanomechanical oscillators coupled to a topological superconductor Stefan Walter, Jan Carl Budich A one-dimensional topological superconductor features a single fermionic zero mode that is delocalized over two Majorana bound states located at the ends of the system. We study a pair of spatially separated nanomechanical oscillators tunnel-coupled to these Majorana modes. Most interestingly, we demonstrate that the combination of electron-phonon coupling and a finite charging energy on the mesoscopic topological superconductor can lead to an effective superexchange between the oscillators via the non-local fermionic zero mode. We further show that this teleportation mechanism leads to entanglement of the two oscillators over distances that can significantly exceed the coherence length of the superconductor. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q36.00012: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q36.00013: Entanglement spectra between coupled Tomonaga-Luttinger liquids: Applications to ladder systems and topological phases Rex Lundgren, Yohei Fuji, Shunsuke Fukuwara, Masaki Oshikawa We study the entanglement spectrum (ES) and entropy between two coupled Tomonaga-Luttinger liquids (TLLs) on parallel periodic chains. This problem gives access to the entanglement properties of various interesting systems, such as spin ladders as well as two-dimensional topological phases. By expanding interchain interactions to quadratic order in bosonic fields, we are able to calculate the ES for both gapped and gapless systems using only methods for free theories. In certain gapless phases of coupled non-chiral TLLs, we interestingly find an ES with a dispersion relation proportional to the square root of the subsystem momentum, which we relate to a long-range interaction in the entanglement Hamiltonian. We numerically demonstrate this unusual dispersion in a model of hard-core bosons on a ladder. In gapped phases of coupled non-chiral TLLs, which are relevant to spin ladders and topological insulators, we show that the ES consists of linearly dispersing modes, which resembles the spectrum of a single-chain TLL but is characterized by a modified TLL parameter. Based on a calculation for coupled chiral TLLs, we are also able to provide a very simple proof for the correspondence between the ES and the edge-state spectrum in quantum Hall systems. Based of arXiv:1310:0829 [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q36.00014: On the spreading rate of entanglement in a many-body localized quantum spin chain Arun Nanduri, Hyungwon Kim, David Huse Although the many-body localized phase does not allow the transport of local observables, the unbounded logarithmic growth of bipartite entanglement entropy, $S$, has recently been observed (Bardarson et al., Phys. Rev. Lett. {\bf 109}, 017202 (2012)). We aim to elucidate the origin of this logarithmic growth through exact diagonalization methods, analyzing an XXZ spin model with random longitudinal fields. Based on a proposed phenomenology of entanglement spreading (Huse and Oganesyan, arXiv:1305.4915v1), we connect the rate of entanglement spreading with the localization length ($\xi$) of the system and the saturated entanglement entropy per spin ($s_{\infty}$). We find that the time dependence of the entanglement spreading takes the form $S\sim \xi s_{\infty} \log t$. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q36.00015: Exotic topological order from quantum fractal code Beni Yoshida We present a large class of three-dimensional spin models that possess topological order with stability against local perturbations, but are beyond description of topological quantum field theory. Conventional topological spin liquids, on a formal level, may be viewed as condensation of string-like extended objects with discrete gauge symmetries, being at fixed points with continuous scale symmetries. In contrast, ground states of fractal spin liquids are condensation of highly-fluctuating fractal objects with certain algebraic symmetries, corresponding to limit cycles under real-space renormalization group transformations which naturally arise from discrete scale symmetries of underlying fractal geometries. A particular class of three-dimensional models proposed in this paper may potentially saturate quantum information storage capacity for local spin systems. [Preview Abstract] |
Session Q37: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization V
Sponsoring Units: DMPChair: Wenjuan Zhu, IBM Watson Research Center
Room: 705/707
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q37.00001: Graphene-MoS2 Hybrid Technology for Large-Scale Two-Dimensional Electronics Lili Yu, Han Wang, Yi-Hsien Lee, Xi Ling, Yong-Cheol Shin, Elton J.G. Santos, Efthimios Kaxiras, Jing Kong, Tomas Palacios Two-dimensional (2D) materials have generated great interest in the last few years as a new toolbox for electronics. This family of materials includes, among others, metallic graphene, semiconducting transition metal dichalcogenides (such as MoS$_{2})$ and insulating Boron Nitride. These materials and their heterostructures offer excellent mechanical flexibility, optical transparency and favorable transport properties for realizing electronic, sensing and optical systems on arbitrary surfaces. In this work, we develop several etch stop layer technologies that allow the fabrication of complex 2D devices and present for the first time the large scale integration of graphene with molybdenum disulfide (MoS$_{2})$, both grown using the fully scalable CVD technique. Transistor devices and logic circuits with MoS$_{2}$ channel and graphene as contacts and interconnects are constructed and show high performances. In addition, the graphene/MoS$_{2}$ heterojunction contact has been systematically compared with MoS$_{2}$-metal junctions experimentally and studied using density functional theory. The tunability of the graphene work function significantly improves the ohmic contact to MoS$_{2}$. These high-performance large-scale devices and circuits based on 2D heterostructure pave the way for practical flexible transparent electronics in the future. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q37.00002: Sources of disorder in double-gated graphene-insulator-graphene tunneling devices Sergio de la Barrera, Tania Roy, Randall Feenstra, Eric Vogel We demonstrate vertical tunneling through layered graphene / hexagonal boron nitride / graphene heterostructures and investigate the resulting non-linear current-voltage characteristics and gated operation of this device. Some devices show negative differential resistance and steep switching due to a novel resonant tunneling mechanism caused by the graphene density of states, while other devices lack this intriguing feature. We explain the differences in experimental findings by modeling the dominant forms of disorder that can occur in these devices. While certain aspects of current experimental results can be explained in terms of finite-size effects and charge impurities in the surrounding materials and substrate, it is evident that additional forms of disorder remain to be understood. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q37.00003: The Coherent Interlayer Resistance of a Single, Misoriented Interface between Two Graphite Stacks Roger K. Lake, K. M. Masum Habib, Somaia Sylvia, Supeng Ge, Mahesh Neupane The coherent, interlayer resistance of a misoriented, rotated interface between two stacks of AB graphite is determined for a variety of misorientation angles ranging from $0^{\circ}$ to $27.29^{\circ}$. The quantum-resistance of the ideal AB stack is on the order of 1 to 10 m$\Omega \mu{\rm m}^2$ depending on the Fermi energy. For small rotation angles $\leq 7.34^{\circ}$, the coherent interlayer resistance exponentially approaches the ideal quantum resistance at energies away from the charge neutrality point. Over a range of intermediate angles, the resistance increases exponentially with primitive cell size for minimum size cells. A change of misorientation angle by one degree can increase the primitive cell size by three orders of magnitude. These large cell sizes may not follow the exponential trend of the minimal cells especially at energies a few hundred meV away from the charge neutrality point. At such energies, their coherent interlayer resistance is likely to coincide with that of a nearby rotation angle with a much smaller primitive cell. The energy dependence of the interlayer transmission is described and analyzed. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q37.00004: Van der Waals heterostructures Invited Speaker: Roman Gorbachev Research on graphene and other two-dimensional atomic crystals is intense and is likely to remain one of the leading topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. In this talk I will review our recent progress on fabrication and investigation of such heterostructures starting from ultrahigh quality graphene encapsulated in h-BN up to complex 6-layer structures comprised of several materials. Significant attention will be paid to Moir\'{e} patterns with associated Hofstadter-like states in graphene. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q37.00005: Electrical transport properties of metal and graphene contacts to MoS$_2$ Yunqiu (Kelly) Luo, Hua Wen, Tiancong Zhu Two-dimensional crystals are an exciting class of materials for novel physics and nanoelectronics. MoS$_2$ and related transition metal dichalcogenides have received tremendous interest due to its native band gap and strong spin orbit coupling. Unlike graphene, the presence of the band gap leads to transistors with high on-off ratios. One important issue is the electrical properties of the contacts to the MoS$_2$. Recent studies have shown the presence of a Schottky barrier and its dependence on the metal workfunction, back gate voltage, and interfacial oxide barriers. In this work, we investigate the interfacial properties of metal to MoS$_2$ contact and graphene to MoS2 contact by studying the junction's Schottky barrier formation and bias dependence. We utilize a polymer based transfer method to precisely position exfoliated graphene flakes onto exfoliated MoS$_2$ flakes. We intensively study various junction combination between monolayer/few-layer graphene and monolayer/few-layer MoS$_2$. Dependence on temperature and back gate will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q37.00006: Carbon nanotube- MoS2 p-n junction: Fabrication and transport properties Udai Bhanu, Muahmmad Islam, Saiful Khondaker Integrating two different nanoscale semicondcutors of opposite carrier types are of great interest for many electronic and optical applications. Few layers molybdenum disulfide (MoS$_{2})$ is an n-type semiconductor while semiconductoing single walled carbon nanotubes (SWNT) show p-type behavior. In this work, we demonstrate a simple technique for integrating these two semiconductors for fabricating a p-n junction. Few layers MoS$_{2}$ device were mechanically exfoliated from a single crystal of MoS$_{2}$ and making electrical contact via electron beam lithography. Another pair of electrodes, which are orthogonal to MoS$_{2}$ device, is deposited and semiconducting reach SWNT(s-SWNT) solution was dielectrophoretically assembled between the second pair of electrodes. The s-SWNT goes over the MoS$_{2}$ and fabricates two p-n junctions. We will discuss the electronic transport properties of the fabricated devices. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q37.00007: Charge transport and optoelectronic process in an atomically thin p-n junction Chul-Ho Lee, Gwan Hyung Lee, Arend van der Zande, Wenchao Chen, Yilei Li, Minyong Han, Xu Cui, Ghidewon Araffe, Colin Nuckolls, Tony F. Heinz, Jing Guo, James Hone, Philip Kim Heterostructures based on atomically thin van der Waals materials provide an unprecedented opportunity in new materials design. In particular, the ability to assemble two-dimensional (2D) materials into artificial heterostructures with atomically sharp interfaces, combined with recent rediscoveries of transition metal dichalcogenides as an atomically thin semiconductor, enables to build the unique 2D semiconductor heterojunction for fundamental studies as well as device applications. In this talk, we present the electronic and optoelectronic processes in an atomically thin p-n junction consisting of vertically stacked WSe$_{2}$ and MoS$_{2}$ monolayers. Unlike conventional p-n junctions, tunneling-mediated recombination governs the overall charge transport, and gate-tunable photovoltaic response is driven by charge transfer at the atomically sharp interface with large band offsets. Furthermore, the fully vdW heterostructured vertical p-n junctions with graphene electrodes will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q37.00008: Graphene/MoS$_{2}$ Schottky diodes and their integration for metal base transistors Amelia Barreiro, Jason Seol, Chul-ho Lee, Inanc Meric, Elton Santos, Lei Wang, Efthimios Kaxiras, James Hone, Ken Shepard, Jing Guo, Philip Kim In this contribution we present an experimental and theoretical investigation of graphene/MoS2 Schottky diodes and MoS2/graphene/MoS2 metal base transistors. We observe that the Schottky barrier height can be modulated by the chemical potential of the graphene and MoS2 layers with the back gate and tuned in the range of 0-450 meV. To extract further information regarding the quality of the graphene/MoS2 interfaces and the conduction mechanism across them, we analyze the ideality factor as a function of temperature and find it can vary from n$=$3 at 270 K to n$=$12.9 at 100 K. We attribute this strong temperature dependence to a spatial variation of the Schottky barrier, caused by 2D electrostatic effects. Moreover, we have fabricated MoS2/graphene/MoS2 metal base transistors that work as a permeable base transistors. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q37.00009: Photo Sensor Devices Based on Atomically Thin TMDCs Nestor Perea, Ana L. Elias, Nihar Phradan, Zhong Lin, Bartolomeu Cruz-Viana, Luis Balicas, Humberto Terrones, Mauricio Terrones Few-layered films of different transition metal dichalcogenides (TMDCs) like MoS2, WS2, and WSe2 were successfully used as light sensors. The samples were structurally characterized by Raman spectroscopy, AFM, SEM, and HRTEM. The produced samples consisted of few layered sheets possessing up to 10 layers obtained by different synthetic or isolation methods including low-pressure CVD, atmospheric-pressure CVD and mechanical exfoliation. Current-voltage (I-V) and photo response measurements carried out by connecting the TMDC layered sample with Au/Ti contacts. The photocurrent measurements were carried out at different wavelengths from 400 to 800 nm. The results indicate that the electrical response strongly depends on the photon energy from the excitation lasers. In addition, it was found that the photocurrent varied non-linearly with the incident power, and the generated photocurrent in the WS$_{2}$ samples varied as a squared root of the incident power. The response time of the devices was measured and resulted in the order of few milliseconds. Because of its fast response, good responsivity and stability few-layered TMDCs are strong candidates for constructing novel optoelectronic devices. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q37.00010: Exploiting Semiconductor to Metallic Phase Transformation in Layered Transition Metal Dichalcogenides for Ohmic contact Contacts Rajesh Kappera, Damien Voiry, Wesley Jen, Sibel Ebru Yalcin, Gautam Gupta, Aditya Mohite, Manish Chhowalla Achieving ohmic contacts to transition metal dichalcogenides (MoS$_{2}$, WS$_{2}$, WSe$_{2}$ and MoSe$_{2})$ has been a challenge for researchers owing to the formation of a large Schottky barrier between metal and semiconductor. This results in low on-currents, mobilities and sub-threshold swings in the devices made with these materials. Here we report a universal strategy using chemical approach to reversibly transform the semiconducting phase (2H) to metallic phase (1T). Taking advantage of the metallic phase, we have fabricated hybrid transistors, which have 1T phase contacts and semiconducting 2H phase of the material as the channel. The metallic phase dramatically reduces the Schottky barrier between the metal and the semiconductor thereby mitigating the high contact resistance issues. This strategy should be applicable to several other applications such as catalysis, supercapacitors and batteries. Detailed synthesis, structural, electrical and optical characterization will be described. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q37.00011: Charge-Density Wave Driven Phase Transitions in Single-Layer MoS$_2$ Houlong L. Zhuang, Michelle D. Johannes, Richard G. Hennig Phase transitions in single-layer MoS$_2$ are frequently observed in experiments. We reveal that charge doping can induce the phase transition of single-layer MoS$_2$ from the 2$H$ to the 1$T$ structure. Further, the 1$T$ structure undergoes a second phase transition due to the occurrence of a charge-density wave (CDW). By comparing the energies of several possible resulting CDW structures, we find that the $\sqrt3a \times a$ orthorhombic structure is the most stable one, consistent with experimental observations. Moreover, we discover that the band structure of the $\sqrt3a \times a$ structure possesses a Dirac cone, which is split by spin-orbit interactions into a bandgap of 50 meV. We show that the underlying CDW transition mechanism is not electronic, but can be controlled by charge doping nonetheless. Finally, we calculate the interface energy and band offsets of a lateral heterostructure formed by the 2$H$ and $\sqrt3a \times a$ structures. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q37.00012: Discovery of new monolayer material Nb$_{3}$SiTe$_{6}$ Jin Hu, Xue Liu, Chunlei Yue, Zhiqiang Mao, Jiang Wei The discovery of atomically-thin materials, such as graphene and monolayer transition metal dichalcogenides, has ushered in a new era of low-dimensional physics. Due to the quantum confinement effect in reduced dimensionality, the electronic structures of monolayer materials are reconstructed, leading to exotic physical properties such as Dirac fermions in graphene, large direct band gap and valley-spin coupling in MoS$_{2}$. Recently we prepared a new monolayer form of a complex material Nb$_{3}$SiTe$_{6}$. Nb$_{3}$SiTe$_{6}$ possesses a tetragonal structure with each Nb-Si lattice sheet sandwiched by two Te layers. The Te-Nb/Si-Te layers are coupled by Van der Waals gap. Similar to MoS$_{2}$, within Te-Nb/Si-Te layers each Nb forms six bonds with Te atoms, forming trigonal prismatic coordination. We successfully obtained mono-layer Nb$_{3}$SiTe$_{6}$ using micro-mechanical exfoliate technique. While bulk Nb$_{3}$SiTe$_{6}$ is metallic, the electronic properties of Nb$_{3}$SiTe$_{6}$ monolayer are expected to be distinct from those of bulk due to the quantum confinement effect. In this talk, we will report the preparation and electronic properties of Nb$_{3}$SiTe$_{6}$ monolayer. This success of preparing Nb$_{3}$SiTe$_{6}$ monolayer provides a new playground for studying low dimensional physics and nanotechnology. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q37.00013: Peierl Transition Temperature and ac Conduction Study in Few Layer Blue Bronze (K$_{0.3}$MoO$_{3})$ Crystals Mehdi Jamei, Oscar Vazquez, Jairo Velasco, Alex Zettl, Michael Crommie K$_{\mathrm{x}}$MoO$_{3}$ (0.24\textless x\textless 0.3) or Potassium Molybdenum Blue Bronze is a monoclinic crystal with a layered structure. The presence of sliding charge-density waves (CDW) in Blue Bronze, and its potential to be cleaved and exfoliated, make this material an interesting candidate to investigate in 2D form. In this study, Blue Bronze crystals were grown by the electrochemical method. This method involves passing a DC current through a mixture of MoO$_{3}$ and K$_{2}$MoO$_{4}$, while the temperature is set right above the melting point. then exfoliated thin flakes (below 100nm) of Blue Bronze from these crystals and fabricated 2, 3 and 4-terminal devices using e-beam lithography. An Indium-Chrome-Gold tri-layer was used as the metal contact. Nitrogen plasma treatment before depositing the metal proved to make a dramatic improvement in the contact resistance. We studied the effect of thickness on the Peierl transition temperature. The ac-conductivity of these crystals was measured in cryogenic temperature. Also we investigated the induction of CDW into Graphene in a Blue Bronze-Graphene stack structure. [Preview Abstract] |
Session Q38: Superconducting Qubits: Amplifiers & Readout
Sponsoring Units: GQIChair: Stefano Poletto, University of Delft
Room: 709/711
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q38.00001: Photon tomography of a Josephson Parametric Amplifier William Kindel, Michael Schroer, Gene Hilton, Leila Vale, Martin Sandberg, Michael Vissers, Jiansong Gao, David Pappas, Lehnert Konrad Josephson Parametric Amplifiers (JPAs) are an important resource for quantum limited measurement, feedback and nonclassical state generation. To study the JPA transformation, we use a superconducting qubit-cavity system to launch single photons or, n=1 Fock states, into a JPA, which measures the state. From repeated measurements, we can infer the state's loss of purity as a results of the JPA transformation. We will present our estimates of the JPA's efficiency as a photon detector along with progress toward measuring arbitrary superpositions of n=0 and n=1 Fock states. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q38.00002: Design and operation of novel Josephson parametric amplifiers for QND supeconducting qubit readout A. Narla, K.M. Sliwa, M. Hatridge, S. Shankar, L. Frunzio, R.J. Schoelkopf, M.H. Devoret Parametric amplifiers based on Josephson junctions are essential tools in superconducting quantum information experiments. However, their integration with current 3D Circuit QED experiments is made challenging by the need to transition between waveguide, coax and printed circuit boards. Moreover, these amplifiers need auxiliary microwave components, like hybrids and directional couplers, that are sources of spurious losses and/or difficult-to-predict impedance mismatch that can limit measurement efficiency. We develop a new architecture for these parametric amplifiers that eliminates superfluous microwave components and interconnects. This simplifies their assembly and integration into experiments. We present an experimental realization of such a device that demonstrates 20 dB of gain with 17 MHz BW at 11.4 GHz, on par with conventional devices. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q38.00003: Qubit readout with a directional parametric amplifier K.M. Sliwa, B. Abdo, A. Narla, S. Shankar, M. Hatridge, L. Frunzio, R.J. Schoelkopf, M.H. Devoret Josephson junction based quantum limited parametric amplifiers play an essential role in superconducting qubit measurements. These measurements necessitate circulators and isolators between the amplifier and qubit to add directionality and/or isolation. Unfortunately, this extra hardware limits both quantum measurement efficiency and experimental scalability. Here we present a quantum-limited Josephson-junction-based directional amplifier (JDA) based on a novel coupling between two nominally identical Josephson parametric converters (JPCs). The device achieves a forward gain of 11 dB with a 15 MHz dynamical bandwidth, but higher gains are possible at the expense of bandwidth. We also present measurements of a transmon qubit made with the JDA, and show minimal measurement back-action despite the absence of any isolator or circulator before the amplifier. These results provide a first step toward realizing on-chip integration of qubits and parametric amplifiers. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q38.00004: Josephson traveling-wave parametric amplifier for superconducting qubit readout Chris Macklin, D.H. Slichter, O. Yaakobi, L. Friedland, V. Bolkhovsky, D.A. Braje, G. Fitch, W.D. Oliver, I. Siddiqi Superconducting parametric amplifiers (paramps) have successfully demonstrated near quantum limited sensitivity, enabling single-shot qubit readout, feedback, and state tracking. However, these amplifiers are commonly limited to narrow bandwidth and modest dynamic range, and most require microwave circulators to separate input and output modes. These limitations stem from the use of a resonant non-linearity to achieve mixing between a signal and pump mode. Our traveling-wave parametric amplifier (TWPA) is based on a superconducting nonlinear Josephson junction transmission line, thereby inherently sidestepping the limitations associated with a cavity structure. We present theoretical predictions and experimental results, including improved gain and noise performance. We discuss transmon qubit readout in the circuit QED architecture using a TWPA. We also comment on promising architectures for chip-level integration and multiplexing. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q38.00005: Development of integrated, on-chip microwave amplifiers for superconducting qubit measurement D.M. Toyli, A. Eddins, E.M. Levenson-Falk, S. Khan, A.A. Clerk, R. Vijay, I. Siddiqi In recent years, superconducting parametric amplifiers (paramps) have become essential tools for quantum-limited measurement of superconducting qubits. Despite the utility of such paramps in quantum measurement, feedback, and metrology, current hardware configurations require that paramps be isolated from qubits by lossy and bulky microwave components that limit their quantum efficiency and scalability. Here we describe progress toward achieving fast, high-fidelity qubit measurement using on-chip microwave amplifiers. Our approach is based on engineering weak nonlinearity into linear circuits conventionally used for circuit QED readout and probing these systems in a manner that enables independent control of the phases of the measurement and amplification processes. We report on device design, performance calculations, and preliminary measurements of integrated qubit-amplifier devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q38.00006: Enhanced Dynamic Range in $N$-SQUID Lumped Josephson Parametric Amplifiers A. Eddins, E.M. Levenson-Falk, D.M. Toyli, R. Vijay, Z. Minev, I. Siddiqi Simultaneously providing high gain and nearly quantum-limited noise performance, superconducting parametric amplifiers (paramps) have been used successfully for high fidelity qubit readout, quantum feedback, and microwave quantum optics experiments. The Lumped Josephson Parametric Amplifier (LJPA) consists of a capacitively shunted SQUID coupled to a transmission line to form a nonlinear resonator. Like other paramps employing a resonant circuit, the LJPA's dynamic range--a potentially key ingredient for multiplexing--is limited. Simple theory predicts that the dynamic range can be increased without any reduction in bandwidth or gain by distributing the resonator nonlinearity over a series array of SQUIDs. We fabricated such array devices with up to 5 SQUIDs and observed a clear increase in the critical power for bifurcation about which parametric gain occurs. We discuss in detail amplifier performance as a function of the number of SQUIDs in the array. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q38.00007: A Broadband Quantum-Limited Josephson Parametric Amplifier, Part I: Exp. T.C. White, R. Barends, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, E. Jeffrey, J. Kelly, A. Megrant, J.Y. Mutus, C. Neill, P. O'Malley, C. Quintana, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, A.N. Cleland, J.M. Martinis While Josephson parametric amplifiers (JPA) have achieved noise performance near the quantum limit, their bandwidth and saturation power is constrained by the resonant design. For a 50 ohm environment the relationship between junction critical current, frequency, and coupled Q means that bandwidth and saturation vary inversely. We present a device in which the coupled Q was lowered by engineering the environment impedance, increasing both bandwidth and saturation power without changing the resonator circuit parameters. The 50 ohm environment was transformed to 15 ohms at the resonator using a hybrid co-planar waveguide/micro-strip transmission line to create a broadband impedance matching network. This device exhibits regions with near quantum-limited bandwidth exceeding 700 MHz and saturation powers as high as -105 dBm. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q38.00008: A Broadband Quantum-Limited Josephson Parametric Amplifier. Part II: Theory Josh Mutus, R. Barends, J. Bochmann, B. Campbell, Y. Chen, Z. Chen, B. Chiaro, A. Dunsworth, E. Jeffrey, J. Kelly, A. Megrant, C. Neill, P. O'Malley, C. Quintana, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T.C. White, A.N. Cleland, J.M. Martinis The quantum-limited nature of the Josephson parametric amplifier (JPA) has enabled exquisite studies of single qubit dynamics. Scaling up to larger quantum systems and higher-power dynamics requires wider bandwidth and higher saturation power. We demonstrate that both bandwidth and saturation power can be increased by an order of magnitude through careful engineering of the frequency dependent impedance environment. We can understand and engineer the interaction between the JPA and this environment using the ``pumpistor'' model, in which the flux-pumped SQUID is treated as a linear circuit element. At extreme low Q this interaction, previously viewed as a parasitic effect, can be used to greatly enhance bandwidth while maintaining the robust noise performance of the JPA. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q38.00009: Squeezing with a flux-driven Josephson parametric amplifier E.P. Menzel, L. Zhong, P. Eder, A. Baust, M. Haeberlein, E. Hoffmann, F. Deppe, A. Marx, R. Gross, R. Di Candia, E. Solano, M. Ihmig, K. Inomata, T. Yamamoto, Y. Nakamura Josephson parametric amplifiers (JPA) are promising devices for the implementation of continuous-variable quantum communication protocols. Operated in the phase-sensitive mode, they allow for amplifying a single quadrature of the electromagnetic field without adding any noise. While in practice internal losses introduce a finite amount of noise, our device still adds less noise than an ideal phase-insensitive amplifier. This property is a prerequisite for the generation of squeezed states. In this work, we reconstruct the Wigner function of squeezed vacuum, squeezed thermal and squeezed coherent states with our dual-path method [L. Zhong et al. arXiv:1307.7285 (2013); E. P. Menzel et al. Phys. Rev. Lett. 105 100401 (2010)]. In addition, we illuminate the physics of squeezed coherent microwave fields. This work is supported by SFB 631, German Excellence Initiative via NIM, EU projects SOLID, CCQED, PROMISCE and SCALEQIT, MEXT Kakenhi ``Quantum Cybernetics,'' JSPS FIRST Program, the NICT Commissioned Research, Basque Government IT472-10, Spanish MINECO FIS2012-36673-C03-02, and UPV/EHU UFI 11/55. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q38.00010: Efficient Qubit Readout Using Josephson Photomultipliers E.J. Pritchett, L.C.G. Govia, C. Xu, M.G. Vavilov, B.L.T. Plourde, R. McDermott, F.K. Wilhelm A Josephson photomultplier (JPM) -- a current-biased Josephson junction operated near its critical bias -- can absorb and detect weak microwave signals with high sensitivity (PRL 107, 217401 (2011)). When strongly coupled to a high-Q transmission line ``cavity,'' the JPM can detect single microwave photons with large bandwidth and with near unit efficiency (PRB 86, 174506 (2012)). The switching of a JPM into its voltage state acts on the adjacent cavity via the backaction of photon subtraction (PRA 86, 032311 (2012)). While a destructive measurement of the microwave cavity, this switching can perform a binary non-demolition measurement of a quantum system coupled to the cavity. We present a protocol by which the presence and subsequent detection of a cavity photon by a JPM conveys information about the state of a superconducting qubit without destroying it, thus performing a quantum non-demolition measurement of the qubit's state. Multi-qubit generalizations of this protocol are discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q38.00011: Josephson parametric phase-locked oscillator: application to dispersive readout of superconducting qubits Zhirong Lin, Kunihiro Inomata, William Oliver, Kazuki Koshino, Yasunobu Nakamura, Jaw-Shen Tsai, Tsuyoshi Yamamoto We present a new qubit readout scheme using a Josephson parametric phase-locked oscillator. The parametric oscillator is the same circuit as the flux-driven parametric amplifier used in Refs. 1 and 2, but is operated at the pump power above the oscillation threshold. The oscillator works as a sensitive binary phase detector and discriminates the dispersive phase shifts in the probe microwave field reflected from a resonator coupled to a qubit. The scheme offers fast and latching-type readout, but requires only a small number of photons in the resonator. Using this scheme, we achieved high-fidelity single-shot readout of a flux qubit with more than 90$\%$ contrast of Rabi oscillations. [1] T. Yamamoto {\it et al.}, APL {\bf 93}, 042510 (2008). [2] Z. R. Lin {\it et al.}, APL {\bf 103}, 132602 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q38.00012: Kinetic Inductance Traveling-wave Parametric Amplifier for Qubit and Detector Readout Jiansong Gao, Mike Vissers, Martin Sandberg, Saptarshi Chaudhuri, Clint Bockstiegel, Christopher Abeles, Kent Irwin, David Pappas A broadband quantum-limited amplifier is desired for multiplexed readout of superconducting qubits and detectors. Kinetic inductance traveling-wave parametric amplifier (KIT) is a new type of amplifier that utilizes the intrinsic nonlinearity of kinetic inductance of superconductor for parametric amplification. By applying dispersion engineering, KIT amplifier can achieve quantum-limited noise over a broad bandwidth. We have designed a KIT amplifier which consists of a 2-m long coplanar waveguide fabricated from 20 nm NbTiN film on Si wafer. We have achieved over 10dB gain in a bandwidth from 5 to 11 GHz. We have found the maximum gain is limited by abrupt breakdown at defects in the transmission line. By cascading two devices, more than 20 dB of gain was achieved from 5 to 12 GHz. We are also designing a travel-wave version of Josephson parametric amplifier with GHz bandwidth by applying dispersion engineering. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q38.00013: In-situ characterization of a SQUID MSA located within the Axion Dark Matter eXperiment Andrew Wagner The Axion Dark Matter eXperiment (ADMX) is designed to detect ultra-weakly interacting relic axion particles by searching for their conversion to microwave photons in a resonant cavity immersed in a high magnetic field. A SQUID micro-strip amplifier (MSA) is used as the first stage amplifier in ADMX to achieve a near quantum limited system noise temperature. The in-situ characterization of a SQUID MSA within this large experiment and high magnetic field environment is presented. The possibility of improving the sensitivity of ADMX with Josephson parametric amplifiers and superconducting qubits is also discussed. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q38.00014: High-Fidelity Qubit Measurement using a Superconducting Low-Inductance Undulatory Galvanometer Microwave Amplifier Ted Thorbeck, David Hover, Shaojiang Zhu, Guilhem Ribeill, Daniel Sank, Rami Barends, John Martinis, Robert McDermott We describe a high-fidelity dispersive measurement of a superconducting Xmon qubit using a microwave amplifier based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). We will show a qubit measurement fidelity of 99{\%} in 700 ns with the SLUG, compared to 60{\%} without the SLUG. The SLUG amplifier has a gain of 19 dB at 6.6 GHZ. It also improves the signal-to-noise ratio by 9 dB, compared the same circuit without the SLUG. Also, the SLUG amplifier has a large dynamic range, with an input saturation power corresponding to around 600 photons in the readout cavity. All of these properties make the SLUG a promising microwave amplifier for more complex quantum circuits. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q38.00015: Tunable resonant and non-resonant interactions between a phase qubit and LC resonator Michael Shane Allman, Jed D. Whittaker, Manuel Castellanos-Beltran, Katarina Cicak, Fabio da Silva, Michael Defeo, Florent Lecocq, Adam Sirois, John Teufel, Jose Aumentado, Raymond W. Simmonds We use a flux-biased radio frequency superconducting quantum interference device (rf SQUID) with an embedded flux-biased direct current (dc) SQUID to generate strong resonant and non-resonant tunable interactions between a phase qubit and a lumped-element resonator. The rf-SQUID creates a tunable magnetic susceptibility between the qubit and resonator providing resonant coupling rates from zero to near the ultra-strong coupling regime. By modulating the magnetic susceptibility, non-resonant parametric coupling achieves rates $>100\,\rm{MHz}$. Nonlinearity of the magnetic susceptibility also leads to parametric coupling at subharmonics of the qubit-resonator detuning. Controllable coupling is generically important for constructing coupled-mode systems ubiquitous in physics, useful for both, quantum information architectures and quantum simulators. [Preview Abstract] |
Session Q39: Superconductivity: Sr2RuO4 and Related Materials
Sponsoring Units: DCMPChair: Maxim Khodas, University of Iowa
Room: Mile High Ballroom 2A-3A
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q39.00001: Zero energy bound state at an interface between s-wave superconductor and disordered normal metal with repulsive electron-electron interaction Christopher Reeg, Dmitrii Maslov In recent years there has been a renewed interest in the proximity effect due to its role in the realization of topological superconductivity. In particular, we study a superconductor-normal metal proximity system with a repulsive interaction between electrons in the normal layer. Due to the change in sign of the superconducting pair potential, a zero energy bound state is trapped at the interface [Fauch\`{e}re et al., Phys. Rev. Lett., {\bf 82}, 3336 (1999)]. Using the quasiclassical theory of superconductivity we investigate the behavior of this zero energy state in the presence of finite disorder and an interfacial barrier. We find that as the mean free path is decreased, the peak in the local density of states is broadened and shifted away from zero energy. In the ballistic limit the presence of this bound state eliminates the mini-gap seen in a non-interacting normal layer and a distinct peak is observed. When the mean free path becomes comparable to the normal layer width the low energy peak is strongly suppressed and the mini-gap begins to develop. In the diffusive limit the mini-gap is fully restored and all signatures of the bound state are eliminated. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q39.00002: Satisfaction of Kohler's rule in the pseudogap phase of the high-temperature superconductor HgBa$_2$CuO$_{4+\delta}$ Yang Ge, M. Chan, M. Veit, C. Dorrow, W. Tabis, M. Greven, B. Vignolle, C. Proust, X. Zhao, N. Bari\v{s}i\'{c} We report on the temperature and magnetic field evolution of the planar resistivity in the simple tetragonal high-temperature superconductor HgBa$_2$CuO$_{4+\delta}$ (Hg1201). Counter to the longstanding view that Kohler's rule is universally strongly violated in the cuprates, we find that it is in fact satisfied in the pseudogap state. The magnetoresistance shows a $T^{-4}$ temperature dependence which, in conjunction with previous work demonstrating $\rho\propto T^2$ [1] as well as quadratic temperature and frequency dependence of the dynamical relaxation rate [2], is indicative of a Fermi-liquid quasiparticle scattering in the pseudogap phase of Hg1201. [1] N. Bari\v{s}i\'c et al., Proc. Natl. Acad. Sci. US 110, 12235 (2013). [2] S.I. Mirzaei et al., Proc. Natl. Acad. Sci. US 110, 5774 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q39.00003: Strong anisotropy in the electromagnetic properties of Na$_{2}$Ti$_{2}X_{2}$O ($X =$ As, Sb) crystals Youguo Shi, Nanlin Wang Na$_{2}$Ti$_{2}X_{2}$O ($X =$ As, Sb) crystals have been grown from the flux method. X-ray diffraction characterization revealed an anti-K$_{2}$NiF$_{4}$-type layered structure (tetragonal, space group $I$4\textit{/mmm}) for both compounds. Magnetic susceptibility ($\chi (T )$) and electrical resistivity ($\rho$($T$) measurements revealed major kinks at 115 K ($T_{s1})$ and 320 K ($T_{s2})$ for Na$_{2}$Ti$_{2}$Sb$_{2}$O and Na$_{2}$Ti$_{2}$As$_{2}$O, respectively, signifying possibly the opening of density wave gaps. Both Na$_{2}$Ti$_{2}$Sb$_{2}$O and Na$_{2}$Ti$_{2}$As$_{2}$O showed remarkably strong anisotropy in their electromagnetic transport properties, and values of $\gamma_{\rho } (\rho_{c}/\rho_{ab})$ even reached 140 and 430, respectively, being much larger than that of iron pnictide BaFe$_{2}$As$_{2}(\gamma_{\rho} =$ 2--5). The $\gamma \rho $of Na$_{2}$Ti$_{2}$Sb$_{2}$O changed slightly with cooling, though a small drop at $T_{s1}$ occurred. In contrast, the $\gamma_{\rho }$ of Na$_{2}$Ti$_{2}$As$_{2}$O changed strikingly by exhibiting not only a small change at $T_{s2}$ but also a sudden decrease of 50 K, reduced nearly 1$/$3. Specific heat measurement indicated that Na$_{2}$Ti$_{2}$Sb$_{2}$O was only partially gapped with $\gamma_{1} =$ 4.1 mJ mol$^{-1}$ K$^{-2}$, though a long-range order was established at $T_{s1}$, while Na$_{2}$Ti$_{2}$As$_{2}$O was fully gapped. The remarkably strong electromagnetic anisotropy revealed in Na$_{2}$Ti$_{2}X_{2}$O suggests the crucial role of the TiO$_{2}X_{4}$ layer for the transport properties of layered titanium oxypnictides. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q39.00004: Anisotropy of transport coefficients in the pseudogap phase of the cuprate superconductor YBCO Olivier Cyr-Choini\`ere, Ga\"el Grissonnanche, Nicolas Doiron-Leyraud, Louis Taillefer, James Day, Ruixing Liang, Doug Bonn, Walter Hardy We recently discovered evidence of a broken rotational symmetry in the pseudogap phase of the cuprate superconductor YBa$_{2}$Cu$_{3}$O$_{y}$ [1]. This broken symmetry was inferred from the onset of a large in-plane anisotropy of the Nernst coefficient $N$ below the pseudogap temperature $T^{\star}$, attributed to an anisotropy in the longitudinal coefficients, i.e. the resistivity $\rho$ and/or the Seebeck coefficient $S$. It was pointed out that an anisotropy in $N$ could also come from an anisotropy of the transverse coefficients, i.e. the Hall and Peltier coefficients, $\sigma_{xy}$ and $\alpha_{xy}$ [2]. We report here a complete study of the anisotropy of all transport coefficients in a YBCO single crystal. The measurements were performed first with the sample oriented along the $b$-axis direction and, then along the $a$-axis direction achieved by rotating the CuO chain direction via detwinning. We therefore extract the anisotropy of the transport coefficients without uncertainty from geometric factors or sample dependence. We discuss the possible implications of these transport anisotropies for the physics of the pseudogap phase. \\[4pt] [1] R. Daou {\it et al.}, {\it Nature} {\bf 463}, 519 (2010).\\[0pt] [2] C. Varma {\it et al.}, {\it arXiv} 1007.1215 (2010). [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q39.00005: Anisotropic breakdown of Fermi liquid quasiparticle excitations in overdoped La2$-x$Sr$x$CuO4 Johan Chang, Martin Maansson, Stephane Pailhes, Oliver Lipscompe, Stephen Hayden, Luc Patthey, Oscar Tjernberg, Joel Mesot High-temperature superconductivity emerges from an un-conventional metallic state. This has stimulated strong efforts to understand exactly how Fermi liquids breakdown and evolve into an un-conventional metal [1,2]. A fundamental question is how Fermi liquid quasiparticle excitations break down in momentum space. Here we show, using angle-resolved photoemission spectroscopy, that the Fermi liquid quasiparticle excitations of the overdoped superconducting cuprate La1.77Sr0.23CuO4 is highly anisotropic in momentum space [3]. Fermi liquid excitations are found in the anti-nodal region whereas conventional Fermi liequid excitations are probed around the nodal point [3].\\[4pt] [1] R. A. Copper \textit{et al}., \textit{Science} \textbf{323}, 603--607 (2009).\\[0pt] [2] K. Jin \textit{et al}., \textit{Nature} \textbf{476}, 73--75 (2011).\\[0pt] [3] J. Chang \textit{et al}., \textit{Nature Communications }\textbf{4}, 2559 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q39.00006: Chiral d-wave superconductivity in SrPtAs: A Weyl superconductor Titus Neupert, Mark Fischer, Christian Platt, Andreas Schnyder, Werner Hanke, Ronny Thomale, Manfred Sigrist, Jun Goryo Recent $\mu$SR measurements suggest that the hexagonal pnictide SrPtAs is a chiral d-wave superconductor that spontaneously breaks time-reversal symmetry in the superconducting state. The $d$-wave order parameter fully gaps all Fermi surfaces, except for point nodes on one Fermi surface sheet near the $K$ and $K'$ points at the Brillouin zone corners. Theses nodal points are Majorana-Weyl fermions in momentum space. We study the topological characteristics of this superconducting phase, which features protected chiral surface states, Majorana-Fermi arcs on the surface and an associated thermal Hall response. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q39.00007: Time-reversal symmetry breaking in superconducting Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$ Lei Shu, Douglas E. MacLaughlin, Kevin Huang, M. Brian Maple Zero-field muon-spin-relaxation ($\mu$SR) experiments were performed on the superconductors Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$ ($x < 0.2$). The results reveal the spontaneous appearance of static internal magnetic fields below the superconducting transition temperature ($T_c$). This observation implies time-reversal symmetry breaking in Pr$_{1-x}$Ce$_{x}$Pt$_4$Ge$_{12}$ ($x < 0.2$) below $T_c$. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q39.00008: Strong Coupling S-wave Superconductivity in Bi$_{4}$O$_{4}$S$_{3}$ - Shruti, Pankaj Srivastava, Satyabrata Patnaik We investigate the superconducting properties and pairing symmetry in recently discovered Bi$_{4}$O$_{4}$S$_{3}$ superconductor. A series of Bi$_{6}$O$_{4}$S$_{4}$(SO$_{4}$)$_{1-x}$ samples were synthesized by solid-state reaction. The optimally doped sample Bi$_{4}$O$_{4}$S$_{3}$ which is 50\% SO$_{4}$ deficient shows maximum T$_{c}$ of 5.3K as confirmed by resistivity and magnetization measurement. The upper critical field at zero temperature is found to be $\sim$ 2.75 T and Ginzburg Landau coherence length is estimated to be $\sim$ 110{\AA}. Hall measurement confirmed the dominant role played by the electrons with charge carrier density of 4.405x10$^{19}$ cm$^{-3}$ at 10 K. The Sommerfeld constant $\gamma$ is calculated to be 1.113 mJ/K$^{2}$mol. Superconducting pairing symmetry and superconducting gap was studied from penetration depth measurement using tunnel diode oscillator technique. It is shown that Bi$_{4}$O$_{4}$S$_{3}$ is a strong coupling s-wave type superconductor with fully developed gap. Below T$_{c}$, superfluid density is best fitted with single gap s wave model with zero-temperature value of the superconducting energy gap $\Delta$$_{0}$= 1.54 meV, corresponding to the ratio 2$\Delta$$_{0}$/k$_{B}$T$_{c}$=7.2 which is much higher than the BCS value of 3.53. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q39.00009: Observation of Time-Reversal Symmetry Breaking in the Non-Centrosymmetric Superconductor Re6Zr Martin R. Lees, Adrian D. Hillier, Bayan Mazidian, James F. Annett, Jorge Quintanilla, Donald McK. Paul, Ravi Singh, Geetha Balakrishnan We have investigated the superconducting state of the non-centrosymmetric compound Re$_{6}$Zr using magnetization, heat capacity, and muon-spin relaxation/rotation ($\mu$SR) measurements. Re$_{6}$Zr has a superconducting transition temperature, $T_c = 6.75\pm0.05$ K. Transverse-field $\mu$SR experiments, used to probe the superfluid density, suggest an $s$-wave character for the superconducting gap. However, zero and longitudinal-field $\mu$SR data reveal the presence of spontaneous static magnetic fields below $T_c$ indicating that time-reversal symmetry is broken in the superconducting state and an unconventional pairing mechanism. An analysis of the pairing symmetries identifies the ground states compatible with time-reversal symmetry breaking. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q39.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q39.00011: How the insulator and pseudogap states coalesce beneath the superconductor dome Alejandro Cabo Montes de Oca, Victor Martinez, Alejandro Cabo-Bizet The effect of hole doping on La$_{2}$CuO$_{4}$ physical properties is investigated by means of a TB model for the CuO planes proposed in previous works which generalizes the Hubbard model. Thus, its former predictions of the insulator and paramagnetic-pseudogap states at half-filling become natural ones as emerging from a more general analysis. The effects of hole doping on the insulator state and the paramagnetic-pseudogap one are investigated at T$=$0 K$^{0}$. The results predict a quantum phase transition in which the insulator state coalesce at a critical doping $\delta_{\mathrm{c}}=$0.2 with the pseudogap state, to become a paramagnetic-metal at higher hole densities. The evolution with small doping of the Fermi surface in the insulator state, shows that the holes tend to become localized at the middle of the sides of the Brillouin zone. Then, when the hole doping passes through the critical value, the holes move to become situated at the corners of the zone, showing a structural change of the Fermi surface at the phase transition point. These results offer a clear answer to the debated question about the existence and nature of a QPT beneath the superconductor Dome. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q39.00012: ABSTRACT WITHDRAWN |
Session Q40: Invited Session: Cell Motility in Three-Dimensions
Sponsoring Units: DBIO DFDChair: Moumita Das, Rochester Institute of Technology
Room: Mile High Ballroom 2B-3B
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q40.00001: Force Fluctuations within Focal Adhesions Mediate ECM-Rigidity Sensing to Guide Directed Cell Migration Invited Speaker: Clare Waterman |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q40.00002: Cancer Cell Migration in 3D Invited Speaker: Denis Wirtz Two-dimensional (2D) \textit{in vitro} culture systems have for a number of years provided a controlled and versatile environment for mechanistic studies of cell adhesion, polarization, and migration, three interrelated cell functions critical to cancer metastasis. However, the organization and functions of focal adhesion proteins, protrusion machinery, and microtubule-based polarization in cells embedded in physiologically more relevant 3D extracellular matrices is qualitatively different from their organization and functions on conventional 2D planar substrates. This talk will describe the implications of the dependence of focal adhesion protein-based cell migration on micro-environmental dimensionality (1D vs. 2D vs.. 3D), how cell micromechanics plays a critical role in promoting local cell invasion, and associated validation in mouse models. We will discuss the implications of this work in cancer metastasis. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q40.00003: Single Cell Traction Microscopy within 3D Collagen Matrices Invited Speaker: Mingming Wu Mechanical interaction between the cell and its extracellular matrix (ECM) regulates cellular behaviors, including proliferation, differentiation, adhesion and migration. Cells require the three dimensional (3D) architectural support of the ECM to perform physiologically realistic functions. However, our current understanding of cell-ECM and cell-cell mechanical interactions is largely derived from 2D traction force microscopy, in which cells are cultured on a flat substrate. It is now clear that what we learn about cellular behavior on a 2D substrate does not always apply to cells embedded within a 3D biomatrix. 3D traction microscopy is emerging for mapping traction fields of single cells embedded in 3D gel, but current methods cannot account for the fibrous and nonlinear properties of collagen gel. In this talk, I will present a forward computation algorithm that we have developed for 3D cell traction measurements within collagen gels. The application of this technology to understanding cancer migration and invasion will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q40.00004: Modeling cell migration on filamentous tracks in 3D Invited Speaker: J.M. Schwarz Cell motility is integral to a number of physiological processes ranging from wound healing to immune response to cancer metastasis. Many studies of cell migration, both experimental and theoretical, have addressed various aspects of it in two dimensions, including protrusion and retraction at the level of single cells. However, the {\it in vivo} environment for a crawling cell is typically a three-dimensional environment, consisting of the extracellular matrix (ECM) and surrounding cells. Recent experiments demonstrate that some cells crawling along fibers of the ECM mimic the geometry of the fibers to become long and thin, as opposed to fan-like in two dimensions, and can remodel the ECM. Inspired by these experiments, a model cell consisting of beads and springs that moves along a tense semiflexible filamentous track is constructed and studied, paying particular attention to the mechanical feedback between the model cell and the track, as mediated by the active myosin-driven contractility and the catch/slip bond behavior of the focal adhesions, as the model cell crawls. This simple construction can then be scaled up to a model cell moving along a three-dimensional filamentous network, with a prescribed microenvironment, in order to make predictions for proposed experiments. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:30PM |
Q40.00005: Tumor cell migration is a superstatistical process Invited Speaker: Ben Fabry Over short time scales, cell migration can be well described as a homogeneous correlated random walk with a fixed average step length and a certain degree of directional persistence. On time scales of up to 24 h, however, the migration process is highly inhomogeneous. Superstatistical fluctuations of step length and directional persistence lead to ``anomalous'' features, such as an exponential step width distribution (SWD) and a superdiffusive mean squared displacement (MSD). These features are quantitatively reproduced by a correlated random walk with temporally varying persistence. By comparing cell migration on planar substrates and in a 3D collagen matrix, we demonstrate that the globally averaged MSD and SWD are not sensitive to the microscopic migration mechanism of the cells and can therefore yield identical results in these different environments. By contrast, the temporal fluctuations of step length and directional persistence, and their mutual correlations, provide a characteristic fingerprint of the migration process in different environments. [Preview Abstract] |
Session Q41: Focus Session: Complex Oxide Thin-Film Growth
Sponsoring Units: DMP DCOMPChair: Stephen Hellberg, Naval Research Laboratory
Room: Mile High Ballroom 3C
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q41.00001: Effect of Growth Induced (Non)Stoichiometry on Interfacial Conductance in LaAlO$_{3}$/SrTiO$_{3}$ Invited Speaker: Lane Martin There has been great interest in the emergence of novel phenomena at the heterointerface between SrTiO$_{3}$ and LaAlO$_{3}$ including the observation of two dimensional conductivity, superconductivity, magnetism, and more. Despite extensive work in the field there is still a debate about the mechanism for and how to deterministically control these exotic states of matter. In this talk we will explore the implications of variations in cation stoichiometry for the crystal structure, dielectric, thermal, and electronic properties of such materials. We will demonstrate a strong link between the growth process, the stoichiometry of the LaAlO$_{3}$, and the resulting interfacial electrical properties. Varying the La-cation stoichiometry by a few atomic percent in films grown at 1 x 10$^{-3}$ Torr results in a 2 and 7 order-of-magnitude change in the 300K and 2K sheet resistance, respectively, with highly conducting states occurring only in La-deficient films. Further reducing the growth pressure results in an increase of the sheet carrier density and a dramatic change in the carrier mobility. We will highlight the relative contributions of \textit{intrinsic} and \textit{extrinsic} effects in controlling the properties of these heterointerfaces and how these factors are controlled by the growth process. We will explore the effect of the growth process on the evolution from 2D to 3D conductance via high magnetic field transport measurements of the Shubnikov-de Haas effect and will explore the thickness evolution of the conducting interface. Overall we will demonstrate a strong link between the growth process, the stoichiometry of the resulting materials, the desired properties of the system, and the implications for understanding the physics and how to engineer these and other materials including nickelates, ferroelectrics, and more. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q41.00002: Hybrid functional calculations for the electronic properties of LaAlO$_3$ Jimmy-Xuan Shen, Andr\`{e} Schleife, Anderson Janotti, Chris G. Van de Walle The perovskite LaAlO$_3$ is a wide-band-gap (5.5--6.5 eV) insulator of high interest in both applied and fundamental research. On the applied side, LaAlO$_3$ has been considered a candidate to replace SiO$_2$ as a gate dielectric in field-effect transistors due to its high dielectric constant (23--25) and near-perfect lattice matching with Si ($<1$\%). On the fundamental science side, two-dimensional electron gases (2DEGs) with high carrier density have been observed at the interface of LaAlO$_3$ and SrTiO$_3$. The origin of the 2DEG and the dependency of its density on the thickness of the LaAlO$_3$ top layer have been highly debated. Interestingly, many of the basic electronic properties of LaAlO$_3$ are still poorly understood. For example, measurements of the band gap vary by as much as 1 eV. Here, we report the electronic structure of LaAlO$_3$ using density functional calculations with a hybrid functional. We compute the electronic structure and the dielectric function of bulk LaAlO$_3$ in the rhombohedral and cubic phases, and compare the optical properties with experiments. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q41.00003: Bandgap engineering of SrTiO$_3$ via Al-substitution Agham Posadas, Chungwei Lin, Stefan Zollner, Alex Demkov Epitaxial SrTiO$_3$ is was originally envisioned as a replacement gate dielectric for scaled CMOS technology because of its very high dielectric constant of $\sim$300 at room temperature. However, one critical issue that prevented this technology to be developed is the zero conduction band offset with Si making it unsuitable for use as a gate insulator. We have epitaxially grown Al-substituted SrTiO$_3$ on Si using molecular beam epitaxy, replacing 10-20\% of the Ti atoms with Al. We observe a 0.3 eV increase in the band gap by both spectroscopic ellipsometry and electron energy loss spectroscopy. Capacitor structures show a dramatic decrease in leakage current by six orders of magnitude. This approach may allow SrTiO$_3$ to become useful as a gate dielectric on silicon. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q41.00004: Structural and electrical properties of strained La-doped SrTiO$_{3}$ films Miri Choi, Agham B. Posadas, Heidi Seinige, Andrew Kellock, Martin M. Frank, Maxim Tsoi, Alexander A. Demkov Structural and electrical properties of La-doped thin (10nm and 20nm) SrTiO$_{3}$ films are investigated. Films with three different La doping concentrations (5, 15 and 25{\%}) are grown by molecular beam epitaxy. Epitaxially strained La-doped SrTiO$_{3}$ films were grown on four different substrates, LaAlO$_{3}$(100), (LaAlO$_{3})_{0.3}$(Sr$_{2}$AlTaO$_{6})_{0.7}$(100), SrTiO$_{3}$(100) and DyScO$_{3}$(110), in order to have different strain conditions. We compare the structural properties of La-doped SrTiO$_{3}$ films using X-ray diffraction as a function of strain and La dopant concentrations. We also determine the electrical properties of strained La-doped films using Hall measurements, electrical resistivity and mobility at room temperature. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q41.00005: Electronic and optical properties of La,Cr co-doped SrTiO$_{3}$ epitaxial thin films Ryan Comes, Hongliang Zhang, Yingge Du, Robert Colby, Mark Bowden, Timothy Droubay, Scott Chambers SrTiO$_{3}$ (STO) is a wide-gap semiconductor well suited for photocatalytic H$_{2}$ production due to the alignment of its band edges with the half-cell energies of the H$_{2}$O redox reactions. However, the wide optical gap of STO (3.3 eV) makes the material an inefficient light absorber in the visible spectrum, preventing formation of electron-hole pairs needed for photocatalysis. Cr dopants on the Ti site have been shown to reduce the optical bandgap if the Cr ion is in the 3$+$ state. However, charge conservation in Cr-doped STO dictates that a Cr ion must either be in the 4$+$ oxidation state or compensate the 3$+$ state via an oxygen vacancy. In each case, defect electronic states occur, reducing the efficiency of electron-hole pair formation. To compensate this, others have shown that doping STO with La and Cr ions in equal quantities can promote the formation of the Cr$^{3+}$oxidation$^{\, }$state. In this work, we examine the electronic and optical properties of La,Cr-doped STO films grown using oxide molecular beam epitaxy. Films were characterized via \textit{in situ} x-ray photoelectron spectroscopy to measure valence band and core level energies, confirming that most Cr ions are in the 3$+$ state. Optical absorption measurements show that the optical bandgap is reduced by 0.8 eV from that of undoped STO. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q41.00006: Ionic Liquid Gating of SrTiO3 Nanowires Terence Bretz-Sullivan, Allen Goldman In recent years, ionic liquid (IL) field effect gating of complex oxides has revealed novel electronic phases in electronic density regimes not easily attainable by chemical doping or by solid gate dielectric field effect tuning. Specifically, ionic liquid gated Strontium Titanate (STO) serves as an ideal system to study due to its relevance to the LaAlO3/SrTiO3 hetero-interface. Nevertheless, IL gating of nanoscale regions of STO has not been extensively explored. In this talk, the results of IL gated nanowires of STO will be discussed. Nanowires, patterned by electron beam lithography, are defined by a narrow channel of width 100nm in the resist PMMA on top of single crystal STO substrates. The IL is confined to this channel and thus by applying a gate voltage will accumulate electrons at the IL/STO interface, i.e. at the channel floor, by the formation on an electric double layer. Non-linear current-voltage characteristics have been observed using a two-terminal geometry over a set of gate voltages and a temperature range of 2K-35K. These characteristics exhibit behavior similar to Coulomb Blockade physics; however, the possibility of other phenomena has not been ruled out. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q41.00007: Momentum-resolved view of mixed 2D and nonbulklike 3D electronic structure of the surface state on SrTiO$_{3}$ (001) N.C. Plumb, M. Salluzzo, E. Razzoli, M. Mansson, J. Krempasky, C.E. Matt, T. Schmitt, M. Shi, J. Mesot, L. Patthey, M. Radovic The recent discovery of a metallic surface state on SrTiO$_{3}$ may open a route to simplified low-dimensional oxide-based conductors, as well as give new insights into interfacial phenomena in heterostructures such as LaAlO$_{3}$/SrTiO$_{3}$. Our recent angle-resolved photoemission spectroscopy (ARPES) study demonstrates that not only quasi-2D but also non-bulklike 3D Fermi surface components make up the surface state. Like their more 2D counterparts, the size and character of the 3D components are fixed with respect to a broad range of sample preparations. As seen in previous studies, the surface state can be ``prepared'' by photon irradiation under UHV conditions. An extremely high fraction of the surface valence states are affected by this process, especially in relation to the stability of oxygen core level intensity during the same exposure, which points to a key role of electronic/structural changes that spread over the surface as the metal emerges. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q41.00008: Implication of structural changes to ferroelectrity in epitaxially grown PbVO3 thin films Seol Hee Oh, Hye-Jin Jin, Ran Hee Shin, William Jo, Yu-Seong Seo, Jai-Seok Ahn PbVO3 (PVO) having a perovskite-type tetragonal structure is an intriguing polar magnetic material because its structural distortion would be linked to enhanced ferroelectric polarization and considered as a candidate of multiferroic materials. However, ferroelectricity of PVO is not experimentally demonstrated yet as a form of thin-films partly due to its large tetragonality (c/a$=$1.23) compared with proper ferroelectric PbTiO3 (c/a $=$ 1.06) [1]. Polarization of PVO is generated by lone pair of Pb2$+$ ions and estimated to $\sim$ 152 $\mu $C/cm2 [3]. We used laser ablation to synthesize epitaxial PVO thin films on LaAlO3 (001) substrates under argon ambient from a stable Pb2V2O7 sintered target. X-ray diffraction was used to investigate the phase formation and texture of the films. Only under an optimized condition with no oxygen partial pressure, epitaxial growth of the PVO films was possible. We confirmed four-fold symmetry of in-plane alignment of the films on PbVO3 [001] // LaAlO3 [001], but the a- and c- lattice constants of the PVO films show changes due to the compressive stress from the substrates. In addition, surface morphology of the films displays drastic changes in accordance with the growth conditions. Some elongated grains are related to the Pb2V2O7 pyrochlore structure. The relation between structural deformation and ferroelectricity in the PVO films was examined by local measurement of piezoresponse force microscopy. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q41.00009: PZT Films Fabricated by Metal Organic Decomposition Method Vladimir Sobolev, Valeriy Ishchuk High quality lead zirconate titanate films have been fabricated on different substrates by metal organic decomposition method and their ferroelectric properties have been investigated. Main attention was paid to studies of the influence of the buffer layer with conditional composition Pb$_{1.3}$(Zr$_{0.5}$Ti$_{0.5})$O$_{3}$ on the properties of Pb(Zr$_{0.5}$Ti$_{0.5})$O$_{3}$ films fabricated on the polycrystalline titanium and platinum substrates. It is found that in the films on the Pt substrate (with or without the buffer layer) the dependencies of the remanent polarization and the coercivity field on the number of switching cycles do not manifest fatigue up to 10$^{9}$ cycles. The remanent polarization dependencies for films on the Ti substrate with the buffer layer containing an excess of PbO demonstrate an fundamentally new feature that consists of a remanent polarization increase after 10$^{8}$ switching cycles. The increase of remanent polarization is about 50{\%} when the number of cycles approaches 10$^{10}$, while the increase of the coercivity field is small. A monotonic increase of dielectric losses has been observed in all cases. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q41.00010: Ferroelectric properties in PbZrO$_{3}$/BaZrO$_{3}$ superlattices: an \textit{ab-initio} study Nabil Al-Aqtash, Ahmad Alsaad, Renat Sabirianov Properties of tetragonal (BaZrO$_{3})_{1}$/(PbZrO$_{3})_{\mathrm{n}}$ ferroelectric superlattices with n $=$ 1--3 are calculated from first principles within the density functional theory. We show that an antiferroelectric PbZrO$_{3}$ displays ferroelectric behavior if deposited on a paraelectric substrate (BaZrO$_{3})$. We have performed total energy calculations to investigate the origins of the ferroelectricity and analyze the polarization of BaZrO$_{3}$/(PbZrO$_{3})_{\mathrm{n}}$ superlattices as function of PbZrO$_{3}$ thickness. The densities of states (DOS) show that there is a strong hybridization between Zr/Pb and O atoms which play important role in stabilizing the ferroelectric ground state in the superlattices. Our calculations show that the polarization and tetragonality ($c/a$ ratio) are reduced in the BaZrO$_{3}$/PbZrO$_{3}$ superlattices with respect to bulk tetragonal PbZrO$_{3}$. Moreover, the tetragonality and polarizations of superlattices increase with increasing the fraction of PbZrO$_{3}$ in the superlattices. The estimated polarization of the (BaZrO$_{3})_{1}$/(PbZrO$_{3})_{1}$ superlattice is (38.46 $\mu $C/cm$^{2})$, while it is (56.82$\mu $C/cm$^{2})$ for the (BaZrO$_{3})_{1}$/(PbZrO$_{3})_{3}$ superlattice and (74.22$\mu $C/cm$^{2})$ for bulk tetragonal PbZrO$_{3}$ These ferroelectric superlattices have good lattice matching with shape-memory NiMnIn Heusler alloys and could be very useful as a ferroelectric substrate to systems. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q41.00011: Ferroelectric properties of BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O3 bilayer thin film Pavel Salev, Chun Yang, Alexei Grigoriev The thin film ferroelectric BaTiO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ bilayer was epitaxially grown on SrRuO$_{3}$/SrTiO$_{3}$ substrate by RF sputtering. Electrical measurements of polarization switching revealed two different switching regimes -- a small ferroelectric hysteresis loop at low applied voltage and a larger loop at a high voltage. The measured dielectric permittivity corresponds to weak electrostatic coupling between two layers according to Landau-Ginsburg-Devonshire theory. This weak coupling may allow for independent polarization states to exist in individual layers. This can lead to stable head-to-head and tail-to-tail polarization domain configurations, which would explain the two switching regimes observed in electrical measurements. The compensation of polarization gradient across the interface can be explained by the enhancement of interface charge carrier density due to strong bending of electron energy bands. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q41.00012: Combinatorial studies in Ba$_{0.45}$Sr$_{0.55}$TiO$_{3}$ thin films for microwave components by radio frequency magnetron sputtering Fikadu Alema, Aaron Reinholz, Konstantin Pokhodnya The optimization of dielectric properties of ferroelectric thin films for microwave applications can be limited due to the time and resources consumption of the corresponding device fabrication and testing for each doping level. We report the use of a combinatorial technique to achieve the optimal doping level of Ba$_{0.45}$Sr$_{0.55}$TiO$_{3}$ (BST) thin film with three dopants, Mg, Nb and lanthanide (Ln) metal. The process uses two R.F. magnetron sputtering BST sources doped with few at. {\%} of Mg$^{\mathrm{II}}$/Nb$^{\mathrm{V}}$ in charge compensating concentration and Ln$^{\mathrm{IV}}$, respectively. The guns were shifted and tilted each by 30$^{\circ}$ in opposite directions to realize the dopants gradient across a static wafer. The film is reactively co-sputtered on the static 4'' platinized Al$_{2}$O$_{3}$ wafer. The film crystallinity and phase purity were analyzed and correlated to its dielectric properties measured on 2432 MIM capacitors that are of lithographically fabricated using Pt top electrode. After electrical testing, the wafer was diced into 22 16x16 mm$^{2}$ samples, and the elemental analysis of each piece was performed. The correlation between the composition and dielectric properties was established and the optimal dopant concentrations for obtaining maximum tunability of 75{\%} and minimum loss of 0.02 were determined. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q41.00013: Interplay of the strain and microstructure in ferroelectric epitaxial CaTiO3 Films Qian He, Qiao Liang, Michael Biegalski, Albina Borisevich CaTiO3 (CTO) was predicted to become ferroelectric under lattice strain. However, other factors such as oxygen octahedral tilts or microstructural details can play a role. In this work, two 20 nm CTO films were grown on LSAT and NGO by PLD. They both show ferroelectricity, with Tc near 140 K on LSAT and near 70 K on NGO, and the remnant polarization at 10K of 5 and 2 $\mu $C/cm, respectively. This is surprising given that the strain of CTO on both substrates is similar. AC-STEM shows two major differences in microstructure between two CTO films: Firstly, the first few nm of CTO on NGO show perfect epitaxial growth, and after that grains start to develop, but the c-axis of CTO remains aligned with the c-axis of NGO, suggesting the presence of 180$^{\circ}$ grain boundaries only. However for CTO/LSAT, grains begin to develop at the interface and their c-axes have two possible orientations, resulting in both 180$^{\circ}$ and 90$^{\circ}$ grain boundaries. These grain boundaries are either dislocations or ferroelastic twins. Secondly, the octahedral tilt behavior at the film/substrate interface is different: CTO/LSAT has a 5-6 unit cell transition region from the untilted LSAT to the tilted CTO, which is not the case in CTO/NGO. The connection between the microstructure, substrate strain and connections to the ferroelectric properties will be discussed in detail. [Preview Abstract] |
Session Q42: Focus Session: Topological Materials, Synthesis and Characterization
Sponsoring Units: DMPChair: Ale Lanzara, Lawrence Berkeley National Laboratory
Room: Mile High Ballroom 4A
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q42.00001: High quality topological insulator Bi2Se3 thin film growth on amorphous SiO2 using MBE Nikesh Koirala, Namrata Bansal, Matthew Brahlek, Seongshik Oh We report on the growth of Bi2Se3 thin films on amorphous SiO2 by molecular beam epitaxy method. These films show high c-axis crystalline order and sharp interface with SiO2 substrate. ARPES measurement shows topologically protected surface states in these films. Comparison among Bi2Se3 thin films grown on Al2O3(0001), Si(111) and amorphous SiO2 shows that films grown on amorphous SiO2 are much better than those grown on Si(111) and roughly comparable to those grown on Al2O3 in terms of their electrical transport properties. This suggests that chemical inertness of substrate is more important than lattice matching in determining the transport properties of Bi2se3 films. Additionally, our study on a thinner film shows significant modulation of longitudinal resistivity and sign reversal of hall coefficient with application of gate voltage indicating tunablility of chemical potential through Dirac point. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q42.00002: High-quality Bi$_{2}$Se$_{3}$ MBE grown films from a cracked selenium source Brian Mulcahy, Gustaf Olson, Victor Chua, Edmond Chow, James N. Eckstein Over the past several years, the topological insulator Bi$_{2}$Se$_{3}$ has become a favorite among experimentalists due to its relatively large bulk band gap and simple electronic structure near the Fermi energy. However, the energy cost for an anti-site defect or selenium vacancy is not large compared to growth temperature. These defects dope the material and lead to bulk conductance which masks the role of the metallic surface states in transport. We report on Bi$_{2}$Se$_{3}$ thin films grown using a thermally cracked selenium source by MBE. Cracked selenium molecules are more reactive and this reduces the defect density and the bulk carrier density. Transport measurements show an asymptotic sheet resistance at T=0 more than an order of magnitude larger than what is obtained from films grown with a standard selenium source. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q42.00003: Magnetotransport measurements on Mn-doped Bi$_{2}$Se$_{3}$ Thin Films Sercan Babakiray, Trent Johnson, Pavel Borisov, David Lederman The intrinsic n type conductivity of bulk topological insulator Bi2Se3 was compensated with Mn dopant to increase the resistivity. In addition, the magnetic character of Mn ions causes a gap opening of the corresponding Dirac cone surface states. We investigated the effect of the Mn on crystal structure as well as the transport and magnetic properties of Bi2-xMnxSe3 thin films grown by molecular beam epitaxy on Al2O3 (0001) substrates. Characteristic features in the form of the Kondo effect and weak anti-localization were observed at different Mn concentrations up to temperatures of 50 K accompanied by enhanced resistance and reduced carrier mobility. The phase coherence length of the two-dimensional sheet conductance decreased with increasing Mn-concentration, however the protected surface states were still present up to x$=$0.063. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q42.00004: Transfer of large-area MBE grown BiSefilms to arbitrary substrates Namrata Bansal, Myung R. Cho, Matthew Brahlek, Nikesh Koirala, Yoichi Horibe, Jing Chen, Weida Wu, Yun D. Park, Seongshik Oh Mechanical exfoliation of bulk crystals has been widely used to exfoliate thin topological insulator (TI) flakes to fabricate devices such as field-effect transistors. However, such a process produces only micro-sized flakes that are highly irregular in shape and thickness. In this work, we developed a process to transfer the entire area of TI Bi2Se3 thin films grown epitaxially on Al2O3 and SiO2 to arbitrary substrates. Ultrathin films of 4 quintuple layers (1 QL $=$ 1 nm) with 1 cm x cm lateral size have been successfully transferred with no defects or cracks, as observed by optical microscopy. Transport measurements on the transferred films show that this process yields films with lower carrier concentrations and comparable or higher mobilities than before the transfer. Atomic force microscopy and transmission electron microscopy further confirm the pristine morphology and crystallinity of the transferred films. Furthermore, utilizing this process, we show that as the Fermi level is tuned into the proximity gap at the Dirac point of an ultrathin film, the film makes a clear metal-insulator transition with more than four orders of resistance change. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q42.00005: Growth and characterization of molecular beam epitaxial Bi2Se3 films and heterostructures Zhiyi Chen, Thor Garcia, Joel Jesus, Lukas Zhao, Haiming Deng, Jeff Secor, Milan Begliarbekov, Lia Krusin-Elbaum, Maria Tamargo Significant bulk conduction in the bulk of topological insulators (TIs) has been a major challenge in the studies of the their spin-helical Dirac surface conduction channels, a problem particularly severe in charge transport. Growth of high quality low-carrier concentration TI films is crucial not only for the fundamental study of TIs, but also for manufacture of heterostructures and devices. Here we report our results on synthesis and characterization of high-quality Bi$_2$Se$_3$ films grown using molecular beam epitaxy (MBE). A superior surface topography (smoothness) of the MBE Bi$_2$Se$_3$ films was obtained by a suitable choice of buffer layers used. A precise control of layer thickness was achieved and layers with good uniformity and surface quality were obtained. Hall measurements showed the films to be \textit{n}-type, with sheet carrier concentrations typically in the $6\sim9\times10^{12} \textrm{cm}^{-2}$ range. Using optimal growth conditions for the best quality Bi$_2$Se$_3$ films, magnetically doped Bi$_2$Se$_3$ and heterostructures such as Bi$_2$Se$_3$/ZnSe were also grown and characterized in transport and with optical measurements. Novel magnetically ordered insulating state induced by magnetic doping, and exotic effects at the interfaces will be presented. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q42.00006: Toward Insulating Behavior in Bi$_2$Se$_3$ Paul Syers, Johnpierre Paglione Research in the area of Topological Insulators has made great progress with Bismuth Selenide in recent years. However, achieving true insulating behavior in bulk samples of Bi$_2$Se$_3$ has proven elusive due to the difficulty in controlling the stoichiometry of this compound during synthesis. Here we report on progress with the synthesis and characterization of high purity, undoped Bi$_2$Se$_3$ crystals with the lowest carrier densities and highest resistivities reported to date. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q42.00007: High-pressure phases of Bi$_2$Se$_3$ C. Stephen Hellberg, Jason R. Jeffries, Igor I. Mazin, Steve M. Young, Nicholas P. Butch, Kevin Kirshenbaum, Paul S. Syers, Johnpierre Paglione We present x-ray diffraction measurements and density functional calculations of Bi$_2$Se$_3$ at pressures up to 80 GPa. Four phases are observed as the pressure is increased. The diffraction pattern of each phase agrees well with diffraction patterns computed from structures determined by density functional theory. The electronic structure of the high-pressure phases will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q42.00008: Epitaxial growth of Bi$_2$Te$_3$ on the Ferromagnetic Insulator Cr$_2$Ge$_2$Te$_6$ L.D. Alegria, H. Ji, N. Yao, R.J. Cava, J.R. Petta We report the experimental realization of a new topological insulator-ferromagnetic insulator (TI-FI) material system: the Cr$_2$Ge$_2$Te$_6$-Bi$_2$Te$_3$ heterostructure. The layered chalcogenide FI Cr$_2$Ge$_2$Te$_6$ exhibits a high Curie temperature of 61 K and a resistivity greater than $10^3$ $\Omega$-cm below 77 K, which suit it well for the study of magnetic proximity effects in TI-FI heterostructures. We fabricate heterostructures by growing single crystalline Cr$_2$Ge$_2$Te$_6$ substrates and depositing Bi$_2$Te$_3$ thin films using metalorganic chemical vapor deposition. Cross-sectional transmission electron microscopy reveals a sharp interface along the (0 0 1) planes of the two crystals, with the structures uniformly oriented as Cr$_2$Ge$_2$Te$_6$[1$\bar 1$0]//Bi$_2$Te$_3$[0$\bar 1$0]. The coupling between the Cr$_2$Ge$_2$Te$_6$ and Bi$_2$Te$_3$ layers is studied via the anomalous hall effect. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q42.00009: Bottom up synthesis of topological crystalline insulator SnTe nanostructures with controlled facets and morphologies Zhen Li, Shuai Shao, Nan Li, Kyle McCall, Jian Wang, Shixiong Zhang Tin Telluride (SnTe) has recently been demonstrated to be a topological crystalline insulator with unique metallic surface states protected by crystalline mirror symmetry. The topological surface properties have been predicted to depend on the surface orientation. By combining synthesis experiments and density functional theory (DFT) calculations, we demonstrate the growth of single crystalline nanostructures of SnTe with controlled facets and morphologies. In particular, by tailoring the growth temperature, we obtained two types of single crystalline nanowires: smooth nanowires dominated by \textbraceleft 100\textbraceright facets at high temperatures, and zigzag nanowires composed of both \textbraceleft 100\textbraceright and \textbraceleft 111\textbraceright surfaces at low temperatures. No \textbraceleft 110\textbraceright facet was observed in any of our nanostructures. The experiment results agree well with our DFT calculations of surface energies. Our device fabrication and preliminary electrical characterizations suggest that both types of nanowires are suitable for transport studies. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q42.00010: Searching for ideal Bi-system topological insulator, Pb-system topological crystalline insulator and their topological superconductor Invited Speaker: Genda Gu The discovery of 3D topological insulator materials and topological superconductor open up a new research field in the condensed matter physics. In order to search for the ideal topological insulator, topological crystalline insulator and topological superconductor, we have grown a large number of the single crystals of Bi-system (Bi-Sb-Te-Se) topological insulator Pb-system (Pb-Sn-In-Te-Se) topological crystalline insulator and their topological superconductor. We have measured the physical properties on these single crystals by various techniques. We have studied the effect of crystal growth condition, impurity and composition on the bulk electrical conductivity of these single crystals. We try to find out which composition and crystal growth condition is the best for the ideal topological insulator, topological crystalline insulator and topological superconductor. We have got the bulk topological superconductor with T$_{\mathrm{c}}=$5K. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q42.00011: Charging Dirac states at grain boundaries in the 3D topological insulator Bi$_{2}$Se$_{3}$ Y. Liu, Y.Y. Li, S. Rajput, D. Gilks, V.K. Lazarov, M. Weinert, L. Li Using scanning tunneling microscopy and transmission electron microscopy, we demonstrate the existence of antiphase boundaries between neighboring grains shifted by a fraction of a quintuple layer in the MBE-grown 3D topological insulator Bi$_{2}$Se$_{3}$ (0001) films [1,2]. Scanning tunneling spectroscopy and first-principles calculations further reveal that these boundaries provide electrostatic fields that locally charge the Dirac states, modulating the carrier density, and shifting the Dirac point by up to 120 meV. This intrinsic electric field effect, demonstrated here near interfaces between Bi$_{2}$Se$_{3}$ grains, provides direct experimental evidence at the atomic scale that the Dirac states are indeed robust against extended structural defects and tunable by electric field. \\[4pt] [1] Y. Liu et al. PRL \textbf{108}, 115501 (2012).\\[0pt] [2] Y. Liu et al. PRL \textbf{110}, 186804 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q42.00012: Quantum Transport of Surface State Dirac Fermions of a 3D Topological Insulator Yang Xu, Ireneusz Miotkowski, Jiuning Hu, Tai-lung Wu, Yong P. Chen A three-dimentional (3D) strong topological insulator (TI) has a fully insulating gap in the bulk and topological surface states of gapless Dirac fermions. However it is a great challenge to eliminate bulk conduction and reveal the transport signatures of the Dirac fermion from surface states in real 3D TI materials. By Bridgman method, we have successfully grown high-quality single crystal of 3D TI $BiSbTeSe_2$ with very low bulk carrier density (p type, less than $1.5\times$$10^{15}$ $cm^{-3}$) and high surface mobility (above 1000 $cm^{2}/Vs$ at low temperature). The insulating bulk and dominated surface conduction are confirmed by transport measurements of samples with various thicknesses (20 nm to 52 $\mu$m). In high magnetic fields (up to 31 T), we studied quantum oscillations and quantum Hall transport from topological surface states in exfoliated flake devices on $SiO_2/Si$ substrates, where the density of the bottom surface can be tuned by a back gate voltage. Our experiements reveal an intrinsic 3D TI material and paves the way for further application of topological quantum electronics. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q42.00013: Electronic structure study of Topological Insulator Bi$_{2}$Se$_{3}$ on graphene substrates Xueping Jiang, Bhanu Mahanti, Saroj Nayak In recent experiments, graphene has been found to be a suitable substrate for Topological Insulators (TIs) due to better lattice match and smaller defect density. Recent studies have also shown that the presence of a substrate can modulate the electronic properties of TIs. One system that has not been well studied is Bi2Se3 supported by graphene. In this work, density functional theory has been used to study the electronic structure of such a heterostructure. Six quintuple layers (QLs) and one QL were considered for Bi2Se3. When six QLs of Bi2Se3 are supported by graphene on either one or both sides, the Dirac cone of the TI is shifted below the Fermi energy and a band gap is opened in graphene. In addition, the influence of the graphene substrate on the topological surfaces states is negligible. The four-fold degeneracy around the Dirac point for Bi2Se3 is maintained when the TI is supported on both sides by graphene, but is split when graphene is deposited on only one side. For one QL Bi2Se3 the electronic structure near the band gap was strongly perturbed due to the interaction with graphene orbitals. These results will be compared with other works of TIs on substrates. [Preview Abstract] |
Session Q43: Metals II
Chair: David Parker, Oak Ridge National LaboratoryRoom: Mile High Ballroom 4B
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q43.00001: Possible Superconductivity in the 2-Dimensional Electride Ca$_{2}$N Jeonghoon Ha, Hongwoo Baek, Duming Zhang, Yeji Kim, SungWng Kim, Young Jae Song, Young Kuk, Fred Sharifi, Joseph A. Stroscio An electride is an ionic compound in which electrons take the place of negative charged ions and the topology of the cavities confining these anionic electrons determines the physical properties of the material. A recent study reported Ca$_{2}$N to have a layered structure with anionic electrons confined to 2-D cavities between the cationic crystal layers. Magneto-resistance measurements confirmed diffusive 2-D transport in electron layers. In the present work, we use an ultra-low temperature STM to investigate the local electronic structure of a cleaved surface of a Ca$_{2}$N crystal. A small energy gap was observed in the tunneling spectrum with a main gap of 0.4 meV. The spectra contain multiple coherence-like peaks suggestive of possible multi-junction superconductivity. Temperature-dependent measurements show a gradual reduction of the gap up to 2 K, but the gap is not suppressed in the presence of a perpendicular magnetic field up to 14.5T, suggesting that if the material is superconducting, then the upper critical field is extremely large compared to the transition temperatue. This can be understood in the context of recent reports on unconventional superconductivity of chalcogenide compounds. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q43.00002: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q43.00003: Flux Growth of Large Single Crystals of YFe$_{2}$Al$_{10}$ by Nucleation Site Reduction Jedediah Kistner-Morris, Liusou Wu, William Gannon, Meigan Aronson The metallic d-electron compound YFe$_{2}$Al$_{10}$ is near a quantum critical point. Large single crystals of this compound are required for inelastic neutron scattering experiments. We synthesized high quality single crystals via aluminum flux growth. A number of adjustments to the growth procedure were required to optimize crystal quality and size. First, the cooling rate of the flux growth was adjusted to produce a thermodynamically favorable environment for YFe$_{2}$Al$_{10}$ growth, which was found to grow around 920$^{\circ}$C. Second, initial composition of the growths were then optimized to avoid the growth of the binary phases, YAl$_{3}$ and Fe$_{4}$Al$_{13}$, as well as to maximize crystal size and reduce site nucleation. Third, site nucleation was further reduced by polishing the alumina growth crucibles with sandpaper and then etching them with aqua regia. The result after optimization is that individual growths produced three to five polyhedral crystals with single facets up to 9mm in width, and mass of about 700mg. The implemented nucleation site reduction techniques can be applied to other flux systems to increase crystal size and mass. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q43.00004: Structural Evolution in Ni-Zr Liquids and Glasses Cooper Sinai-Yunker Metallic alloys have been shown to possess a wide-range of glass-forming abilities (GFA). While multicomponent alloys tend to have higher GFA than simpler alloys, the underlying principles that govern GFA are relatively unknown. While the atomic arrangements in metallic glasses do not possess well-defined long-range atomic order that characterizes crystalline metals they do demonstrate short- (SRO) and medium-range (MRO) atomic order. Previous studies suggest a link between structural evolution and GFA. In this talk we discuss recent results on the temperature evolution of the atomic structures of Ni-Zr liquids and glasses using \textit{in situ} high-energy synchrotron X-ray diffraction. Ni-Zr is an excellent system to explore due to its simplicity but also because it forms the basis for high GFA multicomponent alloys. By utilizing the beamline electrostatic levitation (BESL) technique, several compositions were prepared in both the equilibrium and supercooled state. Reverse Monte Carlo (RMC) fits were conducted whereby 3-D atomic configurations consistent with experimental data were be generated and subsequently analyzed. The fits were constrained by partial pair-correlation functions generated from \textit{ab initio} molecular dynamics simulations. Quantitative results will be discussed which suggest that Ni-Zr has a high degree of MRO, chemical ordering and is a highly \textit{fragile} system. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q43.00005: Pair interaction model based Kinetic Monte Carlo simulation of oxygen and metal diffusion in Ni-based alloys Dominic Alfonso, DeNyago Tafen Investigation of oxygen and metal diffusion processes in Ni-based alloy is a problem of high relevance in the area of understanding corrosion behavior. We explored the use of combined approach consisting of density functional theory to compute migration barriers and kinetic Monte Carlo method to evaluate long-time diffusivities of oxygen and metals in Ni containing Al and Fe. Pair interaction model was used to evaluate the influence of the local environment on the kinetic parameters. For reference, vacancy mediated self-diffusion in pure Ni was examined. A diffusion prefactor and barrier of Do $=$ 2.4 x 10$^{-5}$ m$^{2}$/sec and Q $=$2.90 eV were predicted in very good agreement with experimental values of 9.8 x 10$^{-5}$ m$^{2}$/sec and Q $=$2.88 eV. Results for (i) Al impurity, (ii) Fe impurity and (iii) concentrated Fe diffusion exhibit good agreement with available experimental data. The model was employed for oxygen diffusion in Ni and it was found that the inclusion of vacancy improves the level of agreement with compilation of experimental studies. The presence of either Al or Fe reduces the diffusivity of oxygen. This was attributed to the increase in the excess energy of oxygen in Ni due to these metals. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q43.00006: High through put computation of defect properties in metals Bharat Medasani, Maciej Haranczyk, Mark Asta We combine first principles density functional method and MPWorks, a high throughput framework, to compute the vacancy formation energies in metals. Three different exchange correlation (xc) functionals, PBE, PW91 and LDA are evaluated with respect to the computed formation energies. Bulk and defect structures are relaxed using a mesh of 108000 k-points X atoms to achieve an accuracy of 10 meV or better. Of the three functionals, LDA gives better results compared to the PBE and PW91 (GGA) functionals due to the cancellation of exchange and correlation errors arising due to the presence of internal void surface. PBE and PW91 predict noticeably different vacancy formation energy values even though the lattice constants and cohesive energies predicted by them are very close. Applying surface error correction brings the formation energies computed with the three functionals closer to the experimental values. The surface correction is in general small for LDA and bigger for GGA functionals. Meta-GGA functionals are expected to predict better surface energies and hence better vacancy formation energies. We report the performance of one such meta-GGA functional, revTPSS. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q43.00007: Magnetic and structural properties of transition metals and their alloys Matteo Cococcioni, Koichiro Umemoto, Burak Himmetoglu In this talk I will discuss the relationship between magnetic and structural properties emerged from recent DFT calculations on transition-metals and their alloys. In the first part I will present results on Ni$_2$MnGa, a prototype magnetic Heusler alloy. I will demonstrate that improving the description of localized electrons on the d states of Mn is crucial to fix both the magnetization and the relative stability of the austenite and martensite phases. The larger energy difference between Hubbard bands obtained from DFT+U also proves fundamental to capture the stabilization of the non distorted austenite in alloys with eccess Mn, in agreement with experiments. In the second part I will report our recent discovery of a new phase of bulk Fe. The new allotrope is characterized by a unit cell of six atoms and a crystal structure based on a ``wavy'' pattern of distorted Fe octahedra. Although always metastable, it is more stable than other known phases (e.g., HCP) and transforms into FCC Fe under pressure. In addition, the distorted crystal structure results in a magnetization density about 10\% higher than that of other allotropes which could disclose interesting applications for this materials, including magnetic steels and rare-earth-free permanent magnets. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q43.00008: The elastic properties and martensitic transformation behaviour of Ti$_{50}$Pt$_{\mathrm{50-x}}$Co$_{\mathrm{x}}$ shape memory alloy Hasani Chauke, Rosinah Mahlangu, Phuti Ngoepe Ti-based shape memory alloys (SMAs) such as PdTi, AuTi and PtTi are important in the design for high temperature alloys due to their high martensitic transformation (Ms) of above 673 K. PtTi is the most attractive for the development of high temperature shape memory alloys (HTSMAs) since it has the highest Ms of about 1273 K. Above 1273 K The crystal structure is the ordered cubic B2 phase, and transforms to an orthorhombic B19 phase at lower temperature. The supercell approach method was used to investigate the effect of partial substitution of Pt with Co on the TiPt potential shape memory alloy. The first-principles calculations were carried out within the generalized gradient approximation to determine the stability of the T$_{\mathrm{i5}}$0P$_{\mathrm{t50-}}$xCo for x=6.25, 18.75 and 25. We found that the calculated heats of formation and density of states predicted the 6.25 at.{\%} Co to be the most stable structures compared. The elastic properties and the phonon dispersion results suggest that the partial substitution of Pt with Co increases the Ms of TiPt with the softening of the ' shear moduli. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q43.00009: Calculating the titanium $\alpha$ to $\beta$ transition using tight-binding molecular dynamics: a comparison of methods Jon Williams, Daniel Finkenstadt, A. Shabaev, N. Bernstein, S.G. Lambrakos, M.J. Mehl In this we analyze the Titanium phase transition between HCP and BCC structures. Tight-Binding Molecular Dynamics Simulations were run with 64 and 216 atom lattices over temperatures ranging from 50 to 1500 K. Analysis of the data included the phonon Density of States, time evolution of the lattice structure and temperature, and the Vibrational Free Energy of the system. We also develop a theoretical model of the transition based on a perturbation in the Harmonic Oscillator using the coherent states representation. In addition to the tight-binding simulations, we use density functional theory with variable cell shape molecular dynamics at a range of temperatures to study the transition. Direct simulations shows the transition between the two structures, and we investigate the use of constrained simulation to calculate their free energy differences. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q43.00010: Anisotropic magnetoelastic coupling and magnetocaloric effect in the Fe$_{5}$Si$_{3}$-type hexagonal single crystal Guixin Cao, Satoshi Okamoto, M.A. McGuire, Junjie Guo, Ling Li, Jieyu Yi, David Mandrus, Matthew F. Chisholm, Brian C. Sales, Zheng Gai The structural, magnetic properties, and the magnetocaloric effect (MCE) of Fe$_{5}$Si$_{3}$ single crystal with trace of Mn and P doping (Fe$_{4.83}$Mn$_{0.16}$Si$_{2.91}$P$_{0.09})$ are investigated. A first-order magneto-elastic transition was found at the magnetic transition temperature $T_{C}$, with the magnetic easy axis lying in the ab plane. While the trace of Mn and P doping in the Fe$_{5}$Si$_{3}$ single crystal was found to increase both the maximum magnetic entropy change and relative cooling power from those in polycrystalline Fe$_{5}$Si$_{3}$ compound, indicating the intrinsic broaden entropy change in larger temperature span. The anisotropy in the MCE between H//ab and H//c is observed, which originates from the anisotropic spin-lattice coupling between the ab plane and the c axis. The density functional theory calculations were performed to gain microscopic insights into the experimental findings. Our results suggest hexagonal Fe$_{5}$Si$_{3}$ system may become a new candidate of giant MCE as La-Fe-Si and Fe-Mn-P-Si systems. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q43.00011: Barocaloric effect in rare earth compounds Nilson Antunes de Oliveira The magnetic barocaloric effect, which is characterized by the isothermal entropy change and adiabatic temperature change upon pressure variation can be a very useful to improve the performance of magnetic refrigerator. In this work, we discuss the barocaloric effect in rare earth compounds. To this end we use a model of interacting localized magnetic moments [1]. In the first part of the work we make a systematic analysis in terms of the model parameters, considering the simplest case whose angular momentum is 1/2. Our calculations show that the behavior of the barocaloric quantities can be normal, inverse or anomalous. In the second part of the work, we apply the model to describe the barocaloric effect in the compounds RCo$_{2}$, Gd$_{5}$Si$_{2}$Ge$_{2}$ and Tb$_{5}$Si$_{2}$Ge$_{2}$. Our theoretical calculations for Gd$_{5}$Si$_{2}$Ge$_{2}$ is in a reasonable agreement with the available experimental data [2]. Our calculations for the other compounds, need experimental data to be confirmed. \\[4pt] [1] N. A. de Oliveira, P. J. von Ranke Phys. Rep. 489, 89 (2010).\\[0pt] [2] Yuce et al . Appl. Phys. Lett., 101, 071906 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q43.00012: Greater-than-bulk melting temperatures explained: Gallium melts Gangnam style Nicola Gaston, Krista Steenbergen The experimental discovery of superheating in gallium clusters [1] contradicted the clear and well-demonstrated paradigm that the melting temperature of a particle should decrease with its size. However the extremely sensitive dependence of melting temperature on size also goes to the heart of cluster science, and the interplay between the effects of electronic and geometric structure. We have performed extensive first-principles molecular dynamics calculations, incorporating parallel tempering for an efficient exploration of configurational phase space. This is necessary, due to the complicated energy landscape of gallium. In the nanoparticles, melting is preceded by a transitions between phases. A structural feature, referred to here as the Gangnam motif, is found to increase with the latent heat and appears throughout the observed phase changes of this curious metal. We will present our detailed analysis of the solid-state isomers, performed using extensive statistical sampling of the trajectory data for the assignment of cluster structures to known phases of gallium. Finally, we explain the greater-than-bulk melting through analysis of the factors that stabilise the liquid structures.\\[4pt] [1] G. A. Breaux, et al, Phys. Rev. Lett. 91, 215508 (2003) [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q43.00013: GW quasiparticle calculations with spin-orbit coupling for the light actinides Towfiq Ahmed, Robert C. Albers, A.V. Balatsky, Christoph Friedrich, Jian-Xin Zhu We report on the importance of GW self-energy corrections for the electronic structure of light actinides in the weak-to-intermediate coupling regime. Our study is based on calculations of the band structure and total density of states of Np, U, and Pu within a one-shot and spin-orbit coupling enabled formulation of the GW approximation within a full potential LAPWframework. We also present RPA screened effective Coulomb interactions for the f -electron orbitals for different lattice constants, and show that there is an increased contribution from electron-electron correlation in these systems for expanded lattices. We find a significant amount of electronic correlation in these highly localized electronic systems. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q43.00014: First principles calculations on Ga stabilized $\delta $-Pu alloys Sarah C. Hernandez, Daniel S. Schwartz, Christopher D. Taylor, Asok K. Ray The high temperature face-centered-cubic phase ($\delta )$ of plutonium (Pu) may be stabilized at room temperature with the inclusion of impurities, such as gallium (Ga) or aluminum. The addition of Ga within the $\delta $-phase influences the structural and electronic properties of Pu. Using the full-potential linearized augmented plane-wave density functional theory based method we present a systematic study of bulk Pu-Ga alloys. The goals of these calculations are to understand the evolution of the structural and electronic properties of Ga in a $\delta $-Pu lattice. A 32-atom $\delta $-Pu supercell was used to study Pu-Ga alloys at 3.125, 6.25, and 9.375 at. {\%} Ga concentrations. We observe that regardless of the Ga concentration, the magnetic ground state predicted was anti-ferromagnetic, which is contrary to experimental results. However, the equilibrium lattice constants decrease with increasing Ga concentration, which is in agreement with experimental observations. Furthermore, when more than one Ga impurity is present within the supercell, the Ga atoms prefer to be at third nearest neighbor distance. The local effects of the bond lengths around the Ga atom, formation energies and partial density of states (PDOS) in the lowest energy structures will be discussed in detail. PDOS illustrates a Pu \textit{6d} and Ga \textit{4p} hybridization. Finally, we will also discuss the effects of a hydrogen-vacancy complex within a bulk Pu-Ga alloy. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q43.00015: \textit{Ab }initio study of the electronic and structural properties of Pu-Al alloys Jaime L. Sterrett, Sarah C. Hernandez Existing only at higher temperatures, $\delta $-plutonium (Pu) can be stabilized at room temperature when doped with the so-called $\delta $-stabilizers, which are impurities from the IIIA group, such as gallium or aluminum (Al). This is ideal since the $\delta $-phase is the preferred phase used for technological applications. Using density functional theory, we modeled a 32 atom Pu supercell doped with Al concentrations of 3.125, 6.25, and 9.375 atomic percent. The results to be presented will include energy versus volume curves (E-V) for fixed atomic positions performed at the non-magnetic, ferro-magnetic, and anti-ferromagnetic spin structures. Further optimizations of the internal positions of the lowest energy E-V structures will be discussed. The effects of Al within the $\delta $-Pu lattice, particularly changes in the lattice constants, bulk modulus, preferred location of Al, and the bond lengths between Al and the first nearest neighbor Pu atoms will be discussed in detail. The electronic interactions, specifically the Pu-Al hybridizations, will also be discussed by analyzing the partial electronic density of states. Finally, we will the present relaxation and formation energies of the Pu-Al systems. [Preview Abstract] |
Session Q44: Materials: Synthesis, Growth, & Processing
Sponsoring Units: FIAPRoom: Mile High Ballroom 4C
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q44.00001: PLD growth of multilayered MgO/Ag(001)/MgO photocathode Daniel Velazquez, Zikri Yusof, Linda Spentzouris, Jeff Terry Films of of Ag, MgO and multilayers of these were grown via pulsed laser deposition on clean Si(111) 7x7 substrates. The films were studied using reflection high-energy electron diffraction, Kelvin probe and ellipsometry. Information about crystalline and atomic structure as well as surface condition, work function and film thickness was obtained using these techniques. Deposition at various substrate temperatures and partial oxygen pressures was performed in order to understand the parameter settings that lead to higher quality crystalline films. Epitaxial films of Ag(111) were found to grow at an optimal substrate temperature of 256 $^{\circ}$C (fig 1.). The superstructure Ag(111) $\surd $3 x $\surd $3 occurs when deposition takes place at a substrate temperature of 620 $^{\circ}$C. In addition, MgO films were found to grow with small grain size on both, Si(111) 7x7 and Ag(111)/Si(111) at room temperature with a partial oxygen pressure of 5x10$^{-5}$ Torr (fig. 2). Highly-oriented, polycrystalline growth of MgO films is evidenced by their RHEED pattern. In addition, the obliquely-shaped diffraction spots indicate the growth of secondary phase precipitates, most likely due to oxygen deficit. Measurements of the work function of these multilayers indicate that the Ag(111) work function (4.75 eV) is sharply suppressed with the first few MgO shots and has a quasi-linear increase for the first few monolayers (fig. 3). As the thickness of MgO increases (a few nanometers) the work function drops again and stabilizes at the level of MgO ($\sim$ 4.2 eV). [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q44.00002: Light-stimulated epitaxy of GaAs(100) Charlotte E. Sanders, Daniel A. Beaton, Kirstin Alberi Light-stimulated epitaxy offers the potential to improve adatom kinetics at low growth temperatures. In II-VI semiconductors it has been shown to improve crystalline quality (i.e., to reduce defect density), to alter the growth rate, and to enhance substitutional dopant incorporation. These effects have been attributed primarily to (1) direct interaction between photons and adatoms at the growth front, and (2) participation of photogenerated carriers in bonding processes. Although these proposed mechanisms are presumably applicable to a wide array of epitaxially grown materials systems, the photoassisted approach has received little attention outside the context of II-VI growth. We report on our recent investigation of the effects of irradiation of GaAs(100) during growth by molecular beam epitaxy (MBE). The interaction of light with the growth front provides significant insight into fundamental mechanisms of dopant incorporation, and into the physics of adatom diffusion, nucleation, and desorption underlying the MBE process. This work was supported by the DOE Office of Science, Basic Energy Sciences, under contract DE-AC36-08G028308. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q44.00003: Molecular Beam Epitaxy Growth of GaBi, InBi and InGaBi B. Keen, R. Makin, P.A. Stampe, R.J. Kennedy, L.F.J. Piper, B. McCombe, C.F. McConville, S.M. Durbin Recent interest in bismuth alloys of III-V semiconductors for infrared and far-infrared device applications, specifically GaAsBi and InAsBi, has indicated that further study of the III-Bi family of binary compounds would be of great help in improving the quality of these material systems. While immiscibility issues have so far frustrated the growth of GaBi and AlBi, InBi is less problematic, and we have grown it by molecular beam epitaxy on (001) GaAs substrates. However, regions of varying composition exist across the substrate due to poor wetting of the surface. In an effort to improve film quality we have continued to refine the growth parameters by adjusting substrate temperature, beam flux ratio, and deposition rate. Characterization of these films has been performed by x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS). Additionally, we have explored growth of GaBi and In$_{1-x}$Ga$_{x}$Bi at low Ga mole fractions, and modeled this using molecular dynamics simulations. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q44.00004: Effects of incident short wavelength (UV) light on the morphology of MBE grown GaAs Daniel A. Beaton, Charlotte Sanders, Kirstin Alberi The exploration of novel semiconductor materials increasingly relies on growth techniques that operate far from equilibrium in order to overcome thermodynamic limitations to synthesis. As one example, low temperature molecular beam epitaxy (MBE) offers a pathway to enhance substitutional dopant incorporation over surface segregation but adatom mobility suffers as a consequence and leads to higher concentrations of lattice defects. We explore the use of external stimuli, namely incident UV light, as a means to influence adatom kinetics; UV light is absorbed in the first few atomic layers of the as-growing epitaxial film and the effects of the incident radiation predominantly effect only the surface adatoms. GaAs homoepitaxy by MBE is studied as a model case as a function of illumination conditions under broadband Xe and KrF excimer laser irradiation. In-situ reflective high energy electron diffraction analysis paired with ex-situ atomic force microscopy measurements yields insight into the effects of photon irradiation on surface adatom mobility, morphology and smoothing processes. This work was supported by the DOE Office of Science, Basic Energy Sciences under contract DE-AC36-08GO28308. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q44.00005: The origin and distribution of phosphorus in large size HP-Ge crystals Guojian Wang, Hao Mei, Gang Yang, Jayesh Govani, Mianliang Huang, Yutong Guan, Dongming Mei The high-purity germanium (HP-Ge) crystals with 12 cm in diameter were grown by the Czochralski method in highly pure hydrogen (6N) atmosphere. Phosphorus is one of the important shallow level donors in the grown HP-Ge crystals. The radial and axial distribution of phosphorus in the grown crystals was studied using Hall Effect and Photo-thermal ionization spectroscopy (PTIS). The effect of pulling rate and rotation speed on segregation coefficient of phosphorus in HP-Ge was investigated. The origin of phosphorus was analyzed. We report the results in this paper. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q44.00006: Investigation of influential parameters for zone-refinement of germanium crystals Gang Yang, Jayesh Govani, Yutong Guan, Mianliang Huang, Hao Mei, Guojian Wang, Dongming Mei In zone-refining of high-purity germanium crystals, the influential parameters include vacuum level, container of germanium ingot, ambient gases, zone travel speed, zone length, etc. In the present work, the influences of zone length and zone travel speed on the purity level of the zone-refined ingot have been investigated with many experiments. The impurity level in the zone-refined ingot was characterized by van der pauw hall measurement. The shallow impurities are measured with a photothermal ionization spectroscopy (PTIS), which identifies existence of boron, aluminum and phosphor as three main impurities, in the zone-refined germanium ingot. Utilizing the multiple experiments, we have optimized the zone length and zone travel speed. We demonstrate our experimental results with solidification theory of metals. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q44.00007: Dislocation distribution in large high-purity germanium crystal Hao Mei, Guojian Wang, Dongming Mei, Mianliang Huang, Gang Yang, Yutong Guan We investigated the impacts of growth rate, time-temperature profile, thermal gradient on the dislocation distribution in large high-purity germanium crystal (12 cm in diameter) grown via Czochralski along \textless 100\textgreater orientation. The time-temperature profiles of the crystal grown at different input power were investigated using direct measurements and computational modeling. The effect of crystallization speed on dislocation density is discussed from the context of thermal gradient during growth. Several samples from the grown crystals were used for this investigation. We measured dislocation density across the entire cross-section of the grown crystal through the microscope. By measuring and calculating the dislocation density, we were able to identify the denseness and the type of dislocation, which allows us to study how the thermal stress impacts the dislocation generation and distribution across the large grown crystals. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q44.00008: Characterization of three planar germanium detectors fabricated with the crystals grown at USD Hossain Nazir, Mianliang Huang, Muhammad Khizar, Dongming Mei, Guojian Wang, Hao Mei, Yutong Guan We characterized the performance of planar germanium detectors developed in the University of South Dakota (USD). The planar detectors were made from high purity germanium crystals with amorphous germanium contacts. These detectors were developed possible for the neutrinoless double beta-decay measurements and dark matter search underground. They were tested in a temporary cryostat to investigate the depletion voltage, leakage current, efficiency and resolution using a $^{60}$Co $\gamma $ ray source. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q44.00009: Compositional and Surface Effects of Bismuth Incorporation in GaSb Films C. Ryan Tait, Adam Duzik, Joanna Millunchick III-V-Bi semiconductor films represent a new class of highly mismatched alloys that exhibit interesting properties including large reduction in band gap, giant spin orbit bowing, and preserved electron mobility at the expense of hole mobility. These compounds have proven difficult to grow with most results coming from experimentation with GaAsBi with little known regarding GaSbBi. Various growth conditions were tested for GaSbBi and characterized with scanning electron microscopy, x-ray diffraction, and Rutherford backscattering spectroscopy. The films demonstrate Bi concentrations of up to 12{\%} with as low as 3{\%} droplet coverage. Surface Bi and Ga droplet morphology was shown to be dependent on relative flux ratios of Ga, Sb, and Bi and independent of film growth rate. Additionally it is found that As incorporates into the films with no intentional source and the incorporation being dependent on Bi incorporation. This effect is identified as an auto-compensation mechanism for the strain induced from the introduction of Bi. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q44.00010: Fabrication of quantum wire circuits from MBE-grown InAs Carolyn Kan, Chi Xue, James Eckstein If topological quantum computing using Majorana fermions evolves past tabletop experiments involving manipulation of a small number of qubits, scalability of quantum circuits will become an important consideration. At present, experiments have focused on vertically grown nanowires, which must be laid out on a substrate and electrically contacted ex situ. This severely limits the flexibility of the device geometry due to randomness in how the wires fall, and thus hinders the future scalability of the nanowire architecture. We are fabricating quantum wire circuits by lithographic methods out of thin films grown by MBE. We examine high quality MBE-grown InAs material grown GaAs and GaSb substrates, and consider its potential in building Majorana circuits. In particular, better epitaxy and transport is obtained in films grown on lattice matched GaSb, and substrate conductivity appears to freeze out at low temperatures. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q44.00011: The effect of bismuth surfactants on the group V incorporation in InAsSb Evan Anderson, Wendy Sarney, Stefan Svensson, Adam Lundquist, Joanna Millunchick, Chris Pearson Bismuth is a well known surfactant for strained heteroepitaxial growth in compound semiconductors, however, its effect on the incorporation of different species in alloy systems is not well understood. In this work, we investigate the role of a Bi surfactant on the composition and morphology of lattice-matched InAsSb/GaSb as a function of Bi flux and growth temperature. Rutherford Backscattering Spectrometry confirmed that no Bi was incorporated for Bi fluxes up to 4.94e-7 torr. High resolution x-ray diffraction (XRD), on the other hand, showed that the Sb composition decreased with increasing Bi flux or increasing substrate temperature. XRD also indicates that, for a constant Bi flux, Bi has a diminishing effect on film composition as the substrate temperature is increased. Atomic Force Microscopy shows that the surface RMS roughness tended to increase with increasing Bi flux, though this may be due to the fact that these films are further from the lattice matching condition due to reduced Sb incorporation. We speculate that the decrease in Sb incorporation may be a result of a stronger As-Bi interaction energy. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q44.00012: Effect of annealing on $M$-plane GaN thin films grown by PAMBE on tilt-cut LAO substrate Yu-Chiao Lin, Ikai Lo, Ying-Chieh Wang, Cheng-Da Tsai, Chen-Chi Yang, Shuo-Ting You, Ming-Chi Chou The non-polar GaN thin film is a potential candidate for high-efficient photoelectric devices. In this work, we analyzed the characteristics of \textbf{\textit{M}}-plane GaN thin films which were grown on tilt-cut LiAlO$_{\mathrm{2}}$ (LAO) substrate by plasma-assisted molecular beam epitaxy (PAMBE). A series of samples were grown with different N/Ga flux ratios. The crystal structure and optical property of GaN samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), and photoluminescence (PL) measurements. The peak of 32.2$^{\mathrm{o}}$ in the XRD measurement showed the [1\underline {1}00] oriented (\textbf{\textit{M}}-plane) for the GaN samples. To improve the crystal quality, we performed the thermal treatment by rapid thermal annealing (RTA) system on these samples and analyzed the crystal structure, surface morphology and optical property of the samples after thermal treatment. The effect of annealing on the \textbf{\textit{M}}-plane GaN thin films was under investigation. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q44.00013: Growth of yellow-green In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN quantum wells by plasma-assisted molecular beam epitaxy Chia-Hsuan Hu, Ikai Lo, Wen-Yuan Pang, Cheng-Hung Shih, Yu-Chi Hsu, Ying-Chieh Wang, Chen-Chi Yang In order to achieve yellow-green In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN quantum-wells, we grew the In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN quantum-wells (QWs) with high indium concentrations by the insertion of an InGaN buffer layer between the QW and GaN template using plasma-assisted molecular beam epitaxy. The InGaN buffer layer was grown with a gradient indium content, in which the lattice mismatch between In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN QW and GaN template can be reduced. The optical properties of the samples were analyzed by photoluminescence measurement at room temperature. The wavelength of InGaN/GaN quantum-well is shifted from 495nm to 560nm. The XRD and SEM measurement results show the high quality crystalline and smooth surface. Therefore, the InGaN buffer layer with gradient indium content provides an effective way to reach high indium incorporation for high quality yellow-green In$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$N/GaN quantum-wells. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q44.00014: Photoluminescence study of Be acceptors in GaInNAs epilayers Y. Tsai, B. Barman, T. Scrace, A. Petrou, M. Fukuda, I.R. Sellers, M. Leroux, M.A. Khalfioui We have studied the photoluminescence (PL) spectra from MBE grown GaInNAs epilayers doped p-type with Beryllium acceptors. The measurements were carried out in the 5 K -- 70 K temperature range and in magnetic fields (B) up to 7 tesla. The PL spectra contain two features at T $=$ 5 K: The exciton at 1093 meV [2] and a second broader feature at 1058 meV. The intensity of this feature decreases with increasing temperature and disappears completely by 70K while the excitonic feature persists. The emission at 1058meV is identified as the conduction band to Beryllium acceptor transition. If we take into account the binding energy of the exciton [3] we get a value of 23 meV for the Beryllium acceptor binding energy. The acceptor related transition was studied as a function of magnetic field; the energy of this transition has a linear dependence on B with a slope of 055 meV/T. [2] Y.Tsai et al, Appl Phys Lett \textbf{103}, 012104(2013) [3] K.Kashima et al., Jpn. J. Appl. Phys. \textbf{50}, 06GH09(2011) [Preview Abstract] |
Session Q45: Bionanotechnology and Applications of Polymers and Biomaterials
Sponsoring Units: FIAPRoom: Mile High Ballroom 4D
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q45.00001: Electrochemical impedance spectroscopy for graphene surface modification and protein translocation through the chemically modified graphene nanopore Purushottam Tiwari, Yuping Shan, Xuewen Wang, Yesim Darici, Jin He The multilayer graphene surface has been modified using mercaptohexadecanoic acid (MHA) and 1,2-dimyristoyl-sn-glycero-3-phosphoethanolamine-N-[methoxy(polyethylene glycol)-750] (DPPE-PEG750). The surface modifications are evaluated using electrochemical impedance spectroscopy (EIS). EIS measurements show the better graphene surface passivation with DPPE-PEG750 than with MHA. After modification with ferritin, the MHA modified surface shows greater charge transfer resistance (R$_{ct})$ change than DPPE-PEG750 modified surface. Based on these results the translocations of ferritin through modified graphene nanopore with diameter 5-20 nm are studied. The translocation is more successful through DPPE-PEG750 modified graphene nanopore. This concludes that that the attachment of ferritin to DPPE-PEG750 modified graphene nanopore is not significant compared to MHA modified pore for the ferritin translocation hindrance. These results nicely correlate with the EIS data for respective R$_{ct}$ change of ferritin modified surfaces. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q45.00002: Preparation and characterization of functionalized single walled carbon nanotubes (fSWCNT)/ Hydroxyapatite (HAp)-Nylon hybridized composite biomaterial to study the mechanical properties Suraj Khanal, Theodora Leventouri, Hassan Mahfuz, Adam Rondinone Synthetic hydroxyapatite (HAp) bears poor mechanical properties that limit its applicability in orthopedics. We study the possibility of overcoming such limitations by incorporating functionalized single walled carbon nanotubes (fSWCNT) in a biocompatible/bioactive nano-composite. We present results from synthesis and characterization of samples prepared under different processing parameters. Ultra sonication method was to disperse functionalized single walled carbon nanotubes (fSWCNT) in HAp followed by a simple hot assorting method to incorporate with polymerized $\varepsilon $-caprolactam. The fracture toughness of the composite materials was tested in compliance with the ASTM D-5045 standard. We have found that while the fracture toughness strongly depends on the processing parameters, a value comparable to the one for cortical bone is achieved. Mechanical properties, electron microscopy and crystal structure properties of the composite materials will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q45.00003: Enzyme Entrapment in Polyaniline Biosensors Observed via Fluorescence Anisotropy and Antiquenching Louis Nemzer The entrapment of oxidoreductase enzymes within polyaniline polymer films by inducing hydrophobic collapse using phosphate buffered solution (PBS) has been shown to be a cost-effective method for fabricating organic biosensors. Here, we use fluorescence anisotropy measurements to verify enzyme immobilization and subsequent electron donation to the polymer matrix, both prerequisites for an effective biosensor. Specifically, we observe a three order of magnitude decrease in the ratio of the fluorescence to rotational lifetimes. The observed fluorescence antiquenching supports the previously proposed model that the polymer chain assumes a severely coiled conformation when exposed to PBS. We also find that this collapse is further aided by the enzyme itself. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q45.00004: High throughput transfection of cells: nano-electroporation and mobile magnetic traps M. Howdyshell, D. Gallego-Perez, G. Vieira, V. Malkoc, L.J. Lee, R. Sooryakumar Injection of drugs or genes in vitro into cells is a critical technique for biomedical research; there are currently a number of techniques to perform such injections, but drawbacks include lack of control over dosage rates and sustained cell viability, as well as inability to inject into many cells in parallel. We have previously demonstrated a magnetically actuated nano-channel electroporation technique that multiplexes simultaneous transfection of biomolecules into cells by combining an array of remotely operated micro-magnetic traps with a nano-channel electroporation device. This device allows us to control the dosage delivered to each individual cell and reduce cell death during the experiment. The magnetic traps enable precise positioning of magnetically labeled cells and subsequent relocation of the cells for downstream processing. With this integrated approach, the number of cells transfected simultaneously has been increased nearly tenfold. In the current work, we present recent experiments with different types of cells as well as new multiplexed nano-electroporation device designs that are more high-throughput to streamline the parallel injection process, allowing the device to be implemented for a wider variety of applications. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q45.00005: Intracellular distribution of Fe$_{3}$O$_{4}$ nanoparticles in both human and mouse cells Maheshika Palihawadana Arachchige, Suvra Laha, Amulya Rajagopal, Sanjana Kulkarni, Shuo Wang, Amanda Flack, Chunying Li, Bhanu Jena, Gavin Lawes In recent years there has been an increasing interest in developing Fe$_{3}$O$_{4}$ nanoparticles for biomedical applications including targeted drug delivery and magnetic resonance imaging. Understanding of the intracellular distribution of these nanoparticles is crucial when considering these nanoparticles for specific applications. We have synthesized Fe$_{3}$O$_{4}$ nanoparticles having average size of 14 nm using a co-precipitation technique, which were coated with dextran. We studied the structural and morphological characteristics of the nanoparticles using x-ray diffraction, electron microscopy, dynamic light scattering, and zeta potential measurements. We also characterized the magnetic properties of the nanoparticles. In order to investigate the intracellular distribution of these Fe$_{3}$O$_{4}$ nanoparticles, we functionalized the dextran coated Fe$_{3}$O$_{4}$ nanoparticles with a fluorescent dye, Fluorescein isothiocyanate (FITC), and cultured them with both mouse insulinoma MIN 6 cells and human pancreatic MIA PaCa 2 cells. Using optical microscope we investigated the intracellular distribution of the nanoparticles and the effects on cell growth. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q45.00006: Cell Studies of BiFeO$_{3}$ nanoparticles for multimodal imaging Suvra Laha, Maheshika Palihawadana Arachchige, Amanda Flack, Sagar Paudel, Jaipal Singh, Amulya Rajagopal, Sanjana Kulkarni, Michael Synder, Joe Rakowski, Xuequn Chen, Bhanu Jena, Gavin Lawes There is considerable interest in using nanoparticles as contrast agents to improve diagnostic imaging. BiFeO$_{3}$ nanoparticles may be particularly interesting as multimodal contrast agents for both magnetic resonance imaging and x-ray imaging because these combine a large magnetic susceptibility with high atomic mass constituents. We synthesized BiFeO$_{3}$ nanoparticles using a chemical co-precipitation technique. We measured the structural and morphological characteristics of these nanoparticles using x-ray diffraction, electron microscopy, dynamic light scattering, and zeta potential, and probed the magnetic properties through both ac and dc magnetization studies. In order to investigate the cytotoxicity and intracellular distribution of these BiFeO$_{3}$ nanoparticles, we cultured them with mouse insulinoma MIN 6 cells and used optical microscopy to investigate the distribution and cell growth. We discuss the cytotoxicity of these nanoparticles, which will be crucial factor for determining possible biomedical applications together with a discussion of the cellular distribution of these nanoparticles. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q45.00007: Metallic Glass Wire Based Localization of Kinesin/Microtubule Bio-molecular Motility System K. Kim, A. Sikora, S. Yaginuma, K.S. Nakayama, H. Nakazawa, M. Umetsu, W. Hwang, W. Teizer We report electrophoretic accumulation of microtubules along metallic glass (Pd$_{42.5}$Cu$_{30}$Ni$_{7.5}$P$_{20}$) wires free-standing in solution. Microtubules are dynamic cytoskeletal filaments. Kinesin is a cytoskeletal motor protein. Functions of these bio-molecules are central to various dynamic cellular processes. Functional artificial organization of bio-molecules is a prerequisite for transferring their native functions into device applications. Fluorescence microscopy at the individual-microtubule level reveals microtubules aligning along the wire axis during the electrophoretic migration. Casein-treated electrodes are effective for releasing trapped microtubules upon removal of the external field. Furthermore, we demonstrate gliding motion of microtubules on kinesin-treated metallic glass wires. The reversible manner in the local adsorption of microtubules, the flexibility of wire electrodes, and the compatibility between the wire electrode and the bio-molecules are beneficial for spatio-temporal manipulation of the motility machinery in 3 dimensions. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q45.00008: Molecular motor powered nanotransportation guided by carbon nanotubes. A. Sikora, J. Ramon-Azcon, K. Kim, K. Reaves, H. Nakazawa, M. Umetsu, I. Kumagai, T. Adschiri, H. Shiku, T. Matsue, W. Hwang, W. Teizer Due to a decrease in the channel size of nanodevices, pressure-driven transport is increasingly limited by the fluid viscosity. This can be overcome by utilizing the motor protein kinesin that can walk processively along the microtubule filaments for active transport. However, using the kinesin-based transport system requires the ability to control the location and orientation of microtubules. We introduce functionalized multi-wall carbon nanotube (MWNT) tracks, aligned by dielectrophoresis, to guide kinesin powered microtubule shuttles. In order to resist shear flow and the force exerted by an electric field, the MWNT are attached to the surface via a biotin/streptavidin link. The configuration of the aligned MWNT is investigated using scanning electron microscopy and the guiding performance of the MWNT tracks is studied using fluorescence microscopy. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q45.00009: Effect of nanopore diameter on translocation speed of single-stranded DNA Rena Akahori, Takahobu Haga, Toshiyuki Hatano, Itaru Yanagi, Takeshi Ohura, Hirotaka Hamamura, tomio Iwasaki, Takahide Yokoi, Takashi Anazawa The effect of reducing a nanopore's diameter on the translocation speed of single-stranded DNA (ssDNA) was investigated. Various-sized nanopores (minimum 2.3 nm) were fabricated using transmission electron microscopy and atomic-layer deposition. Reducing the diameter was found to increase the drag force generated from the DNA-nanopore interaction and from viscous drag, thereby slowing down the translocation speed. The drag force of ssDNA was weaker than that of double-stranded DNA (dsDNA). These findings were supported by a molecular dynamics (MD) simulation which predicted that reducing nanopore diameter to almost the same as that of ssDNA (i.e., 1.4 nm) would decrease DNA translocation speed (to 1.4 $\mu $s/base) and decrease its variation. Reducing the nanopore diameter is thus a highly effective means of sequencing nanopore DNA. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q45.00010: Vertical electric field stimulation of neural cells on porous amorphous carbon electrodes Shilpee Jain, Ashutosh Sharma, Bikramjit Basu We demonstrate the efficacy of amorphous macroporous carbon substrates as electrodes to stimulate neuronal cell proliferation in presence of external electric field. The electric field was applied perpendicular to carbon electrode, while growing mouse neuroblastoma (N2a) cells \textit{in vitro}. The placement of the second electrode outside of the cell culture medium allows the investigation of cell response to electric field without the concurrent complexities of submerged electrodes such as potentially toxic electrode reactions, electro-kinetic flows and charge transfer (electrical current) in the cell medium. The macroporous carbon electrodes are uniquely characterized by a higher specific charge storage capacity (0.2 mC/cm$^{\mathrm{2}})$ and low impedance (3.3 k$\Omega $ at 1 kHz). When a uniform or a gradient electric field was applied perpendicular to the amorphous carbon substrate, it was found that the N2a cell viability and neurite length were higher at low electric field strengths ($\le $ 2.5 V/cm) compared to that measured without an applied field (0 V/cm). Overall, the results of the present study unambiguously establish the uniform/gradient vertical electric field based culture protocol to stimulate neurite outgrowth and viability of nerve cells. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q45.00011: Systematic decay of Raman signal on DNA Origami-based SERS substrates: analysis of the hot spots Mauricio Pilo-Pais, Anne Watson, Steven Demers, Thom LaBean, Gleb Finkelstein We studied the decay of Raman signal measured on self-assembled DNA origami-based substrates. Specifically, a rectangular origami ($\sim$ 70x90nm$^2$) was used to selectively attach gold nanoparticles ($\sim$ 5 nm) onto its corners, which were subsequently enlarged using a solution-based silver deposition. Further, 4-aminobenzenethiol (ABT) molecules were covalently attached to the assemblies. The assemblies were engineered to form ``hot spots'' of enhanced electromagnetic field between the nanoparticles, resulting in a significant enhancement of Raman signal compared to ABT molecules attached to individual nanoparticles. The signal systematically decayed as a function of the laser exposure time. The one-particle control samples showed no bleaching. We explain this behavior by the bleaching of the molecules due to the high intensity of the electric field at the hot spots. We further increased the laser intensity, allowing us to progressively burn molecules located in the regions within the hot spots where the field intensity exceeded the critical value. The analysis of the signal decay allow us to analyze the field enhancement in the hot spots and quantify the effectiveness of the DNA-origami-based SERS substrates. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q45.00012: Silk/nano-material hybrid: properties and functions Eden Steven, Victor Lebedev, Elena Laukhina, Vladimir Laukhin, Rufina G. Alamo, Concepcio Rovira, Jaume Veciana, James S. Brooks Silk continues to emerge as a material of interest in electronics. In this work, the interaction between silk and conducting nano-materials are investigated. Simple fabrication methods, physical, electronic, thermal, and actuation properties are reported for spider silk / carbon nanotube (CNT-SS) [1] and \textit{Bombyx mori} / (BEDT-TTF)-based organic molecular conductor hybrids (ET-S). The CNT-SS fibers are produced via water and shear assisted method, resulting in fibers that are tough, custom-shapeable, flexible, and electrically conducting. For ET-S bilayer films, a layer transfer technique is developed to deposit linked crystallites of (BEDT-TTF)$_{2}$I$_{3}$ molecular conductor onto silk films, generating highly piezoresistive semi-transparent films. In both cases, the hybridization allows us to gain additional functions by harnessing the water-dependent properties of silk materials, for example, as humidity sensor and electrical current- or water-driven actuators. SEM, TEM, FT-IR, and resistance measurements under varying temperature, strain, and relative humidity reveal the synergistic interactions between the bio- and nano-materials.\\[4pt] [1] E. Steven, et al. Nat. Commun. 4, 2435 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q45.00013: Cellular Silica Encapsulation for Development of Robust Cell Based Biosensors Robert Johnston, Snezna Rogelj, Jason Harper, Michaelann Tartis In order to detect chemical and biological threats both on the battlefield and in civilian life, development of portable, robust detection systems capable of real-time identification of the chemical and biological agents are needed. Living cell--based sensors have proven effective as sensitive, specific, near real-time detectors; however, living cell-based sensors require frequent cell replenishment due to cell sensitivity to the ex-vivo environment, which limits sensor stability. Incorporation of living cells within a biocompatible matrix that provides mechanical protection and maintains access to the external environment may facilitate the development of long-term stable cell-based biosensors. We are exploring the use of a novel Chemical Vapor into Liquid (CViL) deposition process for whole cell encapsulation in silica. In CViL, the high vapor pressure of common silica alkoxides is utilized to deliver silica into an aqueous medium, creating a silica sol. Mixing of cells with the resulting silica sol facilitates encapsulation of cells in silica while minimizing cell contact with the cytotoxic products of silica generating reactions. Using fluorescence microscopy analysis with multiple silica specific markers, encapsulation of multiple eukaryotic cell types (Saccharomyces cerevisiae, Jurkat, HeLa, and U87 cells) with CViL generated silica is shown, providing a foundation for development of long --term stable cell-based biosensors with diverse sensing capabilities. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q45.00014: Effect of (Ag, Sn) Doping on the Structure and Optical Properties of Au Nanocluster Radhakrishnan Balu, Shashi Karna Noble metal nanoclusters (NCs) consisting of a few to 35 atoms in size in the sub 2 nm range dimension are considered to be nontoxic as opposed to nanoparticles that are cytotoxic. Also, due to the quantum confinement of electrons, these NCs exhibit atom-like energy spectrum and display fluorescent properties useful in a wide range of applications, including medical diagnosis. The unique features of NCs such as size-tunable optical properties, intense fluorescence in the visible, and biocompatibility have stimulated an active area of investigation of noble metal NCs comprised of Au, Ag, Cu, and Pt. Furthermore, the electronic properties of nanoclusters can be modified by combining them with other elements. In this study, we consider the space-filled configuration of Au$_{32}$ NC and investigate the effects of Ag and Sn atom incorporation on geometry and electronic spectrum. Our study suggests that Ag and Sn doping of Au$_{32}$ NC red-shifts the absorption maximum and also reduces the oscillator strength. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q45.00015: A highly sensitive magnetic biosensor for detection and quantification of anticancer drugs tagged to superparamagnetic nanoparticles J. Wingo, J. Devkota, T.T.T. Mai, X.P. Nguyen, P. Mukherjee, H. Srikanth, M.H. Phan A precise detection of low concentrations of biomolecules attached to magnetic nanoparticles in complex biological systems is a challenging task and requires biosensors with improved sensitivity. Here, we present a highly sensitive magnetic biosensor based on the magneto-reactance (MX) effect of a Co$_{65}$Fe$_{4}$Ni$_{2}$Si$_{15}$B$_{14}$ amorphous ribbon with nanohole-patterned surface for detection and quantification of anticancer drugs (Curcumin) tagged to Fe$_{3}$O$_{4}$ nanoparticles. The detection and quantification of Curcumin were assessed by the change in MX of the ribbon subject to varying concentrations of the functionalized Fe$_{3}$O$_{4}$ nanoparticles. A high capacity of the MX-based biosensor in quantitative analysis of the nanoparticles was achieved in the range of 0 - 50 ng/ml, beyond which the detection sensitivity ($\eta )$ remained unchanged. The $\eta $ of the biosensor reached an extremely high value of 30{\%}, which is about 4-5 times higher than that of a magneto-impedance (MI) based biosensor. This biosensor is well suited for detection of low-concentration magnetic biomarkers in biological systems. [Preview Abstract] |
Session Q46: Strongly Correlated Electron Systems: Hubbard Model and other Many Body Theories
Sponsoring Units: DCMPChair: Hae-Young Kee, University of Toronto
Room: Mile High Ballroom 4E
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q46.00001: Accuracy of the downfolding scheme for multiorbital Hubbard models Hiroshi Shinaoka, Philipp Werner, Matthias Troyer Deriving an effective low-energy model from \textit{ab initio} calculations is a grand challenge in condensed matter physics. Recently, the so-called constrained random phase approximation (RPA) has been developed. In that scheme, screening effects by high-energy bands are taken into account in the RPA level to derive screened Coulomb interactions in the low-energy model. The method has been applied to various strongly correlated electronic systems such as transition metal oxides and organic compounds in combination with \textit{ab initio} band calculations. However, the accuracy of the scheme still needs to be clarified. In this talk, we discuss the accuracy of this scheme using a multi-orbital Hubbard model. We first derive a low-energy effective single-orbital Hubbard model using the constrained RPA scheme. We then solve both models using dynamical mean-field theory, compare the results and discuss the accuracy of the downfolding scheme. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q46.00002: A variational cluster study of possible phase separation in square and honeycomb Hubbard lattices Kun Fang, Gayanath Fernando, Alexander Balatsky, Armen Kocharian The Hubbard model is examined for possible electronic phase separation using the variational cluster approximation in square and honeycomb geometries. The phase separation is found when different electronic states with different electronic densities $n$ share the same chemical potential $\mu$, so that these states can coexist at equilibrium and be distributed inhomogeneously throughout the lattice. The phase separation is clearly identified in the square lattice but, surprisingly, it is not discovered in the honeycomb lattice in a similar region of on-site Coulomb interaction and hole doping. The phase separation instability found in the square lattice is signatured by the disappearance of a set of one particle spectra around the $k$-point $(\pi/2,\pi/2)$ in momentum space. The electronic state associated with the set of spectra is due to scattering of electrons at the antiferromagnetic (AF) Brillouin zone boundaries and responsible for the phase separation. To our knowledge, no previous publications reveal such an anomalous state. The honeycomb lattice does not show the corresponding anomalies due to its different geometry, so that there is no such phase separation in the honeycomb lattice. Our VCA provides strong support for phase separation instability driven by electronic cor [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q46.00003: Finite-temperature superconducting correlations in the square lattice Hubbard model Ehsan Khatami, Richard Scalettar, Rajiv R.P. Singh We utilize numerical linked-cluster expansions (NLCE) [1,2] to study superconducting properties of the repulsive Fermi-Hubbard model on the square lattice. Within NLCE, temperature-dependent properties in the thermodynamic limit can be obtained from exact diagonalization of small clusters. We calculate the pairing correlation functions, structure factor, and correlation length for d-wave and extended s-wave symmetries at, and especially away from, half filling for a wide range of interaction strengths. A relatively strong tendency to d-wave pairing away from half filling is revealed after subtracting the uncorrelated contributions. We compare our findings to improved results from the determinantal quantum Monte Carlo simulations on large finite clusters with periodic boundary condition.\\[4pt] [1] M. Rigol, T. Bryant, and R. R. P. Singh, Phys. Rev. Lett. 97, 187202 (2006).\\[0pt] [2] E. Khatami and M. Rigol, Phys. Rev. A 84, 053611 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q46.00004: Principle of Maximum Entanglement Entropy and Local Physics of Correlated many-body Electron-Systems Nicola Lanata, Hugo Strand, Yongxin Yao, Gabriel Kotliar We argue that, because of the quantum-entanglement, the local physics of the strongly-correlated materials at zero temperature is described in very good approximation by a simple generalized Gibbs distribution, which depends on a relatively small number local quantum thermodynamical potentials. We demonstrate that our statement is exact in certain limits, and we perform numerical calculations of the iron compounds FeSe and FeTe and of the elemental cerium by employing the Gutzwiller Approximation (GA) that strongly support our theory in general. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q46.00005: Maximally Entangled Mode, Metal-Insulator Transition and Violation of Entanglement Area Law in Non-interacting Fermion Ground States Mohammad Pouranvari, Kun Yang We study in this work the ground state entanglement properties of two models of non-interacting fermions moving in one-dimension (1D), namely random dimer model and power-law random banded model that exhibit metal-insulator transitions. We find that entanglement entropy grows either logarithmically or in a power-law fashion with subsystem size in the metallic phase or at metal-insulating critical point, thus violating the (1D version of) entanglement area law. No such violation is found in the insulating phase. We further find that characteristics of \emph{single fermion} states at the Fermi energy (which can \emph{not} be obtained from the many-fermion Slater determinant) is captured by the lowest energy single fermion mode of the \emph{entanglement} Hamiltonian; this is particularly true at the metal-insulator transition point. In addition, the inverse-participation ratio of the lowest energy single fermion mode of the {\em entanglement} Hamiltonian is proportional to that of the single fermion state at Fermi energy in all cases. Our results suggest entanglement is a powerful way to detect metal-insulator transitions, \emph{without} knowledge of the Hamiltonian of the system. Results on metal-insulator transition of 3D Anderson model will also be presented. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q46.00006: Minimally entangled typical thermal states of fermions in DMRG++ Gonzalo Alvarez I will discuss the minimally entangled typical thermal states (METTS) algorithm (developed by White in PRL 2009) in the context of fermionic systems such as the Hubbard model. The additional idea here (http://prb.aps.org/abstract/PRB/v87/i24/e245130) is to combine METTS with the Krylov-space approach to evolve the classical product states in imaginary time. The issues to be addressed include ergodicity, ``collapse'' bases, and convergence. For the temperature dependence of the superconducting correlations, METTS will be shown to yield the correct exponential decay with distance, and exponents proportional to the temperature at low temperatures. The talk will conclude with a few remarks about recent directions and future plans for DMRG++ (\verb!https://web.ornl.gov/~gz1/dmrgPlusPlus/!) and related codes. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q46.00007: Dual boson approach to collective excitations in correlated fermion system Hartmut Hafermann, Erik G.C.P. van Loon, Alexey N. Rubtsov, Olivier Parcollet, Alexander I. Lichtenstein, Mikhail. I. Katsnelson We describe the interaction between electrons and collective excitations in strongly correlated fermion systems by means of the so-called dual boson approach. It includes nonlocal corrections to extended dynamical mean-field theory (EDMFT) and is applicable to lattice fermion models with both short- and long-range interaction. We present results for the collective charge excitations in the (extended) Hubbard model and show that through the inclusion of vertex corrections to the polarization operator, the approach correctly describes the long wavelength collective excitations in the strong coupling regime. In particular, we find the zero sound mode when forces are short-ranged and plasmons in presence of a long-range interaction. We further examine the effects of nonlocal correlations in the extended Hubbard model and compute the phase diagram. Results are compared to EDMFT and the random phase approximation. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q46.00008: The Holstein polaron: beyond the standard model Carl J. Chandler, Frank Marsiglio The paradigm for describing the polaron is the Holstein model, where only local interactions between the electron and optical phonon modes are considered. We present several variants of this model and discuss the impact on various observables, such as the effective mass. Possible variations include further than nearest neighbour hopping, longer range interactions, and even models that go beyond the Holstein/Frohlich coupling, i.e. the BLF/SSH (Barisic-Labbe-Friedel/Su-Schrieffer-Heeger) model. Recent progress on these models will be described. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q46.00009: On the separability of dynamical and non-local self-energy effects in correlated materials Jan M. Tomczak We employ Hedin's {\it GW} approximation to correlated metals such as the iron pnictide and chalcogenide superconductors, and the transition metal oxide SrVO$_3$. We find that non-local correlation effects in these systems are non-negligible, and indeed crucial for agreement with experimental observations. This advocates that the gold standard for strongly correlated materials, dynamical mean field theory (DMFT), has to be extended to include non-local self-energy effects even for rather 3D-like systems. However, from our first principles calculations we empirically find the dynamical contribution to the electron self-energy (in particular the quasi-particle weight) to be largely independent of momentum when expressed in a local basis. We substantiate our {\it ab initio} results by calculations for the 3D Hubbard model within the dynamical vertex approximation. The finding that dynamical and non-local correlations are separable has important consequences for advancing theories that go beyond DMFT. I will discuss the implications on the example of our recent {\it GW}+DMFT results for SrVO$_3$. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q46.00010: General interaction-induced density wave states from a symmetry perspective J.W.F. Venderbos We present a symmetry classification of particle-hole condensates, i.e. general density wave states, to show how an organization in terms of translational and point group symmetries provides immediate insight into the electronic properties of such states. We discuss site, bond and flux ordered density wave states in systems with square and hexagonal symmetry. We establish a robust connection between the transformation behavior under lattice symmetries of such density waves and the low-energy description of the electronic properties, which is independent of specific lattices and fully determined by symmetry. In addition, we show how an organization in terms of lattice symmetry is helpful in identifying and predicting electronic states matter with topological quantum numbers. For systems with hexagonal symmetry we propose a new class of time-reversal invariant spin-bond ordered density waves. We address how interactions may induce the formation of these density waves in real materials and make contact with recent works which indicate that symmetric density waves are good variational ground state candidates for interacting lattice fermion models. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q46.00011: Hole binding in Mott antiferromagnets: A DMRG study Zheng Zhu, Hong-Chen Jiang, D.N. Sheng, Zheng-Yu Weng The binding of injected holes in antiferromagnets is studied based on the density matrix renormalization group (DMRG) simulation for the t-J model on square ladders. It is shown that the binding strength is substantially enhanced in a spin background with a short-range spin correlation, in contrast to that with a quasi-long-range spin correlation. However, it is further found that the enhanced pairing strength diminishes once the phase string effect in the hopping term of the t-J ladders is switched off and a coherent quasiparticle behavior is restored for an unpaired single hole. General implications for the nature of pairing in doped Mott insulators will be also discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q46.00012: Wave-function Localization and Impurity-induced First Order Phase Transition in Correlated Liquids Near the Thermal Freezing Point Shahriar Shadkhoo, Robijn Bruinsma A quantum mechanical impurity coupled to an ohmic charged liquid near the crystallization phase transition, can stabilize a local cluster in the liquid. A nonlinear free energy functional is borrowed from Landau-Brazovskii (LB) model; the theory of weak crystallization, where in Gaussian approximation and near the thermal freezing point, the correlation of fluctuations with a characteristic wave vector $q_0$ diverges, hence a crystal with unit cells of the size $q_0^{-1}$ forms. Adding nonlinearities to the free energy, however, opens up a gap in density field (order parameter) across the transition, implying a first order phase transition. We apply the instanton technique to study the first order local phase transition of the charged field from liquid to crystalline phase, induced by the impurity. We demonstrate that the particle, can stabilize the metastable minimum of the free energy slightly above the actual transition point, and facilitate the local transition. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q46.00013: Exploring superconductivity in multi-orbital systems Zi Yang Meng, Hae-Young Kee, Yong Baek Kim We study possible unconventional superconducting states in correlated electronic systems with multi-orbital and strong spin-orbit coupling. In particular we focus the interplay between electronic correlation, spin-orbital coupling and lattice structure in determining a pairing symmetry. To study such systems in a controlled manner, we develop a dynamical mean field theory simulation with hybridization expansion continuous time quantum Monte Carlo impurity solver. We further explore the Parquet formalism in which the momentum dependence of the pairing vertex is explicitly introduced by combining both particle-particle and particle-hole channel contributions, to capture the pairing symmetry. The effects of hole and electron doping will be also discussed. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q46.00014: Chiral sp-orbital paired superfluid of fermionic atoms in a 2D spin-dependent optical lattice Bo Liu, Xiaopeng Li, Biao Wu, W. Vincent Liu Recent progress in realizing synthetic quantum orbital materials in chequerboard and hexagonal optical lattices opens an avenue towards exploiting unconventional quantum states, advancing our understanding of correlated quantum matter. Here, we unveil a chiral $sp$-orbital paired superfluid state for an interacting two-component Fermi gas in a 2D spin-dependent optical lattice. Surprisingly, this novel state is found to exist in a wide regime of experimentally tunable interaction strengths. The coexistence of this chiral superfluid and the ferro-orbital order is reminiscent of that of magnetism and superconductivity which is a long-standing issue in condensed matter physics. The topological properties are demonstrated by the existence of gapless chiral fermions in the presence of domain wall defects, reminiscent of quantum Hall edge states. Such properties can be measured by radio frequency spectroscopy in cold atomic experiments. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q46.00015: Ab initio investigation of ground state magnetic and ferroelectric properties of monoclinic CuCl$_{2}$ multiferroic system Ambesh Dixit Materials with simultaneous magnetic and ferroelectric ordering are getting attentions and are widely investigated to understand the strong lattice-charge-spin coupling in these systems. Also, the strong coupling among different degree of freedoms in these systems may give rise to the novel magnetoelectric phenomenon. Recent experimental studies on monoclinic CuCl$_{2}$ system suggest that system undergoes antiferromagnetic transition $\sim$ 25 K in conjunction with ferroelectric ordering simultaneously. The helimagnetic ordering of Cu ions (S $=$ 1/2 ) along c-axis causes the onset of ferroelectric ordering along b-axis, breaking spatial inversion symmetry. We investigated the ground state magnetic and ferroelectric properties of copper chloride in its monoclinic structure (space group C2/m) using density functional theory. The spin dependent calculations are carried out to understand the magnetic structure and ferroelectric polarization was calculated along different axis. The correlation of magnetic structure and the onset of polarization in CuCl$_{2}$ system will be discussed in the context of magnetoelectric coupling in this system. [Preview Abstract] |
Session Q47: Magnetic Oxides, V2O3 and Related: Experiment
Sponsoring Units: DCMPChair: Dimitri Basov, University of California, San Diego
Room: Mile High Ballroom 4F
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q47.00001: Dynamics of the insulator to metal transition in high quality V2O3 thin films Elsa Abreu, Siming Wang, Jingdi Zhang, Kun Geng, Xiaoguang Zhao, Kebin Fan, Mengkun Liu, Gabriel Ramirez, Xin Zhang, Ivan K. Schuller, Richard D. Averitt Metal-insulator transitions (MITs) are a striking manifestation of the interactions between the various degrees of freedom in complex materials. Vanadium sesquioxide (V2O3) is a prototypical MIT material, transitioning from an antiferromagnetic insulator to a paramagnetic metal at Tc=170K. We present a detailed investigation of the insulator-to-metal dynamics in single crystalline thin films of V2O3 following optical and far-infrared excitation, measured using THz time domain spectroscopy. Conductivity dynamics induced below Tc by ultrafast photoexcitation can be quantitatively described by nucleation and growth of the metallic volume fraction, which eventually gives rise to the full metallic state of V2O3 on a timescale of about 50ps. We will discuss our results in the broader context of phase transition dynamics of the vanadates and related strongly correlated electron materials. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q47.00002: Probing the structural evolution across the metal-to-insulator transition in thin film VO2 using nonlinear optics Hao Chu, Darius Torchinsky, Siming Wang, Juan Gabriel Ramirez, Ivan K. Schuller, David Hsieh Time-resolved pump-probe studies on VO2 have demonstrated that a phase transition in both the electronic and structural degrees of freedom can be induced using sub-picosecond light pulses. However, typically the electronic and structural degrees of freedom are separately measured using time-resolved optical and diffraction based techniques respectively. Therefore the timescales of electronic and structural dynamics are typically compared across different samples and different experimental setups. Here we introduce the use of rotational anisotropy nonlinear optical generation spectroscopy as a method to probe the structural symmetry of VO2 purely optically. We demonstrate that the low temperature monoclinic and high temperature rutile phases in a VO2 thin film grown on r-cut sapphire (1-102) are clearly resolved using our technique and compared favorably with calculations. We will discuss how a simultaneous probe of ultrafast electronic and structural dynamics in VO2 can be realized using this technique. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q47.00003: Nanoscale Structural Evolution of Electrically Driven Insulator to Metal Transition in Vanadium Dioxide Greg Stone, Eugene Freeman, Nikhil Shukla, Hanjong Paik, Jarrett Moyer, Zhonghou Cai, Haidan Wen, Roman Engel-Herbert, Darrell Schlom, Venkatraman Gopalan, Suman Datta We report the evolution of local structural of tensile strained vanadium dioxide thin films during an electrically driven insulator to metal transition by nanoscale hard X-ray diffraction. Evaluation of the Bragg diffraction intensity reveals a narrow metallic filament with rutile structure to be the dominant conduction pathway for the electrically driven insulator to metal transition, while the remainder of the channel area remained the monoclinic M1 phase. The filament dimensions were estimated using simultaneous electrical probing and nanoscale X-ray diffraction measurements. Analysis revealed that the width of the conducting channel can be tuned externally using resistive loads in series allowing for the manipulation of the M1/R phase ratio in the phase coexistence regime. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q47.00004: Temperature-driven and photo-induced MIT behaviors of VO$_{2}$ nanowires Ahrum Sohn, Dong-Wook Kim, Ji-Won Byun, Jeong Min Baik VO$_{2}$ shows a metal-insulator transition (MIT) and structural phase transition (SPT) at critical temperature (Tc) of 343K. It has been known that the MIT and SPT behaviors of VO$_{2}$ can be tuned by external stimuli such as light, electric-field, and strain. We carried out comparative studies of MIT behaviors of VO$_{2}$ nanowires during heating-cooling cycles with and without illumination using several light sources (red, blue, and UV). Light can induce change in Tc and hysteresis width of the resistance change. We have investigated influences of light on SPT during MIT. In this presentation, we will discuss possible physical origins for the photo-induced effects on the MIT behaviors of the VO$_{2}$ nanowires. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q47.00005: Three Dimensional Infrared Nano-Imaging of Stripe Order in Vanadium Dioxide Mengkun Liu, Martin Wagner, Jingdi Zhang, Alexander McLeod, Salinporn Kittiwatanakul, Zhe Fei, Elsa Abreu, Michael Goldflam, Aaron Sternbach, Siyuan Dai, Kevin West, Jiwei Lu, Stuart Wolf, Richard Averitt, D.N. Basov We report the three dimensional landscape of the stripe state in vanadium dioxide (VO$_{2})$ films. This is achieved via direct visualization with scattering-type scanning near-field optical microscope (s-SNOM) [Phys. Rev. Lett. 111 (9), 096602 (2013)]. The VO$_{2}$ films we investigate in this study are epitaxially grown on [100]$_{\mathrm{R}}$ TiO$_{2}$ substrates and exhibit uniaxial strain induced cracking uniformly along the rutile c axis. With s-SNOM, we show that (1) monoclinic-tetragonal crystal symmetry, (2) in-plane rotational symmetry and (3) out-of-plane (z-axis) symmetry have been spontaneously broken in the vicinity of the phase transition. Our results demonstrate s-SNOM as a powerful approach for bringing new insight into mesoscopic physics in strained metal oxide thin films. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q47.00006: Mott transition and Schottky barrier observation by photocurrent measurement in VO$_{2}$ devices Giwan Seo, Minjung Kim, Bong-Jun Kim, Yong Wook Lee, Ahrum Sohn, Dong-Wook Kim, Hyeonsik Cheong, Hyun-Tak Kim As one of the metal-insulator transition (MIT) mechanism, the Mott transition occurs due to Coulomb interactions of electrons in VO$_{2}$. This suggests that VO$_{2}$ does not undergo the structural phase transition (SPT) when MIT occurs. For observing the Mott transition and Schottky barrier in VO$_{2}$ devices, we simultaneously observe the temperature dependence of photocurrent and local structural observation in the two-terminal VO$_{2}$ devices by using scanning photocurrent microscopy, which is a typical method for showing images of nanometer-length scale and Raman spectroscopy, respectively. In particular, the photocurrent between two electrodes through VO$_{2}$ channel increases due to the variation of schottky barrier height. Furthermore, the work functions of VO$_{2}$ thin film is investigated while varying the device temperature, which could assure the Schottky contact formation. We also find the metallic phase with monoclinic structure below a conventional transition temperature of VO$_{2}$ ($\sim$ 68 $^{\circ}$C), indicating the Mott transition of VO$_{2}$. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q47.00007: Evidence of the metal-insulator transition in ultrathin V2O3 films Mariela Menghini, Leander Dillemans, Ruben Lieten, Tomas Smets, Chen-Yi Su, Jean Pierre Locquet We report on the strain state and transport properties of V2O3 single layers and V2O3/Cr2O3 bilayers deposited by Molecular Beam Epitaxy on (0001)-Al2O3 substrates. We find that the metal-insulator transition is strongly attenuated in V2O3 layers of 6 and 4 nm grown coherently on Al2O3. This is in contrast with V2O3 layers grown on Cr2O3 buffer layers which exhibit a metal-insulator transition. Our results provide evidence for the existence of a metal-insulator transition in ultra-thin films. These findings are relevant for the understanding of V2O3 properties in the proximity of interfaces and integration of correlated electron systems in devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q47.00008: In situ diffraction studies of H$_{x}$VO$_{2}$ and D$_{x}$VO$_{2}$ D. Natelson, N. Tumanov, V. Ban, Y. Filinchuk, H. Ji, J. Wei, M.W. Swift, A.H. Nevidomskyy Vanadium dioxide exhibits a first-order phase transition at around 338 K between a high temperature, tetragonal, metallic state (T) and a low temperature, monoclinic, insulating state (M1), driven by electron-electron and electron-lattice interactions. Intercalation of VO2 with atomic hydrogen has been demonstrated, with evidence that this doping suppresses the transition. However, the effects of intercalated H on the crystal structure of the resulting hydride had not been examined in detail. Here we report synchrotron and neutron diffraction studies of this material system, mapping out the structural phase diagram as a function of temperature and hydrogen content. In addition to the original T and M1 phases, we find two orthorhombic phases, O1 and O2, which are stabilized at higher hydrogen content. We present density functional calculations that confirm the metallicity of these states. The intercalation of hydrogen above a critical fraction suppresses the metal-insulator transition entirely. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q47.00009: The M2 phase of vanadium dioxide: a view from infrared and optical spectroscopy T.J. Huffman, Peng Xu, M.M. Qazilbash, Joonseok Yoon, Honglyoul Ju, R. Smith, G.L. Carr Bulk single crystalline vanadium dioxide (VO$_{2}$) undergoes a metal-insulator transition (MIT) at 340K. This thermally-driven MIT is accompanied by a structural phase transition that results in pairing of all vanadium ions in the insulating, monoclinic M$_{1}$ phase. However, there also exists an insulating monoclinic M$_{2}$ phase, usually only accessible via external strain or chemical doping, in which only half of the vanadium chains exhibit pairing. The M$_{2}$ phase of VO$_{2}$ is vital for understanding the roles of electronic correlations and vanadium pairing to the MIT. Recent x-ray diffraction studies show that small pure VO$_{2}$ crystals can exhibit an M$_{2}$ phase below 318K, likely due to internal strain.\footnote{ B.S. Mun et al. Physica Status Solidi (RRL) - Rapid Research Letters 5, 107 (2011).} These crystals undergo phase transitions from M$_{2}$ to M$_{1}$ and from M$_{1}$ to rutile metal upon heating. We have performed reflectance micro-spectroscopy with polarized light and generalized spectroscopic micro-ellipsometry between 12 meV and 5.5 eV on these VO$_{2}$ crystals as a function of temperature, uncomplicated by external strain or chemical doping. We report infrared and optical data on the M$_{1}$, M$_{2}$ and rutile phases and compare electronic and phonon properties of M$_{1}$ and M$_{2}$ phases. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q47.00010: X-ray and neutron scattering studies of microstructure and dynamics in VO$_{2}$ John Budai, Jiawang Hong, Michael Manley, Olivier Delaire, Eliot Specht, Chen Li, Jon Tischler, Ayman Said, Douglas Abernathy, Bogdan Leu, Alexander Tselev, Lynn Boatner, Robert McQueeney Vanadium dioxide is a strongly correlated material that exhibits a well-studied, but poorly understood, metal-insulator transition coupled with a tetragonal-monoclinic structural phase transition near 340 K. We have combined synchrotron x-ray and spallation neutron scattering measurements to investigate microstructure and lattice dynamics in VO$_{2}$. Submicron-resolution x-ray microdiffraction studies reveal local phase coexistence driven by strain effects within individual single-microcrystals. Macroscopic diffuse scattering and inelastic x-ray and neutron scattering measurements reveal unusual features in the rutile phonon dispersions at particular locations in reciprocal space, including temperature-dependent dispersions near the R-point. Comparisons of measurements with ab initio molecular dynamics calculations indicate that anharmonicity plays a central role in determining the lattice vibrations and hence physical properties in this system. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q47.00011: Inelastic Neutron Scattering studies of pure and Mo doped VO$_{2}$ Arnab Banerjee, Garrett E. Granroth, Yuen Yiu, Adam A. Aczel, Alexander I. Koleshnikov, Huxia Luo, Robert J. Cava, Stephen E. Nagler For the last half-century~VO$_{2}$ has been viewed as an~archetypal system for studying~the~metal-insulator~transition (MIT). Moreover, there is currently intense interest in this material arising from its promising use in fast energy efficient electronic devices.~ There are key unresolved issues connected with the origin of ~the MIT, including the role of magnetism arising from the S$=$1/2 V$^{4+}$ ions.~ It is known that below 340 K in undoped VO$_{2}$ the V ions form structural dimers in the insulating M1 monoclinic phase.~ Here we report the results of new inelastic neutron scattering measurements of VO$_{2}$ and V$_{0.75}$Mo$_{0.25}$O$_{2}$. Using the~SEQUOIA chopper spectrometer at the SNS~possible lattice and magnetic excitations for energies up to 600 meV were investigated. We discuss the results in the context of current ideas concerning the MIT in VO$_{2}$. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q47.00012: Atomic hydrogen doping in single-crystal vanadium dioxide Heng Ji, Will Hardy, Jiang Wei, Jian Lin, Hanjong Paik, Darrell Schlom, Douglas Natelson Vanadium dioxide is a strongly correlated material with a bulk metal-to-insulator transition (MIT) near 340 K. Previous experiments in single-crystal nanowires (J. Wei et al., Nature Nano. 7, 357-362 (2012)) have shown that catalytic doping with atomic hydrogen can stabilize the high temperature metallic state. In this experiment, we used a hot filament source to split hydrogen molecules and directly dope atomic hydrogen into VO2 material, including epitaxial films and nanowires, without any catalyst. From observations of the wire samples, we infer the relative diffusion rates of H in the monoclinic and rutile crystal structures. Transport measurements of the doped film samples show no temperature-driven transition, but rather a conducting state down to 2K. We present Hall and magnetoresistance measurements on macroscale and mesoscale devices fabricated from the doped films. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q47.00013: Effect of thickness on the film strain and metal-semiconductor transition of VO$_{2}$ thin films Salinporn Kittiwatanakul, Stuart Wolf, Jiwei Lu Many applications of VO$_{2}$ will benefit from the extensive tunability of the Metal to Semiconductor Transition (MST) of VO$_{2}$, which is very sensitive to the film strain and the oxygen stoichiometry [1,2]. Reactive Bias Target Ion Beam Deposition was used to deposit epitaxial VO$_{2}$ thin films on (100), (011), and (001) TiO$_{2}$ substrates, and highly textured VO$_{2}$ on $c$-Al$_{2}$O$_{3}$ with thicknesses in the range of 5-17 nm. X-ray diffractometry confirmed the single-phase nature of the VO$_{2}$ films, and was also used to determine the lattice parameters. Due to the substrate clamping effect, there are very large strains introduced (up to 3.4{\%}), that affect the transition temperatures (T$_{\mathrm{MST}})$. For the 5nm VO$_{2}$/(100) TiO$_{2}$, T$_{\mathrm{MST}}$ is raised above 430 K, which is much higher than in previous reports. The resistivity of this sample changed about 4 orders of magnitude during the transition from a semiconductor to a metal. \\[4pt] [1] S Kittiwatanakul, et al., Journal of Applied Physics 114 (5), 053703-053703-5 (2013) \\[0pt] [2] K. G. West, et al. Journal of Vacuum Science {\&} Technology A 26(1): 133-139 (2008) [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q47.00014: Strain-induced anisotropy in VO$_{2}$ film for metamaterial resonance switching and frequency tuning Daan Leiva, Terry Dunlop, Raul Torrico, Robert Marvel, Jed Ziegler, Richard Haglund, Yohannes Abate Vanadium dioxide (VO$_{2})$ undergoes fascinating first order insulator-to-metal phase transition (IMT) around 68 $^{\circ}$C. We demonstrate VO$_{2}$ that thin-films grown on Si substrate exhibit strain induced metallic streaks that are unidirectional during IMT. We unveil the nanoscale formation and spatial dynamics of these streaks and further use the unique anisotropic property of the VO$_{2}$ film to dynamically tune the metamaterial resonances. We reveal thermal control of the metamaterial infrared resonances by nanoscale spatial near-field imaging of both the metallic streaks phase evolution on VO$_{2}$ film and the resonance states of the metamaterial. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q47.00015: Disorder-dominated Ultrafast Dynamics in Vanadium Dioxide Nathaniel Brady, Kannatassen Appavoo, Rohit Prasankumar, Richard Haglund, David Hilton We have performed nondegenerate ultrafast pump-probe spectroscopy of the insulator-to-metal phase transition in vanadium dioxide ($\mathrm{VO_2}$) on several different samples of varying disorder as functions of temperature and pump fluence. Our results are inconsistent with the commonly used assumption of homogeneous nucleation and growth of metallic islands in the parent semiconducting phase and indicate the strong role of disorder on the ultrafast dynamical response. [Preview Abstract] |
Session Q48: Superconductivity: Material and Properties
Sponsoring Units: DCMPChair: Martin Nikolo, St. Louis University
Room: Mile High Ballroom 1A-1B
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q48.00001: Unexpected Giant Superconducting Fluctuation and Anomalous Semiconducting Normal State in NdO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$Bi$_{\mathrm{1-y}}$S$_{2}$ Single Crystals Xiyu Zhu, Jianzhong Liu, Delong Fang, Zhenyu Wang, Jie Xing, Zengyi Du, Huan Yang, Hai-Hu Wen Experiments on single crystals of BiS$_{\mathrm{2}}$-based superconductors are highly desired. We report the successful growth of the NdO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$Bi$_{\mathrm{1-y}}$S$_{\mathrm{2}}$ single crystals. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at Tc $=$ 4.83 K, while an unexpected giant superconducting fluctuation appears at temperatures as high as 2-4 kBTC. Analysis based on the anisotropic Ginzbaug-Landau theory gives an anisotropy $\gamma = \sqrt {m_{c} /m_{ab} } \approx 30\sim 45$. Two gap features with magnitudes of about 3.5 $\pm$ 0.3meV and 7.5 $\pm$ 1 meV were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition yielding a huge ratio $2\Delta_{s}^{1} /k_{B} T_{c} =$16.8, the larger gap remains up to about 26 K. The normal state recovered by applying a high magnetic field shows an anomalous semiconducting behavior. All these suggest that the superconductivity in this newly discovered superconductor cannot be formatted into the BCS picture. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q48.00002: Superconductivity in the Zintl intermetallic compound Ca$_{11}$Bi$_{10-x}$ Mihai Sturza, Han Fei, Christos Malliakas, Helmut Claus, Duck Young Chung, Mercouri Kanatzidis The recent discovery of the iron-based superconductors with unconventional superconductivity as a new class of superconductors has attracted great attention and triggered extensive research for new compounds. We report the new superconductor Ca$_{11}$Bi$_{10-x}$, which is in fact a Zintl phase. The structure of Ca$_{11}$Bi$_{10}$ contains three discrete anionic fragments: isolated Bi$^{3-}$ ions, dumbbells of Bi$_{2}^{4-}$ and square planar rings of Bi$_{4}^{4-}$ surrounded by Ca$^{2+}$cations. The Bi$_{4}^{4-}$ squares and the Bi$_{2}^{4-}$ dumbbells interact with one another through Bi---Bi bonding to form an extended 3D framework. The extended three-dimensional Bi-Bi interactions are responsible for the metallic behavior observed above T$_{c}$. Electronic band structure calculations at the density functional theory (DFT) level confirm the metallic character of the material. Defects in the form of vacancies on the Bi-sites were also found using single crystal X-ray analysis. The unexpected finding is that unlike most superconductors Ca$_{11}$Bi$_{10-x}$ has very low carrier density. The Ca$_{11}$Bi$_{10-x}$ system is the first member of the intermetallic class M$_{11}$X$_{10}$ (M$=$Ca, Sr, Ba; X$=$Bi, Sb) that exhibits superconductivity suggesting that a broader family of Bi or Sb-containing superconductors may exist. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q48.00003: Spin-orbit effects on electronic transport in superconductor/normal metal heterostructures Nayana Shah, Kuei Sun Electronic transport in superconductor/normal metal (S/N) heterostructures has been extensively investigated for understanding its coherent nature and application on probing quantum entanglement. Recent works have focused on a semiconductor nanowire with a proximity-induced superconducting gap on its part, which makes such device an effective S/N heterostructure with the presence of spin-orbit couplings (SOC) due to the material properties. The SOC splits the energy spectrum in a momentum-dependent pseudo-spin space and thus plays an essential role on the system's Fermi points as well as the group velocities of propagating particles and holes. We study the effects of Rashba and Dresselhaus SOCs and their interplay with the interfacial barriers on various transport processes, including normal/Andreev reflections on the N side and quasi-particle/hole transmissions on the S side. We obtain analytic results for the small SOC regime and numerically calculate experimental observables such as an I/V curve in a wide parameter space. Our results ought to advance the current study on such systems, especially that exploring Majorana zero modes on superconducting nanowires or spin transport in helical liquids. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q48.00004: Nature of local distortions in newly-discovered disordered superconductor, LaO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{2}$ Anushika Athauda, Bing Li, Shinichiro Yano, Seunghun Lee, Despina Louca, Yoshikazu Mizuguchi LaO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{2}$ is a disordered, non-magnetic superconductor belonging to the novel family of BiS$_{2}$ layered superconductors. The parent phase, LaOBiS$_{2}$, is a band insulator with tetragonal structure (P4/nmm space group). The highest T$_{\mathrm{c}}$ of LaO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$BiS$_{2}$ is attained at x $=$ 0.5, as 10.8 K. Upon F doping or temperature change, little change had been reported in the low-energy portion of the phonon spectrum and the question whether LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ is a BCS superconductor or not remains unsolved. The local atomic structures of LaOBiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ were investigated as a function of temperature by using elastic neutron scattering and the pair density function analysis from 6 to 300 K and 2 to 300K respectively. We present possible models of local structure of LaOBiS$_{2}$ and LaO$_{0.5}$F$_{0.5}$BiS$_{2}$ which qualitatively explain the temperature dependence and composition dependence. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q48.00005: Coexistence of ferromagnetism and superconductivity in CeO0.3F0.7BiS2 Jooseop Lee Neutron scattering measurements have been performed on a polycrystalline sample of the newly discovered layered superconductor CeO0.3F0.7BiS2 with or without a magnetic field. This system exhibits the rare and interesting case of the coexistence of ferromagnetism and superconductivity, which is hardly realized in the conventional phonon mediated superconductors due to their antagonistic nature. The crystal and magnetic structures was investigated together with the spin fluctuations, and the spin Hamiltonian describing the spin dynamics in this system was determined. In addition, we examined the external magnetic field dependence of both magnetic structure and its excitation. It seems that superconductivity is robust against a magnetic field and there is no direct relationship between magnetism and superconductivity in this system. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q48.00006: Hole Doped SrPt3P - Physical Characterization and Analysis BenMaan Jawdat, Bing Lv, Yuyi Xue, Ching-wu Chu In 2012, Takayama et al. [1] reported superconductivity up to 8.4 K in APt$_3$P (A = Sr, Ca, and La). Its crystal structure is similar to that of the noncentrosymmetric superconductor CePt$_3$Si, although it possesses a center of inversion symmetry. Afterwards, Nekrasov et al. predicted in a theoretical work that hole doping the system would increase N($\epsilon_f$) as well as the $\mathrm{T_C}$ [2]. We have therefore decided to investigate Si-doped SrPt$_3$P resistively, magnetically and calorimetrically. We found that 1. the partial replacement of P by Si results in hole-doping evidenced from our Hall measurements, within the solubility of Si in SrPt$_3$P, 2. The Sommerfeld coefficient of the compounds increases with Si-doping, and 3. the $\mathrm{T_C}$ decreases with Si-doping, in contrast to the theoretical prediction. The results will be presented and discussed together with results of other chemical doping in progress. \\[4pt] [1] Takayama, T. et al. Strong Coupling Superconductivity at 8.4 K in an Antiperovskite Phosphide SrPt$_3$P. Physical Review Letters 108, 237001 (2012).\\[0pt] [2] Nekrasov, I. A. \& Sadovskii, M. V Electronic Structure of New Multiple Band Pt-Pnictide Superconductors APt$_3$P 1, 1-5 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q48.00007: Muon spin relaxation and x-ray/neutron total scattering studies of layered superconductor BaTi2(As,Sb)2O Benjamin Frandsen, Yasumasa Nozaki, Emil Bozin, Hiroshi Kageyama, Simon Billinge, Yasutomo Uemura Layered oxy-pnictide systems such as ATi2Pn2O (A $=$ Na2, Ba, (SrF)2, (SmO)2; Pn $=$ As, Sb, Bi) possess interesting electronic and magnetic properties, including spin/charge density wave (S/CDW) ordering and superconductivity. In addition, they share similarities with the cuprate and iron-pnictide high-Tc compounds, such as planar sheets metal-oxygen sheets, electron/hole-symmetric electron configurations (3d1 and 3d9), and close proximity of density wave and superconducting orders, thus making them intriguing systems to study to gain insight into unconventional superconductivity. BaTi2(As,Sb)2O is a prototypical layered oxy-pnictide system known to have either CDW or SDW ordering for all compositions and superconductivity below 1 K for the Sb endmember. However, it has remained unclear whether the order is CDW or SDW. To investigate this, we have performed muon spin relaxation/rotation and x-ray/neutron total scattering measurements on several specimens. Zero-field muon spin relaxation measurements show no significant increase in relaxation rate at the density wave ordering temperature, indicating that the system undergoes CDW rather than SDW order. Pair distribution function analysis of the total scattering data has yielded insight into the structural details of the CDW transition. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q48.00008: Possible interface superconductivity with coherent quantum CDW transport and soliton condensation phase transition in heterogeneously doped ion implanted NbSe$_{3}$ single crystals Kalyan Sasmal, Dharshana Wijesundera, Irene Rusakova, Wei-Kan Chu, John H. Miller, Zhong Tang, Arnold Guloy Aharonov-Bohm quantum interference shows oscillations of period $h$/2$e$ in conductance \textit{vs}. magnetic flux of CDW rings above 77 K, reveals macroscopically observable quantum behavior. CDW transports electrons through a linear chain compound all together as the Peierls gaps displace in momentum space along with the entire Fermi Sea, similar to a superconductor. The dV/dI vs. bias at several temps showing a significant drop in zero-bias resistance below 46 K across an ion-implanted boundary suggests possible interfacial superconductivity or a related phase transition near the boundary between ion-implanted and un-implanted regions of a CDW in NbSe$_{3}$. The data suggests condensation of solitons near the interface. Charge soliton ($\pm$ 2$e$) dislocations could accumulate and condense near the boundary either due to injected charge from non-isoelectronic impurities or due to a sharp gradient in optimum CDW phase between the weakly and strongly pinned regions. Implanted NbSe$_{3}$ also been studied with TEM. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q48.00009: Dynamical Mean-Field Theory Study of Correlated Electronic Structures and the Phase Diagram of Hydrocarbon Superconductors Minjae Kim, Hong Chul Choi, Ji Hoon Shim, B. I. Min We have studied correlated electronic structures and the phase diagram of electron-doped hydrocarbon molecular solids, based on the dynamical mean-field theory. We have determined the phase diagram of hydrocarbon molecular solids as functions of doping and energy parameters including the Coulomb correlation, the Hund coupling, and the molecular-orbital (MO) energy level splitting. We have found that the hydrocarbon superconductors (electron-doped picene and coronene) belong to the multi-band Fermi liquid state, while non-superconducting electron-doped pentacene belongs to the single-band state in the proximity of the metal-insulator transition. The size of the MO energy level splitting plays an important role in deriving the superconductivity of electron-doped hydrocarbon solids. The multi-band nature of hydrocarbon solids from the small MO energy level splitting boosts the superconductivity through the enhanced density of states at the Fermi level. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q48.00010: What is special with molecular valence 3 in metal-intercalated phenacenes? Sumitendra Mazumdar, Tirthankar Dutta Two important observations are pertinent to the newly discovered metal-intercalated polycyclic aromatic hydrocarbon (PAH) superconductors (examples include, phenanthrene, picene, coronene, and dibenzopentacene): (a) Why are the acenes with linearly fused benezene rings not superconducting? (b) Why does T$_c$ in the PAHs peak only at metal doping $\sim$ 3? In order to address these questions, we have developed a correlated-electron minimal model for phenanthrene ions in solid state, in the reduced space of the two lowest unoccupied molecular orbitals of neutral phenanthrene. Our model is general and can easily be extended to normal states of other PAHs, like, picene and coronene, although the computations would be more demanding. Exact many-body finite cluster calculations on phenanthrene show that while the systems with molecular charges of $-$1 and $-$2 are one- and two-band Mott-Hubbard semiconductors, respectively, molecular charge $-$3 gives two nearly $\frac{3}{4}$-filled bands. The carrier density per active molecular orbital in the superconducting aromatics is nearly the same as that in the organic charge-transfer solids, and we believe that this common carrier density may be the key to understanding unconventional superconductivity in these molecular superconductors. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q48.00011: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q48.00012: Manipulation of a two-photon pump in superconductor -- semiconductor heterostructures Peter P. Orth, Paul Baireuther, Ilya Vekhter, Joerg Schmalian We investigate the photon statistics, entanglement and squeezing of a pn-junction sandwiched between two superconducting leads, and show that such an electrically-driven photon pump generates correlated and entangled pairs of photons. In particular, we demonstrate that the squeezing of the fluctuations in the quadrature amplitudes of the emitted light can be manipulated by changing the relative phase of the order parameters of the superconductors. This reveals how macroscopic coherence of the superconducting state can be used to tailor the properties of a two-photon state. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q48.00013: Probing the unconventional superconductivity of PrPt$_4$Ge$_{12}$ through Ce substitution Kevin Huang, Lei Shu, Ivy Lum, Benjamin D. White, Marc Janoschek, Duygu Yazici, James J. Hamlin, Diego A. Zocco, Pei-Chun Ho, Ryan E. Baumbach, M. Brian Maple Superconductivity has been observed in a new class of filled skutterudite compounds of the form $M$Pt$_{4}$Ge$_{12}$ ($M$ = Sr, Ba, La, Pr, and Th), with the Pr member exhibiting a high superconducting transition temperature $T_c$ $\sim$ 7.9 K. Most of these skutterudite compounds seems to exhibit conventional BCS-type superconductivity; however, superconductivity in Pr appears to be unconventional as it exhibits characteristics of time-reversal symmetry breaking. To help clarify the nature of the superconductivity in PrPt$_4$Ge$_{12}$, we studied the response of superconductivity to the substitution of Ce for Pr throughout the series Pr$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$ (0 $\leq$ x $\leq$ 1). Measurements of electrical resistivity, magnetic susceptibility, and specific heat were performed and superconductivity were observed up to a Ce concentration of 50$\%$. A pronounced change was observed in the temperature dependence of specific heat below $T_c$ where PrPt$_4$Ge$_{12}$ exhibits a crossover from a $T^3$ to an exponential temperature dependence when Ce is introduced. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q48.00014: Supermetallic and Trapped States in Periodically Driven Lattices Bala Sundaram, Indubala Satija The gapless metallic phases of periodically driven lattices are shown\footnote{Indubala I. Satija and Bala Sundaram, arXiv:0783264, cond-mat-quant-gas} to exhibit unusual resonant transport characteristics where every resonance is accompanied by a pair of anti-resonances. These anti-resonances describe highly coherent, dynamically localized or {\it trapped} states, in sharp contrast with the resonant or {\it supermetallic} states where the quantum dynamics describes free propagation. The supermetallic states are related to the integer winding of the quasienergy spectrum and are characterized by a band structure that is topologically a circle. In the context of novel phases of matter, our study elucidates important distinctions between static and driven systems that pave the way for engineering a variety of band structures resulting in free, trapped as well as flat band states. [Preview Abstract] |
Session Q49: Focus Session: Conductivity and Its Control at LaAlO3/SrTiO3 Interfaces
Sponsoring Units: DMPChair: Shahal Ilani, Weizmann Institute of Science
Room: Mile High Ballroom 1C
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q49.00001: Local conductivity enhancement due to the tetragonal domain structure in LaAlO$_{3}$- SrTiO$_{3}$ heterointerfaces Invited Speaker: Kathryn Moler Progress in the difficult task of growing oxide heterostructures has enabled the field of oxide interface engineering. The ability to control materials properties through interface engineering is demonstrated by the appearance of conductivity at the interface of certain insulators, most famously the \textbraceleft 001\textbraceright interface of the band insulators LaAlO$_{3}$ (LAO) and TiO$_{2}$-terminated SrTiO$_{3}$ (STO). The prevailing explanation of conduction at the interface is electronic reconstruction due to a `polar catastrophe' in which charge migrates from the top LAO layer to the interface. Transport and other measurements in this system display a plethora of diverse physical phenomena. To better understand the interface conductivity, we used scanning superconducting quantum interference device (SQUID) microscopy to image the magnetic field locally generated by current in an interface. At low temperature, we found that the current flowed in conductive narrow paths oriented along the crystallographic axes, embedded in a less conductive background. The configuration of these paths changed upon thermal cycling above the STO cubic to tetragonal structural transition temperature, implying that the local conductivity is strongly modified by the STO tetragonal domain structure. In this talk, I will summarize these results and also report on measurements of conductivity and diamagnetism in related materials that firmly establish the influence of the STO tetragonal domains on electronic properties. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q49.00002: Direct Imaging of Current Flow at the interface of Strained LaAlO3/SrTiO3 Aaron J. Rosenberg, Eric M. Spanton, John R. Kirtley, Christopher Bell, Yanwu Xie, Hiroki Sato, Masayuki Hosoda, Yasuyuki Hikita, Harold Y. Hwang, Kathryn A. Moler At the interface of band insulators LaAlO3 and SrTiO3, recent scanning SQUID and scanning SET studies show that conductivity and charge carrier density are influenced by the tetragonal domain structure of SrTiO3. SQUID imaging shows that transport current flows along more conductive narrow paths within a less conductive background, but the microscopic mechanism of the enhanced conductivity is still under investigation. We propose to apply tunable stress to the LaAlO3/SrTiO3 interface by while the sample is mounted in a scanning SQUID microscope, and observe how strain changes both the configuration of the more conducting paths and the magnitude of their conductivity enhancement. These studies may shed light on the relationship between strain, conductivity, and electronic structure in this system. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q49.00003: Switching properties of superconducting micron-size channels in LaAlO3 /SrTiO3 hetero-structures Simon Hurand, Cheryl Feuillet-Palma, Johan Biscaras, Nicolas Reyren, Edouard Lesne, Nicolas Bergeal, Jerome Lesueur It has been shown that a superconducting two-dimensional electron gas (2DEG) could form at the interface between two insulators such as LaX(X$=$Al or Ti)O3 and SrTiO3 [1,2,3]. We present low temperature transport measurements on micron-size superconducting channels patterned in LaAlO3/SrTiO3 hetero-structures, whose properties can be modulated by field effect. The current-voltage characteristics measured as a function of gate voltage and temperature show a hysteretic behavior. We analyzed the switching and re-trapping currents for a large number of events in the framework of the Resistively and Capacitively Shunted Junction model (RCSJ). Standard deviation of the switching distribution is found to be constant with temperature, but gate-voltage dependent. The results are consistent with the description of the 2DEG as an inhomogeneous array [4] of moderately damped Josephson junctions in the quantum escape regime. [1] A. Ohtomo et al, Nature 427, 423 (2004), Nature 419, 378 (2002) [2] N. Reyren et al, Science 317, 1196 (2007) [3] J. Biscaras et al, Nature Communications 1,89 (2010), Phys. Rev. Lett. 108, 247004 (2012) [4] J. Biscaras et al, Nature Materials 12, 542--548 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q49.00004: Coulomb blockade phenomena in confined LaAlO$_3$/SrTiO$_3$ nanowires Michelle Tomczyk, Guanglei Cheng, Shicheng Lu, Joshua Veazey, Mengchen Huang, Patrick Irvin, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy The LaAlO$_3$/SrTiO$_3$ interface hosts a rich variety of phenomena, including gate-tunable conductivity, ferromagnetism, and low-temperature superconductivity. Nanowires at the interface are fabricated with conductive AFM lithography; this flexible process allows complex nanostructures to be created to study the various phenomena at the interface. Here, tunneling barriers are created to confine a section of wire. Low temperature transport through these confined wires is gate-tunable, exhibiting superconductivity in the strong coupling regime and Coulomb blockade in the weak coupling regime. The Coulomb peaks exhibit interesting behavior in an external magnetic field. The peaks are insensitive to low fields; however, they begin to split and shift above a critical magnetic field. The curious splitting of each Coulomb peak above a critical field could provide insight into the fundamental magnetic, superconducting, and spin-orbit properties at the interface. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q49.00005: Real-Time Transport Properties of Multiply-Connected LaAlO$_3$/SrTiO$_3$ Nanostructures Alexandre Gauthier, Patrick Irvin, Jeremy Levy Electronic nanostructures can be created at the LaAlO$_3$/SrTiO$_3$ interface using a conductive atomic force microscope technique\footnote{C. Cen, \textit{et al.}, Science \textbf{323}, 1026 (2009)}. These nanowires can be arranged into complex nanostructures such as photodetectors\footnote{P. Irvin, \textit{et al.}, Nature Photonics \textbf{4}, 849 (2010)} and single-electron transistors\footnote{G. Cheng, \textit{et al.}, Nature Nanotechnology \textbf{6}, 343 (2011)}. We have developed a way to characterize multi-terminal devices in real time. AC voltages at distinct frequencies are applied to every electrical contact for the device. Fourier analysis of the current response at each electrode allows the $\textit{N}$-terminal conductance matrix to be determined simultaneously. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q49.00006: Anisotropic superconductivity of nanowires at LaAlO$_3$/SrTiO$_3$(110) heterointerface Jeremy Levy, Mengchen Huang, Guanglei Cheng, Patrick Irvin, Anil Annadi, Kalon Gopinadhan, A. Ariando, T. Venkatesan, Q. Zhang, Bo Gu, Seiji Yunoki, Sadamichi Maekawa A two-dimensional electron gas has recently been discovered at the interface of LaAlO$_3$ grown on the (110) surface of SrTiO$_3$.\footnote{A. Annadi, \textit{et al.}.\textit{Nature Commun.} \textbf{4}, 1838 (2013)} The conductivty of the 2DEG at this interface is anisotropic along different crystallographic directions. Using conducting AFM lithography we write interfacial nanowires along the (001) and (1-10) directions on 3-unit cell LaAlO$_3$/SrTiO$_3$(110). Similar to the anisotropic conductivity found in the normal state, we observe anisotropy of the superconducting properties of the two types of wire: the upper critical magnetic field of nanowires along the (001) direction is higher than those along (1-10) direction. This observation can be related to the anisotropic orbital binding of Ti and O atoms and the differences in the spin-orbit coupling along the two different directions. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q49.00007: High-mobility sketched nanostructures at the Al$_2$O$_3$/SrTiO$_3$ interface Shicheng Lu, Sang Woon Lee, Guanglei Cheng, Feng Bi, Akash Levy, Mengchen Huang, Patrick Irvin, Roy G. Gordon, Jeremy Levy A two dimensional electron gas has recently been demonstrated at the interface between amorphous Al$_2$O$_3$ and TiO$_2$-terminated SrTiO$_3$ by atomic layer deposition (ALO/STO).\footnote{S. W. Lee, \textit{et al.}, Nano Lett. \textbf{12}, 4775 (2012).} Similar to LaAlO$_3$/SrTiO$_3$ heterostructrues, when the ALO thickness exceeds a critical thickness, the interface becomes conducting. By using a conducting atomic force microscope tip to control the metal-insulator transition at nanoscale dimensions, we are able to create nanostructures with exceptionally high mobility. Quasi-two-dimensional written structures exhibit Shubnikov de Haas oscillations and mobilities in excess of 2,000 cm$^2$/Vs. Furthermore, by decreasing the channel width to 10 nm width, the mobility becomes as high as 100,000 cm$^2$/Vs. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q49.00008: Nanoscale control of oxide interface conduction in graphene-complex-oxide heterostructures Mengchen Huang, Sangwoo Ryu, Fereshte Ghahari, Giriraj Jnawali, Jayakanth Ravichandran, Patrick Irvin, Philip Kim, Chang-Beom Eom, Jeremy Levy Graphene is a promising material for high-speed optoelectronic devices such as THz modulators and detectors. Recently, broadband THz emission and detection can be achieved with nanostructures at the LaAlO$_3$/SrTiO$_3$ interface \footnote{Y. Ma, \textit{et al.}, \textit{Nano Lett.} \textbf{13}, 2284 (2013)}. We have mechanically exfoliated single layer and multilayer graphene on top of 3.4 unit cell LaAlO$_3$/SrTiO$_3$ and successfully sketched nanowires in the 2DEG underneath graphene using conductive AFM lithgraphy \footnote{C. Cen, \textit{et al.}, \textit{Nat. Mater.} \textbf{7}, 298 (2008)}. Raman and AFM investigations confirm that the graphene quality and surface morphology remain unaltered by the writing process. These first experimental demonstrations of integrating graphene and LaAlO$_3$/SrTiO$_3$ are promising for future DC-THz photonic applications. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q49.00009: Ionic Liquid/Solid Double Gate Modulation of the LaAlO$_{3}$/SrTiO$_{3}$ Interfacial Electron Gas Zhuoyu Chen, Hongtao Yuan, Yanwu Xie, Di Lu, Yasuyuki Hikita, Christopher Bell, Harold Hwang An intriguing combination of properties including high electron mobility, superconductivity, ferromagnetism, and strong spin-orbit coupling has been observed at the LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO) interfacial quasi-two-dimensional electron gas (q2DEG). To experimentally clarify the electronic band structure of the q2DEG and how these properties evolve with external tuning by the electric field effect is a key challenge. Here we study the transport properties of the q2DEG in a double-gate field-effect transistor geometry utilizing an ionic liquid as the top gate dielectric, and the STO substrate as the back gate. A systematic carrier density and mobility modulation over a previously unobtained parameter range is achieved, providing a clear picture of electrostatic gating in this system. Changes in the carrier density, mobility, and conductivity strongly suggest the filling of heavy- and light- mass subbands in the quantum well as the top gate voltage is increased with respect to the q2DEG. When the heavier-mass electron subbands dominate the conductivity, signatures of negative electronic compressibility were observed, implying the presence of a tunable strong Rashba spin-orbit splitting in the anisotropic heavier-mass bands at this heterointerface. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q49.00010: Nanoscale control of the LaAlO$_3$/SrTiO$_3$ metal-insulator transition using a self-assembled monolayer of APTES Jianan Li, Mengcheng Huang, Patrick Irvin, Jeremy Levy, Sangwoo Ryu, Chang-Beom Eom, Daniel Eichelsdoerfer, Keith Brown, Chad Mirkin Nanoscale control over the metal-insulator transition at oxide interfaces represents an exciting opportunity for science and technology. Nanostructures created from 3-unit-cell LaAlO$_3$/SrTiO$_3$ heterostructures via a conductive AFM technique typically decay within hours under ambient conditions, representing a challenge for some technologies. By chemically modifying the top LaAlO$_3$ surface with a self-assembled monolayer of (3-Aminopropyl)triethoxysilane (APTES), normally conductive 4-unit-cell LaAlO$_3$/SrTiO$_3$ can be made highly insulating. The APTES layer can be locally patterned, revealing a highly stable conductive nanoregion. Four-terminal measurements show that nanowires created by selective desorption of APTES remain conductive indefinitely under ambient conditions. The results suggest a robust mechanism for creating long-lived nanostructures at oxide interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q49.00011: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q49.00012: Tunning the conductivity at \textit{n} --type LAO/STO heterointerface by Au(111) nanoparticles Tra Vu Thanh The observation of a two-dimensional electron gas (2DEG) at the LaAlO$_{3}$/SrTiO$_{3}$ heterostructure is a well-known example of interface physics. While most studies are aiming at exploring new interfaces by combining different materials, another key to real device applications is the interface control through external stimuli. In this paper, we propose a generic approach to use Au(111) nanoparticles on the LAO/STO interface as an external stimules in pumping the free electrons into the 2DEG of the LAO/STO heterointerface. Our results show that the conductivity of the LAO/STO interface increases with the density and size of Au nanoparticles. In order to reveal the insight, the change of the interface band structure was investigated by combining both the ferroelectric pattern assisted x-ray photoelectron spectroscopy and the scanning tunneling spectroscopy with modeling calculations. This study opens a new venue of controlling the conduction of complex oxide interfaces. [Preview Abstract] |
Session Q50: Photonic Structures and Semiconductor Lasers
Sponsoring Units: DCMPRoom: Mile High Ballroom 1D
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q50.00001: Colloidal structures of magnetite particles for electrically tunable photonic crystal/glass device Beom-Jin Yoon, Seung Taek Oh, Haenyung Lee, Young-Seok Kim, Gi-Ra Yi Here we report colloidal structures of magnetite particles designed for electrically tunable photonic crystal/glass devices. The color of reflected light from the photonic crystal/glass was tuned by applied electric field and electrophoretic behaviors of the particles. Colloidal dispersion of magnetite particles in polar and non-polar solvent was prepared. The ordered photonic structures were induced by applied electric field. Photonic band gap, the origin of reflected colors, was modulated by the electric field. The three primary colors (red, green, and blue) of display device were successfully presented and tuned by electric field even if the colloidal dispersion didn't show structural coloration without applied field. Peak position of the reflected color, purity of the color, and operating voltage were determined by particle size and surface charge of magnetite particles. The electrophoretic behavior and optical properties of magnetite particles were quantitatively studied, and the principle of color tuning in photonic crystal/glass devices was investigated. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q50.00002: Photonic band structures of periodic arrays of pores in a metallic host: tight-binding beyond the quasistatic approximation Kwangmoo Kim, David Stroud We have calculated the photonic band structures of metallic inverse opals and of periodic linear chains of spherical pores in a metallic host, below a plasma frequency $\omega_{\mathrm{p}}$. In both cases, we use a tight-binding approximation, assuming a Drude dielectric function for the metallic component, but without making the quasistatic approximation. The tight-binding modes are linear combinations of the single-cavity transverse magnetic (TM) modes. For the inverse-opal structures, the lowest modes are analogous to those constructed from the three degenerate atomic $p$-states in fcc crystals. For the linear chains, in the limit of small spheres compared to a wavelength, the results bear some qualitative resemblance to the dispersion relation for metal spheres in an insulating host, as calculated by Brongersma {\it et al.}\ [Phys.\ Rev.\ B \textbf{62}, R16356 (2000)]. Because the electromagnetic fields of these modes decay exponentially in the metal, there are no radiative losses, in contrast to the case of arrays of metallic spheres in air. We suggest that this tight-binding approach to photonic band structures of such metallic inverse materials may be a useful approach for studying photonic crystals containing metallic components. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q50.00003: Transmission properties of a Fibonacci quasi-crystals containing single-negative materials and their usage as multi-channel filters Ali Charkhesht, Hamid Pashaei Adl, Samad Roshan Entezar One of the interesting phenomena appearing in Fibonacci quasi-crystals is wave localization, so that the field becomes spatially confined in some suitable regions, or delocalized in some other parts. Many theoretical works have been written on this interesting subject. The periodic Fibonacci structure properties lead to a transmission spectrum that exhibits some band gap, and it is possible to control these band gaps by the generation number of this structures. All these properties make Fibonacci quasi-crystals materials very attractive from an optical point of view. Accordingly, the transmission properties of Fibonacci quasi-crystals containing single-negative materials are investigated with the transfer matrix method. It is shown that the periodic structures created by repeating the Fibonacci quasi-crystal generations, have some omnidirectional band gaps at the single-negative frequency region.~~Moreover, it is shown these band gaps depends on the number of Fibonacci photonic crystal unit cell. In other words, when generation number of Fibonacci photonic crystal unit cell increases, some sub band gaps appears within this omnidirectional band gap. In this work by using Fibonacci quasi-periodic structures we demonstrate that by increasing Generation Number of Unit cell, some omnidirectional sub-gaps will appear which can be used as a multichannel filter. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q50.00004: 3D Photonic Crystals from a Forest of ZnO Nano-Pillars Donald Priour The band gap and dispersion curves are obtained for photonic crystals made up of square or hexagonal arrays of ZnO nano-pillars with a periodic modulation of the refractive index along the axis of the pillars. The wavelength of the intra-pillar index of refraction variation and the unit cell of the pillar lattice are in the optical range, on the order of 500 nm. The photonic dispersion curves are calculated with a treatment analogous to the nearly free electron model used in electronic band structure calculations; we expand in terms of wave functions compatible with the symmetry of the Brillouin Zone and thereby avoid discretizing the nano-pillar geometries. We validate our results by comparison with band structures obtained by alternative techniques for special lattice geometries. To examine the effect on salient features of the band structure, and to maximize the band gap, we vary parameters such as the pillar radius, the pillar lattice unit cell size, and the wavelength and amplitude of the intrapillar refraction index modulations. While on the one hand we consider an idealized forest comprised of infinitely tall pillars, we also examine finite pillars, the situation to be studied in experiment. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q50.00005: The relationship between surface impedance and bulk band topological phases and interface state formation in one-dimensional systems Meng Xiao, Zhao-Qing Zhang, C.T. Chan Surface impedance is an important concept in photonic systems such as photonic crystals (PCs) For example, the condition of an interface state formation in the interfacial region of two different PCs is simply $Z_{SL} +Z_{SR} =0$, where $Z_{SL} \left( {Z_{SR} } \right)$ is the surface impedance of the semi-infinite PC on the left- (right-) hand side of the interface. We show a rigorous relation between the surface impedance of a one-dimensional PC and its bulk properties through the geometrical phases of the bulk bands. The existence of an interface state in a particular band gap can be determined by the Zak phases of the two PCs. Our results provide new insights on the relationship between surface properties and the bulk properties and the formation of interface states. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q50.00006: Plasmon Polariton Modes in High Index Dielectric Structures Kyle Hoke, Kodiak Murphy, Brad Johnson, Janelle Leger The need to interface optical signals with increasingly small electronic components has led to an interest in subwavelength waveguides. Surface plasmon polaritons (SPPs) are longitudinal surface charge density oscillations localized to a metal/dielectric interface, and as such are capable of confining energy in a structure which is not diffraction limited. Waveguides based on the excitation of SPPs are promising for short-range applications, but in these structures Ohmic damping significantly limits propagation length due to the bulk of the electric field propagating along a metal interface. Here we demonstrate that through selection of materials with specific optical properties, Ohmic damping can be drastically reduced. Specifically, high index dielectric plasmon polariton modes (HID-PPMs) are surface-constructed waves that exist in structures having a core dielectric layer with a higher refractive index than the substrate supporting them. Modes in this region exhibit oscillatory electric fields with the bulk of their electric field confined in the dielectric layer, similar to a total internal reflection waveguide. Damping and insertion losses may therefore be drastically reduced in such structures. Here we report the demonstrationof HID-PPMs in Au/TiO2/Au MIM devices using attenuated total reflectance measurements. Characterization of these modes was performed for several devices of differing core dielectric thickness. Results are in good agreement with theory. We will discuss the application of these waveguides to several technologies related to solar energy conversion. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q50.00007: Electromagnetic Heat Transfer in Artificial Materials Lilia Woods, David Drosdoff, Anh Phan Electromagnetic energy exchange has found promising new opportunities by greatly enhancing the heat transfer between bodies via radiation in the near-field regime. The greatest heat transfer occurs when the bodies support surface plasmons or polaritons that share the same resonant frequency. It has been shown, however, that 2-D materials such as graphene can have their surface plasmons tuned by modifying the chemical potential and temperature. This allows for tuning its resonance with other systems. In this talk, we investigated the electromagnetic radiation in metamaterials characterized by a strong magnetic response. We study theoretically Pendry-like and magnetically active metamaterial/graphene composites. The possibility for enhancing or inhibiting the heat transfer via the graphene properties is investigated. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q50.00008: Defect-Free nanoscale printing using the Talbot effect Mario Marconi, Wei Li, Victor Martinez Esquiroz, Lukasz Urbanski, Dinesh Patel, Carmen Menoni, Aaron Stein, Weilun Chao, Erik Anderson An Extreme Ultraviolet (EUV) lithography technique that utilizes a compact EUV laser to print nanoscale features on a photoresist is presented. The lithographic method uses the Talbot effect and is based on the self-imaging produced when a periodic transmission mask is illuminated with a coherent light beam. A periodic mask composed of an array of tiles with an arbitrary design produces self images that are used to replicate the mask in the surface of a photoresist. When illuminated with coherent light, the tiled diffractive mask produces images which are 1$\times$ replicas at certain locations (Talbot planes). The self-images are generated by the diffraction of the thousands of cells in the mask. Thus, any defect in any of the unitary cells is averaged over a very large numbers of tiles consequently rendering a virtually defect-free image. This is a unique characteristic of this photolithographic approach. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q50.00009: The effect of capping chemistry on GaSb Quantum Dot shape and photoluminescence Matt DeJarld, Erwin Smakman, Marta Luengo-Kovac, Andrew Martin, Vanessa Sih, Paul Koenraad, Joanna Millunchick GaSb quantum dots are known to disintegrate upon capping with GaAs, leading to an increase Sb incorporation into the wetting layer. Preventing Sb diffusion from the quantum dot could improve the retention of the dot shape and reduce wetting layer thickness. To test this theory, the quantum dots were capped with four different capping layers: 50nm of GaAs, 1 monolayer (ML) of AlAs with 50nm of GaAs, 3ML of AlAs with 50nm of GaAs, and 20nm of Al$_{0.5}$Ga$_{0.5}$As with 30nm of GaAs. A clear improvement in the retention of the shape was observed in the Al-containing caps. In this case, at least 70{\%} of the dots retained their shape and had an average height of 4.5nm, compared to only 45{\%} retaining their shape and having a height of 3nm for the GaAs-capped samples. However, the strain induced by the larger dots caused stacking faults to form. Photolumiescence shows that the wetting layer peak shifted towards higher energies in Al containing samples, consistent with a reduction in wetting layer thickness. The dot peak was at approximately the same position in all four samples, perhaps due to the fact that the defects present near the larger dots made them optically inactive. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q50.00010: Secondary instabilities of transverse patterns in a coherent microcavity polariton fluid M.H. Luk, C.Y. Tsang, P. Lewandowski, Y.C. Tse, N.H. Kwong, A. Luecke, P.T. Leung, R. Binder, S. Schumacher Formation of Turing patterns in interacting coherent polariton fluids has recently been studied theoretically and demonstrated experimentally in semiconductor quantum well microcavities. These patterns originate from modulational instabilities driven by inter-polariton scatterings, leading to translational and rotational symmetry breakings in the laser-pumped polariton field. Competitions among hexagonal and stripe patterns under various conditions have been studied. However, further investigations show that under certain conditions, these simple patterns may be transient and, over time, could undergo secondary instabilities, collapsing or evolving into more complicated states. We will discuss these secondary instabilities of hexagonal and stripe patterns using linear stability analysis and numerical simulations. Furthermore we will discuss optical feedback/filter schemes to stabilize some naturally unstable patterns. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q50.00011: Controlling Turing Patterns in Spinor Polariton Fluids in Semiconductor Microcavities K.P. Chan, P. Lewandowski, V. Ardizzone, Y.C. Tse, N.H. Kwong, M.H. Luk, A. Luecke, M. Abbarchi, E. Baudin, E. Galopin, J. Bloch, A. Lemaitre, P.T. Leung, Ph. Roussignol, R. Binder, J. Tignon, S. Schumacher The formation of Turing patterns in a coherent quantum fluid of polaritons has been achieved by pumping a quantum well placed inside a double-microcavity with a normally incident laser beam. With an external weak control beam or other asymmetries introduced in the system, the Turing patterns could be switched. On the other hand, recent experiments show that the orientation and polarization of the patterns can also be controlled by the polarization of the incident pump. This introduces the pump polarization as a new control, which originates from the spinor nature of the polariton fluid.We will present our theoretical studies on the polarization control mechanisms using numerical simulations and analytic simplified models. In particular, we examine how various spin-dependent physical factors, including inter-polariton interactions, polariton dispersion and photon-exciton coupling strengths,control the polarization and orientation of the pattern as observed in the recent experiment. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q50.00012: Polariton Laser in a Wire-Like Microcavity Based on a Sub-Wavelength Grating Bo Zhang, Seonghoon Kim, Zhaorong Wang, Sebastian Brodbeck, Christian Schneider, Martin Kamp, Sven Hoefling, Hui Deng We report exciton-polariton lasing in a wire-like GaAs microcavity. The microcavity is sandwiched between a high-index-contrast sub-wavelength grating as the top mirror and a distributed Bragg reflector as the bottom mirror. The grating has dimensions 7.5 microns by 30 microns. The photoluminescence spectrum in momentum space shows discrete levels along the more tightly confined direction and quasi-continuous dispersion along the orthogonal direction. Unlike in the zero dimensional devices where polariton lases at ground state, in the wire-like device we observe lasing at a state that emerges between first and second lowest energy bands. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q50.00013: Multi-Color Nanowire Photonic Crystal Laser Pixels Jeremy Wright, Sheng Liu, George Wang, Qiming Li, Alexander Benz, Daniel Koleske, Ping Lu, Huiwen Xu, Luke Lester, Ting Luk, Igal Brener, Ganapathi Subramania Emerging applications such as solid-state lighting and display technologies require micro-scale vertically emitting lasers with controllable distinct lasing wavelengths and broad wavelength tunability arranged in desired geometrical patterns to form ``super-pixels.'' Conventional edge-emitting lasers and current surface-emitting lasers do not produce a viable solution as they require abrupt changes in semiconductor bandgaps or cavity length. Here, we successfully address these challenges by introducing a new paradigm that extends the laser tuning range additively by employing multiple monolithically grown gain sections each with a different emission center wavelength. Using broad gain-bandwidth III-nitride multiple quantum well (MQW) heterostructures and a novel top-down nanowire photonic crystal nanofabrication we obtain single-mode lasing in the blue-violet spectral region(Sci.Rep,\textbf{3},2982(2013)). This has a remarkable 60 nm of tuning (or 16{\%} of the nominal centre wavelength) that is determined purely by the photonic crystal geometry. This approach can be extended to cover the entire visible spectrum. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q50.00014: Electrically pumped near-ultraviolet lasing from ZnO Nanowire Based Heterojunctions Richard Mu, Haiyang Xu, Yichun Liu ZnO with a band gap (3.37 eV) and an exciton binding energy (60 meV) is a promising material for ultraviolet (UV) light-emitting diodes (LEDs) and low-threshold lasing diodes. Much progress has been made recently to enhance band edge emission of ZnO nanowire (NW) structure through surface passivation and local surface plasmon enahncement with metal nanparticles. Efforts have been made to fabricate electrically pumped near-ultraviolet lasing devices with metal/insulator/semiconductor laser diode based on ZnO/MgO core/shell nanowires with and without metal nanoparticle presences. The nanowire diode shows higher emission intensity at relatively low operating current density compared with the planar device. The improved efficiency is attributed to enhanced exciton oscillator strength and superior carrier transport properties of single-crystalline ZnO nanowires, and effective surface passivation by MgO coating. Random laser action was confirmed by the calculation of quality factor and the real-time changes of lasing spectra. The results reveal that the MgO coating serves as electron blocking, hole supplying and surface passivation layer for the nanowire heterostructure. Other approaches will also be presented and discussed in the presentation. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q50.00015: Plasmonic and Photonic Lasers Based on Semiconductor Nanowires: Low-loss and High Mode-tunablity Qihua Xiong, Qing Zhang, Xinfeng Liu, Tzechien Sum Understanding the optical gain and mode-selection mechanisms in semiconductor nanowire (NW) based photonic and plasmonic laser is key to the development of high-performance nanoscale oscillators/amplifiers/lasers. Modification of semiconductor band structure through electric field modulation, or alloying semiconductors has so far gained limited success in achieving output mode tunability of the NW laser due to the considerable optical losses. Herein we demonstrate a new optical self-feedback mechanism based on the intrinsic self-absorption of the gain media and plasmonic enhanced Burstein--Moss effect to achieve low-loss photonic and plasmonic lasing with a high degree of mode selectivity (over 30 nm). Moreover, we demonstrate the first room-temperature ultra-violet ($\sim$ 370 nm) plasmonic nanolaser with an extremely-low-threshold ($\sim$ 3.5 MW/cm$^{2})$. A closed-contact planar semiconductor-insulator-metal interface greatly lessens the extrinsic cavity loss, and efficiently promotes the exciton-plasmon energy transfer thus furnishes adequate optical gain to compensate the loss. Our straightforward approaches are widely applicable in most semiconductor NW plasmonic/photonic cavities. [Preview Abstract] |
Session Q51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties VII
Sponsoring Units: DMPChair: Eugenio Cinquanta, CNR-IMM MDM Laboratory
Room: Mile High Ballroom 1E
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q51.00001: Germananes: Germanium Graphane Analogues Invited Speaker: Joshua Goldberger Graphene's success has shown that it is not only possible to create stable, single-atom thick sheets from a crystalline solid, but that these materials have fundamentally different properties than the parent material. Our interest focuses on the synthesis and properties of Group IV graphane analogues. We have synthesized for the first time, mm-scale crystals of a hydrogen-terminated germanium multilayered graphane analogue (germanane, GeH) from the topochemical deintercalation of CaGe$_{2}$. This layered van der Waals solid is analogous to multilayered graphane. The surface layer of GeH only slowly oxidizes in air over the span of five months, while the underlying layers are resilient to oxidation. We demonstrate that it is possible to covalently terminate the external surface with organic substituents to tune the electronic structure, and enhance the stability. These materials represent a new class of covalently terminated graphane analogues having great potential for a wide range of optoelectronic and sensing applications, especially since theory predicts a direct band gap of 1.53 eV and an electron mobility of 18,000 cm2/Vs which is five times higher than that of bulk Ge. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q51.00002: Unveiling the origin of the linear dispersion in silicene/Ag(111) Mingxing Chen, Michael Weinert The band structure of the recently synthesized (3$\times$3) silicene monolayer on (4$\times$4) Ag(111) is investigated using density functional theory. To compare to recent angle-resolved photoemission spectroscopy (ARPES) experiments [Phys.\ Rev.\ Lett.\ {\bf 108}, 155501 (2012)], including the photon energy ($k_\perp$) dependence of the spectra, we use a $k$-projection technique to unfold the supercell bands of both silicene and the substrate onto the corresponding primitive cells. Our calculations reproduce the observed linear dispersion across the K point of (1$\times$1) silicene observed, but demonstrate that this is not a Dirac state, but rather originates from the Ag(111) substrate, thus resolving the controversy concerning the origin of the linear dispersion in silicene/Ag(111). [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q51.00003: Hexagonal Warping and Quasiparticle Chirality in Silicene Probed by Scanning Tunneling Microscopy Baojie Feng, Lan Chen, Kehui Wu We performed low temperature scanning tunneling microscopy (STM) and spectroscopy (STS) studies on the electronic properties of ($\surd 3 \times \surd 3$)R30 phase of silicene on Ag(111) surface. We found the existence of Dirac Fermion chirality through the observation of 1.5 and 1.0 power law decay of quasiparticle interference (QPI) patterns. Moreover, in contrast to the trigonal warping of Dirac cone in graphene, we found that the Dirac cone of silicene is hexagonally warped, which is further confirmed by density functional calculations and explained by the unique superstructure of silicene. Our results demonstrate that the ($\surd 3 \times \surd 3$)R30 phase is an ideal system to investigate the unique Dirac Fermion properties of silicene. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q51.00004: Microscopic origin of the $\pi$ states in epitaxial silicene on ZrB$_2$(0001) Antoine Fleurence, Yasuo Yodhida, Chi-Cheng Lee, Taisuke Ozaki, Yukio Hasegawa, Yukiko Yamada-Takamura Silicene, the graphene-like allotrope of silicon is the object of a recently raised enthusiasm, due to the perspectives opened by the novelty of its electronic, physical and chemical properties deriving from its $\pi$ electronic system. So far, silicene only exists in epitaxial forms on metallic substrates. In particular, the spontaneous and self-terminating segregation of silicon atoms on the (0001) surface of zirconium diboride (ZrB$_2$) thin films epitaxied on Si(111) gives rise to a wide-scale uniform $(\sqrt{3}\times\sqrt{3})$-reconstructed two-dimensional silicene sheet $[1]$. By means of low-temperature scanning tunneling spectroscopy and density functional theory calculations, we investigated the impact of the buckling of epitaxial silicene at atomic scale on the electronic properties. The microscopic origin of the valence and conduction states was determined and the strong contribution of the p$_z$ orbitals of specific Si atoms to those states demonstrate their $\pi$ character. A clear correlation between the estimated orbital hybridization of the Si atoms and the buckling was also found. $[1]$: A. Fleurence et al., Phys. Rev. Lett., 108, 245501 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q51.00005: Growth and Oxidation of Silicene Nano-Ribbons on Ag(110) Mohamed Rachid Tchalala, H. Enriquez, A. Mayne, G. Dujardin, H. Oughaddou, M. Ait Ali Scanning tunneling microscopy (STM) and high resolution photoemission electron spectroscopy (HR-PES) are used to study the growth and the oxidation of silicene nano-ribbons (NRs) on Ag(110) substrate. Deposition of silicon on Ag(110) induces a self-assembled silicene NRs having a (2x5) superstructure. We find out that the NRs are not reactive to molecular oxygen. However for a certain bias, the STM tip can dissociate the molecular oxygen which reacts then immediately with the NRs. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q51.00006: Unusual Optoelectronic Properties of Hydrogenated Bilayer Silicene: From Solar Absorber to Light-emitting Diode Applications Bing Huang, Hui-Xiong Deng, Hoonkyung Lee, Changwon Park, Mina Yoon, Bobby Sumpter, Feng Liu, Sean Smith, Su-Huai Wei Silicon is arguably the greatest electronic material, but not so good an optoelectronic material. By employing first-principles calculations and cluster-expansion approach, we discover that hydrogenated bilayer silicene (BS) shows promising potential as new optoelectronic materials. Most significantly, hydrogenation will covert the intrinsic BS, a strongly indirect semiconductor, into a direct-gap semiconductor with a widely tunable band gap. At low hydrogen concentrations, four ground states of single- and double-side hydrogenated BS are characterized with dipole-allowed direct (or quasidirect) band gaps in the desirable range from 1 to 1.5 eV, suitable for solar applications. At high hydrogen concentrations, three well-ordered double-side hydrogenated BS structures exhibit direct (or quasidirect) band gaps in the range of red, green, and blue colors, respectively, affording white light emitting diodes. Our findings open a door to the search of new silicon-based light-absorption and light-emitting materials for earth-abundant high-efficiency optoelectronic applications. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q51.00007: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q51.00008: Photo-induced Modulation Doping in Graphene/Boron nitride Heterostructures Jairo Velasco Jr., Long Ju, Edwin Hwang, Salman Kahn, Casey Nosiglia, Hsin-Zon Tsai, Wei Yang, Guangyu Zhang, Takashi Taniguchi, Kenji Watanabe, Yuanbo Zhang, Michael Crommie, Alex Zettl, Feng Wang Van der Waals heterostructures (VDH) provide an exciting new platform for materials engineering, where a variety of layered materials with different electrical, optical and mechanical responses can be stacked together to enable new physics and novel functionalities. We report an emerging optoelectronic phenomenon (i.e. photo-induced modulation doping) in the graphene-boron nitride VDH (G/BN heterostructure). We find it enables flexible and repeatable writing and erasing of charge doping in graphene with optical light. We show that the photo-induced modulation doping maintains the remarkable carrier mobility of the G/BN heterostructure, and it can be used to generate spatially varying doping profiles like pn junctions. Our work contributes towards understanding light matter interactions in VDHs, and introduces a simple technique for creating inhomogeneous doping in high mobility graphene devices. [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q51.00009: Sub-nanometer milling of layered materials by a focused Helium Ion Beam Hongzhou Zhang, Daniel Fox, Yangbo Zhou, Robert O'Connell The modification of the structure and geometry of materials at the nanoscale can be used to tailor their properties. A controllable process which can achieve this is required for the development of next generation nano-devices. We used the highly focused beam of helium ions in a helium ion microscope (HIM) to fabricate nanostructures within various layered materials such as graphene, MoS$_{2}$, TiO$_{2}$ and Mn$_{2}$O$_{3}$. Arbitrary patterns can be defined in order to produce structures such as nanoribbons. The edge configuration of atoms in such structures plays a large role in defining their properties. High resolution transmission electron microscopy (TEM) and scanning-TEM (STEM) were used to analyse the structure of the materials after milling. The direct milling of the materials by the helium ions means this approach is suitable for a wide range of nanomaterials. Complex structures can be realized via sophisticated beam control. This also results in the ability to mill along different directions in a crystal, producing edges with different configurations. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q51.00010: Universal method for creating optically active nanostructures on layered materials Tim Kidd, Rui He, Andrew Stollenwerk, Aaron Oshea, Ben Beck, Kyle Spurgeon, Genda Gu We report a new method for the creating of nanostructures using a scanning electron microscope. Residual organic molecules on the surface of layered materials can be excited by electron beam radiation to burrow into the open spaces between the layers of these materials, and then are broken down further to form photoluminescent carbon nanoclusters. Surface characterization by atomic force microscopy shows the surface is nearly undamaged at the molecular level by this process, and a lack of nanostructure formation in non-layered materials confirms that the structures are created by sub-surface incorporation. The presence of carbon nanoclusters was determined by Raman Spectroscopy and photoluminescence in the visible light range. The nanostructures are react strongly to visible light, making them readily apparent using an optical microscope even for features measuring only a few nanometers tall. This technique can be used on apparently any layered material, with successful results on dichalcogenides, topological insulators, graphite, and high temperature copper oxide superconductors. This technique can create patterned nanostructures with vertical resolution at the nanometer scale and lateral resolution of tens of nanometers depending on beam spot size. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q51.00011: Topological Zero-mode States in Graphene/hBN Superlattices Justin Song, Leonid Levitov Lateral heterostructures comprised of layered Van der Waal's materials, such as graphene on hexagonal boron nitride (hBN) are a new playground in which electronic Hamiltonians can be engineered, and new electronic states can be found. G/hBN heterostructures provide an instructive example wherein the lateral superlattice periodicity can match typical electron wavelengths. As a result, G-hBN coupling, even if weak, strongly affects electronic states. We will describe how novel electronic states can arise in G/hBN superlattices. Importantly, G/hBN exhibits a potential landscape which has a spatially alternating mass (sub-lattice asymmetric) allowing topologically protected 1-D states to form along its nodal lines. These analogs of Topological Insulator surface states have clear manifestations which allow them to be observed. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q51.00012: Novel Electronic Properties of Si-Doped Boron Nitride Monolayers Sanjeev Gupta, Haiying He, Douglas Banyai, Mingsu Si, Ravindra Pandey, Shashi Karna In this work, we address the most critical fundamental question on the effect of doping, especially by Si, on the stability, electronic, magnetic and electron transport properties of the two-dimensional BN monolayers using density functional theory. Si substitutions at B, N and a divacancy site created by the removal of a B-N pair are investigated. The non-equal valence state of the Si dopant leads to a non-zero magnetic moment for the single site substitution. Si-induced gap states in BN monolayer energy spectrum are also observed, which demonstrate profound impact on the electron transport properties of the BN monolayer. Unique features in the device characteristics of Si-doped BN monolayers are predicted including a significant enhancement of current at the Si site, diode-like asymmetric current-voltage response and negative differential resistance. The calculated STM images clearly discern the site-dependence of Si dopants in the monolayer. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q51.00013: Flakes of artificial graphene in magnetic fields Esa Rasanen, Michael Aichinger, Stefan Janecek Artificial graphene [1] (AG) is a man-made nanomaterial that can be constructed by arranging molecules on a metal surface [2] or by fabricating a quantum-dot lattice in a semiconductor heterostructure [3]. In both cases, AG resembles graphene in many ways [1-4], but it also has additional appealing features such as tunability with respect to the lattice constant, system size and geometry, and edge configuration. Here we solve numerically the electronic states of various hexagonal AG flakes similar to those in Ref. [2]. In particular, we demonstrate the formation of the Dirac point as a function of the lattice size and its response to an external, perpendicular magnetic field. Secondly, we examine the complex behavior of the energy levels as functions of both the system size and magnetic field. Eventually, we find the formation of ``Hofstadter's butterfly''-type patterns in the energy spectrum.\\[4pt] [1] For a recent review, see M. Polini et al., Nature Nanotech. 8, 625 (2013).\\[0pt] [2] K. K. Gomes et al., Nature 483, 306 (2012).\\[0pt] [3] M. Gibertini et al., Phys. Rev. B 79, 241406(R) (2009).\\[0pt] [4] E. Rasanen et al., Phys. Rev. Lett. 108, 246803 (2012). [Preview Abstract] |
Session Q52: Physical Properties of Copper-oxide Superconductors
Sponsoring Units: DCMPChair: Antony Carrington, University of Bristol
Room: Mile High Ballroom 1F
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q52.00001: In-plane Anisotropic Superconductivity in YBa$_{2}$Cu$_{3}$O$_{7-\sigma}$/ BiFeO$_{3}$ Heterostructure Yen-Lin Huang, Ying-Hao Chu We demonstrate a correlation between the periodic domain structure of multiferroic BiFeO$_{3}$ thin films and the anisotropic superconducting of YBa$_{2}$Cu$_{3}$O$_{7-\sigma}$/BiFeO$_{3}$ heterostructures. Two types of periodic domain structures in BFO -- 71$^{\circ}$ and 109$^{\circ}$ are used to manipulate the superconductor, YBCO, and an anomalous superconducting behavior, which shows different transition regions parallel or perpendicular to the domain walls of BFO respectively, is observed. The superconducting transition region is much broader when the current goes perpendicularly to the domain walls in BFO than parallel. The difference of Tc is about 40 K, in other words the YBCO shows one dimensional superconducting behavior within 40 K. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q52.00002: Evolution of the Superconductivity Dome in the two dimensional Hubbard Model Kuang-Shing Chen, Ziyang Meng, Shuxiang Yang, Thomas Pruschke, Juana Moreno, Mark Jarrell By means of large-scale dynamical cluster quantum Monte Carlo simulations, we are able to identify the evolution of the $d$-wave superconducting dome in the hole-dope side of the phase diagram, with next-nearest-neighbor hopping ($t'$), chemical potential and temperature as control parameters. To obtain the superconducting transition temperature $T_c$, we employ two-particle measurements of the pairing susceptibilities. As $t'$ goes from positive to negative values, we find the $d$-wave projected irreducible pairing vertex function is enhanced, and the curvature of its doping dependence changes from convex to concave, which fixes the position of the maximum superconducting temperature at the same filling ($n\approx0.85$) and constraints the dome from precisely following the Lifshitz line. We furthermore decompose the irreducible vertex function into fully irreducible, charge and spin components via the parquet equations, and consistently find that the spin component dominates the pairing vertex function in the doping range where the dome is located. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q52.00003: Charge order instabilities in the t-J model Andrea Allais, Johannes Bauer, Subir Sachdev Motivated by the observation of incommensurate charge order in the pseudogap phase of the cuprates, most notably in a series of recent experiments on YBCO, we explore the occurrence of charge-ordering instabilities in an extended t-J model. We allow for on site and bond ordering, with arbitrary ordering wavevector and a number of possible internal wavefunctions. Our results are obtained by a combination of slave boson mean field theory, dynamical mean field theory and variational Monte Carlo. We find instability towards several possible ordering patterns, depending on the choice of parameters. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q52.00004: Dipolon Theory of High $T_C$ Superconductors--Experimental Proofs of Dipolon Excitations Ram Sharma Owing to the success of the dipolon theory [1,2] in explaining and predicting the properties [1-4] of high $T_C$ superconductors (HTSC) some scientists ask whether the dipolons have been observed. Proofs will be revealed that the dipolons have been observed in variety of experiments not only in superconducting state but also in the normal state. The observed photoemission (PE) spectra (broad peak, dip and sharp peak) and a low energy and two high energy kinks (first predicted [3,4] by dipolon theory and observed later on) ocur due to dipolons [3], inherent in dipolon-theory-deduced five principles [4] of PE. The dipolons have been observed as several optical excitations [1] in HTSC. The observed plasmons and their dispersions are due to [5] the dipolons. The observed but unexplained optical phonons in HTSC are predicted to be a consequence of dipolons. In experiments, no dipolons means no superconductivity.\\[4pt] [1] R. R. Sharma, Phy. Rev. {\bf B 63}, 054506 (2001).\\[0pt] [2] R. R. Sharma, Physica {\bf C 439}, 47 (2006).\\[0pt] [3] R. R. Sharma, Physica {\bf C 468}, 190 (2008).\\[0pt] [4] R. R. Sharma, ``Dipolon Theory..,'' in ``.. Cuprates,'' Ed. K. N. Courtlandt, P. 81-100, Nova Sc, Pub., New York, 2009.\\[0pt] [5] R. R. Sharma, Phys. Rev. {\bf B 54}, 10192 (1996). [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q52.00005: Chiral charge order from interlayer tunneling in the hole doped cuprates Akash Maharaj, Srinivas Raghu We show how charge density waves in layered materials can be gyrotropic, {\it i.e.} break spatial inversion and all mirror symmetries. This order is stabilized by coherent interlayer tunneling whose amplitude depends on in-plane momentum. We present mean field calculations which demonstrate the presence of this chiral configuration of charge density waves, and justify these results using a Landau-Ginzburg theory. The implications for recent experiments ({\it e.g.} Kerr, X-ray etc.) in underdoped YBCO are also discussed. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q52.00006: Time reversal symmetry breaking in hole-doped cuprates Andrey Chubukov, Yuxuan Wang We consider charge-density-wave (CDW) instability in weakly hole-doped cuprates. We show that the interaction mediated by spin fluctuations gives rise not only to $d-$wave superconducting pairing but also to (CDW) order with momentum $(\pm Q, 0)$ and $(0,\pm Q)$. We show that this particular order has two components, one symmetric and another anti-symmetric under time reversal. We derive and analyze the corresponding Ginzburg-Landau functional and show that both components appear simultaneously at $T_{CDW}$, i.e., the CDW-ordered state breaks time reversal symmetry. We further show that time-reversal symmetry actually gets broken even before CDW orders develop, as the two CDW components form a (4-fermion) bound state at some $T_{bs} > T_{CDW}$. In between $T_{bs}$ and $T_{CDW}$, time-reversal symmetry is broken, but CDW order does not yet develop. We show that the same result can be obtained by re-expressing the Ginzburg-Landau functional in terms of collective variables and solving saddle-point equations. We discuss experimental consequences of this emerging order. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q52.00007: Evidence for a charge collective mode associated with superconductivity in copper oxides from neutron and x-ray scattering measurements of La$_{2-x}$Sr$_x$CuO$_4$ Seung Ryong Park, T. Fukuda, A.Q.R. Baron, A. Hamann, D. Lamago, L. Pintschovius, M. Fujita, K. Yamada, D. Reznik In superconducting copper oxides some Cu-O bond-stretching phonons around 70meV show anomalous giant softening and broadening of electronic origin and electronic dispersions have large renormalization kinks near the same energy. These observations suggest that phonon broadening originates from quasiparticle excitations across the Fermi surface and the electronic dispersion kinks originate from coupling to anomalous phonons. We measured the phonon anomaly in underdoped (x=0.05) and overdoped (x=0.20,0.25) La$_{2-x}$Sr$_x$CuO$_4$ by inelastic neutron and x-ray scattering with high resolution. Combining these and previously published data, we found that doping-dependence of the magnitude of the giant phonon anomaly is very different from that of the ARPES kink, i.e. the two phenomena are not connected. We show that the phonon anomaly likely originates from novel collective charge excitations as opposed to interactions with electron-hole pairs. Their amplitude follows the superconducting dome so these charge modes may be important for superconductivity. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q52.00008: Fitting of Diverging Effective Mass Obtained by Quantum-Oscillation Measurements in Underdoped YBCO$_{\mathrm{6+x}}$ Systems Hyun-Tak Kim The diverging-effective mass (DEM) in a metallic system is regarded as evidence of strong correlation between fermions in strongly correlated systems. The identity of the DEM still remains to be revealed The effective mass derived from the Brinkman-Rice picture, m*$=$m$_{\mathrm{o}}$/(1-$\rho ^{4})$ [1] where $\rho $ is band filling helps clarify the DEM, m*/m$_{\mathrm{e}}$ analyzed by measurements of quantum oscillation in inhomogeneous underdoped YBCO$_{\mathrm{6+x}}$ systems [2]. As x decreases, m*/m$_{\mathrm{e}}$ increases rapidly like the DEM [2] When it is assumed as the fact that the metal-insulator transition occurs near a critical x$_{\mathrm{c}}\approx $0.45 the data of the DEM [2] is closely well fitted in condition that $\rho =$x$_{\mathrm{c}}$/x increases as x decreases. Its physical meaning is a percolation phenomenon that the extent of the metallic regime is constant and the magnitude of the measurement region, x, increases. It's fitting is performed with m*$=$m$_{\mathrm{e}}$/(1-$\rho^{4})$ where $\rho =$x$_{\mathrm{c}}$/x From the fitting result, we deduce that a correlation strength is greater than 0.95, and conclude that the metal phase of the normal state in YBCO systems is a strongly correlated.\\[4pt] [1] Physica C 341-348(2000)259; e-print arXiv:cond-mat/0110112; Physica C 460--462, 1076 (2007).\\[0pt] [2] PNAS 107 (2010) 6179. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q52.00009: Fermi-surface reconstruction in the cuprate superconductor YBCO via the thermal Hall effect Gael Grissonnanche, Sophie Dufour-Beausejour, Francis Laliberte, Alexis Riopel, Olivier Cyr-Choiniere, Nicolas Doiron-Leyraud, Louis Taillefer, James Day, Brad Ramshaw, Ruixing Liang, Doug Bonn, Walter Hardy, David Graf, Steffen Kramer We recently showed that the thermal conductivity $\kappa_{xx}$ can be used to directly measure the upper critical field H$_{c2}$ in cuprate superconductors [1]. Here we show that the thermal Hall conductivity $\kappa _{xy}$ can be used to probe the nature of the carriers in these materials. We present a study of $\kappa_{xy}$ in YBCO at a doping p $=$ 0.11, as a function of magnetic field up to 35 T down to low temperature. The fact that $\kappa_{xy}$ is negative above H$_{c2} \quad =$ 24 T confirms the presence of an electron-like pocket in the normal-state Fermi surface [2], the result of a reconstruction caused by the emergence of charge order at low temperature [3]. We show how the Fermi-surface reconstruction evolves as a function of field and temperature. [1] G. Grissonnanche \textit{et al.}, arXiv:1303.3856 (2013). [2] D. LeBoeuf \textit{et al}., Nature 450, 533 (2007). [3] F. Lalibert\'{e} \textit{et al}., Nature Communications 2, 432 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q52.00010: A new quantum oscillation frequency in the thermoelectric response of YBa2Cu3Oy Nicolas Doiron-Leyraud, Samuel Rene de Cotret, Francis Laliberte, Louis Taillefer, Brad Ramshaw, Ruixing Liang, Doug Bonn, Walter Hardy The Seebeck and Nernst coefficients of the cuprate superconductor YBa2Cu3Oy were measured in a high-quality single crystal with doping p = 0.11, as a function of magnetic field up to 45 T at low temperatures. Giant quantum oscillations are observed in both thermoelectric coefficients. The dominant frequency Fa = 530 T is in agreement with previously observed oscillations in transport, specific heat, and magnetization, attributed to a closed electron Fermi surface. The Seebeck effect reveals an additional frequency Fb = 95 T [1], with a low effective mass [2]. We propose that this new frequency arises from a hitherto undetected closed hole-like Fermi surface. This can explain the variation with magnetic field of several transport properties, such as the Seebeck, Hall [3], and thermal Hall effects, as well as the doping dependence of the normal-state Seebeck effect at low temperature [4], which is inconsistent with a lone electron pocket. Work performed at the NHMFL Tallahassee. [1] N. Doiron-Leyraud et al., preprint. [2] S. Badoux et al., preprint. [3] D. LeBoeuf et al., Phys. Rev. B 83, 054056 (2011). [4] F. Laliberte et al., Nat. Commun. 2, 432 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q52.00011: Linear magnetic field dependence of the specific heat in underdoped YBCO Jonathon Kemper, Oskar Vafek, Scott Riggs, Jon Betts, Fedor Balakirev, Albert Migliori, Ruixing Liang, Walter Hardy, Doug Bonn, Gregory Boebinger We report the observation of two distinct regimes in the field dependence of the low temperature electronic heat capacity (C) of YBa$_2$Cu$_3$O$_{6.47}$. Measurements were performed in applied magnetic fields ($H$) up to 34.5~T and at temperatures between 1 and 8 K. Below 10 T we observe C$\sim H^{1/2}$ attributable to a d-wave superconducting gap. Above 10 T, C approaches a nearly linear-in-field form. All behavior is clearly observable well below the field-induced resistive transition, and thus, likely closely tied to the nodes in the gap. We rule out the suppression of the superconducting gap by the magnetic field as a possible explanation through a comparison with specific heat data from other dopingsof underdoped YBCO. [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q52.00012: The structure of oxygen-annealed La$_{1.9}$Ca$_{1.1}$Cu$_{2}$O$_{6}$ superconductor Hefei Hu, Yimei Zhu, Ruidan Zhong, John Schneeloch, Tiansheng Liu, Genda Gu, John Tranquada, John Hill, Simon Billinge Effect of annealing under high oxygen pressure on La$_{1.9}$Ca$_{1.1}$Cu$_{2}$O$_{6}$ (2126) is studied and structure change at the nanometer scale is investigated by using transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). The as-grown single crystal is non-superconducting, which is thought to be due to oxygen deficiency. With annealing under a high oxygen pressure, superconductivity is induced with T$_{\mathrm{c}}$ $\sim$ 53 K. While the as-grown 2126 shows homogenous structure at a large scale, after oxygen annealing, the sample develops a secondary phase La$_{\mathrm{2-x}}$Ca$_{\mathrm{x}}$CuO$_{4}$(214) at nanometer scale, which is evidenced by electron diffraction patterns together with EELS analysis. The content of 214 phase is estimated to be around 20{\%} based on the analysis of scanning TEM images. Magnetization measurements indicate that the 214 phase is also superconducting. Chemical change of the remaining 2126 phase after oxygen annealing will also be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q52.00013: Magnetic Penetration Depth in Tl-2201: Disentangling the In- and Out-of-plane Components S. Mahyad Aghigh, Jordan Baglo, James Day, Darren Peets, Pinder Dosanjh, Ruixing Liang, Walter Hardy, Doug Bonn Investigation of Tl$_{2}$Ba$_{2}$CuO$_{6+\delta}$ (Tl-2201) properties is important as it provides access to the overdoped side of the superconducting dome in a material with relatively low quasiparticle scattering rates. As one of the most fundamental characteristics of a superconductor, we have measured the magnetic penetration depth, $\lambda$(T), for a single crystal of Tl-2201 with T$_{c}$= 43 K using a loop-gap cavity perturbation technique well established by our group. Taking advantage of the tetragonal structure of this cuprate, as well as the existing measurements of the absolute value of the penetration depth at zero temperature, the in- and out-of-plane components, $\lambda$$_{ab}$(T) and $\lambda$$_{c}$(T) have been determined. In this talk I will describe the measurement technique, the disentangling procedure, as well as the degree of reliability of the results. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q52.00014: Field-Dependent Specific Heat Study of Chain Superconductivity in YBa$_2$Cu$_4$O$_8$ Philip Walmsley, Antony Carrington A unique property of the Y-based cuprate superconductors is the presence of conducting quasi-1-D CuO chains in addition to the CuO2 planes in which superconductivity is thought to originate. The nature of the superconducting interactions in the chains remains a matter of debate. YBa$_2$Cu$_3$O$_{7-\delta}$ (Y123) has a single b-axis CuO chain per unit cell whereas its stoichiometric relative YBa$_2$Cu$_4$O$_8$ (Y124) has two filled chains per unit. Previously, it has been observed that at low temperature there is an anomalous increase in the superfluid density along the b and c directions in Y124 which does not occur in Y123. The response along the a direction in Y124 is linear as expected for a d-wave nodal gap. This anomalous increase in Y124 is likely caused by proximity coupling to the planes. In this work we present a study of the field dependent specific heat in Y124. The field induced changes in the electronic specific heat due to the Volovik effect are used to deduce the field scale which quenches chain superconductivity. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q52.00015: Charge and magnetic excitations in hole- and electron-doped infinite layer cuprate superconductors G. Dellea, L. Maritato, A. Galdi, P. Orgiani, D.G. Schlom, D. Di Castro, A. Tebano, G. Balestrino, C. Aruta, M. Moretti Sala, N.B. Brookes, C.J. Jia, B. Moritz, T.P. Devereaux, M. Minola, C. Mazzoli, L. Braicovich, G. Ghiringhelli Infinite layers (IL) present the simplest crystallographic structure among layered cuprates. Here we present $Cu-L_3$ resonant inelastic x-ray scattering (RIXS) measurements on insulating and superconducting IL systems. In particular, we compare spectra for the two possible doping mechanisms, n- and p-type. $(CaCuO_2)_m/(SrTiO_3)_n$ superlattices are characterized by hole doping,\footnote{D. Di Castro et al., Phys. Rev. B 86, 134524(2012).} while $Sr_{1-x}La_xCuO_2$ presents electron doping.\footnote{L. Maritato et al., J. Appl. Phys. 113, 053911(2013).} Beside several analogies, the two systems show noticeable differences in evolution with doping level of the charge excitation continuum and of the magnetic peak intensity and damping. These trends can be found also in the corresponding theoretical calculations, in which the spin-spin correlation function is determined using a single band Hubbard model plus 2p core level with spin-orbit coupling.\footnote{C. J. Jia et al., http://arxiv.org/abs/1308.3717(2013).} [Preview Abstract] |
Session Q53: Surfaces, Interfaces, and Thin Films: Molecules on Surfaces
Sponsoring Units: DMPChair: Chenggnag Tao, Virginia Polytechnic Institute and State University
Room: Mile High Ballroom 2C
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q53.00001: End Functional Hydrogen Bonding Modulates Odd-Even Effect in Alkanethiol Monolayer Assembly Kshitij Jha, Yeneneh Yimer, Mesfin Tsige Hydroxyl (-OH) terminated n-alkanethiols on gold show a two-dimensional zig-zag nature for the top hydrogen bonded network. We observe transfer of packing characteristics from top network to the buried ad-atom (sulfur) distribution quantified as occupancy percentages for atop, hollow (fcc and hcp), and bridge sites. Employing validated metal potentials and all-atom molecular dynamics, we also quantify the dynamic correlation between the top layer (end-functional) and ad-layer (thiol) through variance in distribution peaks for nearest neighbors as a function of temperature. The hydrogen bond network and packing of the monolayer increases in strength with chain length for –OH terminated n-alkanethiols. Shorter chain lengths lead to better transfer of packing, for a given network strength. Odd chain lengths, compared to even, have a lower lattice spacing by an average of 0.04 {\AA}. The transfer effect of the top network is not observed, as expected, for methyl (-CH$_3$) terminated n-alkanethiols since there is no hydrogen bonding. Trends in packing and transfer for monolayer assembly could provide design principles for polymer based nanoactuators and sensors. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q53.00002: Imaging the wave functions of adsorbed molecules using angle-resolved photoemmision data Daniel L\"uftner, Thomas Ules, Eva Maria Reinisch, Georg Koller, Serguei Soubatch, F. Stefan Tautz, Michael G. Ramsey, Peter Puschnig The frontier electronic orbitals of molecules are the prime determinants of the respective compounds' chemical, electronic, and optical properties. Although orbitals are very powerful concepts, experimentally only the electron densities and energy levels are directly observable. As has been shown in recent publications, angle-resolved photoemission (ARPES) intensity maps of organic molecular layers are related to the absolute value of the Fourier transform of the initial state molecular orbital. However, the lost phase information impedes the back-transformation of the orbital into real space. Here, we show how molecular orbital images as well as the absent phase information can be retrieved by applying an iterative procedure which takes experimental ARPES maps as input and only assumes spatial confinement of the orbital. The method is demonstrated for several molecular orbitals of two proto-typical pi-conjugated molecules: the LUMO, HOMO, and HOMO-1 of pentacene, and the LUMO and HOMO of PTCDA [1]. The technique is simple and robust and further emphasizes the capabilities of ARPES looking at spatial distributions of wave functions of adsorbed molecules thereby complementing data obtained from scanning probe methods. [1] D.L\"uftner et al., PNAS (accepted) [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q53.00003: Tuning gap states at organic-metal interfaces by quantum size effects Meng-Kai Lin, Yasuo Nakayama, Chin-Hung Chen, Chin-Yung Wang, H.-T. Jeng, Tun-Wen Pi, Hisao Ishii, S.-J. Tang Organic-metal interfaces are key elements to the organic-based electronics. The energy level alignment (ELA) between metal Fermi level and molecule orbital levels determines the injection barriers for the charge carriers at the interfaces, which are crucial for the performance of organic electronic devices. Dipole formation at the interfaces has been regarded as the main factor for ELA and several models were proposed for the mechanism of it in the context of the interface between organic molecules and bulk metal crystal surface, at which surface states (SS) were mostly used to probe the interfacial properties. We show that when the bulk metal crystal is replaced by a uniform metal thin film, another 2-dimensional electronic state, quantum well states , will not only be able to probe but also modify the interfacial electronic structures such as gap states, which don't have the counterpart at the organic-bulk crystal interface. Moreover, thickness-dependent quantum size effects of metal thin films provide a new method for engineering the organic electronic devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q53.00004: Single Molecule Characterization of Conjugated Oligomers Formed through Radical Cyclization at a Surface Hsin-Zon Tsai, Alexander Riss, Sebastian Wickenburg, Liang Tan, Patrick Gorman, Dimas Oteyza, Yen-Chia Chen, Aaron Bradley, Miguel Ugeda, Grisha Etkin, Steven Louie, Felix Fischer, Michael Crommie Conjugated polymers have gained considerable attention due to their potential industrial applications and interesting fundamental properties. Real-space imaging their chemical bonds and understanding their electronic structures at the nanoscale could lead to enhanced control in the synthesis of these polymers for the potential applications in the nanoelectronics. Here, we present the synthesis and characterization of poly-acetylene derivatives resulting from cyclizations of enediyne molecules on an Au(111) surface. We performed non-contact atomic force microscopy (nc-AFM) with sub-molecular resolution to determine the precise chemical structure of cyclized monomers and chemically linked molecular chains. Additionally, STM measurements provide insight into the corresponding electronic structure and reveal a 1D conducting channel along the backbone of the conjugated oligomers, consistent with theoretical predictions. This work demonstrates the unique insight that can be gained by combining nc-AFM and STM to study the chemical and electronic structure of molecular assemblies at surfaces. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q53.00005: Scanned Probe Characterization of Atmospheric Effects on diF TESADT Thin-Film Transistors Cortney Bougher, Shawn Huston, Jeremy Ward, Abdul Obaid, Marsha Loth, John Anthony, Oana Jurchescu, Brad Conrad Single crystal organic semiconductors have been shown to exhibit carrier mobilities comparable to the silicon currently used in photovoltaics. However, during solution deposition of common organic semiconducting materials the resultant thin-film is often polycrystalline. Device performance and electrical properties of organic thin-film transistors are highly dependent on crystal structure and molecular packing. In polycrystalline thin-films, boundary regions between crystal grains can affect the overall performance of devices, as crystal structure and packing may differ from that of the surrounding crystal regions. These boundary regions may also serve as defect sites, allowing environmental factors, such as oxygen content and humidity, to alter local charge transport through devices. We utilize Kelvin Probe Force Microscopy (KPFM) to characterize how grain boundaries alter local conductivity and device performance as a function of doping in 2,8-difluoro-5,11-triethysilylethynyl anthradithiophene (diF TESADT) thin-film transistor surfaces. Device voltage drops at grain boundaries are characterized as a function of both atmospheric dopants and transition time between dopants. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q53.00006: Interfacial energy level shifts in few-molecule clusters of the organic semiconductor PTCDA Sarah Burke, Katherine Cochrane, Agustin Schiffrin, Tanya Roussy Detailed knowledge of the local electronic structure of organic semiconductors near interfaces is crucial for the understanding of a variety of electronic and optoelectronic applications of these emerging materials. However, organic molecules are highly sensitive to the local environment, which abruptly changes at an interface. Here, we present a study on the prototypical organic semiconductor PTCDA by scanning tunneling microscopy and spectroscopic mapping. Nanoscale clusters of varying size and geometry were probed on a bilayer NaCl film on Ag(111). The molecular states, while decoupled from the underlying metal surface, are relatively delocalized within these monolayer islands. Depending on the size of the cluster and arrangement of molecules within the cluster, edge molecules exhibit varying energy level shifts relative to the central molecules, both of which differ from the isolated molecule. For well ordered islands, this leads to a type-1 heterojunction, with a larger band gap at the edge of the cluster differing by as much as 0.5eV. In considering nanoscale structures within multicomponent device architectures, such internal heterostructures established by differences in the local environment are an important consideration, and could even be exploited. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q53.00007: The Observation of an Unoccupied Shockley-Type Surface State at the H$_2$-phthalocyanine/Ag(111) Interface Benjamin Caplins, David Suich, Alex Shearer, Charles Harris A free-electron interface state has been observed at the H$_2$-Phthalocyanine/Ag(111) interface using time- and angle-resolved two-photon photoemission. Energetically the interface state is located $\sim$0.21 eV above the Fermi level and angle-resolved measurements yield an effective mass of 0.5 $m_e$. These measurements, in conjunction with density functional theory calculations allow us to assign the interface state as being a metal/molecule hybrid state derived from the Shockley state of the clean Ag(111) surface. Time-resolved measurements of two different crystalline phases of H$_2$-Phthalocyanine monolayers reveal that the lifetime of the interface state is sensitive to the bonding geometry of the molecule. The results of this study add to a mounting body of evidence that suggests that the Shockley surface state is robust and persists after deposition of organic $\pi$-conjugated materials in the form of an uplifted interface state, instead of being `quenched'. [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q53.00008: Few-layer and symmetry-breaking effects on the electrical properties of ordered CF$_3$Cl phases on graphene Josue Morales-Cifuentes, Yilin Wang, Janice Reutt-Robey, T.L. Einstein An effective pseudopotential mechanism for breaking the inherent sub-lattice symmetry of graphene has been studied using DFT calculations on hexagonal boron nitride.\footnote{Gianluca Giovannetti et al.\ , PRB 76, 073103 (2007)} Electrical detection of CF$_3$Cl phase transitions on graphene shows the existence of a commensurate ordered phase in which this can be tested.\footnote{Yilin Wang et al.\ , APL 103, 201606 (2013)} We study the electronic properties of this phase using VASP ver 5.3.3, with ab initio van der Waals density functionals (vdW-DF1 and vdW-DF2).\footnote{Ji\v{r}\'{i} Klime\v{s} et al.\ , PRB 83, 195131 (2011) } \footnote{Kyuho Lee et al.\ , PRB 82, 081101(R) (2010) }Consistent with a physisorbed phase, binding energies and charge transfer per CF$_3$Cl molecule are calculated to be on the order of 280meV and 0.01e, respectively. By exploring different coverages and orientations of this ordered phase we are able to open a band gap in some configurations; said gap is in the range of 8 to 80meV depending on the strength of the effective pseudopotential. Furthermore, we calculate the screening of these effects in bi-layer and tri-layer graphene. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q53.00009: Covalent Attachment to GaP(110) -- Engineering the Chemical Functionalization of a III-V Semiconductor A.J. Bradley, M.M. Ugeda, Wenjun Liu, Min Yu, T. Don Tilley, Rub\'en P\'erez, Jeffrey B. Neaton, M.F. Crommie With its 2.3 eV bulk bandgap, relatively high conduction band edge, and low chemical reactivity, the (110) surface of GaP is an excellent candidate for many UV and visible light applications, such as photo-catalysis and light-induced chemical reduction. However, the reconstruction and resulting charge transfer of the surface makes it difficult to covalently attach the required molecules. Indeed, very little work has been done to understand either covalent functionalization or passivation of this surface. Here we report on a Staudinger-type, thermally-driven covalent attachment of perfluorophenyl azide (pfpa) to GaP(110). We have studied the adsorption of pfpa molecules by means of high-resolution scanning tunneling microscopy and spectroscopy in combination with first principles calculations. We show a progression from a physisorbed state at room temperature to a covalently attached state after exposure to slightly higher temperatures ($\sim$ 50$^{\circ}$C). The developed approach is expected to be valid for various other functional groups attached to the azide, as well as other III-V semiconductors. \\[4pt] [1] J.L.A. Alves, et. al.\textit{Phys. Rev. B.}, 6188-6198.\\[0pt] [2] M.M. Ugeda, et. al.\textit{J. Phys. Chem. C.}(accepted November 2013). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:30PM |
Q53.00010: Ferrocenes on Calcite: Single-electron tunneling detected at room temperature Philipp Rahe, Ryan Steele, Clayton Williams We present the assembly of a functionalized ferrocene derivative on a truly insulating support, namely the calcite (10\={1}4) surface, and investigate the transfer of single electrons between the molecules and the conductive tip of an atomic force microscope in the absence of a macroscopic tunneling current. Molecules on insulating surfaces attract currently increasing attention [1], stimulated by promising applications in the fields of surface functionalization and, especially, in the context of molecular (opto-)electronics. For isolated atoms and single molecules adsorbed on thin insulating films, the manipulation and storage of single charges has been induced by a tunneling current [2,3]. Our approach, however, is based on single-electron tunneling force microscopy methods [4,5] combined with Kelvin-probe force microscopy. By using this combination of methods we present the measurement and control of the charge state of the ferrocene molecules by injecting and extracting charge on the order of single electrons.\\[4pt] [1] Rahe et al.; Adv. Mater. 2013, 25, 3948;\\[0pt] [2] Gross et al.; Science 2009, 324, 1428;\\[0pt] [3] Leoni el al.; Phys. Rev. Lett 2011, 106, 216103;\\[0pt] [4] Bussmann et al.; Appl. Phys. Lett. 2004, 85, 2538;\\[0pt] [5] Bussmann et al.; Appl. Phys. Lett. 2006, 88, 263108 [Preview Abstract] |
Wednesday, March 5, 2014 4:30PM - 4:42PM |
Q53.00011: Identifying individual chemical bonds in single-molecule chemical reaction products using nc-AFM Sebastian Wickenburg, Dimas G. de Oteyza, Yen-Chia Chen, Alexander Riss, Hsin-Zon Tsai, Zahra Pedramrazi, Aaron J. Bradley, Miguel M. Ugeda, Patrick Gorman, Grisha Etkin, Duncan J. Mowbray, Alejandro Perez, Angel Rubio, Michael F. Crommie, Felix R. Fischer Determining reaction pathways and products is an integral part of chemical synthesis. Ensemble measurements are commonly used, but identifying products of complex reactions at surfaces presents a significant challenge. Here we present a non-contact AFM (nc-AFM) study to directly address this issue[1]. We followed the change of the chemical structures, from reactants to products of enediyne cyclization reactions on metal surfaces. Thermal annealing of enediynes induced a series of cyclization cascades leading to radical species and the formation of dimers. Atomically resolved nc-AFM images reveal the precise chemical structure and the formation of chemical bonds between single molecular units. With the support of DFT calculations, we identified the underlying chemical pathways and barriers, demonstrating the potential of this atomically resolved AFM technique to study unknown reaction products in surface chemistry at the single-molecule level. [1] D. G. de Oteyza et al., Science 340, 1434 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 4:42PM - 4:54PM |
Q53.00012: Electronic Polarization at Pentacene/Polymer Dielectric Interfaces: Imaging Surface Potentials and Contact Potential Differences as a Function of Substrate Type, Growth Temperature, and Pentacene Microstructure Yanfei Wu, Greg Haugstad, C. Daniel Frisbie Interfaces between organic semiconductors and dielectrics may exhibit interfacial electronic polarization, which is equivalently quantified as a contact potential difference (CPD), an interface dipole, or a vacuum level shift. Here we report quantitative measurements by Scanning Kelvin Probe Microscopy (SKPM) of surface potentials and CPDs across ultrathin (1-2 monolayer) crystalline islands of the benchmark semiconductor pentacene thermally deposited on a variety of polymer dielectrics (e.g., poly(methyl methacrylate), polystyrene). The CPDs between the pentacene islands and the polymer substrates are in the range of -10-$+$50 mV, they depend strongly on the polymer type and deposition temperature, and the CPD magnitude is correlated with the dipole moment of the characteristic monomers. Surface potential variations within 2 monolayer (3 nm) thick pentacene islands are approximately15 mV and may be ascribed to microstructure (epitaxial) differences. Overall, the microscopy results reveal both strong variations in interfacial polarization and lateral electrostatic heterogeneity; these factors ultimately should affect the transport properties of these interfaces in devices. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q53.00013: Tunable Charge Transfer Dynamics at Tetracene/LiF/C60 Interfaces Dmitry Yarotski, Siddharth Sampat, Aditya Mohite, Brian Crone, Anton Malko, Antoinette Taylor, Sergei Tretiak Organic conducting polymers offer an attractive alternative to regular semiconductors in both photovoltaic and optoelectronic applications due to their low cost and improved processability. Although current organic devices suffer from relatively low electro-optical conversion efficiency, novel polymer nanocomposites, both organic/organic and organic/inorganic, should provide better control over the material properties and improve the device performance. In such nanocomposites, physical and electronic structures of the interfaces govern carrier generation and transport characteristics yet underpinning mechanisms are still poorly understood. Here, we apply ultrafast optical spectroscopy to observe an interfacial charge transfer dynamics in Tc/LiF/C60 multilayered heterostructures, where charge separation processes compete with parasitic radiative and non-radiative charge transfer exciton recombination. Our studies reveal that the tunneling barrier created by LiF buffer layer between donor and acceptor materials provides means for independent control over the rates of direct and diffusion-induced charge transfer exciton formation and recombination dynamics. These findings might have implications for development of more efficient organic photovoltaic and light-emitting devices. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q53.00014: Tuning energy level alignment at organic/semiconductor interfaces using a built-in dipole in Chromophore-Bridge-Anchor compounds Sylvie Rangan, Alberto Batarseh, Keyur Chitre, Andrew Kopecky, Elena Galoppini, Robert Bartynski We report a new approach, using chromophore-containing molecules designed to contain internal dipoles, for controlling at the molecular level the energy level alignment between an organic chromophore and a transition metal oxide surface. The approach employs a chromophore-bridge-anchor molecular architecture where the three components are electronically decoupled. By introducing electron donor (D) and acceptor (A) groups to the bridge, an intramolecular dipole is introduced between the chromophore and the anchor. When a monolayer of such molecules is bonded to a metal oxide surface, the resulting dipole layer establishes a potential difference that shifts the chromophore levels with respect to those of the substrate. This concept is demonstrated using a chromophore (ZnTPP)-bridge (substituted with an electron donating (NMe2) and electron withdrawing (NO2) groups to create a built-in dipole)-anchor (Isophtalic acid) architecture. Shifts of the chromophore's HOMOs on the order of plus or minus 0.1 eV with respect to the ZnO valence band edge have been observed, without altering the photoabsorption properties of the chromophore or the HOMO-LUMO gap. An important strength of this concept is that it provides a general design applicable to a large number of anchoring functional groups, built-in dipole bridges, and redox-active centers. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q53.00015: Vibrational properties of an adamantane monolayer on a gold surface Yuki Sakai, Giang D. Nguyen, Rodrigo B. Capaz, Sinisa Coh, Ivan V. Pechenezhskiy, Xiaoping Hong, Michael F. Crommie, Feng Wang, Susumu Saito, Steven G. Louie, Marvin L. Cohen We study the vibrational properties of an adamantane monolayer on a Au(111) surface. The IR spectrum of a self-assembled monolayer of adamantane on Au(111) is measured by a newly developed infrared scanning tunneling microscopy (IRSTM) technique. We analyze the IR spectrum of this system by a density functional theory and find that the IR spectrum is severely modified by both adamantane-gold and adamantane-adamantane interactions. One of three gas-phase C-H bond stretching modes is significantly red-shifted due to the molecule-substrate interactions. The intermolecular interactions cause a suppression of the IR intensity of another gas-phase IR peak. The techniques used in this work can be applied for an independent estimate of molecule-substrate and intermolecular interactions in related diamondoid/metal-substrate systems. [Preview Abstract] |
Session Q57: The Fred Kavli Special Symposium: The Many Electron Problem -- Where are we now?
Sponsoring Units: APSChair: Leon Balents, University of California, Santa Barbara
Room: Four Seasons Ballroom
Wednesday, March 5, 2014 2:30PM - 3:06PM |
Q57.00001: Magnetism as the emergent phenomena Invited Speaker: Yoshinori Tokura Versatile emergent phenomena have been observed in strongly correlated electron systems as a consequence of mutual strong coupling among the spin, orbital, and charge degrees of freedom. Here, we would overview the outcomes of topological spin textures in transport, dielectric, and optical properties of correlated systems; these include sciences of colossal magnetoresistance, multiferroics, skyrmions, and topological/quantum-anomalous Hall effects. Impacts of the emergent electric and magnetic fields acting on the electrons in a solid are discussed as well as their possible applications to future devices. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:42PM |
Q57.00002: Deciphering Electron Matter in Novel Superconductors Invited Speaker: Laura Greene Superconductors may be grouped into two major classes. The first is conventional metallic, whose pairing mechanism is explained by the BCS theory and electron-phonon coupling. The pairing mechanism of the second class, driven by electron correlations, is still to be completely worked out. These superconductors have electronic properties that are highly tunable, either by doping or pressure, from a non-superconducting ground state to a superconducting one, thus defining a superconducting ``dome'' in the phase diagram. More than 40 families of such superconductors, including high-temperature cuprate and iron-based, heavy fermion, organic, and transition-metal di-chalcogenide superconductors exhibit this ubiquitous phase diagram. All of these materials show intriguing correlated electron states above the dome, and researches agree that the understanding of this electron matter holds the key to the pairing mechanism, and ultimately predicative design of new superconductors, which hold great promise of revolutionary applications, including energy, information technology, and medicine. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 4:18PM |
Q57.00003: Correlated Electrons in Two Dimensions: The Fractional Quantum Hall Effect and More Invited Speaker: James Eisenstein A collection of electrons confined to move on a plane surface is surely one of the simplest many-body systems imaginable. But in spite of this apparent simplicity, a strong magnetic field applied perpendicular to the plane opens a door to a complex and beautiful world filled with many-body exotica. The magnetic field quenches the kinetic energy, leaving Coulomb interactions in control of the physics. The result has been a revolution in many-body physics comparable to that created by the discovery of superconductivity. Incompressible liquid ground states with fractionally charged quasiparticle excitations exhibit the quantized Hall effect at numerous discrete partial fillings of the lowest and first excited Landau level. The first examples of topological condensed matter, these many-body bulk insulators possess complex families of both conducting and neutral edge states at their boundaries. Highly correlated compressible phases of composite fermions also exist and may be viewed as progenitors of the various families of incompressible states. Multi-component two-dimensional systems with active discrete internal degrees of freedom (spin, layer, valley, etc.) display a wide array of broken symmetry states including ferromagnetism and exciton condensation. Now thirty years old, the field generically dubbed ``the fractional quantum Hall effect,'' remains extraordinarily vibrant. Once confined largely to GaAs/AlGaAs heterostructures, the fractional quantum Hall effect and its many relatives and offspring are now pursued in graphene, various oxide interfaces, and other materials. Some of the most fundamental aspects, including the exotic non-abelian quasiparticle statistics expected of some of the more subtle phases, have hardly been touched experimentally even as their potential for applications to quantum computation is alluring. In this talk, I will try to give a flavor of this enormous field, emphasizing current topics and possible future directions. [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q57.00004: Theoretical Approaches to Correlated Electron Problems Invited Speaker: Steven Kivelson Theoretical studies of the electronic properties of strongly correlated materials can rarely be both “realistic” and “controlled.” Despite the lack of a small parameter, astonishing success has been achieved by realistic approaches using various generalized forms of mean field theory including, most notably, local density approximation. I will instead discuss some of what has been learned by studying simple paradigmatic models, such as the Hubbard model, in limits in which the existence of a small parameter allows asymptotic control of the theory, and of “engineered” models, that are amenable to exact solution. This approach is ideal for establishing, as points of principle, what behaviors can exist. In some cases, invoking the principle of adiabatic continuity, the results can be extrapolated to a physically reasonable regime so that contact with experiment becomes plausible. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:30PM |
Q57.00005: Numerical studies of strongly correlated systems: beating the exponential growth in computation time Invited Speaker: Steven White In simulating strongly correlated systems, where approximate approaches based on small parameters are unreliable, the key problem is the exponential growth in computation time with system size, inverse temperature, or accuracy. For example, in exact diagonalization methods, the size of the vector describing the wavefunction has a length which is exponential in the number of sites. Progress in simulation methods has often involved removing this exponential for a certain class of problems. In quantum Monte Carlo, for example, for unfrustrated, half-filled Hubbard or Heisenberg models, the lack of a fermion sign problem eliminates the exponential, and large systems can be studied with high accuracy. In contrast, for most frustrated or doped systems, the expectation value of the sign falls exponentially and thus the computation time grows exponentially with the system size. The density matrix renormalization group eliminates the exponential for 1D systems. In this approach, the low entanglement of many-body ground states is exploited in a systematically improvable matrix product description of the wavefunction. For 2D systems, one is again faced with an exponential growth, but in a weaker form: an exponential of the width only, not the length. This weaker exponential has made DMRG the current method of choice for many 2D systems with a sign problem. Recently, the first approaches which appear to eliminate the exponential much more broadly have appeared, based on tensor networks such as projected entangled pair states (PEPS), which are closely related to DMRG. These methods also exploit the relatively low entanglement of ground states of realistic Hamiltonians. The computation time of these approaches, while non-exponential, is still quite high. Nevertheless, practical calculations with these methods are now becoming as good as DMRG and other approaches for 2D systems, and the methods are improving at a rapid rate. [Preview Abstract] |
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