Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session D1: Focus Session: High Magnetic Fields, THz Spectroscopy, MRI
Sponsoring Units: GIMS GMAGChair: Charles Mielke, NHMFL Los Alamos National Laboratory
Room: 203
Monday, February 27, 2012 2:30PM - 3:06PM |
D1.00001: Novel magnetic textures in SrCu$_{2}$(BO$_{3})_{2}$ from magnetostriction up to 97.4 tesla Invited Speaker: Marcelo Jaime Quantum magnets are model systems wherein strongly frustrated spin interactions generate a variety of exotic magnetic phases of current interest, including quantum spin ices, spin liquids, spin supersolids and complex magnetic superstructures. SrCu$_{2}$(BO$_{3})_{2}$, the only classic realization of the spin-1/2 Heisenberg antiferromagnet in the Shastry-Sutherland (orthogonal spin dimer) lattice is known to exhibit numerous magnetization plateaus due formation of stripe-like magnetic textures in high fields. However, the fine structure of these plateaus remains controversial on both experimental and theoretical fronts due to the existing limits for achievable magnetic fields in the laboratory, the sensitivity of current magnetization techniques, and the uncontrolled nature of available theoretical approaches for highly frustrated magnetic lattices. This talk will describe how we probe magnetic textures in SrCu$_{2}$(BO$_{3})_{2}$ via a recently-developed \textit{magnetostriction} technique based on optical fiber Bragg gratings [1]. We achieve microstrain (nm-resolution) sensitivity in ultrahigh pulsed fields to 97.4 T using the NHMFL 100 tesla multi-pulse magnet system [2]. The magnetostriction data reveal fine structure corresponding to all magnetization plateaus, and a significant lattice response to the long-predicted 1/2-saturation plateau at 82 T, as well as a new feature at 73.6 T that we attribute to a never before observed structure corresponding to 2/5 of magnetization saturation [3]. These data are complemented by simultaneous magnetocaloric-effect measurements, and are supported by numerical results obtained using a controlled density matrix renormalization group method.\\[4pt] [1] Daou R. et al., \textit{Rev. Sci. Instrum}. \textbf{81}, 033909 (2010).\\[0pt] [2] Sims J.R., et al. IEEE Trans. Appl. Supercond. 18, 587-591 (2008).\\[0pt] [3] M. Jaime et al., submitted. In collaboration with R. Daou, S.A. Crooker, F. Weickert, A. Uchida, A. Feiguin, C.D. Batista, H. Dabkowska, and B. Gaulin. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D1.00002: Superconducting critical current measurements in pulsed magnets Fedor F. Balakirev, Philip J.W. Moll, Nikolai D. Zhigadlo, Janusz Karpinski, Bertram Batlogg Measurements of critical current in single crystals of high temperature superconductor using pulsed magnetic fields are tricky due to short time scale, fast field sweep rate and sheer absolute current values in restricted sample space. We will present a measurement system design that addresses the challenges via a combination of Field Programmable Gate Array (FPGA) fast-response signal generation and detection architecture and Focused Ion Beam crystal shaping. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D1.00003: Development of a high-field electron paramagnetic resonance spectrometer Ekaterina E. Romanova, Franklin H. Cho, Viktor Stepanov, Susumu Takahashi Electron paramagnetic resonance (EPR) spectroscopy is a powerful and versatile technique to study structure and dynamics of biomolecules. Structural investigations of biological molecules begin with site-directed spin labeling (SDSL). Using SDSL, a nitroxide spin label containing a stable unpaired electron is covalently attached at a specific site within a bio-macromolecule. The time resolution and the sensitivity of EPR spectrometer become higher when the system is operated at higher frequencies and magnetic fields. In addition, a fine spectral resolution obtained with a high-field EPR (HFEPR) enables us to study details of the conformation in biological molecule by determining the orientation of a spin-label or the relative orientation of two spin-labels embedded in the molecule. In this presentation, we will report the development of a 115/230 GHz continuous wave (cw) and pulsed EPR spectrometer at USC. The spectrometer is based on a 700/100 mW solid-state source at 115/230 GHz respectively, a 12-Tesla magnet and a superheterodyne detection system. The system also has the 2nd synthesizer for double electron-electron resonance (DEER) spectroscopy. HFEPR measurements with spin-labeled CS DNA will be discussed. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D1.00004: Sensitivity of charge transport measurements to local inhomogeneities Daniel Koon, Fei Wang, Dirch Hjorth Petersen, Ole Hansen We derive analytic expressions for the sensitivity of resistive and Hall measurements to local variations in a specimen's material properties in the combined linear limit of both small magnetic fields and small perturbations, presenting exact, algebraic expressions both for four-point probe measurements on an infinite plane and for symmetric, circular van der Pauw discs. We then generalize the results to obtain corrections to the sensitivities both for finite magnetic fields and for finite perturbations. Calculated functions match published results and computer simulations, and provide an intuitive, visual explanation for experimental misassignment of carrier type in n-type ZnO and agree with published experimental results for holes in a uniform material. These results simplify calculation and plotting of the sensitivities on an $N\times N$ grid from a problem of order $N^5$ to one of order $N^3$ in the arbitrary case and of order $N^2$ in the handful of cases that can be solved exactly, putting a powerful tool for inhomogeneity analysis in the hands of the researcher: calculation of the sensitivities requires little more than the solution of Laplace's equation on the specimen geometry. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D1.00005: Polarization Modulation THz TDS of Topological Insulators Deepu George, Chase Ellis, Tobias Kiessling, John Cerne, Andrea Markelz Optical hall conductivity measurements are powerful alternatives to DC transport measurements in samples in which the latter are challenging. They provide a deeper understanding of interactions in correlated systems and also serve as a measure of disorder in such systems. Aoki et al[1] has studied Quantum Hall Effect in graphene theoretically and has predicted that Optical Hall Conductivity should be measureable with an accurate detection of the Hall angle in the THz regime. Shimano et al [2] has reported evidence for Quantum Hall Plateau in the longitudinal conductivity $\sigma _{xy}$ in the THz region in a 2DEG system. In this work, we have developed a new broadband technique which rapidly measures complex Faraday and Kerr angles. Our technique is capable of measuring the entire complex conductivity tensor with a single scan, with an accuracy of 5mRad in the frequency range 02 to 2.5THz. We have employed this to study topological insulators and have observed a magnetic field dependent absorption around 0.5THz. 1. Morimoto, T., Jour. of Phy: Conference Series, 2009. 150(2). 2. Ikebe, Y., et al., PRL, 2010. 104(25): p. 256802. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D1.00006: Analysis of complex mixtures using a new, high resolution Trapped Ion Mobility Spectrometer -- Mass Spectrometer Francisco Fernandez-Lima, Christopher Thompson, Desmond Kaplan, Melvin Park Over the last decade, a variety of new types of Ion Mobility Spectrometry (IMS) analyzers have been developed (e.g., periodic focusing DC ion guide, segmented quadrupole drift cell, multistage IMS, field asymmetric IMS and transient wave ion guide). High resolution IMS (R$>$50) has been mainly restricted to long IMS cells (1-2 m), where ions are separated based on size-to-charge ratio as they are pushed by an electric field through a stationary bath gas. In the present work we describe a Trapped Ion Mobility Spectrometer (TIMS) and its applications. In as much as TIMS uses an electric field to hold ions stationary in a moving bath gas, it represents a paradigm shift in mobility analysis. This results in an analyzer capable of high resolution mobility separations (R$>$80) in a compact ($<$ 10 cm), low voltage ($<$ 300 V) design. Hybridization with a mass analyser (TIMS-MS) provides versatility for the analysis, separation and structural characterization of a variety of chemical compounds with increasing complexity. In particular, examples of TIMS -- MS separation for complex biological and heteroatom hydrocarbons will be shown. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D1.00007: Measurement of statistical nuclear spin polarization in a nanoscale GaAs sample Fei Xue, Dennis Weber, Phani Peddibhotla, Martino Poggio We measure the statistical polarization of quadrupolar nuclear spins in a sub-micrometer ($0.6\ \mu \mathrm{m}^3$) particle of GaAs using magnetic resonance force microscopy. The crystalline sample is cut out of a GaAs wafer and attached to a micro-mechanical cantilever force sensor using a focused ion beam technique. Nuclear magnetic resonance is demonstrated on ensembles containing less than $5 \times 10^8$ nuclear spins and occupying a volume of around (300 nm)$^3$ in GaAs with reduced volumes possible in future experiments. We discuss how the further reduction of this detection volume will bring the spin ensemble into a regime where random spin fluctuations, rather than Boltzmann polarization, dominate its dynamics. The detection of statistical polarization in GaAs therefore represents an important first step toward 3D magnetic resonance imaging of III-V materials on the nanometer-scale. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D1.00008: Carbon nanotube quantum dots as highly sensitive THz spectrometers Mohamed Rinzan, Greg Jenkins, Dennis Drew, Serhii Shafranjuk, Paola Barbara We show that carbon nanotube quantum dots (CNT-Dots) coupled to antennas are extremely sensitive, broad-band, terahertz quantum detectors. Their response is due to photon-assisted single-electron tunneling (PASET)[1], but cannot be fully understood with orthodox PASET models[2]. We consider intra-dot excitations and non-equilibrium cooling to explain the anomalous response. REFERENCES: [1] Y. Kawano, S. Toyokawa, T. Uchida and K. Ishibashi, THz photon assisted tunneling in carbon-nanotube quantum dots, Journal of Applied Physics 103, 034307 (2008). [2] P. K. Tien and J. P. Gordon, Multiphoton Process Observed in the Interaction of Microwave Fields with the Tunneling between Superconductor Films, Phys. Rev. 129, 647 (1963). [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D1.00009: Ultrafast electro-optic modulation for Terahertz upconversion spectroscopy Matthew DeCamp, Zhiyuan Chen Traditional time-domain terahertz (THz) spectroscopy techniques require the combination of sub-picosecond laser systems and time-delay stages for spectral analysis, making the design of a portable THz spectrometer challenging. In this work, we demonstrate an alternative method of spectrally resolving coherent THz radiation using narrow band optical light and passive optical spectrum analyzers. This method utilizes THz induced electro-optic modulation of a narrow band laser to upconvert the spectral content of the THz radiation to the visible portion of the electro-magnetic spectrum. This device does not require any movable parts and is well suited for spectrally analyzing both broadband and narrowband THz radiation. The spectral resolution of this technique is limited by the bandwidth of the optical radiation and the non-linear medium. Further advancements will include the development of a portable THz spectrometer, suitable for either research or clinical applications. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D1.00010: Detection of internal molecular structural motions using anisotropic spectroscopy Rohit Singh, Deepu George, Timothy Korter, Andrea Markelz The far infrared spectroscopy of molecular crystals reveals both intra and inter molecular vibrational modes [1,2]. With the significant increase in complexity of structures, one finds increasing overlap in the internal modes. As an overall strategy to measure the correlated structural motions in protein, we use anisotropic and birefringent behavior of molecular crystals to develop a new technique called MOSTS (Modulated Orientation Sensitive THz Spectroscopy). We achieve high sensitivity and mode separation by using single molecular crystal such as sucrose and rapid modulation of the relative alignment of the terahertz polarization and the crystal axes by rotating the sample. By locking into the signal at the rotation frequency we determine the polarization sensitive signal and map out the optically active vibrational resonances. To illustrate the technique we compare our measured spectra with the calculated and find a close agreement. \\[4pt] [1] D.G. Allis, J.A. Zeitler, P.F.Taday and T.M.Korter, Chem. Phys. Lett., 463, 84 (2008).\\[0pt] [2] P.U. Jepsen and J.C. Stewart, Chem. Phys. Lett., 442, 275 (2007). [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D1.00011: Ultra-Low Frequency MRI: Novel Imaging Sequences Matthew Nichols, Paul SanGiorgio, John Clarke Our ultra-low field MRI system operates at a field of 0.132 mT with the signal detected by a Superconducting QUantum Interference Device (SQUID) coupled to an untuned, superconducting, second-derivative gradiometer. Operation at such low fields requires that we prepolarize the protons of our specimen in a field of about 150 mT prior to imaging to increase the sample magnetization. With the ultimate goal of \textit{in vivo} imaging of prostate tissue by taking advantage of the enhanced longitudinal-relaxation-time at low fields, we seek to decrease the imaging time and optimize the signal-to-noise ratio of our imaging pulse sequences to make \textit{in vivo} imaging viable in a clinical setting. To achieve this, we begin with standard imaging sequences used in other frequency domains and adapt them to our specific purposes and requirements, in particular the need to prepolarize. We describe modified inversion recovery and multiple echo imaging sequences, specifically Carr-Purcell-Meiboom-Gill (CPMG) pulse train and fully balanced Steady-State Free Precession (SSFP) sequences. We present the results of applying these sequences to imaging agarose gel phantoms. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D1.00012: Millimeter-wave and terahertz spectroscopy for the detection of ionized air and chemical vapor Benjamin Graber, Rongjia Tao, Dong Ho Wu Our previous work has demonstrated that the time domain terahertz spectroscopy (TDTS) of chemical vapor or ionized air produces characteristic spectrum from which one can identify chemicals or an ionization source, such as nuclear isotopes. While the average power of TDTS is only less than micro-Watts, the peak power of terahertz pulse can exceed kilo-Watts. When terahertz pulse is concentrated within a small area this large peak power produces a very large electric field, exceeding several kV/m. We investigate the field strength dependence of TDTS to see how it affects the detection capability of TDTS for chemical vapor and ionized air when the peak power is varied over the range from a few mW to kW. At the same time we performed similar experiments using a CW millimeter-wave spectroscopy over the frequency range from 75 GHz to 110 GHz and the power strength range from a few micro-Watts to several mW. We will present the details of our experimental results and discuss the merits of both systems for accuracy and long range detection of vapors. We will also examine some theory to understand the data. [Preview Abstract] |
Session D2: Invited Session: Harnessing Local Atomic Structure to Control Magnetic Interactions in Complex Oxides
Sponsoring Units: GMAGChair: Steven May, Drexel University
Room: 204AB
Monday, February 27, 2012 2:30PM - 3:06PM |
D2.00001: Interface engineering in manganites: from diodes to transistors Invited Speaker: Harold Hwang Perovskite manganites show strong coupling between charge, spin, and lattice degrees of freedom as exemplified by `colossal magnetoresistance'. The recent advances in thin film growth techniques have enabled the generation of novel phases at oxide heterointerfaces, the atomic control of their interface electronic structure, and their incorporation in novel device platforms. We apply these techniques to manganite thin films, first emphasizing the subtleties in optimizing the growth kinetics and stoichiometry [1,2], which has enabled us to create atomically precise heterostructures exhibiting room temperature metallic ferromagnetism in superlattices composed of just 5 unit cell layers [3]. The interface electronic structure was examined using Schottky junctions formed between La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ and Nb-doped SrTiO$_{3}$, where the band offset (Schottky barrier height) can be controlled by the termination layer at the interface [4]. This band engineering technique was applied in making a metal-base transistor [5], which takes advantage of the strong internal electric field at interfaces. An analysis of many devices enables the quantitative understanding of the evolution from a hot-electron transistor to a permeable base transistor. This structure provides a platform for developing devices incorporating the exotic ground states of perovskite oxides and their interfaces. \\[4pt] [1] J. H. Song, T. Susaki, and H. Y. Hwang, \textit{Adv. Mater.} \textbf{20}, 2528 (2008). \\[0pt] [2] D. A. Muller, L. Fitting Kourkoutis, M. Murfitt, J. H. Song, H. Y. Hwang, J. Silcox, N. Delby, and O. L. Krivanek, \textit{Science} \textbf{319}, 1073 (2008). \\[0pt] [3] L. Fitting Kourkoutis, J. H. Song, H. Y. Hwang, and D. A. Muller, \textit{PNAS} \textbf{107}, 11682 (2010). \\[0pt] [4] Y. Hikita, M. Nishikawa, T. Yajima, and H. Y. Hwang, \textit{Phys. Rev. B} \textbf{79}, 073101 (2009). \\[0pt] [5] T. Yajima, Y. Hikita, and H. Y. Hwang, \textit{Nature Mater.} \textbf{10}, 198 (2011). [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D2.00002: New material design strategies to realize strong coupling in multiferroics and beyond Invited Speaker: Craig Fennie Perovskite ABO$_3$ oxides display an amazing variety of phenomena that can be altered by subtle changes in the chemistry and internal structure, making them a favorite class of materials to explore the rational design of novel properties. In this talk I will review a recent advance in which rotations and tilts of the BO$_6$ octahedra give rise to a novel form of ferroelectricity. Octahedral rotations strongly influence other structural, magnetic, orbital, and electronic degrees of freedom in perovskites and related materials. Thus, I want to discuss the idea that octahedral rotation-driven ferroelectricity has the potential to robustly control emergent phenomena with an applied electric field. As one example, I will show from first principles how these ``functional'' octahedral rotations simultaneously induce ferroelectricity, magnetoelectricity, and weak ferromagnetism in a class of naturally occurring Ruddlesden-Popper (RP) (ABO$_3$)$_2$(AO) layered perovskites and discuss the challenges to realize electric field switching of magnetism in these RP and in (ABO$_3$)/(A'BO$_3$) perovskite superlattice novel multiferroics. \\[4pt] N. A. Benedek and C. J. Fennie, {\it Phys. Rev. Lett} {\bf 106}, 107204, 2011;\\[0pt] J. M. Rondinelli and C. J. Fennie, arXiv 2011; N. A. Benedek and C. J. Fennie, arXiv 2011. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D2.00003: Reversal of Magnetic Interactions by Electric field Invited Speaker: Philip Ryan Direct magneto-electric coupling describes the interaction between magnetic and electric polarization through an intrinsic microscopic phenomenon in a single phase material. Systems which exhibit such coupling are potential candidates for use in a multistate logic memory storage device whereupon magnetic control with electric fields or ferroelectric control with magnetic fields could be used to alter memory bits. I will present x-ray resonant magnetic scattering results providing direct evidence of a magneto-electric cross field effect mediated through strong spin-lattice coupling in a single phase rare earth titinate film. Compressively strained EuTiO$_{3}$ is, as in bulk, an antiferromagnetic-paraelectric material, however through strain the balance of the magnetic interactions, both antiferromagnetic and ferromagnetic, shifts whereby the two approach energetic degeneracy. By applying an electric field \textit{in-situ} one can tip the delicate equilibrium and suppress the long range antiferromagnetic order. This is accompanied by the emergence of short range ferromagnetic order. In addition we have qualitatively replicated the microscopic preferential shift from antiferromagnetic to ferromagnetic order with electric field using first principles density functional calculations. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D2.00004: Strain-induced oxygen defect formation and interfacial magnetic phase separation in SrTiO$_{3}$(001)/La$_{1-x}$Sr$_{x}$CoO$_{3}$ Invited Speaker: Chris Leighton The remarkable functionality and epitaxial compatibility of complex oxides provides many opportunities for new physics and applications in oxide heterostructures. Perovskite manganites and cobaltites provide excellent examples, being of interest for solid oxide fuel cells, catalysis, ferroelectric RAM, gas sensing, resistive switching memory, and oxide spintronics. However, the same delicate balance between phases that provides this diverse functionality also leads to a serious problem - the difficulty of maintaining desired properties close to the interface with other oxides. Although this problem is widespread, manifests itself in several ways, and could present a significant roadblock to the development of heterostructured devices for oxide electronics, there is no consensus as to its origin, or even whether it is driven by electronic or chemical effects. In this work, using SrTiO$_{3}$(001)/La$_{1-x}$Sr$_{x}$CoO$_{3}$ as a model system, we have combined epitaxial growth via high pressure oxygen sputtering with high resolution x-ray diffraction, atomic resolution electron microscopy and spectroscopy, and detailed magnetic, transport, and neutron scattering measurements to determine the fundamental origin of the deterioration in interfacial transport and magnetism. The effect is found to be due to nanoscopic magnetic phase separation in the near-interface region driven by a significant depletion in interfacial hole doping due to accumulation of O vacancies. This occurs due to a novel mechanism for accommodation of lattice mismatch with the substrate based on formation and long-range ordering of O vacancies, thus providing a fundamental link between strain state and O vacancy density. Further impacts of the O vacancy ordering and interfacial magnetic phase separation, such as formation of a spin-state superlattice and an extraordinary coercivity enhancement, will also be discussed. Work in collaboration with M. Sharma, M. Torija, J. Schmitt, C. He, S. El-Khatib, J. Gazquez, M. Varela, M. Laver and J. Borchers. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D2.00005: Directly probing the effect of strain on magnetic exchange interactions Invited Speaker: Kathrin Dorr Thin films of transition metal oxides of the perovskite type ABO$_{3}$ (B = 3d or 4d metal) have revealed abundant examples for strain-driven changes of magnetic ordering. One most popular is the strain-induced ferromagnetic ferroelectric state of otherwise antiferromagnetic paraelectric EuTiO$_{3}$. Another promising example is the strain control of orbital occupation and magnetic coupling at oxide interfaces of SrRuO$_{3}$ with manganites. In spite of strong efforts, the theoretical treatment of magnetic exchange in complex oxides has remained a challenge, and experiments continue to show unpredicted / unexplained large effects of the epitaxial strains in films. In order to provide meaningful experimental data on strain dependences, epitaxial thin films should be grown in various coherent strain states on different substrates without changing anything but the strain. This is inherently difficult: possible problems may arise from a strain-dependent oxidation level or microstructure. As a complementary approach, the in-plane strain of epitaxial oxide films can be controlled reversibly using a piezoelectric substrate, even though the accessible reversible strain of 0.1 -- 0.2{\%} is an order of magnitude smaller. In my talk, I will address reversible-strain studies on La$_{0.7}$Sr$_{0.3}$MnO$_{3}$, La$_{1-x}$Sr$_{x}$CoO$_{3}$ (x = 0, 0.2, 0.3) und SrRuO$_{3}$ films, showing the strain response of the magnetic Curie temperature, the magnetization and the electrical resistance and discussing the current understanding of the strain effects on magnetic ordering. In La$_{0.8}$Sr$_{0.2}$CoO$_{3}$, a strain-driven phase transition between ferromagnetic and spin-glass-like could be established by combining the piezoelectric substrate with a tuned buffer system providing varied as-grown strain states. In SrRuO$_{3}$, a tetragonal tensile strain state shows a suppression of the ordered magnetic moment. Lattice parameters and symmetries of the films were determined by x-ray diffraction. It is noted that the atomic displacements (bond lengths and angles) under strain in these compounds are yet essentially unknown and subject to present research. [Preview Abstract] |
Session D3: Invited Session: Stripe Order and Fermi-Surface Reconstruction in Cuprate Superconductors
Sponsoring Units: DCMPChair: Michael Norman, Argonne National Laboratory
Room: 205AB
Monday, February 27, 2012 2:30PM - 3:06PM |
D3.00001: Fermi-surface reconstruction by stripe order in cuprate superconductors Invited Speaker: Francis Lalibert\'e The origin of pairing in a superconductor resides in the underlying normal state. In the cuprate high-temperature superconductor YBCO, application of a magnetic field to suppress superconductivity reveals a ground state that appears to break the translational symmetry of the lattice, pointing to some density-wave order [1,2,3]. In another cuprate, Eu-LSCO, the onset of stripe order - a modulation of spin and charge densities - at low temperature is well established [4]. By a comparative study of thermoelectric transport in the cuprates YBCO and Eu-LSCO, we show that the two materials exhibit a very similar process of Fermi-surface reconstruction as a function of temperature and doping [5,6]. This strongly suggests that Fermi-surface reconstruction is caused by stripe order in both cases, compelling evidence that stripe order is a generic tendency of hole-doped cuprates.\\[4pt] Work done in collaboration with J. Chang, N. Doiron-Leyraud, E. Hassinger, R. Daou, D. LeBoeuf, M. Rondeau, B. J. Ramshaw, R. Liang, D. A. Bonn, W. N. Hardy, S. Pyon, T. Takayama, H. Takagi, I. Sheikin, L. Malone, C. Proust, K. Behnia and L. Taillefer.\\[4pt] [1] N. Doiron-Leyraud \textit{et al.}, Nature \textbf{447}, 565 (2007).\\[0pt] [2] D. LeBoeuf \textit{et al.}, Nature \textbf{450}, 533 (2007).\\[0pt] [3] D. LeBoeuf \textit{et al.}, Phys. Rev. B \textbf{83}, 054506 (2011).\\[0pt] [4] J. Fink \textit{et al}., Phys. Rev. B \textbf{83}, 092503 (2011).\\[0pt] [5] J. Chang \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 057005 (2010).\\[0pt] [6] F. Lalibert\'e \textit{et al.}, Nat. Commun. \textbf{2}, 432 (2011). [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D3.00002: Competition between charge and superconducting orders in underdoped YBCO Invited Speaker: Marc-Henri Julien We report nuclear magnetic resonance measurements (NMR) showing that high magnetic fields induce a static, unidirectional, modulation of the charge density in the CuO$_{2}$ planes of underdoped YBa$_{2}$Cu$_{3}$O$_{y}$ [T. Wu et al., Nature 477, 191 (2011)]. The appearance of the charge order coincides with the Fermi surface reconstruction inferred from quantum oscillation and other transport measurements. This charge order appears to be most probably the same 4$a$-periodic stripe modulation as in La-214 cuprates. That it develops only when superconductivity fades away (no charge order is observed under strong fields parallel to the planes) and near the same 1/8 hole doping as in La-214 suggests that charge order, although visibly pinned by CuO chains in YBa$_{2}$Cu$_{3}$O$_{y}$, is an intrinsic propensity of the superconducting planes of high-Tc copper oxides. Since field induced stripe order is also compatible with neutron scattering data in La-214 and with STM data in Bi-2212, charge order could be a generic competitor of high Tc superconductivity. \\[4pt] Work performed with T. Wu, H. Mayaffre, S. Kr\"{a}mer, M. Horvatic, C. Berthier (LNCMI Grenoble), W.N. Hardy, R. Liang, D.A. Bonn (University of British Columbia, Vancouver) [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D3.00003: The electronic state of underdoped YBCO at high magnetic fields and low temperatures: evidence from quantum oscillatory phenomena Invited Speaker: Gil Lonzarich Quantum oscillations in bulk and transport properties have been observed in underdoped YBa$_2$Cu$_3$O$_{\rm{6+x}}$ via a range of techniques and by independent researchers in applied magnetic fields above 20T and temperatures below 10K. The consensus is that the oscillations are periodic in the reciprocal of the magnetic field and consist of a number of components with frequencies (fundamental or otherwise) of below 2kT, nearly an order of magnitude lower than that observed in the overdoped state of Tl$_2$Ba$_2$CuO$_{\rm {6+x}}$. Moreover, the temperature dependence of the amplitude of the strongest oscillatory components that can be measured accurately follows closely that expected for elementary excitations of fermionic character. I will discuss a model of the Fermi surface that can potentially account for each of the periodic components observed, and that appears to be consistent with a number of other known properties in the high-field low-temperature state. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D3.00004: Evidence for competing orders in underdoped YBa$_{2}$Cu$_{3}$O$_{y}$ Invited Speaker: Cyril Proust 25 years after the discovery of high temperature cuprate superconductors, the observation of quantum oscillations has deeply changed the theoretical landscape relevant to these materials. The measurements of quantum oscillations on both sides of the phase diagram of cuprates confirm the existence of a Fermi surface with sharply defined excitations on the overdoped side and also show that the Fermi surface has suffered a drastic modification on the other side. The small Fermi pockets inferred from quantum oscillations in the underdoped regime combined with the negative Hall and Seebeck coefficients pointing to electrons as dominant charge carriers, greatly strengthen the case that the Fermi surface of underdoped YBa$_{2}$Cu$_{3}$O$_{y}$ undergoes a reconstruction because the translational symmetry of its lattice is broken at low temperature. Many studies, such those of the Hall and Seebeck effects, point to a reconstruction of the Fermi surface due to stripe order. More recently, NMR measurements discover a static, unidirectional, modulation of the charge density in the CuO$_{2}$ planes that has the 4a-periodic stripe modulation. In this talk, I will show the impact of this charge order on some transport properties, in particular $c$-axis transport. A crossover to a coherent regime of metallic behavior of the $c$-axis resistivity at low temperature coincides with the emergence of the charge order. I will also address the topical question of competition between the charge order and superconductivity in high fields. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D3.00005: Fermi-surface reconstruction by charge-stripe order in a cuprate superconductor Invited Speaker: Steve Kivelson |
Session D4: Long-range Interacting Rydberg and Dipolar Gases
Sponsoring Units: DAMOPChair: Michael Fleischhauer, University of Kaiserslautern
Room: 205C
Monday, February 27, 2012 2:30PM - 2:42PM |
D4.00001: Rydberg-Rydberg interactions in ultracold atomic gases Sebastian Hofferberth, Jonathan Balewski, Stephan Jennewein, Alexander Krupp, Huan Nguyen, Johannes Nipper, Michael Schlagmueller, Christoph Tresp, Robert Loew, Tilman Pfau The giant size and large polarizibility of Rydberg-atoms make them ideal to study many-body collective effects. We present recent results from the ultra-cold Rydberg experiments in our group. Firstly, we discuss studies of F\"orster resonances, which appear when two-body-states are tuned into resonance. The resulting strong dipolar interatomic interactions vary in strength and angular dependency based on the magnetic substates involved. We study these interactions in a Ramsey atom interferometer, where the two interferometer arms are atoms in the ground and in the Rydberg state. Using this phase sensitive tool, F\"orster resonances are studied with unprecedented accuracy. The coherent technique enables to resolve several resonances and study their coherence properties. Secondly, we present our new setup for studying single Rydberg-excitations in optical traps smaller than the Rydberg-blockade sphere. Such ensembles, where all trapped atoms coherently share a single Rydberg-excitation, form a two-level ``superatom" whose Rabi-oscillation is collectively enhanced. Our new apparatus combines single ion-detection, sub-micron optical resolution, and highly flexible optical trapping potentials to study coherent dynamics of individual superatoms as well as interactions between superatoms. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D4.00002: Towards single-photon optical nonlinearities using cold Rydberg atoms Thibault Peyronel, Ofer Firstenberg, Qiyu Liang, Vladan Vuletic, Mikhail Lukin Effects of the Rydberg blockade in cold atomic clouds have been intensively explored over the last few years. Optical fields can be coherently mapped onto atomic states with a Rydberg component using EIT techniques thanks to the long lifetime of the Rydberg states. As the dipole-dipole interaction between Rydberg atoms prevents several polaritons from propagating simultaneously within a Rydberg volume, it gives rise to strong non-linearities which are mapped back on the probe optical field. We aim at bringing the Rydberg-EIT into the single-photon regime in order to produce non-classical highly correlated states of light. Rubidium atoms are loaded in a far off-resonant (1064nm) optical dipole trap, where densities are typically large enough to reach high optical depths within a single blockade volume. In this regime, the outcoming photon-photon correlation function is expected to exhibit highly non-classical behavior, corresponding to trains of spatially separated single-photons. Moreover, EIT techniques together with a high-resolution imaging system allow the observation of Rydberg excitations in the quasi-1D configuration, and should pave the way to in-situ monitoring of strongly correlated many-body states such as the crystallisation of Rydberg atoms. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D4.00003: Altering Photon Statistics using Strong Rydberg Interactions Johannes Otterbach, David Petrosyan, Alexey V. Gorshkov, Thomas Pohl, Michael Fleischhauer, Mikhail D. Lukin The recent advance in coherently controlling and manipulating strong, long-range Rydberg interactions has triggered extensive research in studying interesting many-body effects as, e.g. the use of Rydberg blockade effects for quantum information processing and crystal formation. In this talk I show that Rydberg interactions can be used to alter the photon statistics of a weak probe field after propagating in a coherently prepared atomic Rydberg gas under conditions of Electromagnetically Induced Transparency (EIT). The Rydberg blockade mechanism leads to an effective two-level physics when two photons are separated less than the blockade radius resulting in a strong anti-correlation of two photons separated by an avoided volume. Implications of the formation of such hard-sphere photons for the recent experiment of Pritchard et al. [Phys. Rev. Lett. 105, 193603 (2010)] and the observation of such correlation in future experiments will be discussed. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D4.00004: Collective quantum jumps of Rydberg atoms Tony Lee, Hartmut Haeffner, Michael Cross A quantum system under constant observation may occasionally switch between two metastable states. These quantum jumps are usually observed in a single object, like an atom, electron, or superconducting qubit. We report on a collective type of quantum jump in a group of atoms with long-range Rydberg interaction, laser driving, and spontaneous emission. Over time, the system occasionally jumps between a state of low Rydberg population and a state of high Rydberg population. The jumps are inherently collective and in fact exist only for a large number of atoms. We explain how entanglement and quantum measurement enable the jumps, which are otherwise classically forbidden. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D4.00005: Quantum crystals in a trapped Rydberg-dressed Bose-Einstein condensate C.-H. Hsueh, T.-C. Lin, T.-L. Horng, W.C. Wu Spontaneously crystalline ground states, called quantum crystals, of a trapped Rydberg-dressed Bose-Einstein condensate are numerically investigated. As a result described by a mean-field order parameter, such states simultaneously possess crystalline and superfluid properties. A hexagonal droplet lattice is observed in a quasi-two-dimensional system when dressing interaction is sufficiently strong. Onset of these states is characterized by a drastic drop of the non-classical rotational inertia proposed by Leggett [Phys. Rev. Lett. \textbf{25}, 1543 (1970)]. In addition, an AB stacking bilayer lattice can also be attained. Due to an anisotropic interaction possibly induced by an external electric field, transition from a hexagonal to a nearly square droplet lattice is also observed. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D4.00006: Quantum Magnetism with Polar Molecules: Tunable Generalized $t$-$J$ Model Alexey Gorshkov, Salvatore Manmana, Kevin Kuns, Kaden Hazzard, Gang Chen, Jun Ye, Eugene Demler, Mikhail Lukin, Ana Maria Rey We show that dipolar interactions between ultracold polar molecules in optical lattices can be used to realize a highly tunable generalization of the $t$-$J$ model, which we refer to as the $t$-$J$-$V$-$W$ model. The ``spin" is encoded in the rotational degree of freedom of the molecules, while the interactions are controlled by applied static electric and continuous-wave microwave fields. We show that the tunability and the long-range nature of the interactions in the $t$-$J$-$V$-$W$ model enable enhanced superfluidity in one dimension and controllable preparation of robust d-wave superfluids in two dimensions. The latter may provide fundamental insights into high-temperature superconductivity. [References: Phys. Rev. Lett. 107, 115301 (2011); Phys. Rev. A 84, 033619 (2011); arXiv:1110.5330] [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D4.00007: Tunable Superfluidity with Ultracold Polar Molecules on quasi-1D Optical Lattices Salvatore R. Manmana, Alexey V. Gorshkov, Kevin A. Kuns, Gang Chen, Jun Ye, Eugene Demler, Mikhail D. Lukin, Ana Maria Rey By selecting two dressed rotational states of ultracold polar molecules on an optical lattice, strong electric dipole-dipole interactions allow to directly emulate spin Hamiltonians and a highly tunable generalization of the $t-J$ model, the $t-J-V-W$ model. We present the phase diagram of the simplest experimentally realizable case, the $t-J_\perp$ model with long-range dipolar spin-exchange interactions, on (quasi) one-dimensional chain and ladder systems as obtained from extensive density matrix renormalization group (DMRG) calculations. For the chain, we discuss the possible realization of unconventional quasi-long-range-order caused by the dipolar interactions. While the phase diagram of the dipolar $t-J_\perp$ chain is similar at low filling to that of the standard t-J chain, the superconducting region is strongly enhanced. We approach the ladder systems by coupling square plaquettes and comparing the numerical results to a mean-field description. We discuss the possibility to enhance superconductivity in these ladder systems by the presence of the dipolar interactions. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D4.00008: Dynamical generation and detection of entangled quantum magnetic states in ultracold polar molecules Kaden Hazzard, Salvatore Manmana, Alexey Gorshkov, Ana Maria Rey We show that \textit{existing} ultracold polar molecule experiments in optical lattices may generate strongly correlated many-body states by mimicking far-from-equilibrium dynamics of models of quantum magnetism. Recent theory shows that molecules' rotational states can emulate quantum spins with strong ($100$-$10,000$Hz) ``spin-spin" interactions. Applying external fields generates a zoo of models: spin-1/2 and larger Heisenberg and XXZ models, and well beyond. We consider the dynamics of the easily prepared fully polarized initial state for the XXZ case predicted to be realized in current experiments. Our analytic and DMRG calculations show that the dynamics can: (i) verify and characterize the spin model (XXZ) description of the system, (ii) generate interesting, entangled states (e.g., cat states, GHZ), and (iii) explore behavior where no quantitative theory is presently possible. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D4.00009: Interactions of polar molecules dressed by far-off-resonant light: Entangled dipoles up- or down-holding each other Mikhail Lemeshko, Bretislav Friedrich We show that the electric dipole-dipole interaction between a pair of polar molecules undergoes an all-out transformation when superimposed by a far-off resonant optical field. The combined interaction potential becomes tunable by variation of wavelength, polarization and intensity of the optical field and its dependence on the intermolecular separation exhibits a crossover from an inverse-power to an oscillating behavior. The ability thereby offered to control molecular interactions opens up avenues toward the creation and manipulation of novel phases of ultracold polar gases among whose characteristics is a long-range entanglement of the dipoles' mutual orientation. We devised an accurate analytic model of such optical-field-dressed dipole-dipole interaction potentials, which enables a straightforward access to the optical-field parameters required for the design of intermolecular interactions in the laboratory. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D4.00010: Clustering of cold polar molecules in arrays of one-dimensional tubes Michael Knap, Erez Berg, Eugene Demler Cold polar molecules allow to study exciting new phenomena which arise from the long-range and anisotropic nature of their mutual interactions. Here, we demonstrate that a Wigner crystal of polar molecules confined in planar arrays of one-dimensional tubes can be made unstable with respect to the formation of clusters of particles. By controlling the orientation of the external electric field which aligns the dipolar moments, increasingly complex structures with a varying number of particles per cluster and thus varying periodicity are formed. The spatial agglomeration of multiple polar molecules results from the interaction and can be described classically. However, we show that the effect survives when quantum fluctuations are present. For systems of a finite number of tubes, the result is a sequence of ''clustered`` Luttinger liquid states. Finally, we determine the ratio between the interaction and the kinetic energy which is necessary for the spatial agglomeration of polar molecules. We find that the requirements for clustering are reachable in current experiments with cold polar molecules. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D4.00011: A dielectric superfluid of polar molecules Seth Rittenhouse, Ryan Wilson, John Bohn We consider a Bose-Einstein condensate of heteronuclear molecules in an applied electric field. In the strong field regime, the molecules are fully polarized and produce fields that tend to be weak compared to the applied field. However, in weaker applied fields the internal fields due to the polarization of the molecules can become comparable to the applied field, and the system develops a dielectric character. We derive a set of self-consistent mean-field equations that couple the condensate density to its polarization field, leading to the emergence of polarization modes that are coupled to the quasiparticle spectrum of the condensate. While the roton instability is suppressed in this system, the coupling gives rise to a phonon-like instability that is characteristic of a dielectric material with a negative static dielectric function. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D4.00012: Dipolar gases in two coupled one-dimensional lattices Marianne Bauer, Meera Parish We consider dipolar bosons in two tubes of one-dimensional lattices, where the boson filling fraction is the same in each tube and the dipoles are aligned to be maximally repulsive. In the classical limit of zero inter-site hopping, the bosons arrange themselves into an ordered crystal for any rational filling fraction, forming a complete devil's staircase like in the single tube case [1]. When we turn on hopping within each tube, we obtain a competition between the crystalline Mott phases and a superfluid of defects or solitons. However, in contrast to the single tube case [2], we find that solitons in different tubes can bind into pairs for certain topologies of the filling fraction. This provides an intriguing example of pairing that is purely driven by correlations close to a Mott insulator.\\[4pt] [1] P. Bak and P. Bruinsma, Phys. Rev. Lett. 49, 249 (1982)\\[0pt] [2] F. J. Burnell, M. M. Parish, N. R. Cooper and S. L. Sondhi, Phys. Rev. B, 80, 174519 (2009) [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D4.00013: Paired Phases of Dipoles in a Bilayer System Arghavan Safavi-Naini, Sebnem Gunes Soyler, Guido Pupillo, Hossein Sadeghpour, Barbara Capogrosso-Sansone By means of large scale quantum Monte Carlo simulations we study a system of dipolar lattice bosons in bilayers geometries and with no hopping between layers. We investigate under which conditions pairing (e.g. paired superfluidity, paired supersolidity) is stabilized and make estimates for current experimental setups. We also study temperature effects and possible experimental signatures of such phases. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D4.00014: Bond Order Solid of Two-Dimensional Dipolar Fermions Satyan Bhongale, Ludwig Mathey, Shan-Wen Tsai, Charles Clark, Erhai Zhao Cold atoms provide a promising platform to solve problems that, although computationally infeasible, are of immense importance to condensed matter physics and material science. Ultra-cold bosonic atoms have been quite successful in emulating the Bose-Hubbard model. Experiments are now underway towards mapping out the unknown phase diagram of the Fermi-Hubbard model. Recent experimental advances in cooling dipolar gases to quantum degeneracy provide an unprecedented opportunity to engineer Hubbard- like models with long range interactions. Here, with the aid of functional renormalization group technique, we show that two new and exotic types of order emerge generically in dipolar fermion systems: bond order solids of p- and d-wave symmetry. Similar, but manifestly different, phases of two-dimensional correlated electronic systems have previously only been hypothesized. Our results suggest that these phases can be constructed flexibly with dipolar fermions, using currently available experimental techniques, providing detectable experimental signatures. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D4.00015: Re-entrant first-order phase transitions and anomalous hysteresis of dipolar Bose gases in a triangular optical lattice Daisuke Yamamoto, Ippei Danshita, Carlos S\'a de Melo We study the hysteresis behavior of dipolar Bose gases loaded into a triangular optical lattice. A large-size cluster mean-field approximation is applied to the corresponding extended Bose-Hubbard model to take into account metastable states. We find that the phase transition between supersolid (or solid) and superfluid states is always first-order except for the particle-hole symmetric point. In the phase diagram, the supersolid and solid phases are sandwiched in between the superfluid phase, and the system exhibits a re-entrant behavior from superfluid to solid (to supersolid), and back to superfluid with varying the chemical potential. Our most remarkable finding is that in the hysteresis accompanying this ``re-entrant'' first-order phase transition, the quantum melting transition from supersolid or solid to superfluid can occur while the reverse process is impossible since the superfluid phase remains locally stable for any value of the chemical potential. Moreover, the hysteresis curve of density versus chemical potential does not form a ``hysteresis-loop'' structure unlike the case of the conventional first-order transition. We show that this anomalous behavior of the hysteresis is a common property of systems exhibiting a re-entrant first-order phase transition. [Preview Abstract] |
Session D5: Vanadium Oxides; Metal-insulator Transitions
Sponsoring Units: DCMPChair: Mumtaz Qazilbash, College of William and Mary
Room: 206A
Monday, February 27, 2012 2:30PM - 2:42PM |
D5.00001: Metal-Insulator Transition in ultrathin CaVO$_{3}$ Films Man Gu, Jiwei Lu, Stuart Wolf Bulk CaVO$_{3}$ (CVO) with a 3d$^{1}$ electronic configuration has been found to exhibit metallic and Pauli paramagnetic behavior. We have synthesized epitaxial ultrathin films of CVO on single crystal (100) SrTiO$_{3}$ substrates by pulsed electron deposition. The CVO films were capped with 2.5nm SrTiO$_{3}$ layer. Compared with single crystal CVO, thin film CVO demonstrated very large resistance ratios (RR), e.g., R (300K)/R (2K) more than 3000. The temperature dependent Hall measurement showed mostly that the large RR determined by the change in mobility, we observed the metal-insulator transition at $\sim $ 100K in CVO ultrathin films with thickness below 4nm, which was not observed in either thick CVO films or STO films. Above 100K, ultrathin CVO exhibited the metallic behavior, and below 100 K, it became an insulator. The emergence of MIT could be attributed to a pseudogap that appeared at Fermi surface with decreasing film thickness, indicating a transition from 3D metal to 2D insulator transition in ultrathin CVO films at temperatures lower than 100K. A metal-insulator transition was further characterized by I-V measurements, the insulator phase was only observed with driven current below 100K and 2$\times $10$^{-6}$A. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D5.00002: Thermoelectric response in the incoherent transport region near Mott transition: the case study of La$_{1-x}$Sr$_x$VO$_3$ Masaki Uchida, K. Oishi, Y. Onose, J. Fujioka, Y. Tokura, M. Matsuo, M. Mori, S. Maekawa, W. Koshibae, S. Miyasaka We report a systematic investigation on the high-temperature thermoelectric response in a typical filling-control Mott transition system La$_{1-x}$Sr$_x$VO$_3$. In the vicinity of the Mott transition, incoherent charge transport appears with increasing temperature and the thermopower undergoes two essential crossovers, asymptotically approaching the limit values expected from the entropy consideration, as known as Heikes formula. By comparison with the results of the dynamical mean field theory, we show that the thermopower in the Mott critical state mainly measures the entropy per charge carrier that depends on electronic degrees of freedom available at the measurement temperature. Our findings verify that the Heikes formula is indeed applicable to the real correlated electron systems at practical temperatures ($T>200$ K). [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D5.00003: First-principles calculations of phonons in VO$_2$ Eric J. Walter, Henry Krakauer, Tyler Huffman, M.M. Qazilbash Vanadium dioxide (VO$_2$) undergoes a metal-insulator transition at 340K. This is accompanied by a structural transition from a metallic, high-temperature rutile phase to a low-temperature monoclinic insulating phase. Recently, it has become possible to produce single crystal platelets of VO$_2$ deposited on a oxidized silicon substrate. These micro-crystals are under strain which can potentially alter their properties compared to bulk samples. Infrared micro-spectroscopy on these samples permits accurate measurements of their electronic and phonon properties as the mircro-crystals are driven reversibly across the temperature-driven insulator-to-metal transition (IMT). We present {\it ab-initio} calculations of phonons in the rutile and monoclinic phases of VO$_2$. These calculations were performed using first-principles density functional theory using both LDA and LDA+U. The effect of the Hubbard parameters and strain on both phases is discussed. We compare our results to the single crystal measurements and previous experimental results. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D5.00004: Infrared micro-spectroscopy of strained VO$_2$ micro-crystals M.M. Qazilbash, T. Huffman, E.J. Walter, H. Krakauer, Jiang Wei, D.H. Cobden, H.A. Bechtel, M.C. Martin, G.L. Carr, D.N. Basov The temperature-driven insulator-to-metal transition (IMT) in vanadium dioxide (VO$_2$) is accompanied by a structural instability (SI). The IMT and SI lead to a drastic change in the electronic properties, crystal structure, and lattice dynamics. We performed infrared micro-spectroscopy on single crystal platelets of VO$_2$ deposited on oxidized silicon substrate by physical vapor deposition. The firm attachment of these micro-crystals to the substrate causes strain which can alter their properties compared to bulk samples. We report infrared data on these micro-crystals and demonstrate both their electronic and phonon properties in the monoclinic M1 phase and the rutile phase. We also compare their infrared conductivity to that of bulk single crystals and thin films. Finally, we compare infrared-active phonon features to first-principles density functional theory calculations. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D5.00005: Ultrafast Pump Probe Transmission Spectroscopy of VO$_2$ Nathaniel Brady, Kannatassen Appavoo, Minah Seo, Prashanth Upadhya, Joyeeta Nag, Richard Haglund, Rohit Prasankumar, David Hilton We have performed nondegenerate pump-probe transmission spectroscopy, pumping with an above the band gap (1.5 eV) 50 fs pulse and probing with a 0.4 eV (below the band gap) pulse to monitor the dynamics of the formation of the metallic phase in vanadium dioxide ($\mathrm{VO_2}$). Below the percolation threshold ($<$ 330 K), we find an initial drop in transmission consistent with electron-hole generation across the band gap, while in the fully metallic phase ($<$ 365 K), we see an initial rise in transmission due to transient heating of electrons at the Fermi surface. In the transitional region, the data show complex time dependence consistent with the nucleation and growth of metallic domains in the semiconducting phase and ultrafast heating of metallic precursors in the insulating phase. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D5.00006: Ultrafast Spectroscopy and Optically-Induced Phase Transitions of Single Crystal VO2 Aaron Jones, Jae Park, Jim Coy, David Cobden, Xiaodong Xu We investigate the metal-insulator phase transition (MIT) of single crystal VO2 platelets using non-degenerate optical pump-probe spectroscopy. The pump pulse is at 800 nm and the probe pulse varies between 1.3 $\mu $m and 2.4 $\mu $m, covering the optical gap ($\sim $2 $\mu $m) of VO$_{2}$. We observe ultrafast carrier relaxation on the timescale of 0.5 ps or less in the insulating phase. Higher pump powers induce coherent acoustic phonon oscillations which we explore by adjusting probe wavelength and pump fluence. At temperatures just below the transition, we observe optically induced MIT at a time scale less than 300 fs (pulse width limited), and we investigate the dependence on crystal size, wavelength, and temperature. The properties of the insulator do not seem consistent with a conventional band semiconductor. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D5.00007: THz induced insulator to metal transition in VO$_{2}$ metamaterial Mengkun Liu, Harold Hwang, Hu Tao, Andrew Strikwerda, Kebin Fan, George Keiser, Aaron Sternbach, Kevin West, Salinporn Kittiwatanakul, Jiwei Lu, Stuart Wolf, Fiorenzo Omenetto, Xin Zhang, Keith Nelson, Richard Averitt We use metamaterial enhanced high field terahertz (THz) pulses (up to $\sim $4MV/cm) to induce the insulator-to-metal transition in vanadium dioxide (VO$_{2})$ thin films at 320K. Ultrafast THz field enhancement in the gaps of metamaterial split ring resonators releases free electrons in VO$_{2}$ by the Poole-Frenkel effect. The accelerated hot electrons transfer energy to the lattice via electron phonon coupling inducing the persistent metallic phase. A large nonlinear signature is observed in VO$_{2}$ as modulations of the metamaterial resonance on a picosecond time scale. Our results provide insight into electric field induced phase transitions in VO$_{2}$ and paves the way for studying nonlinear high THz field effects in many other strongly correlated materials. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D5.00008: Optical control of structural evolution of ultrathin VO$_2$ film following a photoinduced metal-insulator phase transition Haidan Wen, Eftihia Barnes, June H. Lee, Yuelin Li, Donald A. Walko, Eric M. Dufresne, Martin Holt, Darrell G. Schlom, Venkatraman Gopalan, John W. Freeland Using ultrafast x-ray diffraction technique, we have demonstrated that the structural symmetry in VO$_{2}$ thin film across the metal-insulator transition (MIT) can be controlled by the optical pump fluence. After an 800 nm, 50 fs laser excitation, the structural evolution of a 13nm-thick, MBE-grown VO$_{2}$ film on TiO$_{2}$ substrate was monitored by a synchrotron based x-ray diffraction with 100 picosecond (ps) time resolution. By adjusting the pump fluence, the VO$_{2}$ film can be prepared in selected structural state 100 ps after excitation, and transits through the known structural phases (tetragonal, monoclinic M2, monoclinic M1) in tens of nanoseconds time scale. When pumping at high fluence (~28 mJ/cm$^{2}$), a new transient state that exhibits lower symmetry than the thermal equilibrium tetragonal phase has been identified and its evolution pathway has been measured. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D5.00009: In-situ measurements of Stress Relaxation across Metal Insulator Transition in Correlated Oxide Thin Films Viswanath Balakrishnan, Shriram Ramanathan Stress relaxation across the metal-insulator transition in correlated oxide thin films such as VO$_{2}$ and SmNiO$_{3}$ is of great importance since it could be directly related to the symmetry breaking structural component of the transition and also affects the properties and performance of the electronic devices significantly. We present in-situ stress relaxation measurements across the thermally triggered metal insulator transition and its impact on the transition characteristics and stability. Mesoscopic size effects, micro-patterning and geometrical confinement effects on the metal insulator transition and associated stress relaxation will be addressed. Correspondence between onset of the electrical transition with stress relaxation leads to several interesting observations regarding the transition dynamics and will be discussed. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D5.00010: In-Plane Impedance Spectroscopy measurements in Vanadium Dioxide thin films Juan Ramirez, Edgar Patino, Rainer Schmidt, Amos Sharoni, Maria Gomez, Ivan Schuller In plane Impedance Spectroscopy measurements have been done in Vanadium Dioxide thin films in the range of 100 Hz to 1 MHz. Our measurements allows distinguishing between the resistive and capacitive response of the Vanadium Dioxide films across the metal-insulator transition. A non ideal RC behavior was found in our thin films from room temperature up to 334 K. Around the MIT, an increase of the total capacitance is observed. A capacitor-network model is able to reproduce the capacitance changes across the MIT. Above the MIT, the system behaves like a metal as expected, and a modified equivalent circuit is necessary to describe the impedance data adequately. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D5.00011: Photoemission Study of VO2 Above and Below the Transition Temperature Luca Moreschini, Young Jun Chang, Davide Innocenti, Andrew L. Walter, Jonathan Denlinger, Aaron Bostwick, Eli Rotenberg Angle-resolved photoemission (ARPES) experiments on VO$_2$ have traditionally been hindered by the quality of cleaved single crystals. The lack of a clear metal-insulator transition (MIT) in low photon energy measurements has even lead to the assumption of a surface region with a different electronic structure. WIth the \textit{in situ} pulsed-laser-deposition (PLD) system available on beamline 7.0.1 at the Advanced Light Source we have grown VO$_2$(001) films on a TiO$_2$ substrate and measured the band structure above and below the transition temperature. We discuss our results in comparison with the available calculations, and we show that the MIT is clearly visible for photon energies within the UV range. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D5.00012: Photoemission measurements of strained VO2 Jude Laverock, Andrew Preston, Dave Newby, Kevin Smith, Salinporn Kittiwatanakul, Jiwei Lu, Stuart Wolf, Mats Leandersson, Balasubramanian Thiagarajan The metal-insulator transition of VO$_2$ has been a textbook example for many years, despite a clear understanding of its microscopic origins proving elusive. Recently, the promise towards novel applications of high-quality thin films, in which the properties of the transition can be tailored by applied strain, has thrust VO$_2$ back into focus. Here, we report photoemission measurements of strained VO$_2$ thin films epitaxially grown on TiO$_2$(110) and TiO$_2$(100) substrates. The applied strain for these two films lead to moderate and large compressive rutile $c$-axis strains, respectively. By making use of the incident photon polarization, we observe the changes in polarization anisotropy both across the transition and as a function of applied strain, and demonstrate how we can use this to learn more about the origin of the MIT in VO$_2$. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D5.00013: Correlations of local electronic properties in vanadium dioxide thin films Adam Pivonka, Kevin O'Connor, Alex Frenzel, Changhyun Ko, Shriram Ramanathan, Eric Hudson, Jennifer Hoffman We probe the local electronic properties of a vanadium dioxide thin film using scanning force microscopy. We scan a conductive cantilever in contact mode across the surface of the sample. At each point, we sweep the voltage applied to the sample, obtaining current versus voltage curves with nanonscale resolution while inducing a transition from the insulating to metallic state. We identify individual grains of $\sim $50-100 nm, and extract the electronic properties of each grain, such as transition voltage, hysteresis, dielectric constant, and metallic state resistance. We discuss the correlations between these properties. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D5.00014: Correlation of Valence State and Transport Properties in VO$_{2}$ Films Salinporn Kittiwatanakul, Jude Laverock, Stuart Wolf, Jiwei Lu Vanadium dioxide (VO$_{2})$ exhibits a metal semiconductor transition (MST) that is accompanied by the abrupt change in the electrical conductivity, optical transmittance and reflectance in infrared region, which can be used in electronic devices such as temperature sensors and electric switches. VO$_{2}$ thin films were grown on c-plane Al$_{2}$O$_{3}$ substrates with different O$_{2}$ flow rates. The XRD scans have been performed to confirm the single phase and highly textured VO$_{2}$ films despite the change in the growth parameter, i.e. the oxygen partial pressure. The valence state of vanadium on different films was investigated by soft x-ray spectroscopy. The Hall bars were fabricated for electrical transport and Hall measurements. As the O$_{2}$ flow rate increases, the XRD results show decreasing lattice parameter, hence increasing compressive strain along b-axis of monoclinic VO$_{2}$; the transport measurements also show the increasing transition temperature (T$_{MST})$ and the increasing change in resistivity associated with the strain and valence state of vanadium. Hall measurements reveal a sudden increase in carrier concentration from 10$^{20}$ to 10$^{24}$ cm$^{-3}$ around T$_{MST}$, while the mobility remains constant before and after MST. The correlation among the valence state, the strain and MST in VO$_{2}$ will be discussed. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D5.00015: Metal-insulator transition in VO2 macro single domain crystals due to phase boundary motion Honglyoul Ju, Bongjin Mun, Kai Chen, Catherine Dejoie, Nobumichi Tamura, Martin Kunz, Zhi Liu, Sung-Kwan Mo, Joonseok Yoon, Changwoo Park Metal insulator transition (MIT) characteristics of macro-size VO$_{2}$ single domain crystals, exhibiting a high resistance ratio of $\sim $ 10$^{5}$ within as small as-10$^{-3} \quad ^{\circ}$C in the vicinity of MIT temperature, were investigated by temperature-dependent electrical transport, optical microscopy, and synchrotron-based polychromatic x-ray micro-diffraction measurements. Our results clearly show that MIT initiated via inhomogeneous nucleation, proceeds with the propagation of sharp phase boundary between the metallic (R) and insulating (M1) phases, along the rutile c axis. In this talk, we will present evidences of MIT of single domain VO$_{2}$ crystals with sharp phase boundary motion and discuss the implications of our findings on to the origin of MIT and related phenomena. [Preview Abstract] |
Session D6: Focus Session: Graphene Devices - Bolometers and Optical Response
Sponsoring Units: DMPChair: Tom Timusk, McMaster University
Room: 206B
Monday, February 27, 2012 2:30PM - 2:42PM |
D6.00001: Probing Dirac electron transport in graphene by far-infrared spectroscopy Jie Shan, Keliang He, Liang Zhao, Kin Fai Mak, Nick Petrone, Jim Hone, Tony Heinz The transport properties of Dirac fermions in graphene are a subject of intense interest. While various scattering mechanisms including impurities, graphene phonons and substrate phonons have been examined by dc transport measurements,\footnote{Das Sarma, S., Adam, S., Hwang, E. H. \& Rossi, E. Rev. Mod. Phys. 83, 407-470, (2011).} direct determination of the scattering rates under different experimental conditions remains challenging.\footnote{Horng, J. et al. Phys. Rev. B 83, 165113, (2011).} In this paper we report on the far-infrared optical conductivity spectrum of monolayer graphene samples obtained by Fourier transform infrared spectroscopy. From the frequency dependence of the optical conductivity we determine both the Drude weight and the carrier scattering rate. The dependence of these transport parameters on temperature and electrostatic doping will be presented, and the importance of many-body Coulomb interactions between Dirac electrons will be discussed. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D6.00002: Ultrathin dual-gated graphene p-n junction photodetectors Nityan Nair, Nathaniel Gabor, Qiong Ma, Kenji Watanabe, Takashi Taniguchi, Wenjing Fang, Jing Kong, Pablo Jarillo-Herrero Optoelectronic devices composed of atomically thin graphene and boron nitride membranes yield great promise for next-generation photonics and optoelectronic research, yet numerous fabrication challenges remain. We use chemical vapor deposited (CVD) graphene to produce atomically thin, local bottom-gates for high-quality exfoliated graphene optoelectronic devices. By incorporating CVD graphene instead of the more conventional silicon bottom-gate electrodes, we create very low-profile dual-gated field effect p-n junction devices with hexagonal boron nitride as the insulating gate dielectric layer. Combining electron-beam and photolithography techniques, we can shape the bottom-gates to locally modulate the carrier density in the active graphene layer. In addition to avoiding optical transmission through thick top-gate electrodes, our approach allows us to perform temperature dependent photoresponse measurements over various device length scales and with direct control of local electronic carrier densities. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D6.00003: Graphene-based Optical Modulator Ming Liu, Xiang Zhang Data communications have been growing at a speed even faster than Moore's Law, with a 44-fold increase expected within the next 10 years. Data Transfer on such scale would have to recruit optical communication technology and inspire new designs of light sources, modulators, and photodetectors. The past decade has seen the flourish of researches in silicon-based optical modulators. However, their performance is limited by the weak refractive index changes in silicon, and consequently large footprint and stringent fabrication tolerance are required. Here we raise a totally new mechanism for optical modulation. Instead of changing the refractive index in silicon, we use graphene as an active layer and change its absorption coefficient by turning on/off the interband transitions. This turning is realized through shifting the Fermi level by simply a back gate. In this way, we can operate the optical modulator at a relatively high speed (1.2 GHz) over a broad range (1.3 to 1.6 $\mu$m), while keep the smallest footprint ($\sim $25 $\mu$m$^2$). More details of the device will be discussed in the talk. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D6.00004: Tunable optical properties of graphene Invited Speaker: Feng Wang Graphene, a single layer of carbon atoms, exhibits novel two-dimensional electronic behavior. Optical spectroscopy provides a powerful toolkit study graphene physics. In this talk, I will show how we can use infrared spectroscopy to probe gate-dependent interband transitions as well as intraband transitions. I will also discuss how we can use electrical gating to control inelastic light scattering processes in graphene. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D6.00005: Photoconductivity of electro-oxidized epitaxial graphene Feihu Wang, Mikhail Itkis, Elena Bekyarova, Robert Haddon We report the enhanced photosensitivity of epitaxial graphene (EG) after electrochemical oxidation in nitric acid. The onset of photoconductivity appears at a photon energy of $\sim $ eV1.7 while the responsivity reaches 200 A/W in the UV spectral range (3.5 eV, 350 nm). The observed photoresponse is attributed to the formation of deep traps at the electro-oxidized EG interface, which release charge carriers under illumination and a significantly prolonged life time of photoexcitations due to the effect of the traps on the recombination dynamics. The enhanced photosensitivity and high selectivity in the UV spectral range make electro-oxidized EG an interesting alternative to the less spectrally selective Si for UV detection, although further optimization of the chemistry is required to shorten the photoresponse time while preserving the high sensitivity. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D6.00006: Photo-controlling electrical properties of graphene-based field-effect transistor with photo-reactive copolymer Chi-Yuan Lin, Forest Shih-Sen Chien, Chen-Shiung Chang, Chia-Chen Hsu Here, we presented a photo-controlling graphene-based FET device. The tri-stable current of graphene device can be achieved in room temperature using the photo-assisted poling (PAP) and photo bleaching (PB) on photo-reactive copolymer PMMA-DR1. PMMA-DR1 was used as a dielectric film between graphene and top gate, where the top gate was the ITO film for applied voltage and excited laser passing. PAP operation created a polarization of PMMA-DR1 film, while the current of graphene FET device was varied due to Fermi energy of graphene was directly influenced by the polarization. The strength of polarization was associated with the gate voltage and laser power during the process of PAP operation. In contrast to the PAP operation, the PB operation was able to destroy the polarization of PMMA-DR1 film, while the current of graphene FET device returned to the initial value. Combing with PAP and PB operations, three current states identified as 1, 0 and -1 states can be achieved in room temperature. The current change ratios were 150{\%} and 50{\%} for 1 state and -1 state, comparing with initial state 0. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D6.00007: Imaging of Polarization-dependent Photocurrent in Graphene Photodevices Minjung Kim, Duhee Yoon, Ho Ang Yoon, Sang Wook Lee, Hyeonsik Cheong Recently, a metal-graphene-metal photodetector for high-speed optical communications was reported. In addition, a graphene-based photodetector was reported to be able to absorb broadband light owing to the unique band structure of graphene [Mueller et al., Nature Photonics 4, 297 (2010)]. We investigated the polarization dependence of the photocurrent generated in metal-graphene-metal junctions. The graphene photodevice was fabricated by depositing Pd/Au and Ti/Au electrodes on single-layer graphene samples. When the polarization of incident laser beam is rotated with respect to the metal-graphene-metal junction, the photocurrent is significantly modulated. In addition, we measured the exact positions where the photocurrent is generated by measuring the photocurrent and Raman images of the graphene photodevices simultaneously. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D6.00008: Supercollisions and Electron-Lattice Cooling in Graphene Justin Song, Michael Reizer, Leonid Levitov Hot carrier proliferation is dependent on slow cooling between electron and lattice systems. In graphene, these cooling rates can be slow due to a small Fermi surface size and the momentum-conserving character of electron-phonon scattering. This in concert with the small ratio between the sound velocity, $s$, and the Fermi velocity, $v_F$, such that $s/v_F= 1/100$ produces a phase space constrained by the Fermi wavevector, $k_F$. In each of these first order scattering events, the energy exchanged between electronic and lattice systems is of the order $T_{\rm BG} = s\hbar k_F$ which at typical doping densities can be many times smaller than $k_BT$; non-thermal phonons are the dominant contributors to scattering dramatically suppressing cooling power. This constraint can be lifted by considering ``supercollisions" that utilize the full thermal distribution of phonons. While the frequency of supercollisions may be lower than the first order process, energy transfer for supercollisions is now on the order of $k_BT$ which is many times larger than the first order process. We we will show that this large exchange of energy allows supercollisions to give a dramatic boost to the cooling rate dominating over the first order process. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D6.00009: Hot Carrier-Assisted Intrinsic Photoresponse in Graphene Nathaniel Gabor, Justin Song, Qiong Ma, Nityan Nair, Thiti Taychatanapat, Kenji Watanabe, Takashi Taniguchi, Leonid Levitov, Pablo Jarillo-Herrero Graphene is considered an excellent candidate for photodetection and energy harvesting applications due to its broadband optical response and high internal quantum efficiency, yet measurements have not clearly determined the photocurrent generation mechanism. Here, we report on the intrinsic photoresponse of dual-gated monolayer and bilayer graphene p-n junction devices. Local laser excitation of wavelength 850 nm at the p-n interface leads to striking six-fold photovoltage patterns as a function of bottom- and top-gate voltages. These patterns, together with the measured spatial and density dependence of the photoresponse, provide strong evidence that non-local hot carrier transport, rather than the photovoltaic effect, dominates the intrinsic photoresponse in graphene [1,2] The hot carrier regime manifests as a strong photo-thermoelectric effect in which the photogenerated carrier population remains hot while the lattice stays cool. [1] Science v. 334, p. 648-652 (2011). [2] Nano Lett. ASAP 10.1021/nl202318u (2011). [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D6.00010: Graphene Terahertz Photodetector Xinghan Cai, Greg Jenkins, Andrei Sushkov, Jun Yan, H.D. Drew, Michael S. Fuhrer A graphene photodetctor device is fabricated using mechanically exfoliated single layer graphene on SiO2/Si substrate contacted by two dissimilar metal electrodes (chromium and gold) using standard electron beam lithography. The graphene is etched into a strip shape with specific width and coupled to a bow tie antenna structure to improve coupling to long-wavelength radiation and enhance the electric field in the center of the device. We have observed the response of the graphene photodetector to optical (632.8nm) and infrared laser (118um) radiation as a function of gate voltage and device width. Experimental results and comparison to a model of graphene plasmon-enhanced photodetction will be discussed. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D6.00011: Bolometric photo response of dual-gated bilayer graphene Jun Yan, M.-H. Kim, J.A. Elle, A.B. Sushkov, G.S. Jenkins, H.M. Milchberg, H.D. Drew, M.S. Fuhrer We study the photo response of dual-gated bilayer graphene devices under infrared radiation. By comparison to Joule heating measurements using a second harmonic transport technique, we determine that the photo response is bolometric instead of photoconductive. The measured large electron-phonon heat resistance of our device is in good agreement with theoretical estimates in magnitude and temperature dependence, and enables our graphene bolometer operating at a temperature of 5 K to have a low noise equivalent power (33 fW/Hz$^{1/2}$) and fast response time (sub nanosecond). [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D6.00012: GHz response of bilayer graphene hot electron bolometer M.-H. Kim, J. Yan, J.A. Elle, A.B. Sushkov, G.S. Jenkins, H.M. Milchberg, M.S. Fuhrer, H.D. Drew An intrinsic GHz speed of a hot-electron bolometer (HEB) based on dual-gated bilayer graphene (BLG) was recently reported (J. Yan, arXiv:1111.1202). The thermal response time is governed by the weak electron acoustic phonon scattering which also results in a high thermal resistance for the lattice cooling of the hot electrons. The time response of BLG HEB was measured as a function of temperature, bias current, and laser power using two time delayed pulses from a 1.03 $\mu$m pulsed laser. In addition, we probed the energy gap dependence of the time response revealing information about the electron density in gapped BLG. We report the temperature dependence of the heat capacity and thermal resistance obtained from these measurements. This work is supported by IARPA grant \#W911NF1010443 [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D6.00013: Non-linear plasma transport in graphene channels and application to the detection of terahertz signals Sergey Rudin, Greg Rupper The non-linear electron plasma response to electromagnetic signal applied to a gated graphene conduction channel can be used to make a graphene based Dyakonov-Shur terahertz detector. The hydrodynamic model predicts a resonance response to electromagnetic radiation at the plasma oscillation frequency. With less damping and higher mobility, the graphene conduction channels may provide higher quality plasma response than possible with semiconductor channels. Our analysis of plasma oscillations in a graphene channel is based on the hydrodynamic equations which we derive from the Boltzmann equation accounting for both electrons and holes, and including the effects of viscosity and finite mobility. [Preview Abstract] |
Session D7: Quantum Hall Effect in Mono-, Bi- and Trilayer Graphene
Sponsoring Units: DCMPChair: Erik Henriksen, California Institute of Technology
Room: 207
Monday, February 27, 2012 2:30PM - 2:42PM |
D7.00001: Correlated random hopping disorder in graphene at high magnetic fields: Landau level broadening and localization properties Ana L.C. Pereira, Caio H. Lewenkopf, Eduardo R. Mucciolo Disorder is key to understand the electronic transport properties in graphene, particularly in the quantum Hall regime. There is still some debate on the most relevant disorder mechanisms for transport in graphene. Among those, ripple disorder is believed to play an important role. Static ripples give rise to random correlated hopping disorder, which is the disorder mechanism analyzed in this work. We study the density of states and localization properties of the lowest Landau levels of graphene at high magnetic fields, focusing on the effects caused by correlated long-range hopping disorder. We find that the broadening of the lowest Landau level shrinks exponentially with increasing disorder correlation length. The broadening also grows linearly with magnetic field and with disorder amplitudes. More importantly, we observe that the ratio between the n=1 and n=0 Landau level widths depends only on the correlation length and is rather insensitive to the disorder strength and to the magnitude of the magnetic field. This allows a closer contact of our results with experiments. In addition, the lowest Landau level peak shows a robust splitting (inferred from the analysis of the participation ratio), whose origin we identify as the breaking of the sublattice (valley) degeneracy. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D7.00002: Electrically tunable quantum anomalous Hall effect in graphene decorated by $5d$ transition-metal adatoms Hongbin Zhang, Cesar Lazo, Stefan Bl\"ugel, Stefan Heinze, Yuriy Mokrousov Based on first-principles calculations, we predict that $5d$ transition-metals on graphene present a unique class of hybrid systems exhibiting topological transport effects that can be manipulated effectively by external electric fields [1]. The origin of this phenomenon lies in the exceptional magnetic properties and the large spin-orbit interaction of the $5d$ metals leading to significant magnetic moments accompanied with colossal magnetocrystalline anisotropy energies. A strong magneto-electric response is predicted that offers the possibility to switch the spontaneous magnetization direction by moderate electric fields, enabling an electrically tunable quantum anomalous Hall effect.\\[4pt] [1] preprint: http://arxiv.org/abs/1108.5915 [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D7.00003: Scanning gate microscopy on graphene in the quantum Hall regime James Hedberg, Yoichi Miyahara, Michael Hilke, Guillaume Gervais, Peter Gr\"utter, Julien Renard, Joshua Folk Scanning Gate Microscopy was performed on monolayer graphene devices in the quantum hall regime. The devices studied consisted of exfoliated graphene deposited on SiO2, etched in a Hall Bar configuration, and electrically contacted by standard lithographic techniques. Using a custom built scanning probe microscope (SPM), operating at liquid Helium temperatures and below, in magnetic fields up to 16 T, we spatially mapped the position dependent effects of a movable gate, i.e. the charged tip of the SPM, on the conductivity of the graphene device. Using a global backgate to modulate the carrier density, we can visually observe the transitions between filling factors. Striking features are observed in the resistance versus position maps, offering insights into the microscopic properties of graphene in the quantum Hall regime. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D7.00004: Confining potential and Landau level edge states in graphene Guohong Li, Adina Luican-Mayer, Eva Y. Andrei Two-dimensional electron systems in the Quantum-Hall (QH) regime host gapless one-dimensional chiral edge states which are responsible for the quantization of the Hall conductivity. In the regime of the fractional QH effect the edge states form a chiral Luttinger liquid which presents unusual quantum properties such as fractionally charged excitations and interference patterns that could serve as building blocks for QH qubits. Observing and exploiting these properties requires precise control of the edges, but in semiconductor-based 2DES were edge states were studied thus far, achieving the necessary control was difficult. This is because the smooth confinement potential in these systems imposes a length scale which is much larger than the magnetic length leading to incompressible strips and to non-universal behavior. We will show that this limitation is not present in graphene. Using scanning-tunneling-microscopy and spectroscopy to follow the spatial evolution of Landau levels toward an edge, we demonstrated that in graphene it is possible to control the edge states by varying the distance to the screening plane and by controlling the confinement geometry. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D7.00005: Unconventional Sequence of Fractional Quantum Hall States in Suspended Graphene Benjamin Feldman, Benjamin Krauss, Jurgen Smet, Amir Yacoby Graphene provides a unique platform to study many-body correlations due to the relativistic nature of its charge carriers and their fourfold degeneracy. We report local electronic compressibility measurements of a suspended graphene flake performed using a scanning single-electron transistor. Between filling factors $v$ = 0 and 1, our measurements reveal incompressible fractional quantum Hall states at $v$ = 1/3, 2/3, 2/5, 3/5, 3/7, 4/7 and 4/9, which clearly follow the standard composite fermion sequence. In contrast, between $v$ = 1 and 2, incompressible states occur only at $v$ = 4/3, 8/5, 10/7 and 14/9. These fractions correspond to a subset of the composite fermion sequence involving only even numerators, suggesting a robust underlying symmetry. We extract the energy gaps of each fractional quantum Hall state as a function of magnetic field and find that $v$ = 1/3, 2/3, 4/3, and 8/5 are strongest at low field, persisting below 1.5 T. Our results provide insight into the interplay between electronic correlations and SU(4) symmetry in graphene. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D7.00006: Quantum Hall effect on centimeter scale chemical vapor deposited graphene films Tian Shen, Wei Wu, Qingkai Yu, Curt Richter, Randolph Elmquist, David Newell, Yong Chen We report observations of well developed half integer quantum Hall effect on mono layer graphene films of 7 mm by 7 mm in size. The graphene films are grown by chemical vapor deposition on copper, then transferred to SiO$_{2}$/Si substrates, with typical carrier mobilities $\approx $4000 cm$^{2}$/Vs. The large size graphene with excellent quality and electronic homogeneity demonstrated in this work is promising for graphene-based quantum Hall resistance standards, and can also facilitate a wide range of experiments on quantum Hall physics of graphene and practical applications exploiting the exceptional properties of graphene. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D7.00007: Spin-skyrmion in graphene Wenchen Luo, Ren\'{e} C\^{o}t\'{e} In the presence of a magnetic field, the band structure of the two-dimensional electron gas in graphene consists in a series of Landau levels with energy $E_{n}=\pm \sqrt{2} v_{F}\sqrt{\left\vert n\right\vert }/\ell .$ Each Landau level is 4-fold degenerate including valley and spin degrees of freedom. Working in the Hartree-Fock approximation and a finite Zeeman coupling, we compute the energy required to excite a spin-skyrmion (or antiskyrmion) in Landau levels $n=1,2$ at filling factors $\nu =4,8.$ We show that a skyrmion-antiskyrmion pair has lower energy than an electron-hole pair at these two filling factors. We compare our results with a field-theoretical calculation [1] and with a recent experimental measurement of the transport activation gap in this system. \\[4pt] [1] Kun Yang, S. Das Sarma and A. H. MacDonald, Phys. Rev. B \textbf{74}, 075423 (2006). [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D7.00008: Coulomb impurity under magnetic field in graphene: a semiclassical approach Yuhui Zhang, Yafis Barlas, Kun Yang We address the problem of a Coulomb impurity in graphene in the presence of a perpendicular uniform magnetic field. We show that the problem can be solved below the supercritical impurity magnitude within the WKB approximation. Without impurity the semiclassical energy correctly reproduces the Landau level spectrum. For values below the supercritical impurity magnitude the energy spectrum still evolves as square root B with a renormalized fine structure constant. For a given Landau level the WKB energy depends on the absolute value of angular momentum in a way which is consistent with the exact diagonalization result. Below the supercritical impurity magnitude, the WKB solution can be expanded as a convergent series in powers of the effective fine structure constant. Relevance of our results to validity of the widely used Landau level projection approximation is discussed. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D7.00009: Chern-Simons theory of an anomalous metallic state in half-filled monolayer graphene Pallab Goswami, Bitan Roy, Kun Yang The ground state of half-filled monolayer graphene undergoes a novel metal-insulator transition with increasing strength of applied magnetic field. In a weak magnetic field the ground state at half-filling corresponds to a critical metallic state, that governs the $\nu=-2$ to $\nu=2$ quantum Hall plateau transition. In the strong magnetic field regime this critical state gives way to an interaction driven quantum Hall insulator state. Currently there is no satisfactory theoretical explanation of the insulating phase and the phase transition. Motivated by this issue, we investigate the nature of the ground state in clean half-filled monolayer graphene, using a lattice Chern-Simons theory. In contrast to the results obtained previously by mean-field calculations in the Landau level basis, our analysis in the unpolarized regime shows the existence of an anomalous semimetallic state up to a critical strength of magnetic field, and the critical strength is determined by non-universal details of interaction strength. In the polarized regime the dynamics of the relevant metallic state changes dramatically, and new insulating phases do emerge. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D7.00010: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 4:30PM - 4:42PM |
D7.00011: Bilayer graphene as a helical quantum Hall ferromagnet Rene Cote, Jeremie P. Fouquet, Wenchen Luo The two-dimensional electron gas (2DEG) in a graphene bilayer supports a variety of uniform broken-symmetry ground states in Landau level $N=0$ and at integer filling factors $\nu \in \left[ -3,4\right] .$ When a bias is applied between the layers at filling factors $\nu =1,3$, the ground state evolves from an interlayer coherent state at small bias to a state with orbital coherence at higher bias where \textit{electric} dipoles associated with the orbital pseudospins order spontaneously in the plane of the layers. We show that by further increasing the bias the 2DEG goes first through an electron crystal with an orbital pseudospin texture at each site and then into a helical state where the pseudospins rotate in space. The pseudospin textures in the crystal and the helical states are due to the presence of a Dzyaloshinsky-Moriya interaction in the effective pseudospin Hamiltonian when orbital coherence is present in the ground state. We study in detail the electronic structure these nonuniform states as well as their collective excitations and compute their electromagnetic absorption. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D7.00012: Theory of integer quantum Hall effect in bi-layer graphene Bitan Roy, Oskar Vafek Bi-layer graphene in a quantizing external magnetic field exhibits plateaus of Hall conductivity at various integer fillings. Moreover, electron-electron interactions in suspended doubly gated bi-layer graphene appear sufficiently strong (and short-ranged) to result in a finite gap persisting down to zero magnetic field. In this talk we will demonstrate the competition of various orders within the zeroth Landau level and how their interplay is influenced by the filled Landau levels lying below the Fermi energy. Scaling behavior of the gap at the neutrality point will be discussed. Besides the splitting of the zeroth Landau level, degeneracy lifting of the rest of the Landau levels will be discussed. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D7.00013: Electronic transport in ABA trilayer graphene E.A. Henriksen, D. Nandi, J.P. Eisenstein We present measurements of the electronic transport in ABA trilayer graphene field-effect devices fabricated with both a back and top gate, at zero, low and high magnetic fields. While the zero field resistivity exhibits a saddle point as a function of the two gate voltages that is similar to bilayer graphene, the low-field Hall data are consistent with a two-band system having a band overlap characteristic of semimetals. At high magnetic fields the quantum Hall effect is clearly observed, and displays features which can be traced to the underlying band overlap, as well as a lifting of the lattice mirror symmetry. Overall the transport in ABA trilayers is that of a semimetal in which the band structure can be strongly modified via the electric field effect. This work is supported by the DOE under grant No. DE-FG03-99ER45766 and the Gordon and Betty Moore Foundation. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D7.00014: Quantum Hall liquid to Charge density wave phase transitions in ABC-trilayer graphene Yafis Barlas, Rene Cote, Maxine Rondeau We study interaction driven states within ABC-stacked trilayer graphene's 12-fold degenerate Landau level which appear near the neutral system Fermi level. The 12-fold degeneracy of the zero-energy LL is due to spin and valley degeneracy along with a degenerate set of triplet ($n=0,1,2$) LL orbitals. We predict that at filling factors $\nu = - 5,-2, 1,4$ a quantum phase transition from a quantum Hall liquid state to a triangular charge density wave occurs as a function of the single-particle induced LL orbital splitting. This transition is preceded by a softening of the magneto-roton minima of the quantum Hall liquid which appears at $ql_{B} \sim 2.4$. The charge density wave is a manifestation of the LL orbital pseudospin textures with nonzero winding numbers. The phase diagrams at other filling factors along with the experimental consequences of our theoretical predictions will also be addressed. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D7.00015: ABSTRACT WITHDRAWN |
Session D8: Organo-metallics: Magnetic and other Properties
Sponsoring Units: GMAGChair: Vivien Zapf, Los Alamos National Lab
Room: 208
Monday, February 27, 2012 2:30PM - 2:42PM |
D8.00001: Polymorphism in Low-dimensional Molecule-based Materials: Synthetic Control of Magnetic and Electronic Structure Adora Baldwin, Jamie Manson, John Schlueter Competition for hydrogen bonds in molecule-based materials frequently leads to the stabilization of polymorphic structures with similar energies. Herein, we report that among magnetic coordination polymers selective crystallization of the [Cu(HF$_{2})$ (pyrazine)$_{2}$]SbF$_{6 }$and [Cu$_{2}$F(HF)(HF$_{2})$ (pyrazine)$_{4}$](SbF$_{6})_{2}$ polymorphs can be achieved through control of pressure and temperature during the crystallization process. Through this manner, orbital order and magnetic dimensionality (2D vs. 1D) can be controlled in this system. Similarly, in the analogous [Co(HF$_{2})$ (pyrazine)$_{2}$]SbF$_{6}$ system, selective crystallization of the [CoF$_{2}$ (pyrazine)$_{2}$]SbF$_{6}$(H$_{2}$O)$_{3}$(H$_{3}$O) polymorph has been achieved. While these cobalt materials exhibit a 2D magnetic structure, the interlayer coupling is diminished in the hydrated material. Among cation radical salts, five polymorphic phases in the [bis(ethylenedithio)tetrathiafulvalene]$_{2}$Ag(CF$_{3})_{4}$(1,1,2-trichloroethane) system have been crystallized, four of which have superconducting ground states. Among the superconducting salts, the ones with intercalated charge ordered layers exhibit a five-fold enhancement of T$_{c}$. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D8.00002: Ground-state properties of EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ by many-variable variational Monte Carlo method Satoshi Morita, Ryui Kaneko, Masatoshi Imada The organic Mott insulator EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ is a strongly correlated electron system on a nearly-regular triangular lattice and regarded as a spin liquid material. We investigate its effective low-energy model derived from first principles calculations and band+dimensional downfolding. The {\it ab initio} effective Hamiltonian is given in the form of the two-dimensional single-band extended Hubbard model on an anisotropic triangular lattice with short-ranged Coulomb and exchange interactions. Its ground state is calculated by the many-variable variational Monte Carlo method with quantum-number projection and multi-variable optimization. We draw the ground-state phase diagram as a function of scaling parameters for the interactions and the geometrical frustration by extending the {\it ab initio} model. We discuss Mott transitions and magnetic properties. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D8.00003: Reduced Influence of Electronic Correlations in a Half-Filled System: Metallic Properties and Charge Order Transition in $\kappa$-(ET)$_2$Hg(SCN) $_2$Cl Natalia Drichko, Rebecca Beyer, Martin Dressel, Harald Jeschke, John Schlueter, Elena Zhilyaeva, Rimma Lyubovskaya The family of $\kappa-$(ET)$_2$Hg(SCN)$_x$Cl$_{2-x}$ can be described within a model of a metal with a half-filled conductance band. Our calculations indicate that the value of the on-site Columb repulsion U is relatively small in these system. We investigate the effects of lower U on the optical properties of $\kappa$-(ET)$_2$Hg(SCN)$_2$Cl in the metallic state and below the metal-insulator transition. Optical conductivity spectra in the metallic state above 35 K show a behavior already observed for half-filled metals on the edge of the Mott insulating state: an optical transition between Hubbard bands at frequencies equal to U is present in spectra at all temperatures, being a precursor of a Mott transition, while a coherent carriers response develops below T*=150 K. We note an influence of the lower U value both on the position of the ``U-band'' and on a relatively high T*. Untypically for a half-filled system, a metal-insulator transition at 35 K is found that is of a charge-order nature. A possibility of a charge-order ground state in a half-filed system in case of low U and relatively high nearest-neighbor Columb repulsion V is discussed. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D8.00004: Non-monotonic magnetic field dependence of the staggered-moment in two-dimensional frustrated antiferromagnets Mohammad Siahatgar, Burkhard Schmidt, Peter Thalmeier We present an efficient method to identify the degree of frustration in quasi-two-dimensional (2D) antiferromagnets described by the $J_1$-$J_2$ quantum Heisenberg model. The frustration ratio $J_2/J_1$ is commonly obtained from the analysis of magnetic susceptibility, specific heat, and saturation field. We show that the field-induced suppression of quantum fluctuations causes a nonlinear and non-monotonic magnetic field dependence of the staggered moment which depends strongly on the frustration ratio. This serves as a powerful criterion to determine $J_2/J_1$ using a combination of the exact diagonalization method on finite clusters and the spin-wave analysis. We apply this method to the quasi-2D compound $\rm{Cu(pz)_2(ClO_4)_2}$ obtaining an intermediate ratio of $J_2/J_1 \simeq 0.2$ for the frustration. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D8.00005: Effects of Pressure on the Magnetic Properties of Prussian Blue Analogues M.K. Peprah, E.S. Knowles, M.F. Dumont, J.S. Xia, M.W. Meisel, C.H. Li, D.R. Talham Prussian blue analogues (PBAs) have been shown to exhibit interesting magnetic characteristics under various external stimuli: charge transfer induced spin transition (CTIST),\footnote{N. Shimamoto \textit{et al.} Chem. Lett. \textbf{31}, (2002) 486.} a temperature effect; persistent photo-induced magnetism (PPIM),\footnote{O. Sato \textit{et al.} Science \textbf{272} (1996), 704-705.} an optical effect; and pressure induced electron transfer (PIET).\footnote{V. Ksenofontov \textit{et al.} Phys. Rev. B \textbf{68} (2003) 024415.} Our research has established that photocontrol of the magnetic response of CoFe-PBA, can be extended to higher temperatures, accompanied by a decrease in magnetization, using heterostructured PBA particles composed of Rb$_a$Co$_b$[Fe(CN)$_6$]$_c\cdot$$m$H$_2$O cores surrounded by shells of K$_j$Ni$_k$[Cr(CN)$_6$]$_l\cdot$$n$H$_2$O.\footnote{M. F. Dumont \textit{et al.} Inorg. Chem. \textbf{50} (2011) 4295.} This effect is attributed to strain in the shell, induced by a lattice expansion in the core during illumination. We report a similar effect in the core-shells accompanied by PIET under application of external pressure and probe the similarities between optical and pressure effects. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D8.00006: Effect of hydrostatic pressure on a low dimensional S=1/2 gapped quantum antiferromagnet by inelastic neutron scattering Tao Hong, C. Stock, I. Cabrera, C. Broholm, Y. Qiu, J.B. Leao, S.J. Poulton, J.R.D. Copley We report an inelastic neutron scattering study of a quasi-two-dimensional $S$= 1/2 dimer system piperazinium hexachlorodicuprate (PHCC) under hydrostatic pressure up to $P$=9.0 kbar. The spin gap $\Delta $ decreases with increasing pressure. The observation of three fold degenerate triplet excitation at $P$=6.0 kbar is consistent with the theoretical prediction and the bandwidth of the dispersion relation is unaffected within the experimental uncertainty. At $P$=9.0 kbar the spin gap is reduced to $\Delta $=0.55 meV from $\Delta $=1.0 meV at ambient pressure [1]. \\[4pt] [1] T. Hong \textit{et. al}., Physical Review B 82, 184424 (2010) [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D8.00007: Density functional theory study of alkali doped picene Milan Tomic, Hunpyo Lee, Roser Valenti, Harald O. Jeschke We employ density functional theory methods to determine the equilibrium structures of $A_x$picene where $A={\rm Na}$, K, Rb, Cs and $x=1,2,3$. We find that alkali doping with one, two and three alkali ions per picene molecule leads to subsequent filling of the LUMO and LUMO+1 derived bands of picene, leading to quarter, half and three quarter filled systems. We analyse the electronic structures using tight binding methods to derive the kinetic energy part of the underlying Hubbard Hamiltonian. As the interaction strength $U$ on the picene molecules is expected to be large compared to the band width, we also employ manybody methods on the resulting Hamiltonian. We compare our results to photoemission experiments. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D8.00008: First principles investigation of the Fabre salts Anthony Jacko, Roser Valenti, Harald O. Jeschke The Fabre salts (TMTTF)$_2X$ are one dimensional charge transfer salts with a very rich phase diagram including charge ordered, Mott insulating, antiferromagnetic and spin Peierls phases. The structure involves stacks of TMTTF molecules with varying degree of dimerization. Open questions involve the role of the anions in the phase transitions. We investigate the electronic structure of compounds with a number of different anions like $X={\rm AsF}_6$, PF$_6$ and Br. We discuss the properties of the underlying Hubbard Hamiltonian for these materials. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D8.00009: Scaling Theory of the Mott Transition and Breakdown of the Gr\"uneisen Scaling Near a Finite-Temperature Critical End Point Lorenz Bartosch We discuss a scaling theory of the lattice response in the vicinity of a finite-temperature critical end point. The thermal expansivity is shown to be more singular than the specific heat such that the Gr\"uneisen ratio diverges as the critical point is approached, except for its immediate vicinity. More generally, we express the thermal expansivity in terms of a scaling function which we explicitly evaluate for the two-dimensional Ising universality class. Recent thermal expansivity measurements on the layered organic conductor $\kappa$-(BEDT-TTF)$_2 X$ close to the Mott transition are well described by our theory.\\[2mm] [1] Lorenz Bartosch, Mariano de Souza, and Michael Lang, Physical Review Letters {\bf 104}, 245701 (2010). [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D8.00010: Tuning the magnetic dimensionality by charge ordering in the molecular TMTTF salts Kazuyoshi Yoshimi, Hitoshi Seo, Shoji Ishibashi, Stuart Brown Low-dimensional (D) organic conductors TMTTF$_{2}$X are considered as a prototypical system for investigations of D-crossover phenomena, since the transfer integrals connecting the conduction chains are readily controlled by applied pressure [1]. In addition, recent NMR experiments in TMTTF salts imply the existence of another type of D-crossover which accompanies a multiferroic property; decreasing the ferroelectric-type charge ordering (FCO) by pressure is associated with the suppression of the antiferromagnetic (AF) transition temperature [2]. In this work, we theoretically investigate the interplay between FCO and magnetic states in TMTTF salts [3], and show that FCO increases 2-D AF spin correlation, whereas in the 1-D regime two different spin-Peierls states are stabilized. By performing first-principles band calculations for different salts and comparing our results with experiments, we identify the controlling parameters in the experimental phase diagram to be not only the inter-chain transfer integrals but also the amplitude of the FCO. [1] D. Jerome, Science 252, 1509 (1991). [2] W. Yu \textit{et al., }Phys. Rev. B. \textbf{70 }121101 (2004). [3] K. Yoshimi, H. Seo, S. Ishibashi, and S. E. Brown, arXiv:1110.3573 and arXiv:1110.3575. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D8.00011: Charge dynamics in Mott-insulating and charge-ordered organic crystals: a possibility of the Berezinskii-Kosterlitz-Thouless transition Yamaguchi Takahide, Tohru Watanabe, Yoshihiko Takano, Kouta Kodama, Shinya Uji, Hiroshi M. Yamamoto The organic Mott insulator $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl exhibits power-law current-voltage characteristics in a low current range at temperatures below $\approx$ 10 K. The power-law current-voltage characteristics are similar to those observed in the charge-ordered organic crystals $\theta$-(BEDT-TTF)$_2$MZn(SCN)$_2$ (M = Cs and Rb) and are accounted for in the same way in terms of exciton excitations: The electric field assists the thermal unbinding of pairs of a doublon and a holon that attract each other due to the two-dimensional long-range Coulomb interaction. The two-dimensional Coulomb interaction arises from the layered structure of the organic crystals and their large dielectric anisotropy. The in-plane dielectric constant can be understood within the context of the same model in terms of polarization of the bound pairs. We discuss a possibility of the Berezinskii-Kosterlitz-Thouless transition in these layered organic crystals. $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl has a much smaller magnetoresistance (25 $\%$ in 10 T at 0.3 K) than $\theta$-(BEDT-TTF)$_2$CsZn(SCN)$_4$ ($\approx$10000 $\%$ in 10 T at 0.1 K), which probably reflects different spin states of the excitations for the Mott and charge-ordered states. Yamaguchi Takahide et al. PRB 84, 035129 (2011) [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D8.00012: $^{1}$H NMR study on X-ray irradiated $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br Kazuya Miyagawa, Takahiko Sasaki, Naoki Yoneyama, Norio Kobayashi, Kazushi Kanoda The $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br ($\kappa$-Br) is a quasi-two dimensional superconductor with $T_{c}$ of 12 K. By substituting Cl for Br in the insulating layer, the system becomes a Mott insulator, k-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl ($\kappa$-Cl), with the Neel temperature of 25 K. So, $\kappa$-Br salt is situated close to the Mott transition. Recently, Sasaki {\it et al.,} have reported the transformation from a metallic (superconducting) state to an insulating state by the x-ray irradiation. The 500h-irradiated thin sample shows insulating behavior in the temperature dependence of resisitivity. We report the $^{1}$H NMR studies on the 500h-irradiated $\kappa$-Br salt. Below 150 K, where the nuclear relaxation is dominated by electron spins, 1/$T_{1}$ of the irradiated sample increases from that of the non-irradiated sample and is even larger than the value of $\kappa$-Cl. Nevertheless, there is no manifestation of magnetic ordering; that is, neither a line broadening nor a divergent peak in 1/$T_{1}$ down to 1.5 K. We will discuss the electronic state of irradiated $k$-Br salt based on experimental results. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D8.00013: TCP, a new quasi-one-dimensional conducting platinate: DC and NMR studies J.A. Alexander, R.I. Leatherbury, O. Gafarov, A.A. Gapud, A.P. Weber, L. Pham, R.E. Sykora, A. Khan, A.P. Reyes, P. Kuhns Cs$_{4}$[Pt(CN)$_{4}$](CF$_{3}$SO$_{3})_{2}$, or TCP, is the newest member of the family of quasi-one-dimensional conducting platinates that includes the widely studied K$_{2}$[Pt(CN)$_{4}$]Br$_{0.30}$3(H$_{2}$O) (KCP) -- best-known for its metal-insulator transition consistent with a Peierls instability. Unlike KCP, however, we have found properties unique to TCP. X-ray diffraction shows longer Pt-Pt separations, and undergoes subtle change with cooling. DC resistivity measurements presented technical challenges that had to be resolved, but in the end revealed a more highly insulating phase at room temperature, and the temperature dependence of resistivity has an anomalous ``peak'' at around 150 K. NMR also presented a technical challenge in that the $^{195}$Pt nucleus, which had been successfully used for NMR studies on KCP, did not produce a usable signal in TCP, wherein we utilized the peripheral $^{133}$Cs nuclei instead. Quadrupole splitting of spin states of $^{133}$Cs measured as a function of orientation showed consistency with the angular dependence expected of the known symmetry axes of $^{133}$Cs. Preliminary measurements of longitudinal relaxation time T1 also reveal an anomalous temperature dependence in the vicinity of 150 K. All these considerations point to a possible structural transformation, as will be discussed. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D8.00014: Experimental and theoretical study of the temperature dependent structural and electronic properties of the spin liquid candidate $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ John A. Schlueter, Mariano de Souza, Rudra Sekhar Manna, Michael Lang, Roser Valenti, Harald O. Jeschke The organic charge transfer salt $\kappa$-(BEDT-TTF)$_2$Cu$_2$(CN)$_3$ is of great experimental as well as theoretical interest due to the interplay of the effects of electronic correlations, low dimensionality and spin frustration. Besides the spin liquid property at very low temperatures, the material has interesting anomalies at 6K, 60K and 150K. Even though the structure of the material has been determined several times over the past 20 years, complete structural data are only available at room temperature. In our work, we fill this gap by precisely determining the structure at 300K, 250K, 200K, 150K, 100K and 20K. We analyze the structures using density functional theory and tight binding methods. We show that the triangular lattice Hubbard Hamiltonian parameters are temperature dependent, with the interaction strength increasing with decreasing temperature and with the frustration going through a minimum at 150 K. Our results point to the fact that even in the absence of structural phase transitions, experimental determination of structures at various temperatures may be important as a starting point for realistic many-body theoretical investigation of complex materials. [Preview Abstract] |
Session D9: Magnetic Devices and Applications
Sponsoring Units: GMAGChair: Dario Arena, Brookhaven National Laboratory
Room: 209
Monday, February 27, 2012 2:30PM - 2:42PM |
D9.00001: Dual microbead-labeled DNA manipulation with magnetic traps in a microfluidic device M. Howdyshell, M. Simon, M. Poirier, R. Sooryakumar Biomolecular responses to mechanical force underlie many critical functions in the context of cellular physiology. In order to develop the technology to apply forces on individual biomolecules, we utilize an array of ferromagnetic disks on a silicon surface to trap and manipulate tethered DNA molecules. The force activation is achieved through remotely controlled programmable weak external magnetic fields that do not damage the biological entity. Moreover, to exploit both hydrodynamic and magnetic forces, the magnetic disks are imprinted within microfluidic channels, while a tethered microbead attached on each end of the DNA strand provides convenient force transmitting handles. Two separate approaches that are utilized involve use of two superparamagnetic beads or a superparamagnetic and nonmagnetic bead pair. The independently controlled hydrodynamic and magnetic forces allow for manipulation of the DNA in all directions within a horizontal plane. Hundreds of magnetic traps are readily patterned onto a single channel, providing the potential to multiplex an ensemble of individual molecules within the device. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D9.00002: Testing of a Helmholtz Microcoil in a Diamond Anvil Cell NMR Ching Lin, Sam Weir, Samuel Weir, Nicholas Curro A new designed, multi-turn tungsten Helmholtz micro-coil has been constructed and tested on the solid-state bulk NMR experiment. A Helmholtz micro-coil with diameter 950~$\mu $m is~embedded on diamond culet and produces a nearly uniform AC magnetic field inside a sample space. A Fluorine polycrystal will be used to test our Helmholtz micro-coil, and the measured NMR data will be compared with the ones produced by other type of diamond anvil cell coils. The Helmholtz micro-coil will be used for high pressure NMR and future investigation of magnetic properties of heavy fermion superconductors. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D9.00003: Magneto-mechanical resonant detection of superparamagnetic microbeads trapped by magnetic domain walls Elizabeth Rapoport, Daniel Montana, Geoffrey Beach Manipulation of superparamagnetic (SPM) beads with magnetic domain walls (DWs) is of interest for lab-on-chip applications. DWs can trap SPM beads and tagged entities, enabling remote manipulation with nanoscale precision [1, 2]. Previously, we have demonstrated DW driven capture and transport of single microbeads at speeds approaching 1000 $\mu$m/s [3]. Here, we demonstrate that the strong magnetostatic bead-DW binding leads to a unique magneto-mechanical resonance [4]. We show experimentally that this resonance can be used to distinguish bead populations based on their size, presenting a new mechanism for bead metrology. Moreover, the bead-DW interaction can be used to sense and characterize magnetic beads without the need for sensor surface functionalization. Exploiting the dual functionality of DWs as both bead carriers and sensors, we present an integrated device capable of high-speed transport and electrical sensing of the magneto-mechanical resonance of individual trapped beads.\\[4pt] [1] G. Vieira et al., Phys. Rev. Lett. 103, 128101 (2009).\\[0pt][2] M. Donolato et al., Adv. Mater. 22, 2706 (2010).\\[0pt] [3] E. Rapoport, G.S.D. Beach, Appl. Phys. Lett. in press.\\[0pt] [4] E. Rapoport, G.S.D. Beach, J. Appl. Phys. in press. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D9.00004: Cascaded Magneto-Optical Ring Resonator Structures for Tunable Faraday Rotation and Reduced Isolator Footprint Mehmet Cengiz Onbasli, Juejun Hu, Lei Bi, Gerald F. Dionne, Caroline A. Ross On-chip optical isolators are indispensible components of integrated optics, and can be modified to enable four-port and multi-port circulators and modulators. We have implemented an on-chip optical isolator by placing a racetrack resonator next to a single mode waveguide and coating half of the resonator with a uniformly magnetized magneto-optical film, which breaks the time-reversal symmetry of light propagation and provides different refractive indices and phase shifts for forward and backward propagating waves. At every pass, the optical mode inside the resonator accumulates Faraday rotation in addition to phase shift due to propagation. The transmission from the output port of the waveguide has a Lorentzian dip due to the resonance peak of the resonator. Light can only propagate in the clockwise direction inside the resonator. Here we model how cascading multiple ring resonators can increase the overall quality factor of the isolator and narrow the resonance linewidth, due to the longer photon lifetime inside the cavity. As a result of better control of Faraday rotation, the isolation ratio is enhanced and the device footprint is reduced with respect to Mach-Zehnder waveguide isolators. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D9.00005: On the Energy Transfer Performance of Mechanical Nanoresonators Coupled with Electromagnetic Fields: Applications with magnetic nanoparticles H. Javaheri, B. Barbiellini, G. Noubir The energy transfer performance in electrically and magnetically coupled mechanical nanoresonators is studied [1]. Using the resonant scattering theory, we show that magnetically coupled resonators can achieve the same energy transfer performance as for their electrically coupled counterparts, or even outperform them within the scale of interest. Magnetic and electric coupling are compared in the \emph{Nanotube Radio}, a realistic example of a nano-scale mechanical resonator. The energy transfer performance is discussed for magnetic coupling in magnetite (Fe$_3$O$_4$) nanoparticles. \\[4pt] [1] H. Javaheri, B. Barbiellini, G. Noubir, arXiv:1108.0633. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D9.00006: Increased Sensitivity of Magnetoelectric Sensors at Low Frequencies Using Magnetic Field Modulation Jonathan Petrie, Dwight Viehland, David Gray, Sanjay Mandal, Gollapudi Sreenivasulu, Gopalan Srinivasan, Alan Edelstein Magnetoelectric (ME) laminate sensors are vector magnetometers that can detect pT magnetic fields at 1 kHz, although sensitivity may be reduced at lower frequencies. These passive sensors consist of alternating layers of magnetostrictive and piezoelectric materials. A magnetic field causes the magnetostrictive layer to strain the piezoelectric material and create measurable charge. We have shown\footnote{To be published in Journal of Applied Physics.} that since the strain response is a nonlinear function of the bias field, sweeping the magnetic bias on the magnetostrictive layer can modulate the ME response and increase the operating frequency of the sensor. This upward shift lowers the $1$/$f$ noise and increases the signal amplitude if the new operating frequency is near a mechanical resonance mode of the sensor. Using this modulation technique, the low frequency sensitivity has been improved by more than an order of magnitude and we have achieved a detectivity of 7 pT/$\surd $Hz at1 Hz. In addition to increasing the magnetic signal frequency, we can use magnetic modulation to increase the operating frequency of acoustic signals detected by these sensors. This occurs because the ME sensors are nonlinear devices. In these cases using magnetic field modulation, the signal appears as sidebands around the modulation frequency. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D9.00007: Bipolar exchange bias modulation in La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/ BiFeO$_{3}$ heterostructure based field effect devices Stephen Wu, Shane Cybart, James Parker, Pu Yu, R. Ramesh, R.C. Dynes We have fabricated and performed electrical transport measurements on a multiferroic field effect device with a BiFeO$_{3}$ (antiferromagnetic/ferroelectric) gate dielectric and a La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (ferromagnetic) channel to investigate the effect of BiFeO$_{3}$ (BFO) polarization on interfacial magnetism by using exchange bias as a diagnostic tool. A reversible static shift in exchange bias through zero applied magnetic field is observed by electrically poling BFO. This bipolar exchange bias modulation behavior strongly suggests that interfacial magnetization is being reversed through the application of electric field. To investigate this phenomenon further we have measured temperature dependent exchange bias, temperature dependent resistivity, and Hall Effect coefficients on multiple devices. Also a comparison to an identical control device using a Pb(Zr$_{0.2}$Ti$_{0.8})$O$_{3}$ (ferroelectric) gate dielectric and La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) channel is offered, which provides insight into the interfacial magnetic interactions uniquely occurring in the BFO/LSMO system. We analyze these results in the context of proposed models, which suggest that we are modulating both carrier density and interfacial magnetic coupling strengths; both of which have a strong effect on the bipolar modulation of exchange bias. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D9.00008: Effect of the exchange bias field on the magnetoimpedance response in multilayered FeNi-IrMn films and CoFeSiB-IrMn ribbons Kerim Ture, Mehmet Yumak, Gulen Aktas, Juan Manuel Florez, Patricio Vargas, Caroline A. Ross, Carlos Garcia The magnetoimpedance effect (MI) has been widely used in sensitive magnetic field (MF) sensors, but its intrinsic nonlinear properties are disadvantageous for sensor applications near zero field. The combination of ferromagnetic (FM) and antiferromagnetic (AFM) layers produce an asymmetrical MI (AMI) peak positions which are shifted to higher MF as the probe frequency increases, so linear MI behavior can be obtained around zero external field by tuning the frequency. Here, AMI was extensively studied in multilayer strips of exchange-coupled FeNi-IrMn. The effect of the thickness of the FM layer and the angle dependence in three stripe samples with EB induced; parallel, perpendicular and forming an angle of 45 with the direction of the wire were studied. MI ratio raise with increasing thickness of the FM layer is attributed to the stronger pinning of the FM adjacent to the IrMn. Also, a combination of the EB angle affects and direction of the applied MF can tune; the number of peaks in the MI response, the asymmetry between peaks and the shift of the MI response. Besides, an antiferromagnetic layer was deposited on the top of CoFeSiB amorphous ribbons enhancing the MI effect at low frequencies and shifting the MI response at higher frequencies. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D9.00009: Tailoring Giant Magneto-impedance Effect in Ultrasoft Ferromagnetic Microwires A. Chaturvedi, A. Ruiz, P. Mukherjee, H. Srikanth, M.H. Phan, V.S. Larin Research on soft ferromagnetic microwires exhibiting giant magneto-impedance (GMI) effect, which is a large change of the ac impedance of a ferromagnetic conductor in a static magnetic field, for advanced magnetic sensor applications is an area of topical interest. In this study we show how the GMI effect and its field sensitivity are optimized in Co-B-Si-Mn microwires by varying the magnetic core to glass shell diameter ratio ($d)$. The microwires have been fabricated by the glass-coated melt spinning method. The largest values of GMI (245{\%}) and its field sensitivity 25{\%}/Oe are achieved at $f$ = 13MHz for the microwires with $d $= 0.86. The $d$ dependence of the magneto-impedance has been analyzed based on those of the magneto-resistance and magneto-reactance. Our studies indicate that the microwires with optimized GMI response are attractive candidate materials for structural health self-monitoring and magnetic biosensing applications. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D9.00010: Soft ferromagnetic microribbons with enhanced GMI effect for advanced magnetic sensor applications A. Ruiz, A. Chaturvedi, P. Mukherjee, H. Srikanth, M.H. Phan Soft ferromagnetic ribbons with giant magneto-impedance (GMI) effect are attractive candidate materials for high-performance magnetic sensor applications. GMI is a large change in the ac impedance of a ferromagnetic conductor subject to a dc magnetic field. There is a need for further improving GMI response of existing materials, as well as reducing the size of a GMI-based sensor for use in micro-sensing systems. In this work, we report the enhancement of GMI in soft ferromagnetic ribbons (Metglas{\textregistered} 2714A) at high frequencies by reducing the width of the ribbon to the micrometer scale. This finding is of practical importance, as sensors with enhanced field sensitivity and reduced size find wider ranging applications. The origin of the enhanced GMI effect in the microribbon is explained in terms of the skin and demagnetization effects. The relative contributions to the magneto-impedance from the magneto-resistance and magneto-reactance have been analyzed and discussed in detail. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D9.00011: Designing New Metal-Semiconductor Hybrid Structures With Large Geometric Magnetoresistance Lisa Pugsley, L.R. Ram-Mohan, S.A. Solin The extraordinary magnetoresistance (EMR) in metal-semiconductor hybrid structures was first demonstrated using a four-contact configuration for a circular semiconductor wafer with a concentric metallic inclusion in it. The EMR effect, which is observed at room temperature, is very suitable for use in read heads in magnetic data storage devices. This effect depends on the orbital motion of carriers in an external magnetic field, and the remarkably high magnetoresistance response (the change in resistance with a magnetic field) observed suggests that the geometry of the metallic inclusion can be optimized to significantly enhance the EMR. The theory and simulations to achieve this goal are considered by comparing various 2D structures in an external magnetic field to evaluate the EMR in them using finite element analysis and geometric optimization. For a 10 $\mu$m square semiconductor wafer with a square metallic inclusion we see a range in resistance from -400 to 400 $\Omega$ for -1 T $\leq B \leq$ 1 T. This response can be optimized by changes in contact orientation and the size and shape of the metallic region. Extension to 3D is being investigated at present, which would allow for modeling of magnetic field sensors that also provide the direction of the field. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D9.00012: Ultra wideband, high sensitivity magneto-optic field sensor Dong Ho Wu, Anthony Garzarella, Vince Fratello Using the bismuth rare-earth iron garnet thick films we have demonstrated a magneto-optic (MO) field sensor. The sensor made of all dielectric materials is nearly noninvasive, and is operated at room temperature. The sensor's sensitivity is scalable; the same sensor design can be used for a low-field sensor to measure fields below nano-Tesla or for a high-field sensor to measure several hundred Tesla. The highest sensitivity that we have achieved with the sensor is about 30 pico-Tesla/(Hz)$^{1/2}$. Presently its frequency range is limited from DC to 2 GHz. We have carried out several different experiments with this sensor to explore a few interesting applications, such as electromagnetic signal interception tests over a very broad frequency range. In this presentation we will report our experimental results obtained from this MO field sensor. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D9.00013: Diamagnetic repulsion, the method of magnetic images {\&} suitability of the solenoid and dipole models Ming Yin, Huaizhou Zhang, Timir Datta The repulsion of a permanent magnet from a diamagnetic region was investigated. A magnet of moment m can be described by two models (i) solenoid - a circulating current of appropriate value; second (ii) a magnetic dipole comprising of a pair of north and south poles of separated by a distance.The magnetic field (B) of a permanent magnet was measured. The magnet was modeled as a solenoid with a circulating surface current. The Biot-Savart law field (B) was of computed in Matlab. The experimental data of was in excellent agreement with the Matlab results. However, for computing the repulsion force (F) between the magnet and its diamagnetic image by the direct integration of the current-current interaction require detailed knowledge of the two current densities. However such knowledge is not essential if image is modeled as a dipole. When the magnet is a distance z above the diamagnetic interface then the image current I2 gives rise to a image dipole m2 and the F $\sim $ m2div B, where the div of the holding field is computed at the distance 2z below the magnet. In this model F is directly proportional to both m' and the derivative of the field and a negative slope indicates repulsion, all three were confirmed. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D9.00014: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 5:18PM - 5:30PM |
D9.00015: Magnetic Field Assisted assembly using programmable array of solenoids - A Manufacturing Approach Vijay Kasisomayajula, Anthony Fiory, Michael Booty, Nuggehalli Ravindra The use of an array of programmable solenoids to implement a magnetic field driven assembly of ~heterogeneous micro-components onto a substrate is studied. ~A lower limit of component size, the upper limit of the rate of assembly and the efficiency of the assembly from various perspectives is presented. Various statistical tests are performed on the assembly process to determine its feasibility. A comparison is made between this method of assembly and established assembly techniques in the literature. [Preview Abstract] |
Session D10: Invited Session: Advancements in Computational Physics using NSF's TeraGrid/XSEDE Resources
Sponsoring Units: DCOMPChair: Ralph Roskies, University of Pittsburgh
Room: 210A
Monday, February 27, 2012 2:30PM - 3:06PM |
D10.00001: Benchmark Calculations of Atomic Collision Processes Invited Speaker: Klaus Bartschat The rapid development of computational resources has resulted in enormous improvements in the accuracy of numerical calculations of atomic collision processes. This talk will concentrate on recent advances in the computational treatment of charged-particle and intense short-pulse laser interactions with atoms, ions, and small molecules. Examples include electron collisions with heavy complex targets that are of significant importance in many modelling applications in plasma and astrophysics, fundamental studies of highly correlated 4-body Coulomb processes such as simultaneous ionization with excitation, and the accurate solution of the time-dependent Schr\"odinger equation in the presence of intense femto/attosecond laser fields, which paves the way for quantum dynamic imaging and coherent control. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D10.00002: Prediction and Design of Materials from Crystal Structures to Nanocrystal Morphology and Assembly Invited Speaker: Richard Hennig Predictions of structure formation by computational methods have the potential to accelerate materials discovery and design. Here we present two computational approaches for the prediction of crystal structures and the morphology of nanoparticles. Many materials properties are controlled by composition and crystal structure. We show that evolutionary algorithms coupled to ab-initio relaxations can accurately predict the crystal structure and composition of compounds without any prior information about the system. We will discuss results for various systems including the prediction of unexpected quasi-1D and 2D electronic structures in Li-Be compounds under pressure [1] and of the crystal structure of the superconducting high-pressure phase of Eu [2]. The self-assembly of nanocrystals into mesoscale superlattices provides a path to the design of materials with tunable electronic, physical and chemical properties for various applications. The self-assembly is controlled by the nanocrystal shape and by ligand-mediated interactions between them. To understand this, it is necessary to know the effect of the ligands on the surface energies (which tune the nanocrystal shape), as well as the relative coverage of the different facets (which control the interactions). Density functional calculations for the binding energy of oleic acid-based ligands on PbSe nanocrystals determine the surface energies as a function of ligand coverage. The Wulff construction predicts the thermodynamic equilibrium shape of the PbSe nanocrystals as a function of the ligand coverage. We show that the different ligand binding energies on the {100} and {111} facets results in different ligand coverages on the facets and predict a transition in the equilibrium shape from octahedral to cubic when increasing the ligand concentration during synthesis. Our results furthermore suggest that the experimentally observed transformation of the nanocrystal superlattice structure from fcc to bcc is caused by the preferential detachment of ligands from particular facets, leading to anisotropic ligand coverage [3]. \\[4pt] [1] J. Feng, R. G. Hennig, N. W. Ashcroft and Roald Hoffmann. Nature 451, 445 (2008). \\[0pt] [2] W. Bi, Y. Meng, R. S. Kumar, A. L. Cornelius, W. W. Tipton, R. G. Hennig, Y. Zhang, C. Chen, and J. S. Schilling. Phys. Rev. B 83, 104106 (2011). \\[0pt] [3] J. J. Choi, C. R. Bealing, K. Bian, K. J. Hughes, W. Zhang, D.-M. Smilgies, R. G. Hennig, James R. Engstrom, and Tobias Hanrath. J. Am. Chem. Soc. 133, 3131 (2011). [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D10.00003: Scalable DAG-Based PDE Frameworks for Multi-Scale Multi-Physics Problems Invited Speaker: Martin Berzins The task-based approach to software and parallelism is well-known and has been proposed as a potential candidate, named the silver model, for exascale software. This approach is not yet widely used in the large-scale multi-core parallel computing of complex systems of partial differential equations. The central idea is to use a Directed Acyclic Graph (DAG) based approach to express the structure of the underlying software. The aim of this talk is to explore the usefulness of DAG based approaches, using recent developments in the parallel Uintah software framework for partial differential equations to assess how well the DAG type approach works on present-day large-scale architectures for complex multi-physics multiscale applications up to 200K cores. As a result of these investigations, a preliminary and tentative evaluation of the DAG type approach for PDE software infrastructures will be given. The conclusion is that these approaches show great promise for petascale but that considerable algorithmic challenges remain. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D10.00004: Simulations of Strongly Correlated Systems Invited Speaker: Juana Moreno The field of strongly correlated systems is one of the most active areas in condensed matter physics. This interest is motivated, in part, by a variety of complex emergent phenomena, including high-temperature superconductivity, quantum criticality and complex phases induced by electron-phonon couplings. The recent very rapid development of high performance heterogeneous computer platforms together with a similar emergence of highly accurate many-body algorithms allow the treatment and modeling of complex correlated material systems which were intractable just a few years ago. Important progress has been made by the development of finite size methods, including exact diagonalization and Quantum Monte Carlo techniques. However, due to the minus sign problem, these methods are limited to small lattice sizes. Another successful approach is the dynamical mean field approximation and its cluster extensions, which treat the local or short-ranged dynamical correlations exactly and non-local or long-ranged correlations in a mean field approximation. Due to the effective medium, the Fermion minus sign problem is much milder than that found in finite sized simulations. However, it is still the primary limitation of these methods. To address this problem, multiscale approaches are used which treat only the correlations at the shortest length scales with exact cluster solvers, intermediate length scales are treated using a diagrammatic approach, such as the parquet equations or the dual-fermion formalism and, the longest length scales are captured by the mean field. I will discuss how these new algorithms impact a few model systems including our understanding of quantum criticality in the Hubbard and Anderson model, new phases in ultracold quantum gases, spintronics materials and the role of electron-phonon interaction. I will conclude discussing recent algorithm redesign motivated by the evolution towards hybrid multicore architectures employing graphical processing unit (GPU). [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D10.00005: Accessing correlated electron motion on the attosecond timescale Invited Speaker: Johannes Feist In the last decade, there have been tremendous advances in the production of coherent ultrashort light pulses as short as 80 attoseconds (1 as = $10^{-18}$ s). The availability of these pulses has led to the development of the field of attosecond physics, which aims to follow and control electron motion on its natural timescale (1 atomic unit of time is about 24 attoseconds). One of the major goals of attosecond physics is to access correlated electron dynamics. This requires a description of the target system that goes beyond the commonly used single-active-electron approximation. The large bandwidth of ultrashort pulses and many-photon absorption in strong infrared fields make such a description extremely challenging. I will discuss our work on the full numerical solution of the two-electron Schr\"odinger equation for helium, which already displays rich correlation effects. I will focus on two applications: The first is attosecond streaking, in which temporal information about the photoionization process in an attosecond pulse is mapped into a momentum shift by a synchronized infrared pulse. This promises to give access to the Eisenbud-Wigner-Smith time delay of photoionization. I will discuss the additional effects that are induced by the infrared field, and how these have to be taken into account for attosecond streaking to fulfill its promise. I will then discuss the possibility of accessing two-electron wave packet dynamics in doubly excited states of helium by an attosecond pump-attosecond probe setup. Such experiments have been called the ``holy grail'' of attosecond physics and should come within reach in the near future. I will discuss our recent proposal of using two-photon absorption from a single pulse as a coherent reference wave, which can be used to increase the experimental signal by almost two orders of magnitude. This provides direct access to time-dependent observables (e.g., the distance between the two electrons) of the two-electron wave packet. [Preview Abstract] |
Session D11: Focus Session: Graphene Structure, Stacking, Interactions: Mesoscopics
Sponsoring Units: DMPChair: Shaffique Adam, NIST
Room: 210B
Monday, February 27, 2012 2:30PM - 2:42PM |
D11.00001: Electronic localization in rotated graphene multi-layer Guy Trambly de Laissardiere, Omid Faizy Namarvar, Didier Mayou, Laurence Magaud Rotated graphene bilayers show an interesting electronic structure with a tendency to layer decoupling at large rotation angles and a stronger electronic mixing at small angles, associated with a strong decrease of the velocity ([1] and Refs. therein). These inter-layer mixed states allow us [2] to address the long lasting problem of the origin of the Moir\'{e} pattern observed on STM images. For large and intermediate rotation angles, we present analytical and numerical studies of the local density of states in the Moir\'{e} that compare well to STM spectra. For very small angles, the inter-layer mixed states ultimately lead to electronic confinement in AA stacking regions in an energy range close to the Dirac point. In graphene multi-layer (up to 10 layers) containing twisted intercalated layers [3], we found both bands with a strong velocity reduction, and bands without velocity reduction. This could explain [2] why velocity renormalization is not observed experimentally in rotated multi-layers on SiC [3,4]. \\[4pt] [1] G. Trambly de Laissardi\`{e}re et al., Nano Lett. 10, 804 (2010). \\[0pt] [2] G. Trambly de Laissardi\`{e}re et al., in preparation. \\[0pt] [3] M. Sprinkle et al., J. Phys. D: Appl. Phys. 43, 374006 (2010). \\[0pt] [4] M. Sprinkle et al., Phys. Rev. Lett. 103, 226803 (2009). [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D11.00002: Interferences and Fabry-Perot Oscillations in ABA Trilayer Graphene Leonardo Cristiano Campos, Kawin Surakitbovorn, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero ABA trilayer graphene (TLG) is a 2D system with multiple sub-bands which can be divided into one monolayer graphene-like sub-band and one bilayer graphene-like sub-band, therefore comprising a system wherein massive and massless bands coexist. Here, we study electronic transmission in the ballistic regime through a 70nm-wide tunable electrostatic potential barrier on exfoliated ABA trilayer graphene encapsulated by hexagonal Boron Nitride. We report Fabry-Perot oscillations of the conductance caused by multiple reflections of the charges in the potential barrier, and we also present a study of their magneto dependence. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D11.00003: Quantum Pumping in Graphene Nanoribbons Tejinder Kaur, Liliana Arrachea, Nancy Sandler The interest in the development of devices at the nanoscale has intensified the search for mechanisms that provide control of transport properties while reducing effects of heat dissipation and contact resistance. Charge pumping, in which dc currents are generated in open-quantum systems by applying local de-phased time-dependent potentials, may achieve these goals. We analyze the properties of non-equilibrium zero-bias currents through nano-ribbons using a tight-binding Hamiltonian and the Keldysh formalism beyond the adiabatic and perturbative regimes. Using a numerical implementation with two local single-harmonic time-dependent potentials, we first contrast results for 1d and 2d metallic systems (square lattice). Next, we focus on quasi-1d systems and graphene ribbons, and discuss the role of reservoirs. We analyze the dependence of the dc pumped current as a function of different parameters such as chemical potential, pumping amplitude, and frequency. We observe a considerable increase of pumped current with the dimensionality of the system. Furthermore, we show that lattice mismatch with reservoirs favors the pumping mechanism for graphene ribbons. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D11.00004: Conductance fluctuations in graphene as a probe of broken-symmetry and fractional quantum Hall states Invited Speaker: Jurgen Smet The observation in macroscopic transport studies of the interaction induced broken symmetry and fractional quantum Hall states in graphene normally requires very clean samples and/or strong magnetic fields. Here we report that even when these fragile states are not developed well enough to produce any of the quantum Hall signatures, they are strongly visible in differential conductance fluctuations that appear as a result of charge carrier localization when the system breaks up into compressible islands with incompressible areas in between when the corresponding integer or fractional filling factor is approached. The conductance fluctuations give access to local information even though a macroscopic measurement is performed. The existence of a landscape of compressible islands is unambiguously proved in Coulomb blockade phenomena in the quantum Hall insulating regime where all other conduction channels are switched off. This work has been carried in collaboration with D.S. Lee, V. Skakalova, R.T. Weitz, K. von Klitzing. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D11.00005: Interference of electronic states between the two misoriented crossed graphene nanoribbons K.M. Masum Habib, Roger Lake In a twisted bilayer graphene, where one layer is rotated with respect to the other one, for rotation angles more than 20 degrees each layer remains electronically decoupled even though they are vertically separated by only a fraction of a nanometer. A relative rotation between two graphene nanoribbons (GNRs), with one place on top of the other, creates a crossed GNR (xGNR) with an overlap region that is neither AA nor AB stacking. The geometry of the overlap region of the xGNR is that of a twisted bilayer graphene. Calculations, based on the extended Huckel theory and the non-equilibrium Green's function formalism, show that the electronic states of the individual GNRs of an unbiased xGNR are decoupled from each other similar to the decoupling that occurs in a twisted bilayer graphene. The decoupling occurs due to the quantum mechanical interference between the electronic states of the individual GNRs. This results in a strong suppression of inter-GNR transmission over a large energy of window. The transmission is, however, a strong function of the potential difference between the GNRs and the atomistic geometry of the overlap region which depends on relative translation and rotation angle between the GNRs. Thus, a bias can be used to control the inter-GNR current. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D11.00006: Gap opening in graphene by 1D and 2D periodic corrugations Ivan Naumov, Alexander Bratkovsky Using first-principles methods and symmetry arguments, we show that a graphene monolayer, which is periodically corrugated in one or two direction(s), can be either semimetal or semiconductor, depending on how strong corrugation is or how the initial symmetry is broken. In the case of 1D periodic ripples, a gap at the Dirac points opens up only due to (i) breaking of the inversion symmetry or equivalence between A and B sublattices and/or (ii) merging of two inequivalent Dirac points, \textbf{D} and -\textbf{D}. Since breaking the inversion symmetry has only relatively modest effect, a tangible gap can be mainly induced by mutual annihilation of the Dirac points, which requires large corrugations, close to mechanical breaking point. In contrast to 1D, the 2D ripples can additionally induce a semiconducting gap via mixing of electronic states belonging to two different $\textbf{K}$, \textbf{K}$'$ valleys. In this case, a gap on the order of 0.5 eV can be opened up at strains safely lower than the graphene failure strain [1]. \\[4pt] [1] I.I. Naumov, A.M. Bratkovsky, arXiv:1104.0314v1. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D11.00007: Electronic structure and energetics of graphene antidot lattice Masahiro Sakurai, Susumu Saito, Yasutami Takada We have made a systematic study of the electronic structure and the energetics of graphene with periodic array of vacancy clusters (graphene antidot lattice) in the framework of the density-functional theory (DFT). We find that the electronic property of the system is well controlled by its lattice periodicity. More specifically, this system can be either metallic or semiconducting, depending on its lattice geometry. Interestingly, some of them are predicted to be direct-gap semiconductors. For example, graphene sheet with high-symmetry arrangements of periodic circle-shape vacancies always has a direct fundamental gap [1]. The DFT total-energy calculations indicate that the geometry of hole edges plays an important role in determining its stability. [1] ``Electronic properties of graphene and boron-nitride based nanostructured materials'' M. Sakurai, Y. Sakai, and S. Saito, J. Phys.: Conf. Ser. 302 (2011) 012018. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D11.00008: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 4:30PM - 4:42PM |
D11.00009: Localization and Conductivity of Graphene with Adsorbates Didier Mayou, Guy Trambly de Laissardiere We compute the conductivity of graphene for two models of resonant and non resonant adsorbates. Away from the Dirac point the conductivity is well given by the semi-classical Drude formula and localization lengths are exponentially large. For some energies, near the Dirac point, the conductivity is well represented by $\sigma \simeq $4e$^{2}$/$\pi $h-$\alpha $Log(Li/Le) as a function of the inelastic scattering length Li and the elastic mean free path Le $<$ Li. This implies a magneto-conductivity that varies also linearly with Log(B) where B is the magnetic field. Our results suggest that the divergence of the localization length which is expected close to the Dirac point affects only a narrow energy region. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D11.00010: Effects of disorder on recombination and relaxation processes via acoustic phonons in graphene Fedir Vasko, Vladimir Mitin, Andrei Sergeev Recombination and relaxation processes via acoustic phonons are allowed in a disordered graphene because of violation of the energy-momentum conservation requirements. These processes are analyzed taking into account the interference of electron-phonon and electron-impurity scattering mechanisms. The recombination and relaxation rates are calculated for the cases of intrinsic and heavily-doped graphene. The transient evolution of nonequilibrium carriers is described by the exponential fit dependent on doping conditions and disorder level. The obtained electron recombination and relaxation rates are compared with available experimental data. [1] F. Vasko et al., Phys. Rev. B 84, 155445 (2011). [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D11.00011: Phase coherent states in graphene heterostructure with chiral asymmetry Junhua Zhang, Enrico Rossi The chiral nature of the fermionic excitations in graphene, bilayer graphene, and topological insulators, induces unique electronic properties in these materials. In bilayer heterostructures the interlayer interaction can induce the formation of an interlayer phase coherent state. An interesting class of heterostructures is constituted by bilayers in which the electrons in the two layers have different chirality. An example of such a system that can be realized experimentally is the heterostructure formed by single layer graphene and bilayer graphene. In this talk I will discuss the conditions for the realization of an interlayer phase-coherent state in chiral-asymmetric heterostructures. In particular I will show how the voltage difference between the two layers affects the conditions for the realization of the phase coherent state, and its properties. I will then discuss the relevance for experiments of our results. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D11.00012: Visualizing Electronic Chirality and Berry's Phases in Graphene Systems Using Photoemission with Circularly Polarized Light Yang Liu, Guang Bian, Tom Miller, Tai-Chang Chiang Electronic chirality near the Dirac point is a key property of graphene systems, which is revealed by the spectral intensity patterns as measured by angle-resolved photoemission spectroscopy under various polarization conditions. Specifically, the strongly modulated circular patterns for monolayer (bilayer) graphene rotate by +-90 degrees (+-45 degrees) in changing from linearly to circularly polarized light; these angles are directly related to the phases of the wave functions and thus visually confirm the Berry's phase of pi (2pi) around the Dirac point. The details are verified by calculations. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D11.00013: Electronic Properties of Curved Graphene-Ring Structures Daiara Faria, Andrea Latg\'e, Sergio Ulloa, Nancy Sandler Recently, deformed graphene in the form of bubbles have been produced on different substrates in a variety of controllable shapes [1]. These findings raise the possibility of changing the electronic properties of these structures allowing for band-structure engineering. We have undertaken a study of the electronic properties of graphene-ring systems with circularly symmetric Gaussian curvature in the Dirac approximation. We obtain energy spectra and wave functions using perturbation theory on the gauge field amplitude describing the curvature. We further analyze the competition between curvature-induced magnetic field and real external fields and the resulting persistent currents generated in their presence. As expected, the results depend on the boundary conditions describing the confined ring edges. In addition, we discuss the effects of the angular asymmetry in the probability density due to the curvature [2] on single rings and more complex confined annular geometries.\\[4pt] [1] T. Georgiou et al., APL 99\\[0pt] [2] Wakker et al., PRB 84. [Preview Abstract] |
Session D12: Graphene Transport: Disorder Scattering
Sponsoring Units: DMPChair: Arthur F. Hebard, Prof. Dr.
Room: 210C
Monday, February 27, 2012 2:30PM - 2:42PM |
D12.00001: Theory of 2D Transport in Graphene for Correlated Disorder Qiuzi Li, Euyheon Hwang, Enrico Rossi, Sankar Das Sarma We theoretically revisit graphene transport properties as a function of carrier density, taking into account possible correlations in the spatial distribution of the Coulomb impurity disorder in the environment. We find that the charged impurity correlations give rise to a density-dependent graphene conductivity, which agrees well qualitatively with the existing experimental data. We also find, quite unexpectedly, that the conductivity could increase with increasing impurity density if there is sufficient interimpurity correlation present in the system. In particular, the linearity (sublinearity) of graphene conductivity at lower (higher) gate voltage is naturally explained as arising solely from impurity correlation effects in the Coulomb disorder. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D12.00002: Simultaneous Raman and electrical transport measurements of disordered graphene in situ in ultra-high vacuum Jacob Tosado, Vince Ballarotto, William G. Cullen, Michael S. Fuhrer Resonant Raman scattering in graphene gives unique information about disorder, as the D peak is observed only in the presence of disorder which produces intervalley scattering of the electrons. The nature of disorder in graphene prepared by various techniques and on various substrates of the subject of significant research, with significant attention being paid to scattering by charged impurities; resonant scatterers due to vacancies, chemisorbed impurities, etc.; and non-resonant short-range impurities. In order to study the effect of these types of disorder on graphene's electronic properties and Raman spectra simultaneously, we have developed a facility combining thermal deposition, ion bombardment, electrical transport and micro-Raman measurements in an ultra high vacuum environment. We will discuss the capabilities of this facility and present the results of Raman and electrical transport measurements on controllably disordered graphene. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D12.00003: Creating Vacancies in Supported and Suspended Graphene Field Effect Transistors Shuren Hu, A.K.M. Newaz, Anthony Hmelo, Kirill Bolotin We have studied the creation of vacancy defects in both graphene samples supported on SiO$_2$ substrate and suspended graphene devices. The defects were created by irradiating graphene with a 30KeV Ga$^+$ ion beam in UHV; the effect of these defects on electrical transport in graphene was measured {\it{in situ}}. We find that the number of defects created in supported devices is dramatically higher in comparison with suspended devices. Using Monte-Carlo and SRIM simulations, we identify the formation of defects by secondary ions in the case of supported devices as a likely explanation of these results. We have also observed that the transport quality degrades drastically with respect to vacancy defect density and follows the same general trend for both supported and suspended graphene. These results may be important in designing devices for high radiation working environments, such as space electronics. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D12.00004: Scattering mechanisms in graphene suspended in liquids. II. Flexural phonons (Theory) Yevgeniy Puzyrev, A.K.M. Newaz, Bin Wang, Kirill Bolotin, Sokrates Pantelides Recent experiments reported strong scattering of charge carriers by flexural phonons in suspended graphene in vacuum.\footnote{Castro E. V., \textit{et al.}, ``Limits on Charge Carrier Mobility in Suspended Graphene due to Flexural Phonons,'' Phys. Rev. Lett. 105, 266601, 2010. } Our experimental data (previous talk) show that the carrier mobility observed for devices immersed in non-polar liquids, namely toluene and hexane, are significantly larger than the mobility limitation due to scattering by flexural phonons. We performed molecular dynamics simulations of graphene sheets suspended in hexane, toluene, and in vacuum at room temperature. We find that the interaction of molecules of the liquid with graphene suppresses the amplitude of the phonons by $\sim $50{\%}. We show computationally that this suppression is equivalent to an effective increase of the bending rigidity\footnote{Fasolino A., Los J. H., Katsnelson M. I., ``Intrinsic ripples in graphene.,'' Nature Mat. 6, 858, 2007.} of graphene from a free-space value $\sim $1.3 eV in vacuum to $\sim $3.6 eV in liquids. Therefore, we demonstrated that scattering by out-of-plane flexural phonons is reduced by the presence of liquids. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D12.00005: Scattering mechanisms in graphene suspended in liquids. I. Coulomb scattering (Experiment) A.K.M. Newaz, Yevgeniy Puzyrev, Bin Wang, Sokrates Pantelides, Kirill Bolotin Enhanced dielectric screening of charged impurities by high-$\kappa$ environment of graphene is predicted to improve the electronic quality of graphene devices by suppressing Coulomb scattering. However, experiments reported so far demonstrate that electronic transport in graphene is only modestly modified by a high-$\kappa$ environment. Here we fabricate large area multiterminal graphene devices suspended in liquids and study electronic transport in graphene as a function of liquid's dielectric constant. We observe a rapid increase of mobility $\mu$ with $\kappa$ due to dielectric screening in non-polar solvents ($\kappa\leq 5$). We also find that charged ions present in polar solvents ($\kappa \geq 18$) cause a drastic drop in mobility counteracting the gains by dielectric screening in polar high-$\kappa$ liquids. Furthermore, molecular dynamics simulations establish that scattering by out-of-plane flexural phonons is suppressed by the presence of liquids (next talk). We expect that our findings may provide avenues to control and reduce carrier scattering in future graphene-based electronic devices. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D12.00006: Short range disorder in Graphene: A Green's function based approach Sankalpa Ghosh, Neetu Aggarwal (Garg), Manish Sharma Electrons at the Fermi level in graphene monolayer behave like massless Dirac fermions. Using Green\'is function based technique we study the transport of such electrons in the presence of randomly located electrostatic impurities in different geometries and compare such transport in graphene with the transport in conventional semiconductors. This study would eventually be used to mimic short range disorder that would be superimposed on a regular structure. Comparison with the optical phenomena will be used to understand such transport. We further extend this technique to study the transport in the presence of magnetic scatterers. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D12.00007: Universal conductance fluctuations in Dirac materials in the presence of long-range disorder E. Rossi, J.H. Bardarson, M.S. Fuhrer, S. Das Sarma We study quantum transport in Dirac materials with a single fermionic Dirac cone (strong topological insulators and graphene in the absence of intervalley coupling) in the presence of long-range disorder [1]. We show, by calculating the conductance fluctuations, that in the limit of very large system size and disorder strength, quantum transport becomes universal. However, a systematic deviation away from universality is obtained for realistic system parameters. By comparing our results to existing experimental data on $1/f$ noise, we suggest that many of the graphene samples studied to date are in a non-universal crossover regime of conductance fluctuations, and provide an explanation for the origin of the 1/f noise in Dirac materials and in graphene in particular. \\[4pt] [1] E. Rossi, J. H. Bardarson, M. S. Fuhrer, S. Das Sarma, {\it Universal conductance fluctuations in Dirac materials in the presence of long-range disorder.} arXiv:1110.5652v1 (2011) [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D12.00008: Diamagnetism of Graphene with Long-Range Scatterers Masaki Noro, Mikito Koshino, Tsuneya Ando We study weak-field orbital susceptibility of graphene containing scatterers with long-ranged potential. The effects of scattering from such impurities are taken up self-consistently by using Green's function technique within a self-consistent Born approximation. To see dependence on the potential range, we consider scatterers with a Gaussian potential and screened charged impurities. Because Green's function or self-energy for long-range impurities strongly depends on wave vector, we have to numerically calculate Green's function and vertex functions. In graphene, the susceptibility diverges at the Dirac point as a delta function of the Fermi energy. For the Gaussian potential, results show that the delta function in the ideal graphene is broadened by disorder, due to the mixing between the states at the Dirac point and other states. The susceptibility as a function of the Fermi energy rapidly decreases away from the Dirac point, with effective width determined by the potential range. As the scattering strength increases, the peak at the Dirac point is less prominent and the susceptibility has a long tail, corresponding to the strong mixing among states due to scattering. These behaviors are obtained only when the vertex corrections are properly taken into account. For charged impurities, the susceptibility shows a double-peak structure caused by the strong variation of the effective scattering strength as a function of the Fermi energy. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D12.00009: Current decay rate due to electron--electron scattering in graphene Ben Yu-Kuang Hu Electron--electron scattering in graphene does not conserve electrical current, because of the linear dispersion of the bands in graphene near the Dirac point. In graphene, when two electrons with initial momenta $\mathbf k_1$ and $\mathbf k_2$ undergo electron--electron scattering to final states $\mathbf k_1'$ and $\mathbf k_2'$, in general the total current $\mathbf v(\mathbf k_1) + \mathbf v(\mathbf k_2) \ne \mathbf v(\mathbf k_1') + \mathbf v(\mathbf k_2')$ [see {\em e.g.}, Li {\em et al.}, Appl.~Phys.~Lett. {\bf 97}, 082101 (2010)]. We calculate the electric current relaxation rate due to the electron--electron scattering of an electron that is injected into extrinsic graphene at low temperature. When the energy of injected electron relative to the Fermi energy is small compared to the Fermi energy, the current decay rate is small due to phase-space restrictions. The current decay rate grows monotonically as the energy of the injected electron increases. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D12.00010: Electron energy relaxation in disordered graphene via e-phonon interaction Wei Chen, Aashish Clerk Motivated by recent experiments, we study theoretically the energy relaxation of hot electrons in disordered graphene via electron-phonon interactions. In contrast to previous treatments [1], we explicitly treat the effects of electronic disorder. Using the Keldysh diagram technique, and including vertex corrections, we identify various mechanisms through which disorder can significantly change the magnitude and temperature dependence of the electronic energy relaxation rate. [1]Felix Von Oppen, Francisco Guinea, Eros Mariani, Phys. Rev.B 80, 075420 (2009) [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D12.00011: Electron scattering at graphene edges S.-J. Choi, Sunghun Park, H.-S. Sim We theoretically study the reflection of electrons at edges in monolayer graphene in a low-energy limit. We consider both of zigzag and armchair edge cases. We find that in the case of zigzag edge, the reflection phase is determined by the rotation of electron pseudospin during the reflection, which is attributed to the chirality between the pseudospin and the electron momentum. On the other hand, in the case of armchair edge, the reflection phase does not contain the information of pseudospin rotation, because of intervalley mixing. The pseudospin rotation can be detected via the measurement of reflection phase in an interferometry setup in graphene with zigzag edge. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D12.00012: Highly Defective Graphene: The thinnest insulating membrane Aurelien Lherbier, Stephan Roche, Oscar A. Restrepo, Yann-Michel Niquet, Arnaud Delcorte, Jean-Christophe Charlier The electronic structure and transport properties of two-dimensional highly defective sp2 graphene are investigated theoretically. Using classical molecular dynamics, large planes of highly defective graphene-based sheets are first generated. An accurate empirical tight-binding Hamiltonian is then elaborated, allowing the prediction of elastic mean free paths, conductivities, and charge mobilities using a real-space order-N Kubo-Greenwood method. In sharp contrast to pristine graphene, the highly defective sp2 carbon sheet displays high density of states close to the Dirac energy. However, the dynamics of the corresponding electronic wavepackets reveals extremely short mean free paths (below 1 nanometer) and quantum interferences, both yielding to particularly strong localization phenomena. Consequently, these highly defective graphene-based sheets, although exhibiting a metallic character through the density of states, are from an electronic transport perspective among the most insulating two-dimensional one-atom-thick membrane ever made. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D12.00013: Boundary scattering in quasi-ballistic graphene/hexagonal boron nitride mesoscopic wires Kazuyuki Iguchi, Satoru Masubuchi, Takehiro Yamaguchi, Masahiro Ohnuki, Kenji Watanabe, Takashi Taniguchi, Tomoki Machida In a quasi-ballistic transport regime where the mean free path is larger than the width of conduction channel, diffusive boundary scattering results in an anomalous positive magnetoresistance due to a megnetic commensurability effect between cyclotron motion and sample width. In this work, we fabricate a high-mobility two terminal graphene mesoscopic wire on hexagonal boron nitride with a mean free path comparable to sample width $\sim $ 1 $\mu $m. Magnetoresistance of the graphene mesoscopic wire shows a peak structure at a magnetic field which scales with the ratio of the cyclotron radius $R_{c}$ to the wire width $w$. The peak field increases with back-gate voltage as a consequence that the ratio $w$/$R_{c}$ is modified due to the change in $R_{c}$. These results indicate the quasi-ballistic transport and diffusive boundary scattering in graphene on hexagonal boron nitride. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D12.00014: Effects of disorder induced scattering in chemical vapor deposited Graphene. Ming-Yang Li, Chiu-Chun Tang, Lain-Jong Li, Cheng-Chung Chi, Jeng-Chung Chen The effect of the short-range scatters in chemical vapor deposited (CVD) graphene on the quantum interference effect of carrier scattering remains to be an interesting question. We study the magneto-resistance and low-frequencies noise of our CVD graphene by varying carrier density and temperature. Unlike previous studies of exfoliated clean graphene flakes, we have found in the vicinity of the Dirac-point (DP) WL signal cannot be fully described in terms of breaking the valley symmetry due to trigonal warping of the bands and atomically sharp disorder [1,2]. The discrepancy regime is coincident with the suppression of noise figures and the vanishing of Hall coefficient. Our data suggest that in low mobility CVD graphene an extra inter-valley elastic scattering process should be considered under the theoretical basis in Ref.1. More detailed experimental results and theoretical analysis will be presented and discussed. Ref[1]: E. McCann, et al., Phys. Rev. Lett. 97 146805(2006) Ref[2]: J.Phys. : Condens. Matter 22 205301 (2010) [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D12.00015: Efros-Shklovskii Variable Range Hopping in Reduced Graphene Oxide Sheets Daeha Joung, Saiful I. Khondaker Reduced graphene oxide (RGO) sheets consist of highly ordered graphene domain and structural defects including oxidized carbon atoms and topological defects. Charge transport properties of RGO sheets are strongly influenced by the degree of disorderliness which can be tuned by varying the amount of the reduction. We studied the hopping conduction of the RGO sheets with different reduction efficiency. We show that the low temperature charge transport properties of the RGO with various reduction efficiency can be well described by Efros-Shklovskii variable range hopping (ES VRH), $\rho $ $\sim $ exp[-(T/T)$^{1/2}$]. We will discuss how the localization length varies with the degree of reduction. The result indicates that the coulomb interactions between graphene domains play an important role in the charge transport of the RGO sheets. [Preview Abstract] |
Session D13: Focus Session: Magnetic Nanostructures-Characterization (Scanning Probe, X-ray and Neutron)
Sponsoring Units: DMP GMAGChair: Yimei Zhu, Brookhaven National Laboratory
Room: 211
Monday, February 27, 2012 2:30PM - 2:42PM |
D13.00001: STM Studies of $\mbox{Mn}_{12}\mbox{-Ph}$ on Highly Oriented Pyrolytic Graphite K. Reaves, K. Kim, K. Iwaya, T. Hitosugi, Y.G. Kim, K. Itaya, H. Zhao, K.R. Dunbar, H.G. Katzgraber, W. Teizer $\mbox{Mn}_{12}\mbox{-Ph}$ displays tunneling of quantized magnetization below 3K. In other $\mbox{Mn}_{12}$ ligand variants this magnetic behavior can alter the electronic behavior of the molecule making it a good candidate for a molecular logic gate. $\mbox{Mn}_{12}\mbox{O}_{12}\mbox{(}\mbox{C}_6\mbox{H}_5\mbox{COO)}_{16}$ ($\mbox{Mn}_{12}\mbox{-Ph}$) has a $\mbox{Mn}_{12}$ core and 16 Phenyl ligands and is deposited onto the surface of highly oriented pyrolytic graphite (HOPG). The samples are then studied via scanning tunneling microscopy in air at 300K and in ultra high vacuum at 300K and 4.2K. At 300K, film formation is studied to optimize samples for subsequent low-temperature studies. Isolated objects are observed via STM on the surface, clearly distinct from the underlying graphite lattice. Topographic data are analyzed in an attempt to correlate apparent features to the internal molecular structure of $\mbox{Mn}_{12}\mbox{-Ph}$. Voltage spectra of locations thought to be associated with the molecular core are compared to other locations thought to be the HOPG and Phenyl. Spectroscopic data indicate a bias voltage dependence at locations associated with the internal molecular structure thought to be related to the metallic-core of the molecules. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D13.00002: Room-temperature spin-polarized scanning tunneling microscopy of antiferromagnetic Mn$_{3}$N$_{2}$(001) nanopyramids Kangkang Wang, Arthur Smith Antiferromagnets play a key role in spintronic applications owing to the exchange bias effect. As devices miniaturize in size and dimension, novel magnetic structures dramatically different from the bulk often emerge. Here we apply spin-polarized scanning tunneling microscopy (SP-STM) at room temperature to study the local magnetization of antiferromagnetic nitride nanostructures. Mn$_{3}$N$_{2}$(001) thin films have been grown on MgO(001) substrates using molecular beam epitaxy and transferred~\textit{in situ}~to a home-built SP-STM~for magnetic imaging. Results show that the surface consists of alternating chemically in-equivalent atomic terraces. Using SP-STM with~\textit{dI/dV}~mapping, different layers can be clearly discriminated due to their different conductances. These differences in conductance are a result of not only the different chemical environments, but also the spin ordering and broken symmetry at the surface. Contrary to expectations, a layer-wise alternating surface anisotropy in these nanopyramids is observed. The presented study enables further investigations of the interplay between growth defects and the formation of intriguing antiferromagnetic domains. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D13.00003: Atomic Force Microscopy Incorporated with Magnetic Sample Modulation: a new approach to detect the magnetic nanomaterials Jing-Jiang Yu, Jayne Garno A new imaging strategy using atomic force microscopy (AFM) for detecting magnetic nanomaterials with much higher spatial resolution and sensitivity than the traditional magnetic force microscopy (MFM) technique is developed [1,2]. This AFM-based imaging mode is referred to as magnetic sample modulation (MSM), since the flux of an AC-generated electromagnetic field is used to induce physical movement of magnetic nanomaterials on surfaces during imaging. The AFM is operated in contact mode using a soft, nonmagnetic tip to detect the physical motion of the sample. By slowly scanning an AFM probe across a vibrating area of the sample, the frequency and amplitude of vibration induced by the magnetic field is tracked by changes in tip deflection. Thus, the AFM tip serves as a force and motion sensor for mapping the vibrational response of magnetic nanomaterials. The investigations are facilitated by nanofabrication methods combining particle lithography with organic vapor deposition and electroless deposition of iron oxide to prepare designed test platforms of magnetic materials at nanometer length scales. Examples of detecting magnetic nanoparticles and magnetic biospecies at single molecular level will be presented [3,4]. \\[4pt] [1] Li et al. Analytical Chemistry, 2009, 81, 4792-4802 [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D13.00004: Uncompensated Magnetization in Antiferromagnets, and New Classification of Exchange Bias Systems Invited Speaker: Igor V. Roshchin Exchange bias (EB) is typically observed in a bilayer consisting of a ferromagnet (FM) and an antiferromagnet (AF) as a horizontal shift of the FM hysteresis loop. It is attributed to exchange coupling across the interface. Several experimental findings demonstrate, and most models agree that uncompensated magnetization (UM) in the AF plays an important role in EB. However, the origin of UM remains unknown for most EB systems. Using magnetometry and polarized neutron reflectivity (PNR) we observe UM in antiferromagnet-only, (110)-FeF$_2$ epitaxially grown on MgF$_2$, thin-film samples. The PNR reveals the spatial distribution of the UM. This UM exhibits the so-called ``\textit{intrinsic exchange bias}'': a shift of the hysteresis loop of UM. This effect is similar to the ``classical'' EB observed in bilayers, except that here, it is observed in a single layer material. The surface is responsible for the macroscopically broken time-reversal symmetry, uncompensated magnetization (UM) in a nominally compensated antiferromagnet [1], and, ultimately, for a new magnetic state. In this magnetic state, zero remanent magnetization cannot be obtained isothermally, because the origin (M(H=0)=0) is outside of the major hysteresis loop. Using symmetry group arguments [1] and results of \textit{ab-initio} calculations [2], we argue that it is an equilibrium state. Below $T_N$, the UM in FeF$_2$ is coupled to the bulk antiferromagnetic order parameter as supported by several experimental results, including high value of EB field, its temperature dependence, and the absence of the training effect. Based on the proposed origin of the UM and experimental observations for different EB systems, we discuss a new classification of exchange bias systems into two categories, explaining differences in the observed properties.\\[4pt] This work is done in collaboration with K. E. Badgley (TAMU), M. Zhernenkov (LANL and ANL), M. R. Fitzsimmons (LANL), M. Erekhinsky, I. K. Schuller (UCSD), K. D. Belashchenko (UNL), and A. H. Romero (CONACyT), and supported by Texas A\&M University, TAMU--CONACyT Collaborative Research Grant Program, DOE, AFOSR, and NSF-9976899. PPPROALMEX-DAAD-CONACyT bi-national program. TACC at UT--Austin is acknowledged for providing HPC resources. \\[4pt] [1] K. D. Belashchenko, Phys. Rev. Lett. \textbf{105}, 147204 (2010).\\[0pt] [2] S. L\'{o}pez-Moreno \textit{et al.}, Phys. Rev. B (submitted). [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D13.00005: Modeling Scanning SQUID Magnetometry Images of Magnetic Dipoles Lisa Qian, Beena Kalisky, Brannon Klopfer, Bo Dwyer, Kathryn Moler Scanning superconducting quantum interference devices (SQUIDs) with sub-micron sized pick-up loops are the most sensitive detectors of local magnetic flux and can have spin sensitivities down to 100 mu{\_}B/sqrt(Hz). ~This makes them the ideal candidate for detecting magnetic dipole signals from individual nanomagnets. However, because the image kernel of the SQUID is not usually well known, quantitative analysis of magnetometry images can often be difficult. By using similarly measured SQUID magnetometry of superconducting vortices, we show that it is possible to fit images of magnetic dipoles by combining images of two monopoles.~This fitting technique allows us to extract the magnetic moment as well as the information on the spatial extent of the imaged dipole.~To quantify the statistical errors of the fit and the systematic errors of the measurement, we fabricated and measured nanomagnet bars of different lengths This analysis technique, in conjunction with scanning SQUID microscopy, can be used to study individual nanomagnets in a wide variety of fields, ranging from biology to condensed matter physics.~ [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D13.00006: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 4:06PM - 4:18PM |
D13.00007: Chemical Segregation in GdFeCo: An X-ray view on Magnetic Coercivity Alexander Reid, Catherine Graves, Benny Wu, Tianhan Wang, Alexey Kimel, Andrei Kirilyuk, Arata Tsukamoto, A. Itoh, Joachim St\"{o}hr, Theo Rasing, Hermann D\"{u}rr, Andreas Scherz The magnetic coercivity in intermetallic alloys is known to be dominated by microscopic inhomogeneities. These control the characteristics of magnetic switching as they provide nuclei for magnetic domain formation, and the pinning sites governing domain wall propagation. However, such regions exist on nanometer length scales with weak magnetic contrast to their surroundings; their characterization has therefore remained illusive. Here we demonstrate how resonant x-ray scattering is intrinsically sensitive to magnetic changes in a segregated phase. We utilizes the fact that magnetic scattering asymmetry directly probes regions where this phase segregation occurs. Our measurements on GdFeCo show strongly temperature dependant magnetic canting in the segregated regions due to local changes in magnetic anisotropy. Understanding the origin and importance of these chemically segregated regions will allow a better understanding of the magnetic switching process in GdFeCo. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D13.00008: Using X-Ray Diffraction Microscopy for Imaging Magnetic Domain Structures of Magnetic Thin Films Oleg Krupin, J.J. Turner, X. Huang, K.A. Seu, D. Parks, S. Kevan, E. Lima, K. Kisslinger, I. McNulty, R. Gambino, S. Mangin, S. Roy, P. Fischer We report the application of iterative phase retrieval from magnetic x-ray diffraction for imaging magnetic domain structures of magnetic thin films. Using coherent x-ray scattering at the x-ray photon energy corresponding to the L$_{3,2}$ absorption edges of the 3d material Co, we demonstrate that linearly polarized soft x rays can be used to obtain the element specific information about both the amplitude and the phase of magnetic domain structures existing in thin films. We successfully recovered an image of the magnetic structure of an amorphous terbium-cobalt thin film with a spatial resolution of about 75 nm and could differentiate between the magnetization directions, finding qualitative agreement with soft x-ray microscopy images recorded with Fresnel zone plate optics having better than 25 nm spatial resolution. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D13.00009: Magnetic order and fluctuations in Fe$_{3}$O$_{4}$ nanoparticles via coherent X-ray magnetic scattering Karine Chesnel, Matea Trevino, Yanping Cai, Andrew Matthew, Roger Harrison, Andreas Scherz Magnetite (Fe$_{3}$O$_{4})$ particles exhibit a superparamagnetic behavior when their sizes are in nanometer scale. Such nanoparticles could potentially be used for applications in the medical field. We are interested in investigating the magnetic order and fluctuation dynamics in self-assemblies of such nanoparticles. Our Fe$_{3}$O$_{4 }$nanoparticles are prepared by an organic route and range from 5 nm to 50 nm in size. They are deposited on membrane where they self-assemble. We have been studying the magnetic order using X-ray resonant magnetic scattering (XRMS) at the SSRL synchrotron facility in Stanford. This unique technique, combined with X-ray Magnetic Circular Dichroism (XMCD), provide information about the spatial distribution of the particles and their magnetic order (1). In addition, the use of coherent light at the SSRL beamline, combined with the application of magnetic field in-situ at different temperatures, allows for studying local magnetic disorder (2) and dynamics of fluctuations near the blocking temperature. \begin{enumerate} \item J.B.Kortright et al., PRB \textbf{71}, 012402 (2005) \item K. Chesnel et al., PRB \textbf{83}, 054436 (2011) \end{enumerate} [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D13.00010: Magnetic x-ray scattering, transport and MFM study of strongly correlated La$_{1-x}$Sr$_{x}$MnO$_{3}$ nanowires Xiaoqian M. Chen, Tyler R. Naibert, Nick Bronn, James C.T. Lee, Shu Wang, James N. Eckstein, Nadya Mason, Raffi Budakian, Peter Abbamonte, Xiaofang Zhai, Anand Bhattacharya Artificial patterning is a promising new approach to studying strongly correlated materials, since a boundary acts as a perturbation that can tip the balance among various competing ground states. We have fabricated large, periodic arrays of 80 nm wide nanowires from epitaxially grown La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) thin films. Their electronic and magnetic properties were studied with resonant soft x-ray scattering (RSXS), transport measurements and magnetic force microscopy (MFM). RSXS measurements revealed a series of structural diffraction peaks that arise from the periodic wire structure. Below the Curie temperature we also observed a series of magnetic superlattice reflections, indicating collective ordering of the magnetic moments into a pattern with a spatial period of five wires. Transport measurements also showed anomalous ``telegraph'' switching noise at temperatures below 15K, and MFM revealed unusual domain formation. We interpret these results as arising from unusual, boundary-induced magnetic domains interacting via long-ranged, classical magnetic dipole coupling. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D13.00011: Local structure and magnetic properties of ultrathin Mn films grown on Si(001) Samer Kahwaji, Theodore Monchesky, Daryl Crozier, Robert Gordon We report on the structural and magnetic properties of ultrathin Mn layers deposited onto Si(001) by molecular beam epitaxy (MBE) at low temperature. X-ray absorption fine structure (XAFS) studies reveal that the structure of the silicide layer that forms depends on the growth temperature of the capping layer. A capping layer grown at 200 $^{\circ}$C on 0.35 monolayer (ML) Mn results in a metastable MnSi phase with a B2-like (CsCl) structure, whereas a cap grown at room temperature on 0.5 ML followed by annealing at 200 $^{\circ}$C produces a lower coordinated MnSi phase with a B20-like structure. Increasing the Mn thickness from 0.5 to 4 monolayers does not trigger a structural transformation but drives the structure closer to MnSi-B20. Using SQUID magnetometry, we show that the sample with B2-like structure has the largest Mn magnetic moment of 0.33$\mu _{B}$/Mn at T=2 K, and a Curie temperature $T_{C}$ above 250 K. MnSi-B20 layers showed lower moments and much lower $T_{C}$'s, in-line with those reported for MnSi-B20 thin films. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D13.00012: Probing the surface magnetic properties via Auger-photoelectron coincidence spectroscopy Gian Paolo Brivio, Guido Fratesi, Mario Italo Trioni, Roberto Gotter, Gianni Stefani Auger-photoelectron coincidence spectroscopy (APECS) via a dichroic effect is a suitable tool to study complex systems such as magnetic thin films and multilayers. We present clear evidence for such a dichroic effect in the M$_{3}$VV Auger line shape of Fe films on Cu(001) measured by angle resolved APECS showing final state spin selectivity (triplet vs. singlet components). Using the Fermi Golden rule and the density functional theory formalism, the Auger spectrum and its angular distributions are computed. For magnetic systems, the spin dependence of the Auger matrix elements allows one to work out the individual multiplet contributions to the Auger spectrum. For an accurate interpretation of experiments we also take into account the valence hole-hole interaction affecting the Auger line shape by the Cini-Sawatzky theory but considering a spin dependent on-site interaction U. The calculated angular distribution, in case of a non-spherical ionized core level (e.g. l=1, m=0), follows a non-trivial behavior in agreement with experiment. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D13.00013: Polarized small angle neutron scattering of MnO/Mn$_{3}$O$_{4}$ nanocrystals L. Dedon, Y. Ijiri, R. Booth, K. Krycka, J.A. Borchers, W.C. Chen, S. Watson, J.J. Rhyne, S.A. Majetich Monodisperse magnetic nanoparticles are of great interest for biomedical and data storage applications, particularly in cases where the core and shell can be carefully controlled to alter properties like magnetic anisotropy. However, it is often difficult to determine the underlying moment arrangements and correlations in these systems. Here, we focus on manganese (II) oxide/manganese (II,III) oxide core/shell nanoparticles, using polarized small angle neutron scattering (SANS) to probe the magnetic intra and interparticle interactions. The 30nm diameter particles with 4-5nm shell were prepared by solution chemistry methods and self-assembled into 3D nanocrystals. SANS measurements were conducted in magnetic fields from remanence-1T and temperatures from 10-300K. Magnetic and structural scattering components were separated using an algorithm previously described in [1]. The magnetic signature depended on the field and temperature history of the sample. Modeling work has been done to further quantify the interparticle length scales and the effects of crystal packing. This work was supported in part by NSF grants DMR-0454672, -0704178, -0804779, -1104489, and DOE grant DE-FG02-08ER40481. [1] K.L. Krycka, et al. Phys. Rev. Lett. 104, 207203 (2010). [Preview Abstract] |
Session D14: Focus Session: Spins in Carbon - Spins in Graphene
Sponsoring Units: DMP GMAGChair: Ezekiel Johnston-Halperin, Ohio State University
Room: 212
Monday, February 27, 2012 2:30PM - 3:06PM |
D14.00001: Spin transport in graphene Invited Speaker: Barbaros \"Ozyilmaz Conventional electronic transistors involve the control of electronic charge at the nanoscale to realize memory, logic and communication functions. All these electronic charges, however, also carry a spin that remains unutilized in present commercial devices. This has motivated the search for new materials that propagate spin-polarized currents over large distances. Among the most promising materials for spintronics has been graphene. Micron-scale spin relaxation lengths have been previously demonstrated in single-layer graphene. Recently, we showed that bilayer graphene is a far more interesting candidate for spintronics. By fabricating spin valves on bilayer graphene we have achieved at room temperature spin relaxation times up to 2 nanoseconds, which are an order of magnitude higher than for single layer graphene [1]. Furthermore, the spin-relaxation time scales inversely with the mobility of BLG sample. This indicates the importance of D'yakonov-Perel' spin scattering in BLG. Last not but least, the presence of an electric field tunable band gap in bilayer graphene makes it particularly appealing. Our work provides fundamental insight into the unique properties of bilayer graphene for spintronic applications. Remarkably, a similar difference between single layer and bilayer graphene is also observed in large area graphene grown by the CVD method on copper. These results demonstrate the potential of CVD graphene in realistic spintronics devices [2]. \\[4pt] [1] T - Y. Yang et al., Observation of Long Spin-Relaxation Times in Bilayer Graphene at Room Temperature, PRL (2011). \\[0pt] [2] A. Avsar et al., Towards Wafer Scale Fabrication of Graphene Based Spin Valve Devices, Nano Lett. (2011). [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D14.00002: Spin Transport in Epitaxial Graphene on SiC (0001) J. Abel, A. Matsubayashi, C. Dimitrakopoulos, D.B. Farmer, Ali Afzali, A. Grill, C.Y. Sung, V.P. LaBella Long spin lifetimes in graphene make it an ideal candidate for the channel material in future spintronic devices. The long spin lifetimes arise due to the small intrinsic spin orbit coupling and low hyper-fine interaction of the electron spins with the carbon nuclei. Spin lifetimes are measured in epitaxially grown graphene on SiC from IBM. The spin lifetime is measured with non-local Hanle measurements to observe spin precession in the graphene. Spin lifetimes are then extracted by fitting to the solution of the Bloch equation with a diffusion term. A comparison will be presented between an injection and detection contact structure with a bare Co/graphene interface and a Co/HfO$_2$/graphene interface, where it was found that the HfO$_2$ interface results in an increased spin lifetime. Temperature dependent spin lifetimes and Hall mobilities were measured, and a comparison of the two measurements will also be presented to provide insight into the spin scattering mechanism in epitaxial graphene on SiC. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D14.00003: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 3:30PM - 3:42PM |
D14.00004: Spin Dependent Scattering from Gated Obstacles in Graphene Systems Mahmoud Asmar, Sergio Ulloa We study scattering of Dirac fermions in the presence of both intrinsic and Rashba spin orbit interactions (SOIs). We use the analytical form of eigenstates in a system with cylindrical symmetry to calculate useful quantities for the scattering of Dirac particles such as phase shifts, and both transport and total cross sections, as well as the corresponding scattering times. At low energies the scattering from a gated obstacle in the absence of SOIs is anisotropic and predominantly forward $[1]$. In contrast, for energies close to the intrinsic SOI amplitude, the scattering becomes \emph{isotropic}, which can be seen as arising from the effective Dirac mass generated by the SOI interaction. In the presence of Rashba fields we find that the spin-flip scattering is isotropic while it remains anisotropic and predominantly forward for spin-preserving scattering, leading to persistent spin polarization in the forward direction. At high energies, we find a series of resonances in the elastic scattering times, associated with particle trajectories orbiting the obstacle and characterized by long lifetimes. The Rashba SOI is found to double the number of long lived states in both spin-preserving and spin-flip scattering channels. [1]M.Monteverde et al., PRL \textbf{104},126801 (2010) [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D14.00005: Tunnel spin injection into graphene through ALD-grown tunnel barrier Takehiro Yamaguchi, Satoru Masubuchi, Kazuyuki Iguchi, Rai Moriya, Tomoki Machida Graphene is a promising material for spintronics devices because of its long spin relaxation time due to weak spin-orbit interaction and hyperfine interaction. For the spintronics, it is very essential to develop a reliable method to inject spin polarized electrons into graphene from ferromagnetic electrodes. In this study, between ferromagnetic electrodes and graphene we fabricated a new type of Al$_{2}$O$_{3}$ tunnel barrier grown by atomic layer deposition (ALD). Before ALD of Al$_{2}$O$_{3}$, we functionalized the surface of graphene with a self-assembled monolayer of 3, 4, 9, 10 perylene tetracarboxylic acid (PTCA) to improve adhesion and growth of Al$_{2}$O$_{3}$. Using ALD-Al$_{2}$O$_{3}$/PTCA composite barrier, large nonlocal magnetoresistance of 30 $\Omega $ was observed at 45 K. Nonlocal magnetoresistance reached maximum around charge neutrality point, and $I-V$ characteristics of the contacts are nonlinear. These results indicate the achievement of tunnel spin injection into graphene, revealing potentially high performance of ALD-Al$_{2}$O$_{3}$/PTCA tunnel barrier [1]. [1] T. Yamaguchi et al., J. Magn. Magn. Mater. (2011), doi: 10. 1016/j.jmmm. 2011. 09. 031 [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D14.00006: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 4:06PM - 4:18PM |
D14.00007: Spin waves in finite graphene ribbons Andrea Latge, Francisco Culchac, Antonio Costa In a previous work we have shown that the spin excitations of a graphene zigzag ribbon have a dispersion relation predominantly linear for large wave lengths, due to the antiferromagnetic coupling between the magnetizations of the opposite edges. Although the excitations are weakly damped in electrically neutral nanoribbons, the damping can be enhanced by the application of gate voltages. This allows control of the spin relaxation times by purely electrostatic means. Here we investigate spin excitations and also electronic transport in finite zigzag ribbons, connected to graphene-like metallic leads. The ground state is described self-consistently within a mean-field scheme. The spin excitations are extracted from transverse dynamic susceptibility. As a general result we found conductance gaps populated with localized states that are swept out as the length of the conductor increases. The magnetic moment is site dependent, differently from the infinite case, and diverse spin wave excitation modes are exhibited. We study the spin wave behavior and the dynamic susceptibility dependence on the coupling intensity between ribbon and leads. We analyze the role played by the coupling on the spin wave life times and the effects of external doping. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D14.00008: GMAG Dissertation Award Talk: Spin Injection and Relaxation in Graphene Invited Speaker: Wei Han Graphene is a unique and promising candidate for spintronics due to its high mobility, low intrinsic spin-orbit and hyperfine couplings, which should lead to long spin lifetimes and relaxation lengths. Experimentally, the gate-tunable spin transport has been achieved at room temperature. However, the spin injection efficiency has been low and the spin lifetime is still much shorter (50- 200 ps) than expected theoretically ($\sim $micro seconds). To fulfill the potential of graphene for spintronics, two major advances are needed to be accomplished; enhance the spin injection efficiency and extend the spin lifetime. In this talk, I will focus on the contributing results for these advancements in graphene spintronics during my Ph. D. study. First, I develop a method to grow atomically smooth MgO tunnel barrier using Ti seeding layer prior the MgO growth on graphene. Then tunneling spin injection into graphene is achieved. The nonlocal spin signal is observed to be as high as 130 ohms at 300 K, with a spin injection efficiency of 30{\%}. Second, using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at 300 K, 1.0 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation mechanisms are found in SLG and BLG. Third, the spin lifetimes on the same SLG spin valve with tunable mobilities are investigated. These results are important advances for graphene to be used for spin computing or spin logic applications. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D14.00009: Magnetism in neutron irradiated graphene samples Yancen Li, Joel Therrien Recent work in our lab has shown that graphene can become ferromagnetic by way of the addition of hydrogen. The graphene shows a weak but distinct magnetization loop at room temperature. Magnetic force microscopy shows that the magnetic effect can be added and subsequently removed by exposure to a cold hydrogen plasma followed by annealing at 400 Deg C. An outstanding question has been whether the effect observed is due to the interaction of the hydrogen with the graphene, or the addition of defects from the hydrogen. The role of the hydrogen vs. defects was studied using raman microscopy and magnetic force microscopy on graphene samples exposed to neutron irradiation for comparison of a sample containing defects without adatoms. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D14.00010: Magnetic Moment Formation in Hydrogenated and Defected Graphene Kathleen McCreary, Adrian Swartz, Wei Han, Roland Kawakami Recent experimental observations of magnetic moment formation and magnetic ordering in graphene and graphite have excited both theorists and experimentalists. Magnetic ordering in carbon based materials would provide an alternate material to the conventional d and f metals employed in current technologies and could contribute to new applications in nanotechnology, spintronics, medicine, and telecommunications. While still a young and controversial field, previous experimental and theoretical works suggest the presence of magnetic moments in carbon materials is attributed to impurities, boundaries, reduced dimensionality, or defects. In this study, we perform spin transport measurements on graphene devices in order to investigate magnetic moment formation in doped graphene. The graphene surface is modified inside an ultrahigh vacuum chamber through a variety of methods including hydrogen adsorption, Ar sputtering, and molecular beam deposition of transition metals. We observe signatures of paramagnetic moment formation associated with dopants and defects in graphene. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D14.00011: Quantum Monte Carlo study of magnetic impurity in bilayer grephene J.H. Sun, F.M. Hu, H.K. Tang, Hai-Qing Lin It is expected to observe many different properties in bilayer graphene when compared with single layer graphene due to the differences in crystal structure. Additionally, bilayer system offers a freedom of inducing a gap in the energy band by applying a shift in the electrochemical potential to two graphene layers. In this work, we study the magnetic properties of an Anderson magnetic adatom in Bernal stacking bilayer graphene and compare the results with those of single layer counterpart. Several different cases such as different adatom position and different potential bias of two layers are studied using the quantum Monte Carlo method. In all the cases, we find that the impurity local magnetic moment can be switched between relatively large and small values by tuning the chemical potential. We apply MaxEnT method to compute impurity spectral density and find its behavior to differ from that of an impurity in a single layer graphene. We also calculate various correlation functions and make comparisons. [Preview Abstract] |
Session D15: Focus Session: Spins in Metals - Ultra Fast Dynamics
Sponsoring Units: DMP FIAP GMAGChair: Shufeng Zhang, University of Arizona
Room: 213
Monday, February 27, 2012 2:30PM - 3:06PM |
D15.00001: Ultrafast Magnetism of Multi-component Ferromagnets and Ferrimagnets on the Time Scale of the Exchange Interaction Invited Speaker: Ilie Radu Revealing the ultimate speed limit at which magnetic order can be controlled, is a fundamental challenge of modern magnetism having far reaching implications for the magnetic recording industry [1]. Exchange interaction is the strongest force in magnetism, being ultimately responsible for ferromagnetic or antiferromagnetic spin order. How do spins react after being optically excited on a timescale of or even faster than the exchange interaction? Here, we demonstrate that femtosecond (fs) measurements of ferrimagnetic and ferromagnetic alloys using X-ray magnetic circular dichroism provide revolutionary new insights into the problem of ultrafast magnetism on timescales pertinent to the exchange interaction. In particular, we show that upon fs optical excitation the ultrafast spin reversal of GdFeCo - a material with antiferromagnetic coupling of spins - occurs via a transient ferromagnetic state [2]. The latter emerges due to different dynamics of the Gd and Fe magnetic moments: Gd switches within 1.5 ps while it takes only 300 fs for Fe. Thus, by using a single fs laser pulse one can force the spin system to evolve via an energetically unfavorable way and temporarily switch from an antiferromagnetic to a ferromagnetic type of ordering. In order to understand whether the observation of this temporarily decoupled and element-specific dynamics is a general phenomenon or just something strictly related to the case of ferrimagnetic GdFeCo, we have investigated the demagnetization of the archetypal ferromagnetic NiFe alloys. Essentially, we observe the same distinct magnetization dynamics of the constituent magnetic moments: Ni demagnetizes within $\sim $300 fs being much faster than the demagnetization of Fe of $\sim $800 fs. This distinct demagnetization behavior leads to an apparent decoupling of the Fe and Ni magnetic moments on a few hundreds of fs time scale, despite the strong exchange interaction of 260meV ($\sim $16 fs) that couples them. These observations supported by atomistic simulations, present a novel concept of manipulating magnetic order on different classes of magnetic materials on timescales of the exchange interaction [3]. \\[4pt] [1] A. Kirilyuk, A.V. Kimel and Th. Rasing, \textit{Rev. Mod. Phys.} \textbf{82}, 2731 (2010). \\[0pt] [2] I. Radu \textit{et al.}, \textit{Nature} \textbf{472}, 205 (2011). \\[0pt] [3] I. Radu \textit{et al.}, \textit{submitted} (2011). [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D15.00002: Uncovering the Ultrafast Angular Momentum Transfer Channels on the Nanoscale in GdFeCo Catherine Graves, Alex Reid, Benny Wu, Tianhan Wang, Sanne de Jong, Ilie Radu, Sasha Epp, Robert Hartmann, Arata Tsukamoto, Ryan Coffee, Mina Bionta, Joshua Turner, William Schlotter, Yves Acremann, Alexey Kimel, Andrei Kirilyuk, Joachim St\"{o}hr, Theo Rasing, Hermann D\"{u}rr, Andreas Scherz The ultrafast control of electron spins is of both fundamental scientific and technological interest. Recent experiments have shown that femtosecond laser excitation can act as a stimulus to switch the magnetization direction in ferrimagnetic GdFeCo, called all-optical switching. However, how angular momentum is transferred to result in a switched state remains unknown. To further understand this mechanism, we use 80fs x-ray pulses from LCLS to study how angular momentum transfer is triggered in GdFeCo by fs laser excitation using time-, element- and spatially-resolved x-ray resonant magnetic scattering. We present here the first-ever measurement of the fs magnetic response in GdFeCo with spatial resolution down to 10nm. Our results reveal drastically different behaviors on the nanoscale as compared to the bulk and provide insight into the angular momentum transfer channels. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D15.00003: Ultrafast Demagnetization Measurements using Extreme Ultraviolet Light: Comparison of Electronic and Magnetic Contributions Chan La-o-vorakiat, Emrah Turgut, Carson A. Teale, Henry C. Kapteyn, Margaret M. Murnane, Denis Lvovsky, Roman Adam, Patrik Grychtol, Claus M. Schneider, Stefan Mathias, Martin Aeschlimann, Justin M. Shaw, Hans Nembach, Thomas J. Silva Ultrashort pulses from high-harmonic generation provide new capabilities for uncovering coupled charge, spin, and phonon dynamics in magnetic materials by combining elemental selectivity with ultrafast time resolution in a tabletop source. In this talk, we address an important question in magneto-optics that has implications for understanding femtosecond magnetism: is the signal from the transverse magneto-optical Kerr effect at the M-shell absorption edges of a magnetic material purely magnetic or perturbed by non-magnetic optical artifacts? We conclusively show that high harmonics sensitively probe the magnetic state, with negligible contributions from electronic effects because of hot-electron dynamics. Finally, our measurements are in excellent agreement with conventional visible-wavelength magneto-optics probes and illustrate the power of high harmonics for probing the dynamics in magnetic materials. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D15.00004: Theory of spin hot pockets in laser-induced demagnetization in ferromagnetic nickel Guoping Zhang, Mingsu Si, Yihua Bai, Thomas F. George We will first review the current theory of the demagnetization process, with a particular focus on two distinctive contributions: (a) the optical dipole interaction (ODI) between a laser field and a magnetic system and (b) the spin expectation value change (SEC) between two transition states. We then introduce a new optical spin operator, a product of SEC and ODI between transition states. In ferromagnetic nickel, our first-principles calculation demonstrates that the larger the value of optical spin operator is, the greater the dynamic spin moment change is. This simple operator is very useful, as it directly links the time-dependent spin moment change $\Delta M_{z}^{\bf k}(t)$ at every crystal-momentum ${\bf k}$ point to its intrinsic electronic structure and magnetic properties. Those hot spin pockets should be the focus of future research. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D15.00005: Ultrafast Transport of Laser-Excited Spin-Polarized Carriers in Metallic Multilayers Invited Speaker: Alexey Melnikov The ultrafast spin dynamics induced by a transport of spin polarized carriers is a hot topic motivated by the fundamental interest in magnetic excitations and applications like spintronics and data storage. To understand underlying elementary processes typically occurring on femtosecond time scales, we have developed a time-of-flight-like approach that probes the spin dynamics induced by hot carriers (HC) and demonstrated a \textit{spin polarized} HC transport through an epitaxial Au/Fe/MgO(001) structure. Using a back pump-front probe configuration, we establish that HC induced in Fe by the pump laser pulse can form a nearly \textit{ballistic} spin current (SC) in Au. Optical second harmonic (SH) generated at the Au surface by the probe pulse monitors the transient surface HC density and spin polarization (SP). Since HC with different SP are excited in Fe to different final energies and consequently have different lifetimes in Au, the HC pulse has steep leading part formed by ballistic HC with the negative SP and shallow trial part formed by diffusive HC with the positive SP. This leads to the SC sign change within the 1~ps overall SC pulse duration. We also make a step towards understanding the origin of laser-induced ultrafast demagnetization overcoming the limited ability of conventional pump-probe schemes to distinguish photon-, electron-, and phonon-mediated effects. Comparing the SH response of Fe to the direct optical excitation with that to the excitation by hot carriers generated in Au, we rule out coherent effects of the pump pulse electromagnetic field and show that the HC-induced spin dynamics is responsible for the ultrafast demagnetization. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D15.00006: First-principles dynamical calculation of a pump-probe scenario for the spin flip on NiO Georgios Lefkidis, Wolfgang H\"{u}bner Using a fully ab-initio approach we calculate in a dynamic way the time-dependent probe signal of a spin flip scenario on the antiferromagnetic NiO surface. We start from a first-principles calculation of the highly correlated, relativistic, electronic states of a doubly embedded NiO$_{5}^{-8}$ cluster followed by the time-propagation of the system under the influence of the spin-flipping pump pulse \emph{and} the detecting probe pulse. This way we treat both pulses on equal footing and, for the first time, consider the effects of the electronic non-equilibrium due to the concurrent presence of the pulses. Our time-resolved calculations reveal the subtle influence of the probe pulse itself on the detection signal, which cannot be completely treated solely by the time propagation of the pump pulse and the subsequent calculation of the static susceptibility tensor [1,2]. We also analyze the angular-momentum conservation and its distribution among the system and both laser pulses [3].\\ $[1]$ G. P. Zhang, W. H\"{u}bner, G. Lefkidis, Y. Bai, and T. F. George, Nature Physics {\bf 5}, 499 (2009)\\ $[2]$ G. P. Zhang, G. Lefkidis, W. H\"{u}bner and Y. Bai, J. Appl. Phys. {\bf 109}, 07D303 (2011)\\ $[3]$ G. Lefkidis, G. P. Zhang and W. H\"{u}bner, Phys. Rev. Lett. {\bf 103}, 217401 (2009) [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D15.00007: Non-equilibrium coupling and femto-second oscillatory spin torque in non-collinear F/N/F magnetic multilayers Jianwei Zhang, Qiang Wang By employing time-dependent diffusion theory, we study time-evolution behaviors of spin torque and spin current in non-collinear Ferromagnetic/Normal/Ferromagnetic tri-layers structure. We find in ferromagnetic layer spin toque has femto-second oscillatory in its initio building-up process, which indicates that excited itinerant electron's spin process alone background magnetization in femto-second period near the interfacial region. We also find even mismatch of background magnetization at each F/N interfaces is not enough to compensate discontinuity of spin current near interface, however, by introducing non-equilibrium coupling between two F/N interfaces, one can produce a continuous spin current state across both two interfaces without mandating any boundary conditions. In our study, we find a new universal time-dependent propagator to generate all directions continuous spin current across two F/N interfaces. This new spin propagator is also closely related to spin flip scatter at interface. In addition, we also find the coupling of two N/F interfaces enlarges transverse spin diffusion channels into magnetic layer. Finally, our time-dependent spin current and spin torque states also match our pervious self-consistent steady state results. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D15.00008: Probing the timescale of the exchange interaction in a ferromagnetic alloy Emrah Turgut, Chan La-O-Vorakiat, Mark E. Siemens, Margaret M. Murnane, Henry C. Kapteyn, Stefan Mathias, Patrick Granitzka, Steffen Eich, Martin Aeschlimann, Patrik Grychtol, Roman Adam, Claus M. Schneider, Justin M. Shaw, Hans T. Nembach, Thomas J. Silva The underlying physics of all ferromagnetic behavior is the cooperative interaction between individual atomic magnetic moments that results in a macroscopic magnetization. In this work, we use extreme ultraviolet pulses from high-harmonic generation as an element-specific probe of ultrafast, optically driven, demagnetization in a ferromagnetic Fe-Ni alloy (Permalloy). We show that for times shorter than the characteristic timescale for exchange coupling, the magnetization of Fe quenches more strongly than that of Ni. Then, as the Fe moments start to randomize, the strong ferromagnetic exchange interaction induces further demagnetization in Ni, with a characteristic delay determined by the strength of the exchange interaction. We can further enhance this delay by lowering the exchange energy by diluting the Permalloy with Cu. This measurement probes how the fundamental quantum mechanical exchange coupling between Fe and Ni in magnetic materials influences magnetic switching dynamics in ferromagnetic materials relevant to next-generation data storage technologies. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D15.00009: Driving coherent spin reorientation transition with femtosecond laser pulses Vladimir Stoica, Roy Clarke Ultrafast studies of spin reorientation transition provide insight in magnetization switching processes and are important for the magnetic recording technology. Using femtosecond optical techniques, we demonstrate coherent control of the magnetization vector in epitaxial Fe films. These films feature uniaxial anisotropy that is thermally modulated by an optical pulse. We observe an optically-induced spin reorientation transition of first-order that provides an efficient route to ultrafast coherent magnetization switching. The switching is found to be a three-step temporal process: a coherent reorientation ($\sim $ 100 ps) is followed by a spin precession in a newly created metastable state ($\sim $ 300 ps), which evolves into a dual domain state that undergoes relaxation within $\sim $ 2 - 4 ns. We provide a model to explain the experimental data and predict further applications of this technique. The details of the experiments compare favorably with the simulated magnetization trajectories, opening new pathways for coherent control of magnetic dynamics with pulsed lasers. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D15.00010: Theory of the inverse Faraday effect driven by non-coplanar spin structures Katsuhisa Taguchi, Gen Tatara We show theoretically that a new mechanism of the inverse Faraday effect exists in the presence of non-coplanar spin structures in metals even without the spin-orbit interaction. The spin density generated by the effect is proportional to the circular polarization of the light, ($\mathbf{\mathcal{E}}\times \mathbf{\mathcal{E}}^*$) ($\mathbf{\mathcal{E}}$ is the complex amplitude vector of the electric field of the circular light), and $\nabla \times \mathbf{j}_{\rm{s}}^\alpha$, where $\mathbf{j}_{\rm{s}}^\alpha \equiv (\mathbf{n}\times \mathbf{\nabla}\mathbf{n})^\alpha$ is the spin current carried by the spin structure ($\mathbf{n}$ is the unit vector of the localized spin). The effect turns out to be larger than the conventional inverse Faraday effect, and the present mechanism is expected to be useful for the magnetization flip of the Skyrmion. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D15.00011: Examination of spin waves in a two-dimensional magnetic superlattice Glade Sietsema, Michael Flatt\'e We have studied the properties of spin waves in two-dimensional periodic superlattices of magnetic materials.[1] Frequencies and linewidths are calculated for square and hexagonal symmetry superlattices from the Landau-Lifshitz-Gilbert equations. Large differences in the saturation magnetizations and exchange stiffness constants of the materials are shown to be capable of producing gaps in the magnonic spectrum across the entire superlattice Brillouin zone. For example, with a hexagonal superlattice of Fe and YIG we find gaps of 0.5THz and 1THz within the lowest four bands. Additionally, calculations of the system's Green's functions are used to examine the superlattice's response to pulse excitations, such as from a spin torque oscillator.\\[4pt][1]arXiv:1111.2506v1 [Preview Abstract] |
Session D16: Strongly Correlated Numerics and Theory
Sponsoring Units: DCMPChair: Andre-Marie Tremblay, Universite de Sherbrooke
Room: 251
Monday, February 27, 2012 2:30PM - 2:42PM |
D16.00001: Local and non-local correlations in nanoscopic systems Giorgio Sangiovanni, A. Valli, G. Rohringer, A. Toschi, K. Held, H. Das, T. Saha-Dasgupta Tools for reliably treating nanoscopic systems, like coupled quantum-dots, ad-atoms on surfaces, macromolecules, etc., in the presence of electronic correlations are either missing or prohibitively expensive. We have implemented a new computational scheme based on a self-consistently defined set of local problems [1]. Our method scales linearly with the number of sites and allows us to perform large-scale sign-problem free Quantum Monte-Carlo simulations. We have studied the behavior of a single-atom junction formed upon stretching a metallic wire and found that a metal-insulator crossover is induced when the wire is about to break up. The combination with ab-initio techniques allowed us to study size-dependent properties of Manganite nano-clusters [2]. The simplest implementation of our method includes only local self-energy effects. We recently went beyond this and applied the resulting more sophisticated version of our method to an exactly solvable model finding results in remarkable agreement with the exact solution. \\ \\ \noindent [1] A. Valli, G. Sangiovanni, O. Gunnarsson, A. Toschi and K. Held, PRL {\bf 104}, 246402 (2010) \\ \noindent [2] H. Das, G. Sangiovanni, A. Valli, K. Held and T. Saha-Dasgupta, PRL {\bf 107}, 197202 (2011) [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D16.00002: Quantum impurity solver based on truncated ED (RASCI) wave function expansion Dominika Zgid, Emanuel Gull, Chris Marianetti, Andrew Millis, David Reichman, Garnet Chan Quantum impurity models appear in many applications, including nanoscience and the dynamical mean field approximation (DMFT). Many physically relevant impurity models are too large to be solved by exact diagonalization (ED), lack the interaction and hybridization structure required for quantum Monte Carlo (QMC) simulations, or suffer from a severe sign problem. We present an alternative impurity solver inspired by configuration interaction (RASCI) techniques of quantum chemistry and based on a controlled truncation of a wave function expansion. The method can access larger impurity models (impurities with 5 $d$-orbitals and 20 bath orbitals can be easily calculated on a single processor) than can ED and avoids the sign problems of QMC methods. The performance is demonstrated for a cluster DMFT approximation to the two dimensional Hubbard model and for the problem of a Co adatom on a Cu(111) surface. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D16.00003: Modified Iterated perturbation theory in the strong coupling regime and its application to the 3d FCC lattice Louis-Fran\c{c}ois Arsenault, Patrick S\'{e}mon, B. Sriram Shastry, A.-M.S. Tremblay The Dynamical Mean-Field theory(DMFT) approach to the Hubbard model requires a method to solve the problem of a quantum impurity in a bath of non-interacting electrons. Iterated Perturbation Theory(IPT)[1] has proven its effectiveness as a solver in many cases of interest. Based on general principles and on comparisons with an essentially exact Continuous-Time Quantum Monte Carlo (CTQMC)[2], here we show that the standard implementation of IPT fails when the interaction is much larger than the bandwidth. We propose a slight modification to the IPT algorithm by requiring that double occupancy calculated with IPT gives the correct value. We call this method IPT-$D$. We show how this approximate impurity solver compares with respect to CTQMC. We consider a face centered cubic lattice(FCC) in 3d for different physical properties. We also use IPT-$D$ to study the thermopower using two recently proposed approximations[3]$S^*$ and $S_{Kelvin}$ that do not require analytical continuation and show how thermopower is essentially the entropy per particle in the incoherent regime but not in the coherent one.[1]H.Kajueter et al. Phys. Rev. Lett. 77, 131(1996)[2]P. Werner, et al. Phys. Rev. Lett. 97, 076405(2006)[3]B.S. Sriram Shastry Rep. Prog. Phys. 72 016501(2009) [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D16.00004: Magnetic properties of the Hubbard model on the fcc lattice Hao Shi, Shiwei Zhang As a possible model for ferromagnetism, we study the magnetic properties of the Hubbard model on an fcc lattice. Near-neighbor and next-near-neighbor hopping parameters are included to examine the effect of band structure. We use exact diagonalization and the Constraint Path Monte Carlo (CPMC) \footnote{S.~Zhang, J.~Carlson, and J.~Gubernatis, Phys.~Rev.~B {\bf 55}, 7464 (1997); C.-C.~Chang and S.~Zhang, Phys.~Rev.~B {\bf 78}, 165101 (2008).} methods. Several methodological improvements in CPMC, for example the release of the constraint, will be discussed. We present benchmark quality results on the paramagnetic ground state and partially polarized states, as a function of interaction strength. A magnetic phase diagram is obtained from our many-body calculations, and comparison will be made with results from Dynamical Mean Field theory \footnote{ M.~Ulmke, The Eur. Phys. J.~B.~{\bf 1}, 301 (1998) }. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D16.00005: Dual fermion dynamical cluster approach for strongly correlated systems Shuxiang Yang, Herbert Fotso, Hartmut Hafermann, Ka Ming Tam, Juana Moreno, Thomas Pruschke, Mark Jarrell A multi-scale many-body approach is developed for strongly-correlated electron systems by combining the dynamical cluster approximation (DCA) and the recently introduced dual fermion formalism. This approach systematically incorporates non-local corrections to the DCA by employing an exact mapping from a real lattice to a DCA cluster of linear size Lc embedded in a dual fermion lattice. The Green function in the dual space serves as a small parameter enabling the use of a diagrammatic perturbation calculation on the dual fermion lattice. For example, the dual fermion self-energy calculated with simple second-order perturbation theory scales as ${\cal{O}}(1/L_c^3)$. We demonstrate the effectiveness of the approach by applying it to the 2D Hubbard model. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D16.00006: Extended Correlation in Strongly Correlated Systems, Beyond Dynamical Cluster Approximation Herbert Fotso, Shuxiang Yang, Hartmut Hafermann, Ka-Ming Tam, Juana Moreno, Thomas Pruschke, Mark Jarrell We present a new multi-scale approach for strongly correlated systems that combines the Dynamical Cluster Approximation and the recently introduced dual-fermion formalism. This approach employs an exact mapping from a real lattice to a DCA cluster of linear size $L_c$ embedded in a dual fermion lattice. The short-length-scale physics is addressed by DCA cluster calculations, while the longer-length-scale physics is addressed diagrammatically using dual fermions. The bare and dressed dual fermionic Green functions scale as ${\cal{O}}(1/L_c)$, so perturbation theory on the dual lattice converges very quickly. E.g., the dual Fermion self-energy calculated with simple second order perturbation theory is of order ${\cal{O}}(1/L_c^3)$, with third order and three-body corrections down by an additional factor of ${\cal{O}}(1/L_c)$. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D16.00007: Extension of dual-fermion formalism towards disordered systems Hanna Terletska, Shuxiang Yang, Zi Yang Meng, Sandeep Pathak, Ka Ming Tam, Juana Moreno, Mark Jarrell To study the correlation effects in disordered materials, we extend the recently developed dual fermion approach [1] to include systems with disorder. In particular, we consider the effect of nonlocal disorder-induced correlations in the non-interacting Anderson model. Within this method, such nonlocal effects are included in a systematic way as an expansion to the coherent potential approximation (CPA). The ability to properly treat the nonlocal correlations and provide non-local corrections to the CPA, is crucial for the description of the electron localization. In our analysis, we consider the density of sates and localization effects and compare them with the existing results. [1] A.N. Rubtsov, et. al., Phys. Rev. B 79, 045133 (2009). [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D16.00008: Application of the Dual Fermion-Dynamical Cluster Approach to the 1D Falicov Kimball Model Sandeep Pathak, Zi Yang Meng, S.-X. Yang, Mark Jarrell, Juana Moreno The Falicov Kimball model is the simplest model for correlated electrons. It was introduced to study metal-insulator transitions. In one dimension, it is known to possess a charge density wave (CDW) instability at zero transition temperature ($T_c$). However, finite cluster methods like Dynamical Mean Field Theory (DMFT), Dynamical Cluster Approximation (DCA), Cellular Dynamical Mean Field Theory (CDMFT) , etc. show finite temperature CDW transition. In this paper, we study the model using the recently developed Dual Fermion-Dynamical Cluster approach that takes into account large length scale correlations through the auxiliary particles known as dual Fermions. We find that $T_c$ obtained from this method is lower than that obtained from the cluster methods. In particular, we study the scaling behavior of $T_c$ with the linear cluster size and also the scaling of other one-particle and two-particle quantities near the criticality. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D16.00009: Phase Separation and Charge-Ordered Phases of the d = 3 Falicov-Kimball Model at T$>$0: Temperature-Density-Chemical Potential Global Phase Diagram from RG Theory A. Nihat Berker, Ozan S. Sariyer, Michael Hinczewski The global phase diagram of the spinless FK model in d=3 is obtained by renormalization-group theory, exhibiting 5 distinct phases. Four of these phases are charge-ordered (CO) phases, in which the system forms two sublattices with different electron densities. The CO phases occur near half filling of the conduction electrons, for the entire range of localized electron densities. Phase boundaries are second order, except for the intermediate and large interaction regimes, where a first-order phase boundary occurs in the central region of the phase diagram, resulting in phase coexistence near half filling of both localized and conduction electrons. These two-phase or three-phase coexistence regions are between different charge ordered phases, between charge-ordered and disordered phases, and between dense and dilute disordered phases. The second-order phase boundaries terminate on the first-order transitions via critical endpoints and double critical endpoints. The first-order phase boundary is delimited by critical points. The phase diagram cross-sections with respect to the chemical potentials and densities of the localized and conduction electrons, at all representative interaction strengths, hopping strengths, temperatures, are calculated and exhibit 10 distinct topologies. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D16.00010: Collective Excitations and Stability of the Excitonic Phase in the Extended Falicov--Kimball Model Denis I. Golosov We consider the excitonic insulator state (often associated with electronic ferroelectricity), which arises on the phase diagram of an extended spinless Falicov--Kimball model (FKM) at half-filling. Within the Hartree--Fock approach, we calculate the spectrum of low-energy collective excitations in this state up to second order in the narrow-band hopping and/or hybridisation. This allows to probe the mean-field stability of the excitonic insulator. The latter is found to be unstable when the case of the pure FKM (no hybridisation with a fully localised band) is approached. The instability is due to the presence of another, lower-lying ground state and {\it not} to the degeneracy of the excitonic phase in the pure FKM. The excitonic phase, however, may be stabilised further away from the pure FKM limit. In this case, the low-energy excitation spectrum contains new information about the properties of the excitonic condensate (likely including the critical temperature). [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D16.00011: Propagation of entanglement in one-dimensional models of many-body localization Jens H. Bardarson, Frank Pollmann, Joel E. Moore An important and still unanswered fundamental question is whether a closed quantum system of many interacting particles can be localized by disorder. The time evolution of an initially unentangled state is studied for a 1D random-field XXZ Hamiltonian. Even for weak interactions, when the system is thought to be in a many-body localized phase, entanglement shows neither localized nor diffusive behavior: interactions act as a singular perturbation on the truly localized state with no interactions. The logarithmic time dependence of entanglement observed rather suggests a broad range of time scales typical of glassy behavior. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D16.00012: Fidelity Spectrum in Quantum Phase Transitions Wing Chi Yu, Shi-Jian Gu, Hai-Qing Lin A quantum phase transition (QPT) is incarnated by an abrupt change in the qualitative structure in the ground state wavefunction of a many-body system as the external driving parameter varies. The ground state fidelity, which is a measure of similarity between two states, is expected to show a sudden drop across the transition point and its possibility as a witness to QPTs has raised much interest in recent years. However, the ground state fidelity does not capture much information about the contribution of the low-lying excitations. In this presentation, we introduce the concept of fidelity spectrum, i.e. the matrix elements of $M=|\Psi(\lambda)\rangle\langle\Psi(\lambda+\delta\lambda)|$, where $\lambda$ is the external driving parameter and $\Psi(\lambda)$ is the wavefunction of the system at $\lambda$. By studying the fidelity spectrum, we hope to shed light on the role of excited states played in QPTs. We investigate the fidelity spectrum in two many-body systems, namely the one-dimensional transverse-field Ising model and the two-dimensional Kitaev model defined on a honeycomb lattice. We found that in different phases, as well as at the critical points, the fidelity spectrum shows significant different behaviors. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D16.00013: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 5:06PM - 5:18PM |
D16.00014: Quantum Monte Carlo calculation of reduced density matrices Lucas Wagner Quantum Monte Carlo(QMC) methods offer an efficient way to approximate the interacting ground state and some excited states of realistic model Hamiltonians based on the fundamental Coulomb interaction between electrons and nuclei. Many highly accurate results have been obtained using this method; however, it is often a challenge to extract the important correlations that the QMC wave function contains. I will describe some new results using the reduced density matrices(RDM's) to understand the electron correlation in the many-body wave function. The RDM's have both informative usage for describing correlation and pragmatic uses in further improving the variational wave function. [Preview Abstract] |
Session D17: Focus Session: Surfaces and Interfaces in Nonoxide Nanostructures: Growth, Structure, and Characterization - Growth Dynamics
Sponsoring Units: DMPChair: Dan Dougherty , North Carolina State University
Room: 252A
Monday, February 27, 2012 2:30PM - 3:06PM |
D17.00001: Factors that control the morphology of ice films on metal surfaces Invited Speaker: Norman Bartelt Examination of the equilibrium nanoscale morphology of ice films provides important clues about the energetics of water-metal interactions [1]. In this talk I compare STM and DFT results for the structure of ice films on Pt(111) and Ni(111). Because the lattice constants of Ni and Pt differ by 10{\%}, this comparison allows us to probe the effect of lattice misfit on ice nucleation and growth. On both substrates, STM suggests a first molecular water layer very different from bulk ice: besides the usual hexagonal rings they also both contain a motif of pentagons and heptagons [2]. Furthermore, at 140K, thicker films on both substrates dewet the substrate to lower interfacial energy by forming 3-dimensional ice crystallites several nanometers thick [3]. However, despite these similarities, there are striking differences in the submonolayer structure and in the kinetics of the dewetting process. Using DFT calculations as a guide, I will discuss the how these differences can be related to substrate lattice constant and draw conclusions about the processes that control ice film morphology. \\[4pt] [1] A. Hodgson and S. Haq, Surf. Sci. Rep. 64, 381 (2009). \\[0pt] [2] S. Nie, P. J. Feibelman, N. C. Bartelt and K. Th\"{u}rmer, Phys. Rev. Lett. 105, 026102 (2010). \\[0pt] [3] K. Th\"{u}rmer and N. C. Bartelt, Phys. Rev. B 77, 195425 (2008); Phys. Rev. Lett. 100, 186101 (2008). [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D17.00002: Structure and dynamics of water nano-droplets on graphene Ming Ma, Angelos Michaelides, Gabriel Aeppli The wettability of graphene and the diffusion of water droplets across it is of central importance to many emerging applications in nanofluidics. Here we report an extensive set of molecular dynamics simulations for water clusters of varying sizes on graphene (20 to 2,000 water molecules), using force field parameters fitted to recent ab initio quantum Monte Carlo data [J. Ma, A. Michaelides, D. Alfe, L. Schimka, G. Kresse, and E. G. Wang, Phys. Rev. B 84, 033402 (2011)]. The contact angle for the water droplets obtained here, with our ab initio water - carbon interaction, is in very good agreement with experiments. A strong size dependence in the diffusion of the water droplets across the surface is also observed. This work is a step towards understanding surface transportation in carbon based nanofluidics. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D17.00003: Large-scale simulations of glancing-angle deposition Jacques Amar, Xuejing Liu While thin-films grown via glancing-angle deposition have interesting structural, mechanical, and optical properties, the large range of time- and length-scales makes realistic simulations difficult. Accordingly, while activated relaxation processes may be important at long time-scales, here we focus on the deposition process since we expect the effects of shadowing and deposition-induced relaxation to dominate for large deposition angles. In particular, by taking advantage of the speed of recently developed graphical-processing-units (GPUs) we have carried out ``large-scale'' GPU-enhanced MD simulations of Cu/Cu(100) growth up to 20 monolayers (ML) for deposition angles $\theta$ (corresponding to the angle with respect to the substrate normal) ranging from 50$^{\circ}$ to 85$^{\circ}$ and for both random and fixed azimuthal angles. In general, we find good agreement with experimental results for the dependence of thin-film porosity on deposition angle and film-thickness. Results for the dependence of the surface roughness, lateral correlation length and microstructure (e.g. defect density, vacancy density, and strain) on the deposition angle and film thickness will also be presented. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D17.00004: Shape transitions in strained Cu islands on Ni(100): kinetics versus energetics Yunsic Shim, Jacques Amar We examine the shape transition from compact to ramified islands observed in submonolayer Cu/Ni(100) growth. Recently, it has been argued that this transition is not due to a growth instability but can be understood in terms of energetic arguments. In order to determine the responsible mechanisms we have carried out energetics calculations as well as temperature-accelerated dynamics (TAD) and kinetic Monte Carlo (KMC) simulations. Our results indicate that the shape transition cannot be explained by equilibrium arguments, but is instead due to kinetic effects which are mediated by strain. In particular, by calculating the relevant line-tension and strain energies, we find that the equilibrium critical island-width is at least four orders of magnitude larger than the experimentally observed arm-width. In contrast, our TAD simulations indicate that unexpected concerted motions occurring at step edges are responsible. The energy barriers for these concerted motions decrease with increasing island size and appear to saturate for islands larger than 300 - 400 atoms. By including these strain-induced kinetic processes in our KMC simulations of island-growth, we have been able to explain both the temperature- and coverage-dependence of the island morphology. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D17.00005: Dewetting Processes in Ultra-thin Epitaxial Ag Films on Si(111) Charlotte E. Sanders, Jisun Kim, C.K. Shih The authors have recently reported the development of a technique to grow large-area, single-crystal, atomically smooth Ag films on Si(111), and have demonstrated the utility of such films for plasmonics applications: the films support surface plasmon polaritons with extremely low damping. Although the authors have observed that films with thickness on the order of several tens of nanometers can be relatively stable against dewetting--at least on a time scale long enough for fabrication and EOT probing under ambient conditions--they have also seen that very thin Ag films (e.g., 5 nm) start dewetting under ambient conditions within about 24 hours. This raises an important question: how and why does dewetting occur? The authors have now undertaken a detailed and systematic study of the mechanism of dewetting in epitaxial Ag films on Si(111) as a function of film thickness. The current presentation will focus on this work, and will attempt to shed light on the apparent robustness of films grown using their method. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D17.00006: Diffusion of a Ga adatom on the GaAs(001)-\emph{c}(4 $\times$ 4)-heterodimer surface: A first-principles study Sanjay Khare, Jason Roehl, Sandeep Aravelli, Ray Phaneuf The adsorption and diffusion behavior of a Ga adatom on the $\textrm{GaAs(001)-\emph{c}(}4\times4\textrm{)}$-heterodimer surface were studied by employing ab initio density functional theory computations in the local density approximation. Structural and bonding features of the $\textrm{\emph{c}(}4\times4\textrm{)}$-heterodimer reconstruction surface were examined. A comparison with the $\textrm{\emph{c}(}4\times4\textrm{)}$-ss reconstruction\footnote{J. L. Roehl et al, Phys. Rev. B 82, 165335 (2010).} was performed. Minimum energy sites (MES) on $\textrm{\emph{c}(}4\times4\textrm{)}$-heterodimer surface were located by mapping the potential energy surface for a Ga adatom. Barriers for diffusion of a Ga adatom between the neighboring MES were calculated by using top and exchange diffusion mechanisms. We proposed two unique diffusion pathways for a Ga adatom diffusing between the global minimums of two neighboring unit cells. Signature differences between electronic structures of top- and exchange- diffusion mechanisms were studied for relevant atoms. We observed a higher diffusion barrier for exchange mechanism compared to top hopping.\footnote{Supported by NSF DMR 0705464, CNS 0855134.} [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D17.00007: Voronoi Cell Patterns: theoretical model and application to submonolayer growth Diego Luis Gonz\'alez, T.L. Einstein We use a simple fragmentation model to describe the statistical behavior of the Voronoi cell patterns generated by a homogeneous and isotropic set of points in 1D and in 2D. In particular, we are interested in the distribution of sizes of these Voronoi cells. Our model is completely defined by two probability distributions in 1D and again in 2D, the probability to add a new point inside an existing cell and the probability that this new point is at a particular position relative to the preexisting point inside this cell. In 1D the first distribution depends on a single parameter while the second distribution is defined through a fragmentation kernel; in 2D both distributions depend on a single parameter. The fragmentation kernel and the control parameters are closely related to the physical properties of the specific system under study. We apply our model to describe the Voronoi cell patterns of island nucleation for critical island sizes $i$=0,1,2,3. Experimental results for the Voronoi cells of InAs/GaAs quantum dots are also described by our model. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D17.00008: Aberration corrected Low Energy Electron Microscopy for Surface and Interface Studies Invited Speaker: Rudolf Tromp Correction of spherical and chromatic aberrations of the electron microscope objective lens constitutes one of the most significant and far-reaching breakthroughs in electron optics in the last 20 years. For instance, with the TEAM microscope it is now possible to image atoms with a spatial resolution of 50 picometers, providing a detailed real-space view of the carbon atoms in a single sheet of graphene. Similarly, the resolution on Low Energy Electron Microscopy (LEEM) has improved from a typical value of 5 nm, to less than 2 nm, at an electron energy of just a few eV. Photo Electron Emission Microscopy (PEEM) has recently achieved a resolution of 5 nm. In this talk I will discuss the successful implementation of electron-mirror based aberration correction in LEEM. Some of the details of the electron optical implementation will be discussed, in particular the unique optical properties of the electron mirror, and its mode of operation. Quantitative methods to verify proper control of the optical parameters and successful aberration correction have been developed and implemented in this new instrument. Spatial resolution has improved by more than a factor 2 as compared to the uncorrected instrument, and an ultimate spatial resolution below twice the wavelength of the electron at the sample appears to be achievable. In comparison, the highest resolution Tranmission Electron Microscopes have a spatial resolution of about 20 electron wavelengths. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D17.00009: LEEM and STM studies of Ag on Ge (110) Bret Stenger, Cory Mullet, Marshall van Zijll, Emilie Huffman, Dylan Lovinger, Shirley Chiang The growth of Ag deposited on Ge(110) was studied with low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). The LEEM studies showed the formation of long one dimensional islands as Ag was deposited above 430\r{ }C. Island nucleation proceeded from defects in the Ge substrate. During deposition, the length of the islands increased while the width remained constant. The size and distribution of the islands was dependent on the substrate temperatures during deposition. At 480\r{ }C, islands were 100 nm wide and 1-20 $\mu $m long at 9 ML of coverage. At 530\r{ }C, islands were 200nm wide and 1-3 $\mu $m long at 9 ML of coverage. STM images showed that the islands were composed of Ag and that the surface regions between the islands exhibited a reconstruction which is characteristic of pure Ge. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D17.00010: LEEM Observations of Ag/Ge(111) Structural Phases and Phase Transformations Shirley Chiang, Cory Mullet We use low energy electron microscopy (LEEM) to study the growth of, and transformations between structural phases of Ag deposited on Ge(111) above and below the Ag desorption temperature. Ag deposited on Ge(111) forms three main surface phases above 100 \r{ }C: (4x4), ($\surd $3x$\surd $3)R30\r{ }, and (3x1). For deposition between 540-575 \r{ }C, a (3x1) phase grows. Upon the completion of the growth of the (3x1) phase, a ($\surd $3x$\surd $3)R30\r{ } phase grows. For sufficiently high Ag deposition rates, we observed the same growth sequence above the Ag desorption temperature of 575 \r{ }C , up to 640 \r{ }C. Desorption above 575 \r{ }C proceeds with the reverse sequence: the ($\surd $3x$\surd $3)R30\r{ } phase desorbs followed by desorption of the 3x1 phase. Above 640 \r{ }C, we observed the growth of the (3x1) phase but not the ($\surd $3x$\surd $3)R30\r{ }. For 4x4 and ($\surd $3x$\surd $3)R30\r{ } surfaces prepared by deposition between 200-500 \r{ }C we observe the transformation of these phases to a 1x1 disordered phase at the desorption temperature (575 \r{ }C), with desorption proceeding from the edges of disordered 1x1 domains. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D17.00011: Crystalline Structure of the Pb/Si(111)7x7 Stable Wetting Layer M. Gramlich, S.T. Hayden, Yiyao Chen, C. Kim, M.C. Tringides, P.F. Miceli The wetting layer formation in the Pb/Si(111)7x7 system has attracted extensive interest because of anomalously fast kinetics, which enables the formation of quantum size effect (QSE) nanoislands [Jeffrey et al. PRL \textbf{96}, 106105 (2006)]. However, previous studies of the wetting layer by x-ray diffraction and scanning-probes have led to \textit{inconsistent} structural models; thus, the structure of this wetting layer has been unsolved. Furthermore, a recent investigation has revealed that the wetting layer is out-of-equilibrium over a surprisingly broad temperature range [Gramlich et al., PRB \textbf{84}, 075433 (2011)]. Using \textit{in situ} x-ray scattering methods, we have solved the \textit{stable}, low temperature annealed structure of the wetting layer. It exhibits a strained atomic layer where Pb atoms are in transition, from Si-7x7 sites towards 8x8-sites, with some Pb-atoms vertically closer to the Si-7x7. Interestingly, the Si adatoms shift to the edges of the unit cell. Funding is acknowledged from NSF DMR-0706278 (PFM, MWG, STH, YC, and the Ministry of Knowledge Economy of Korea 2009-F014-01 (CK). The experiments were performed on the 6IDC beam line, supported by the US-DOE (through Ames Lab, W-7405-Eng-82), at the Advanced Photon Source (US-DOE, W-31-109-Eng-38) located at Argonne National Laboratory. [Preview Abstract] |
Session D18: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- What is a good conductor for Metamaterials and Plasmonics
Sponsoring Units: DMPChair: Costas Soukoulis, Ames Lab/Iowa State University
Room: 252B
Monday, February 27, 2012 2:30PM - 3:06PM |
D18.00001: Manipulating Plasmons Using Graphene for One-Atom-Thick Optical Signal Processing Invited Speaker: Nader Engheta In this talk, we provide an overview of our recent theoretical work on merging the field of graphene with metamaterials and transformation optics. In particular, we show that the control and variation of graphene's conductivity, spatially and temporally, may offer a one-atom-thick platform for manipulation of optical signals propagating along the single sheet of graphene as the surface Plasmon polariton (SPP) surface waves. Since the phase velocity of the SPP surface wave directly depends on the graphene conductivity, tailoring the conductivity can be regarded as a ``knob'' to control and shape the propagation of such SPP across the graphene. We have shown that such inhomogeneous distribution of conductivity across the graphene gives rise to the notion of one-atom-thick transformation optics and metamaterials---essentially thinnest possible metamaterials [A. Vakil, N. Engheta, \textit{Science}, 332, 1291 (2011)]. Additionally we have demonstrated, theoretically and using computer simulations, that one can achieve optical signal processing functions such as Fourier transforming across a single layer of graphene [A. Vakil, N. Engheta, ``Fourier Optics on Graphene'' \textit{ArXiv} 1108.5218 (2011)]. The graphene-based Fourier optics suggests prospects for multilayer signal processing, allowing for design of ultra-compact nanoscale signal processing systems. We are also investigating other optical manipulation, such as waveguiding, field confinement in cavities, graphene antennas, optical spectrometry, and optical mirror reflection, all on one-atom-thick structures. In this talk, we will present some of our results on these topics. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D18.00002: Graphene, superconductors, and metals: What is a good conductor for metamaterials and plasmonics? Invited Speaker: Philippe Tassin Recent developments in the field of metamaterials and plasmonics have promised a number of exciting applications, in particular at terahertz and optical frequencies. Most metamaterials consist of carefully designed metallic structures that replace atoms in their role as the basic unit of interaction with electromagnetic radiation. Unfortunately, the noble metals are not particularly good conductors at optical frequencies, resulting in significant dissipative loss in metamaterials. In this communication, we address the question of what is a good conductor for use in metamaterials and in plasmonics. We develop a model based on the quasistatic response of the metamaterial constituents to an incident electromagnetic field in order to derive a figure of merit for conductors. We find (1) it is the resistivity of the material-rather than the conductivity or permittivity-that provides direct information on the dissipative loss in the metamaterial, and (2) the dissipative loss depends on certain geometric aspects of the system, such as the layer thickness. Subsequently, we apply the model to graphene, to superconductors (Nb and YBCO), to several noble and transition metals, and to some conducting oxides (like ITO). [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D18.00003: Quantum Plasmonics Reinier Heeres, Leo Kouwenhoven, Valery Zwiller Surface plasmon polaritons allow confinement of light to sub-wavelength length-scales. Due to the confinement the electro-magnetic fields involved are stronger, which can be used to enhance optical interactions. We use this fact to realize a plasmonic beam-splitter based on a directional-coupler geometry, i.e. two waveguides close to each other coupled by their evanescent fields. This beam-splitter can be much smaller than conventional dielectric structures. Integrated Niobium-Nitride superconducting single-photon detectors (SSPDs) allow to probe the plasmons directly in the near-field. This makes it possible to study the structure on the quantum level using photon pairs created in a spontaneous parametric down-conversion process. Our aim is to observe Hong-Ou-Mandel interference, a true quantum effect which causes indistinguishable photons arriving at the same time at both inputs to exit through the same port, i.e. bunch. This will prove the quantum nature of surface plasmons and could be used to build sub-wavelength quantum logic gates. We also show that resonant plasmonic antennas can greatly enhance the absorption and therefore detection efficiency of SSPDs. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D18.00004: Application of metasurface description for multilayered metamaterials and an alternative theory for metamaterial perfect absorber Jiangfeng Zhou, Hou-Tong Chen, Thomas Koschny, Abul Azad, Antoinette Taylor, Costas Soukoulis, John O'Hara Recently, the metamaterial perfect absorber has attracted intense interest in metamaterial community. The impedance matching mechanism based on effective bulk permittivity and permeability is widely used to explain such structures. However, this model has difficulties, in particular, because such systems are usually asymmetric, assigning homogenous effective material parameters to these systems may lead to unphysical results. In our work, we use an effective medium model that treats each layer of the metamaterial as a metasurface with unique effective surface electric and magnetic susceptibility, $\chi_{se}$ and $\chi_{sm}$ . We then use a transfer matrix method to analyze the overall EM properties of multilayered metamaterials using the effective material parameters (surface susceptibilities) of each layer. We find that the functional mechanism is the Fabry-Perot interference resulting from the multiple reflections in the cavity bounded by two metamaterial layers. This contrasts with previous explanations based on bulk effective medium theory. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D18.00005: Active Core-Shell Nanowire Optical Antenna Absorbers Carlos Aspetti, Chang-Hee Cho, Ritesh Agarwal Core-shell dielectric-metal nanoparticles have demonstrated tunability of their absorption properties due to the size- and shape-dependence of the surface plasmon resonance. Recently, the core-shell semiconductor-insulator-metal nanowire was examined as a platform for manipulating the core emitter lifetimes due to the highly confined and intense electromagnetic fields mediated by whispering gallery surface plasmon polariton modes. Combining these two concepts we realize an active semiconductor-insulator-metal optical antenna, which demonstrates a highly tunable absorption spectrum. By directly contacting the semiconductor core, photocurrent data is coupled with simulations to show highly tunable, significant broad-band absorption enhancement; a general result for a range of material systems. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D18.00006: Genetic Optimization of Optical Nanoantennas Carlo Forestiere, Alyssa Pasquale, Antonio Capretti, Sylvanus Lee, Giovanni Miano, Antonello Tamburrino, Luca Dal Negro Metal nanostructures can act as plasmonic nanoantennas (PNAs) due to their unique ability to concentrate the light over sub-wavelength spatial regions. However engineering the optimum PNA in terms of a given quality factor or objective function. We propose a novel design strategy of PNAs by coupling a genetic optimization (GA) tool to the analytical multi-particle Mie theory. The positions and radii of metallic nanosphere clusters are found by requiring maximum electric field enhancement at a given focus point. Within the optimization process we introduced several constraints in order to guarantee the physical realizability of the tailored nanostructure with electron-beam lithography (EBL). Our GA optimization results unveil the central role of the radiative coupling in the design of PNA and open up new exciting pathways in the engineering of metal nanostructures. Samples were fabricated using techniques and surface-enhancement Raman scattering measures were performed confirming the theoretical predictions. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D18.00007: Plasmonic Anti-Hermitian Coupling for Nano-Manipulation of Light Ziliang Ye, Shaung Zhang, Yuang Wang, Yongshik Park, Guy Bartal, Xiaobo Yin, Xiang Zhang Open quantum systems consisting of coupled bound and continuum states have been studied in a variety of physical systems. In these systems, the effects of the continuum decay channels are accounted for by indirect anti-Hermitian couplings among the bound states. Here we propose a general scheme to control light in a nano-plasmonic system by utilizing the anti-Hermitian coupling between the individually designed resonances of each plasmonic element in the system. As a specific example, we experimentally show a realistic coupled plasmonic dipole antenna array with $\lambda$/15 separations, in which selective excitation of an individual antenna can be achieved by tuning the frequency of the incident light. Without the anti-Hermitian coupling, these antennas are indistinguishable from each other. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D18.00008: Broadband scattering reduction using a hybrid inertial metamaterial design Theodore Martin, Gregory Orris The ability to hide an object from an external wave through scattering reduction is one of the most sought-after goals of the metamaterials community. Using transformational optics, Pendry [1] demonstrated that a wave can be bent around an object using a conformal map that reduces the object's scattering cross section to zero. The transformational method has now been extended to transformational acoustics [2], but the traditional inertial method requires infinite mass at the boundary of the hidden object, which cannot be easily approximated in practice. Scattering reduction can also be obtained over a more limited bandwidth using wrapping layers that cancel some of the modal coupling between the object and the exterior environment [3]. Using multiple scattering theory, we demonstrate that a combination of an ``imperfect'' conformal map with a scattering cancellation layer can achieve improved scattering reduction over a broad bandwidth in an aqueous acoustic environment. Our ``hybrid'' design is amenable to a parameter-space constrained to within an order of magnitude of the background fluid in order to obtain a solution with realistic material properties. The introduction of a cancellation layer enables us to optimize performance over targeted frequency bands with only a small impact on the overall size of the design. \\[4pt] [1] J. B. Pendry, D. Schurig, and D. R. Smith, \textit{Science} \textbf{312}, 1780 (2006). \\[0pt] [2] S. A. Cummer and D. Schurig, \textit{New J. Phys.} \textbf{9}, 45 (2007). \\[0pt] [3] A. Al\`{u} and N. Engheta, \textit{Phys. Rev. Lett.} \textbf{100}, 113901 (2008). [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D18.00009: Surface plasmon excitations in multicoaxial metamaterial cables: Zero magnetic field Bahram Djafari-Rouhani, Manvir Kushwaha By using an elegant response function theory, which does not require matching of the messy boundary conditions, we investigate the surface plasmon excitations in the multicoaxial cylindrical cables made up of negative-index metamaterials in the absence of an applied magnetic field. The multicoaxial cables with {\em dispersive} metamaterial components exhibit rather richer (and complex) plasmon spectrum with each interface supporting two modes: one TM and the other TE for (the integer order of the Bessel function) $m \ne 0$. The cables with {\em nondispersive} metamaterial components bear a different tale: they do not support simultaneously both TM and TE modes over the whole range of propagation vector. The computed local and total density of states enable us to substantiate spatial positions of the modes in the spectrum.\footnote{M.S. Kushwaha and B. Djafari-Rouhani, J. Opt. Soc. Am B {\bf 27}, 605 (2010).} Such quasi-one dimensional systems as studied here should prove to be the milestones of the emerging optoelectronics and telecommunications systems. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D18.00010: Surface Plasmon Based Engineering of Semiconductor Nanowire Optics Chang-Hee Cho, Carlos O. Aspetti, Michael E. Turk, James M. Kikkawa, Sung-Wook Nam, Ritesh Agarwal Emission from unthermalized (hot) excitons can be observed from high-quality crystals and quantum-well structures due to decreases in the exciton lifetimes but typically with low yields. By employing a plasmonic nanocavity, we observe efficient hot-exciton emission in core-shell CdS-SiO$_{2}$-Ag nanowires with intensities surpassing those from thermalized excitons [1]. These new spectral characteristics are mediated by whispering gallery plasmonic modes that yield highly intense electromagnetic fields. As a result, the exciton radiative lifetime is decreased by several orders of magnitude. The introduction of a high-quality hybrid plasmonic nanocavity structure significantly changes the photophysics of the host material, demonstrating an approach applicable to other material systems. \\[4pt] [1] Chang-Hee Cho, \textit{et al}, Nature Materials, \textbf{10}, 669 (2011). [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D18.00011: Plasmonic nanostructures for multiscale light amplification Aiqing Chen, Ulrich Welp, Vitalii Vlasko-Vlasov, Ryan Miller, Elena Shevchenko, Leonidas Ocola, Stephen Gray, Eugene DePrinceIII We demonstrate experimentally a multiscale plasmonic design for giant light amplification using gold nanoparticles self-assembled in gratings on a metal mirror with thin dielectric spacer. The successive increase of the light enhancement factors upon addition of individual plasmonic elements in the design is tested by measurement of the Raman signal from R6G and benzenethiol molecules on clusters of nanoparticles, their ordered arrays on dielectric, semiconducting, and metal substrates, and on metal substrate with gratings. High fidelity of our structures as SERS substrates are confirmed by areal maps of the Raman response. FDTD numerical calculations are in a good agreement with our experimental measurements. [Preview Abstract] |
Session D19: Invited Session: Novel Phases in Strongly Correlated Iridates
Sponsoring Units: DCMP DMPChair: Gang Cao, University of Kentucky
Room: 253AB
Monday, February 27, 2012 2:30PM - 3:06PM |
D19.00001: Spin orbit coupling, electron correlations and exotic magnetism in 5d complex Ir oxides Invited Speaker: Hidenori Takagi In 5d Iridium oxides, a large spin-orbit coupling of $\sim $ 0.5 eV, inherent to heavy 5d elements, is not small as compared with the width of d bands and often modifies the landscape of the electronic structure substantially. This is distinct from those of 3d transition metal oxides and gives rise to a variety of novel electronic phases. Layered Ir$^{4+}$ perovskite Sr$_{2}$IrO$_{4}$ is recently revealed to be a novel J$_{eff}$=1/2 Mott insulator [1,2], where even a moderate Coulomb U can open up a correlation gap because of the large spin-orbit coupling. In the three dimensional analogue of Sr$_{2}$IrO$_{4}$, SrIrO$_{3, }$ the large spin-orbit coupling manifests itself in a contrasted way, where the interplay of strong spin-orbit coupling and lattice distortions brings the system almost to a band insulator. SrIrO$_{3}$ is in fact a very low carrier density semimetal with unusual transport and magnetic properties. If J$_{eff}$=1/2 Ir$^{4+}$ is placed on a honeycomb lattice or a geometrically frustrated lattice such as pyrochlore lattice, even more exotic states might be anticipated, including a correlated topological insulator [3] and a Kiteav magnet [4]. Our attempt to explore such spin-orbit coupling induced states will be reported.\\[4pt] Work done in collaboration with T. Takayama, B.J.Kim, S.Fujiyama, K.Ohashi, J.Matsuno, H.Osumi and T.Arima. \\[4pt] [1] B.J.Kim et al., Phys Rev Lett 101, 076402 (2008). \\[0pt] [2] B. J. Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H. Takagi, and T. Arima, Science 323, 1329 (2009). \\[0pt] [3] Shitade et al., Phys. Rev. Lett. 102, 256403 (2009). \\[0pt] [4] J. Chaloupka, G.Jackeli, and G.Khaliullin, Phys. Rev. Lett. 105, 027204 (2010) [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D19.00002: Topological and spin-liquid phases in strongly correlated iridates Invited Speaker: Yong Baek Kim Recently, the emergence of topological phases in interacting electron systems such as topological insulators and spin liquid phases, has been a subject of intensive research. In particular, much attention has been given to 5d transition metal oxides where the strong spin-orbit coupling and intermediate strength of the electron interaction provide an ideal playground for the emergence of a number of interesting topological phases. We summarize recent theoretical efforts in this direction in the context of Iridates, or iridium oxides. We make connections to the existing and future experiments on a variety of iridates materials including pyrochlore iridates, honeycomb-lattice iridates, and hyperkagome-lattice systems. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D19.00003: Possible proximity of the Iridates (Na,Li)$_2$IrO$_3$ to a topologically ordered Mott insulator: Phase diagram of the Heisenberg-Kitaev model Invited Speaker: Simon Trebst Motivated by the recent experimental observation of a Mott insulating state for the layered Iridates (Na,Li)$_2$IrO$_3$, we discuss possible ordering states of the effective Iridium moments taking into account the extreme sensitivity of these 5d transition metal oxides to crystal field effects and strong spin-orbit coupling. The microscopic exchange has been argued [1,2] to be a combination of isotropic Heisenberg and highly anisotropic Kitaev exchange, which can be tracked back to the spin and orbital components of the effective momenta. Depending on the relative strength of these two couplings, the system exhibits either various types of conventional magnetic order or a more exotic gapless spin-liquid ground state. Carefully studying [3] the stability of these phases at finite-temperatures -- and the role of frustration, i.e. a considerable suppression of the ordering temperature from the Curie-Weiss temperature -- allows us to connect back to thermodynamic experiments [4] on the Iridates (Na,Li)$_2$IrO$_3$ and possibly estimate microscopic coupling parameters. Finally, we discuss the effects of a magnetic field applied in the [111] direction -- perpendicular to the hexagonal lattice formed by the Iridium moments -- and show that a topologically ordered ground state is found over a small range of coupling parameters [5], also indicating the existence of an exotic critical point whose location might not be far from actual material parameters.\\[4pt] Work done in collaboration with H.C. Jiang, Z.C. Gu, X.L. Qi, J. Reuther, and R. Thomale.\\[4pt] [1] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205 (2009). \\[0pt] [2] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010). \\[0pt] [3] J. Reuther, R. Thomale, and S. Trebst, Phys. Rev. B 84, 100406(R) (2011).\\[0pt] [4] Y. Singh, S. Manni, and P. Gegenwart, arXiv:1106.0429\\[0pt] [5] H.C. Jiang, Z.C. Gu, X.L. Qi, and S. Trebst, Phys. Rev. B 83, 245104 (2011). [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D19.00004: Honeycomb lattice spin-orbit Mott insulators Invited Speaker: Philipp Gegenwart Iridates displaying a Mott insulating state caused by the interplay of electronic correlations and strong spin-orbit coupling have recently attracted considerable attention. We focus on the honeycomb material A$_{2}$IrO$_{3}$ (A=Na, Li), in which the topology of the underlying lattice leads to interesting magnetic properties [1]. The strong spin-orbit coupling in this 5d transition metal system is expected to result in orbital-dependent highly anisotropic magnetic in-plane exchange [2]. The combination of J$_{eff}$ = 1/2 and the underlying honeycomb lattice makes A$_{2}$IrO$_{3}$ a promising candidate for the Kitaev model, which is exactly solvable and has a spin-liquid ground state. Our experimental data on Na$_{2}$IrO$_{3}$ prove a Mott insulating state of effective J=1/2 moments with predominant antiferromagnetic coupling, indicated by a Weiss temperature of $\theta =-$120 K. A bulk antiferromagnetic transition occurs at a much reduced temperature of T$_{N}$ = 15 K and the reduced magnetic entropy suggests strong magnetic frustration and/or low-dimensional magnetic interactions. The nature of the ordered phase has also been studied by resonant x-ray spectroscopy near the Ir-L3 edge, providing evidence for an unconventional, most-likely zig-zag-type spin ordering [3]. The latter may be related to next-nearest neighbour exchange and/or a substantial Kitaev contribution in the Heisenberg-Kitaev model [2]. Upon replacing Na with the smaller Li, one may enhance the relative importance of the Kitaev contribution. For Mott insulating Li$_{2}$IrO$_{3}$ we observe a similar ordering temperature of 15 K, while the negative Weiss temperature is drastically reduced. These observations are compatible with an enhancement of the Kitaev contribution compared to the Na-system, suggesting that Li$_{2}$IrO$_{3}$ could be located close to the Kitaev limit [5]. \\[4pt] [1] Yogesh Singh and P. Gegenwart, Phys. Rev. B. 82, 064412 (2010). \\[0pt] [2] G. Jackeli and G. Khaliullin, Phys. Rev. Lett. 102, 017205, (2009). \\[0pt] [3] X. Liu et al., Phys. Rev. B 83, 220403(R) (2011). \\[0pt] [4] Yogesh Singh, S. Manni, P. Gegenwart, arXiv:1106.0429v1. \\[0pt] [5] J. Reuther, R. Thomale, S. Trebst, Phys. Rev. B 84, 100406 (2011). [Preview Abstract] |
Session D20: Invited Session: Advanced Electromagnetic Imaging and Remote Sensing: From DC to Daylight
Sponsoring Units: FIAPChair: Peter Weichman, BAE Systems
Room: 253C
Monday, February 27, 2012 2:30PM - 3:06PM |
D20.00001: Advanced EM Modeling in Support of Buried Target Detection and Identification Invited Speaker: Eugene Lavely The detection and discrimination of buried unexploded ordnance is essential for restoration of millions of acres of munitions testing grounds to public use. Public safety is paramount so high detection probability is essential, and accurate discrimination is key for economically feasible remediation. Time domain electromagnetic (TDEM) induction methods in principle have the capability to address these criteria. The transmitter loop current pulse generates a magnetic field in the target region. This changing applied field, especially as the pulse terminates, induces currents in the target, generating a scattered magnetic field. The decaying scattered field, following pulse termination, induces the measured voltage in the receiver loop. There are three different regimes that one may identify in the voltage time traces: early, intermediate, and late time. At very early time, immediately following pulse termination, the currents are confined to the immediate surface of the target. The initial diffusion of these currents into the target interior leads to a 1/2 power law decay for nonferrous targets, (3/2 for ferrous targets). At intermediate time, as the currents penetrate the deeper target interior, the power law crosses over to a multi-exponential decay, representing the simultaneous presence of a finite set of exponentially decaying modes. Finally, at late time only the single, slowest decaying mode survives. At intermediate- to late-time our mean field algorithm models the dynamics by computing as large a number as possible of the modes, and determining the excitation level of each. At early time, the power law arises from a superposition of an essentially infinite number of exponentials, and a complementary theory, based on the detailed dynamics of the initial very thin surface current sheet, has been developed instead. We pre-compute intrinsic properties of modes containing all discrimination information of targets, and use this for nearly real-time inversion of TDEM measurement for target properties and subsequent discrimination. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D20.00002: Through-wall microwave imaging: some applications of physics to urban reconnaissance Invited Speaker: Peter Weichman I will review some recent physics-based modeling approaches in support of microwave through-wall building tomography. Building layout estimation is a nonlinear inverse problem with a large number of degrees of freedom (geometry, location, and scattering properties of major building elements, such as walls, floors, and ceilings, plus many other smaller elements such as windows, doorways, and stairways). The physics of microwave propagation in such environments is very complex, involving multiple reflection, transmission, and diffraction events. Careful control of measurement protocol, using well-focused and directed transmitter and receiver arrays, can mitigate this to some degree. However, even under the most optimistic scenarios, the number of interactions increases exponentially as the signal penetrates more deeply into the building. Multiple overlapping returns from different building elements quickly overwhelm one's ability to disambiguate their sources. To explore the fundamental limitations on solutions to the inverse problem, efforts to create physics-based models that capture the signal complexity as accurately as possible will be described. These models remain an approximate description of reality, but nevertheless enable one to understand the effects of the explosion of multiple scattering events on the inversion, and quantify the limits of the inversion quality under even the most optimistic scenarios for data diversity and precision. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D20.00003: Advanced methods in synthetic aperture radar imaging Invited Speaker: Thomas Kragh For over 50 years our world has been mapped and measured with synthetic aperture radar (SAR). A SAR system operates by transmitting a series of wideband radio-frequency pulses towards the ground and recording the resulting backscattered electromagnetic waves as the system travels along some one-dimensional trajectory. By coherently processing the recorded backscatter over this extended aperture, one can form a high-resolution 2D intensity map of the ground reflectivity, which we call a SAR image. The trajectory, or synthetic aperture, is achieved by mounting the radar on an aircraft, spacecraft, or even on the roof of a car traveling down the road, and allows for a diverse set of applications and measurement techniques for remote sensing applications. It is quite remarkable that the sub-centimeter positioning precision and sub-nanosecond timing precision required to make this work properly can in fact be achieved under such real-world, often turbulent, vibrationally intensive conditions. Although the basic principles behind SAR imaging and interferometry have been known for decades, in recent years an explosion of data exploitation techniques enabled by ever-faster computational horsepower have enabled some remarkable advances. Although SAR images are often viewed as simple intensity maps of ground reflectivity, SAR is also an exquisitely sensitive coherent imaging modality with a wealth of information buried within the phase information in the image. Some of the examples featured in this presentation will include: (1) Interferometric SAR, where by comparing the difference in phase between two SAR images one can measure subtle changes in ground topography at the wavelength scale. (2) Change detection, in which carefully geolocated images formed from two different passes are compared. (3) Multi-pass 3D SAR tomography, where multiple trajectories can be used to form 3D images. (4) Moving Target Indication (MTI), in which Doppler effects allow one to detect and geolocate moving targets within SAR images. (5) Real time video SAR, where one forms a continuously updated SAR image by ``staring'' at an area of interest. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D20.00004: Multiple-input Multiple-output Ground Moving Target Indicator Radar: Theory and Practice Invited Speaker: Dan Bliss Multiple-input multiple-output (MIMO) extensions to radar systems enable a number of advantages compared to traditional approaches. These advantages include improved angle estimation and target detection. In this paper, an overview of MIMO radar is provided, and the concept of coherent MIMO radar is defined. The principle focus of the paper is the discussion of MIMO ground moving target indication (GMTI). For GMTI radar modes, the advantages of a coherent MIMO architecture include improved angle estimation and enhanced slow speed target detection. To illustrate this, the concept of coherent MIMO radar is introduced and performance comparisons made between MIMO GMTI and traditional radar GMTI. These comparisons are supported by theoretical bounds, simulations, and experimental results for GMTI angle estimation accuracy and minimum detectable target velocity. For some applications, these results indicate significant potential improvements in clutter-mitigation, signal-to-noise ratio (SNR) loss, and reduction in angle-estimation error for slow-moving targets. The important effects of waveform characteristics is addressed. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D20.00005: Image Formation in Bio-optical Sensing Invited Speaker: Eric Miller Over the past two decades a number of optical sensing methods have emerged with potential to provide complementary information to traditional medical imaging modalities in application areas ranging from basic science to disease diagnosis and treatment monitoring. Though still largely in the research and development stage, modalities including diffuse optical tomography (DOT), fluorescence molecular tomography (FMT), photo-acoustic tomography (PAT), and bio-luminescence tomography (BLT) have excited much interest due to their natural functional imaging capability, their relatively low cost, and the fact that none required the use of ionizing radiation. These advantages however are tempered by a number of challenges associated with the processing of these data. Specifically, these data types all rely in one way or another on the interaction of light with tissue. The diffusive nature of this interaction inherently limits the spatial resolution of these modalities. As a result the process of forming an image is a far more delicate task than is the case with more standard imaging modalities such as X-ray computed tomography (CT). Two basic methods have been explored to address the ill-posedness of these problems in order to improve the information content in the resulting images. The optical data may be augmented either through the use of spectral diversity or by attempting to integrate optical data types with information from other modalities such as CT or MRI. Alternatively, a mathematical technique known as regularization can be used to impose physically-based constraints on the reconstruction. In this talk, I shall provide an overview of the work in my group in optical image formation within the contexts of DOT for breast cancer imaging and FMT for small animal imaging. The focus of the talk will be on methods that integrate data augmentation and mathematical regularization. In the case of FMT, we shall discuss our work in combining the optical data with information provided by CT concerning the structural distribution of tissue classes within the region of interest. Here, we have developed a number of spatially-varying regularization methods which use the CT data to help constrain the FMT reconstruction and obtain imaging results that are substantially improved over classical regularization techniques. For DOT, we have recently been considering the use of hyperspectral data sets in which information from over 100 near infrared wavelengths is made available to the processing. When combined with a regularization scheme based on parameterizing the images in a geometric manner, we believe that it will be possible to produce a standalone DOT system with spatial resolution that is today only achieved by combining DOT with e.g., CT. [Preview Abstract] |
Session D21: Multiband Superconductivity, mostly MgB2
Sponsoring Units: DCMPChair: Morten Ring Eskildsen, University of Notre Dame
Room: 254A
Monday, February 27, 2012 2:30PM - 2:42PM |
D21.00001: STS studies of the pi-band superconductivity in MgB$_2$ in a transverse field C. Griggs, M.R. Eskildsen, N.D. Zhigadlo, J. Karpinski Since being discovered MgB$_2$ has become the paradigm for two-band/two-gap superconductivity. Early scanning tunneling spectroscopy (STS) measurements, showed a rapid suppression of the superconductivty in the isotropic $\pi$-band for modest applied fields $H \parallel c$. These measurements were performed with the tunnel current ($I_t$) parallel to the crystalline $c$-axis which couple, almost exclusively, to the $\pi$-band, and with the suppression attributed to vortex core overlap. Here we report STS measurements performed in a transverse field, such that $I_t \parallel c \perp H$. In this configuration no vortices are cutting through the image plane, and instead the superconducting phase is affected by the Meissner currents running within one penetration depth of the sample surface. Within this field orientation we observe far less suppression of the superconducting state in the $\pi$-band compared to the earlier measurements with $H \parallel c$. A clear gap is seen up to $H= 0.9$~T. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D21.00002: Measurement of penetration depth of MgB$_{2}$ using DC SQUID Daniel Cunnane, Ke Chen, X.X. Xi Penetration depth is an important parameter for the design of Rapid Single Flux Quantum (RSFQ) circuits. For MgB$_{2}$, different values from 40 nm to 150 nm have been reported by different groups. We have measured the penetration depth of MgB$_{2}$ using MgB$_{2}$ DC SQUIDs. The SQUID was made using MgB$_{2}$/MgO/MgB$_{2}$ Josephson junctions with epitaxial MgB$_{2}$ electrodes deposited via Hybrid Physical-Chemical Vapor Deposition. The MgO was deposited by RF Magnetron Sputtering of an MgO target. The device shows good voltage modulation above 150 $\mu$V. The working temperature for these SQUIDs ranges from below 10K to 37K, with the optimum voltage modulation near 35K. The results will be discussed in comparison to the penetration depth reported by other measurement techniques. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D21.00003: Effect of scattering on the gap distribution in tunneling spectra of MgB$_{2}$/I/Pb junctions Wenqing Dai, Qi Li, Ke Chen, Xiaoxing Xi We fabricated MgB$_{2}$/Native oxide/Pb planar tunneling junctions using the MgB$_{2}$ films grown by the hybrid physical-chemical vapor deposition technique (HPCVD). Both $\pi $ ($\sim $1.8 meV) and $\sigma $ gaps ($\sim $7.9 meV) were observed in spectra due to tunneling from side of $c$-oriented MgB$_{2}$ grains on film surface on SiC (0001) substrates. We have previously observed a distribution of energy gap values within both bands in clean HPCVD films with long electron mean free path. Here we report an investigation of the tunneling spectra by systematically varying the MgB$_{2}$ film thickness and the thickness of native oxide barrier. We found the MgB$_{2} \quad \pi $ gap distribution range narrows from $\sim $1.7 meV to $\sim $1.4 meV together with a loss of fine peak structures, as the MgB$_{2}$ film thickness decreases from 100 nm to 33 nm, in which the electron mean free path is limited by the thickness. In addition, fine peak structures also smear out when the junction resistance is large. The results show electron scattering from both inside the film and on the surface can smear out the gap distribution structures in tunneling spectra. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D21.00004: Vortex Lattice Transition Dynamics in MgB$_2$ C. Rastovski, P. Das, K. Schlesinger, M.R. Eskildsen, W.J. Gannon, C.D. Dewhurst, N.D. Zhigadlo, J. Karpinski We present small-angle neutron scattering (SANS) studies of the vortex lattice (VL) in MgB$_{2}$ with $H \parallel c$. This material has three different VL phases, all with triangular symmetry but oriented differently with respect to the crystalline axes. Furthermore, a high degree of metastability between the VL phases of MgB$_{2}$ has been observed as the sample is cooled or heated across the equilibrium phase transitions. Here we present detailed studies of how the metastable (MS) VL phases transition to the ground state (GS), either driven by small changes of the DC magnetic field or by a transverse AC field. Our results show that the MS VL is not due to vortex pinning, and results are inconsistent with predictions based on the Bean model. Instead, we speculate that a ``jamming'' of counter rotated VL domains is responsible for the VL metastability. This is further supported by a power law dependence of the GS VL domain population upon the number of applied AC cycles. This work was supported by the Department of Energy, Basic Energy Sciences under Award No. DE-FG02-10ER46783. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D21.00005: Non-linear Time Effects in Superconducting Polycrystalline MgB$_{2}$ Murat Olutas, Atilgan Altinkok, Atilla Kilic, Kivilcim Kilic The time evolution of the voltage response (V-t curves) to a symmetric bi-directional square wave (BSW) current with long periods (P) applied to the MgB$_{2}$ sample was investigated at low dissipation levels. It was observed that regular sinusoidal-like voltage oscillations evolve at certain ranges of the amplitude (I$_{BSW})$ and period (P$_{I})$ of the BSW current, external magnetic field (H), and temperature (T). The regular sinusoidal-like oscillations were interpreted in terms of the dynamic competition between pinning and depinning processes. In this process, the interaction between flux lines and pinning centers could be elastic coupling because of rigidity of flux line lattice in MgB$_{2}$. We suggest that the oscillating mode can be related to the drifting of flux lines which are in motion. In this case, due to the strong pinning and complex relaxation effects, the majority of flux lines traversing the sample does not leave it entirely and remains in an oscillating mode by moving forth and back as a function of time. We suggest that the flux dynamics associated with the oscillating mode is the ordered motion of defective flux line system. Fast Fourier Transform analysis of V-t oscillations showed that the period of oscillations is comparable to that of the BSW current. The oscillating mode was also discussed in terms of sliding charge density waves (CDWs). [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D21.00006: Study of MgB$_{2}$ Films for RF Cavity Applications Teng Tan, Chenggang Zhuang, Alex Krick, Ke Chen, Xiaoxing Xi Magnesium diboride (MgB$_{2})$ is a promising superconducting material for RF cavity applications due to its high critical temperature $T_{c}$ and large thermodynamic critical field $H_{c}$. Using Hybrid Physical-Chemical Vapor Deposition (HPCVD), we have grown 2"-diameter MgB$_{2}$ films on sapphire and metal substrates, including molybdenum, niobium, tantalum, and stainless steel. Measured by DC magnetization, the $T_{c}$'s of these films were between 38.2 to 39.2 K; the upper critical field $H_{c2}$'s were about 7 T, in consistent with previously reported value of clean MgB$_{2}$ films; the zero-field critical current density $J_{c}$'s were above 10$^{7}$ A/cm$^{2}$ and were suppressed rapidly by increasing applied magnetic field, indicating a lack of pinning in clean MgB$_{2}$ films. Multilayered MgB$_{2}$/MgO films were also investigated to prevent vortex penetrating the MgB$_{2}$ layer and increase the vortex penetration field (H$_{c1})$ following Gurevich's theoretical work [1]. The RF properties of these films were studied. \\[4pt] [1] A. Gurevich, \textit{Appl. Phys. Lett.} 88, 012511 (2006). [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D21.00007: Mesoscopic Nonlinear Electrodynamic Response of the Two Band Superconductor MgB$_{2}$ Tamin Tai, Teng Tan, Xiaoxing Xi, Behnood Ghamsari, Steven Anlage Multi-gap superconductors are expected to have exotic physics associated with internal coupling between the multiple order parameters. 10 years after its discovery, MgB$_{2}$ is a two gap superconductor that still attracts much attention for both fundamental and practical reasons. A microwave measurement of the temperature dependent third harmonic response P$_{3f}$ (T) is performed by applying a strong and localized (mesoscopic) RF magnetic field on high quality MgB$_{2}$ films. Comparing to the P$_{3f}$ (T) result on Nb thin films, the low temperature nonlinearity of MgB$_{2}$ shows a complete harmonic cancelation at a temperature near the transition temperature of the proximity-induced $\pi $ band. One possibility is that this phenomenon is due to an additional intrinsic nonlinearity arising from Josephson coupling between the $\sigma $ and $\pi $ bands. This nonlinear response then interferes with other nonlinear mechanisms coming from moving vortices and the intrinsic nonlinear Meissner effect of the two-gap system. To investigate further, we measure the magnitude and phase of the nonlinear response and compare to models of the three nonlinearity mechanisms. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D21.00008: Superconducting-to-Normal Switching and I-V Experiments showing Multiple Tunneling Channels in a Multi-Gap Superconductor Steve Carabello, Joseph Lambert, Jerome Mlack, Wenqing Dai, Yi. Shen, Qi Li, Daniel Cunnane, C.G. Zhuang, Ke Chen, X.X. Xi, Roberto Ramos Magnesium diboride is a BCS superconductor with many interesting properties, notably its two superconducting energy gaps associated with the disconnected sheets of its Fermi surface. A heterojunction, using the dual-gap MgB$_{2}$ and a single-gap superconductor as its superconducting electrodes, provides a system well-suited for exploring the unique properties of MgB$_{2}$, by exhibiting multiple tunneling channels. We present data from superconducting-to-normal state switching experiments and tunneling spectroscopy experiments to temperatures as low as 20mK that indicate multiple tunneling channels in this multiple-gap superconductor. We describe features of escape rates and I-V curves that may be consistent with recent published theoretical work on macroscopic quantum tunneling in multi-gap superconductors. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D21.00009: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 4:18PM - 4:30PM |
D21.00010: Multilayer Structures in MgB2 superconducting thin films: influence on doping, pinning, and connectivity Mike Sumption, Mike Susner, Scot bohnenstiehl, Ted Collings Pinning force density, connectivity, and doping were investigated for MgB$_{2}$ superconducting thin films. Pulsed Laser Deposition (PLD) was used to produce MgB$_{2}$ thin films on Al$_{2}$O$_{3}$ (0001), MgO (111), YSZ (111), and SiC (0001) substrates. The MgB$_{2}$ target was manufactured through high pressure and high temperature induction heating that produced a highly dense material. ZrB$_{2}$, SiC, or C targets were alternated with the MgB$_{2}$ target to produce films with various levels of doping and/or pinning centers. X-ray diffraction (XRD) was used to determine the substitution, strain, and epitaxy. Surface microstructures and grain sizes were compared via scanning electron microscopy (SEM). Transmission electron microscopy (TEM) was employed on ion-milled samples representing cross-sections of the film to determine the oxide content of the films as well as to confirm if the added dopants modified the superconducting properties of the MgB$_{2}$ via atomic substitution or increases in strain. Normal-state resistivity measurements were taken on the films to determine the connectivity as compared to literature single crystal values. Magnetic $J_{c}$s were taken of the films to determine the influence of the microstructure on the pinning properties. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D21.00011: Effect of Carbon Nanotubes on Transport Properties of MgB$_{2}$ Derek Caplinger, Danhao Ma, Ruwantha Jayasingha, Kofi Adu, Gamini Sumanasekera Carbon nanotubes (CNT) and magnesium diboride (MgB$_{2})$ are two fascinating materials; with CNT exhibiting unique quantum electrical properties due to its 1-D structure and MgB$_{2}$ being a superconducting with transition temperature (T$_{c})$ at $\sim $40 K. We report preliminary results on the effect of carbon nanotubes on the temperature dependent (300K to 4K) thermoelectric power (TEP) and resistivity of MgB$_{2}$. The normalized resistance (R-R$_{300})$/R$_{300}$ shows very little dependence on the sample composition; however, the absolute resistance increases with increasing CNT concentration. At high CNT composition, the TEP mimic the characteristic TEP of CNT. However at low CNT composition, we see an upturn at about 30K. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D21.00012: Applicability of two-component Ginzburg-Landau model in two-band systems Mihail Silaev, Egor Babaev We report a microscopic derivation of the conditions under which the two-band superconductors can be described by a two-component Ginzburg-Landau (GL) field theory. We also investigate the conditions when multicomponent GL-like expansions fail and one should resort to a microscopic description. We show that besides being directly applicable at elevated temperatures, a version of a minimal two-component GL theory in certain cases also gives an unexpectedly accurate description of certain aspects of a two-band system even substantially far from $T_c$. This shows that two-component GL model can be used for addressing a wide range of questions in multiband systems. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D21.00013: Inter-band scattering effects in the Chevrel phase superconductors Anson Cheung Recent scanning tunneling spectroscopy on the Chevrel phases SnMo$_6$S$_8$ and PbMo$_6$S$_8$ by Petrovi\'{c} et al.~(PRL \textbf{106}, 017003 (2011)) demonstrates clear signatures of multi-band superconductivity. In contrast with MgB$_2$, where there is extremely weak scattering between the $\sigma$ and $\pi$ bands, the inter-band scattering rate is relatively high compared to the intra-band scattering rate in the Chevrel phases. We calculate the quasi-classical Green's function in the presence of strong inter-band scattering and derive comparable results to the measured density of states. We make predictions for proposed measurements on the vortex cores. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D21.00014: Collective modes in three-band superconductors with repulsive interband interactions Valentin Stanev I consider a simple model of a three-band superconductor with repulsive interband interactions. In such a system frustration associated with the odd number of gaps leads to the possible existence of intrinsically complex time-reversal symmetry breaking (TRSB) order parameter. I show that in this state the fluctuations of the \emph{different} gaps are strongly coupled and this leads to the development of novel excitations, which mix the phase and amplitude oscillations. This is due to the non-trivial relative phase angle between the gaps. The energy of these excitations is less than $2 \Delta$ and thus they are true collective modes of the system. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D21.00015: Cryomagnetic STM imaging of the subsurface vortex lattice in the multiband superconductor $2H$-NbSe$_2$ Igor Fridman, V. Lukic, C. Kloc, C. Petrovic, J.Y.T. Wei Using cryomagnetic scanning tunneling microscopy (STM) in a novel field geometry, we studied single crystals of the multiband superconductor $2H$-NbSe$_{2}$ under diamagnetically-induced superfluid momentum. Spectroscopy and conductance imaging were performed at 300 mK and in a field of up to 9 T, applied in the $ab$-plane.[1] Spatial maps of the Doppler effect on the quasiparticle tunneling spectrum revealed distinct stripe patterns that originate from in-plane vortices whose cores are buried in the bulk.[2] The stripe separation varies systematically as a function of the applied field. Our results are interpreted in terms of the interaction between vortical and screening currents, and demonstrate a general method for probing subsurface vortices, especially in emerging multiband superconductors such as the ferro-pnictides/chalcogenides.\\[4pt] [1] I. Fridman \emph{et al.}, arXiv:1110.6490 (2011) \\[0pt] [2] I. Fridman \emph{et al.}, Appl. Phys. Lett. \textbf{99}, 192505 (2011) [Preview Abstract] |
Session D22: Focus Session: Fe-based Superconductors - Crystal Growth, Structure, and Properties of K_xFe_1-ySe Phases
Sponsoring Units: DMP DCOMPChair: Nicholas Butch, Lawrence Livermore National Laboratory
Room: 254B
Monday, February 27, 2012 2:30PM - 3:06PM |
D22.00001: Superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ Invited Speaker: Cedomir Petrovic Single crystal alloys KxFe$_{2-y}$Se$_{2-z}$S$_{z}$ offer valuable insight into the strength of electronic correlations in the normal state and structural characteristics associated with superconductivity. I will discuss the evolution of the superconducting and magnetic ground states as a function of sulfur concentration $z$ and some noticeable changes in the average and local crystal structure associated with this [1-4]. Conductivity and magnetic properties coincide with stoichiometry changes and with particular local environment of Fe atoms on the two Fe sites in the crystal structure. The ratio of superconducting T$_{c}$ and Fermi temperature T$_{F}$ is also suppressed by sulfur doping, indicating the suppression of electronic correlations. The superconductivity persists with relatively high T$_{c}$ even when electronic correlations in the normal state are greatly reduced. The results for z = 0 will be compared with other experimental techniques that probe nanoscale phase separation and degree of vacancy order [5-6]. It will be shown that local structure and population of particular Fe sites is rather important for obtaining the bulk superconducting phase. Superconducting volume fraction and homogeneity of superconducting phase is in direct competition with Fe vacancy order [7].\\[4pt] [1] Hechang Lei et al., Phys. Rev. Lett. 107, 137002 (2011)\\[0pt] [2] Hechang Lei et al., Phys. Rev. B 83, 180503 (2011)\\[0pt] [3] Kefeng Wang et al., Phys. Rev. B 174503 (2011)\\[0pt] [4] Kefeng Wang et al., Phys. Rev. B 84, 054526 (2011)\\[0pt] [5] Z. Wang et al., Phys. Rev. B 83, 140505 (2011)\\[0pt] [6] Y. J. Yan et al., arXiv:1104.4941 (2011)\\[0pt] [7] Hyejin Ryu et al., arXiv:1111.2597. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D22.00002: Crystal growth and detailed structural characterization of superconducting and non-superconducting phases in the K$_{1-x}$Fe$_{2-y}$Se$_2$ system Daniel Shoemaker, Duck Young Chung, Melanie Francisco, Helmut Claus, Sevda Avci, Anna Llobet, Hefei Hu, Jian-Min Zuo, Mercouri Kanatzidis Amid the flurry of activity on K$_{1-x}$Fe$_{2-y}$Se$_2$ superconductors, it remains established that the stoichiometric compound K$_2$Fe$_4$Se$_5$ is an antiferromagnetic semiconductor. This raises the question of whether subtle Fe$^{1+/3+}$ doping causes K$_{1-x}$Fe$_{2-y}$Se$_2$ to become a bulk superconductor, and if so, is there a structural distinction between superconducting and non-superconducting phases? We have grown K$_{1-x}$Fe$_{2-y}$Se$_2$ samples that show superconductivity with $T_C$ = 31 K, even when growth conditions are starkly different from those reported in the literature. Here we present high-resolution synchrotron X-ray diffraction measurements, alongside single-crystal x-ray and electron diffraction, to elucidate the phase space in this system. Combined with magnetometry, heat capacity, and transport measurements, our structure-property relations help prescribe how chemical composition and heat treatment induce superconductivity and vacancy ordering in the K$_{1-x}$Fe$_{2-y}$Se$_2$ system. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D22.00003: Suppression of superconductivity and spin-glass behavior in Cu-doped K0.8Fe2Se2 Rongwei Hu, Johnpierre Palione, Shanta Saha, Richard Greene Single crystals with nominal compositions of K0.8Fe2-xCuxSe2 were grown and studied with low temperature electrical transport and magnetic susceptibility measurements. We show that the superconductivity present in undoped K0.8Fe2Se2 crystals with transition temperature of 31 K is very quickly suppressed with Cu doping into the Fe site, and the system very quickly becomes insulating. We discuss anomalous behavior at higher doping, including spin-glass like behavior with further Cu doping. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D22.00004: What is the true parent state in alkali-doped Iron Selenides Wei Li, Shuai Dong, Chen Fang, Jiangping Hu By performing first-principles electronic structure calculations and analyzing effective magnetic model of alkali-doped iron selenides, we show that the materials without iron vacancies should approach a novel checkerboard phase in which each four Fe sites group together in tetragonal structure. The checkerboard phase is the ground state with a block antiferromagnetic (AFM) order and a small charge density wave order in the absence of superconductivity. Both of them can also coexist with superconductivity. The results explain mysterious 2 by 2 ordered patterns and hidden orders observed in various different experiments, clarify the missing link between AFM and superconducting phases, suggest that the block-AFM state is the parent state, and unify the understanding of various observed phases in alkali-doped iron selenides. (Reference: Wei Li, et al, arxiv:1110.0372 (2011) [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D22.00005: Structural and Superconducting Properties of K$_{0.8}$Fe$_{1.6+x}$Se$_{2 }$ single crystals Su Jung Han, Genda Gu, Qiang Li We report structural and superconducting properties of K0.8Fe1.6+xSe2 single crystals. Superconducting properties were studied by using transport, specific heat and magnetization measurements, and compared with that of FeSe$_{1-x}$Te$_{x}$. We found the $c$-axis resistive transition and specific heat behavior are distinctively different in these two classes of materials. Structural properties were studied via transmission electron microscopy and energy-dispersive X-ray spectroscopy. We found large scale structural and chemical disorder in the K0.8Fe1.6+xSe2 samples. The relationship between structural and superconducting properties in K0.8Fe1.6+xSe2 will be discussed. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D22.00006: Imaging the coexistence of superconductivity and a charge density modulation in K$_{0.73}$Fe$_{1.67}$Se$_{2}$ superconductor Peng Cai, Cun Ye, Wei Ruan, Xiaodong Zhou, Aifeng Wang, Meng Zhang, Xianhui Chen, Yayu Wang We report scanning tunneling microscopy studies of the local structural and electronic properties of the iron selenide superconductor K$_{0.73}$Fe$_{1.67}$Se$_{2}$ with $T_{C}$ = 32K. On the atomically resolved FeSe surface, we observe well-defined superconducting gap and the microscopic coexistence of a charge density modulation with $\sqrt 2 \times \sqrt 2 $ periodicity with respect to the original Se lattice. We propose that a possible origin of the pattern is the electronic superstructure caused by the block antiferromagnetic ordering of the iron moments. The widely expected iron vacancy ordering is not observed, indicating that it is not a necessary ingredient for superconductivity in the intercalated iron selenides. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D22.00007: Local Atomic and Electronic Structure of K$_{0.8}$Fe$_{1.6+x}$Se$_{2}$: Structural Order and Disorder Trevor Tyson, Tian Yu, Su Jung Han, Mark Croft, Genda D. Gu, Ivo Dimitrov, Qiang Li The local structure of superconducting single crystals of K$_{0.8}$Fe$_{1.6+x}$Se$_{2}$ was studied by x-ray absorption spectroscopy. Near-edge spectra reveal that the average valence of Fe is 2+. The structure about the Se and Fe sites shows a high degree of order in the nearest neighbor Fe-Se bonds. On the other hand, the combined Se and K local structure measurements reveal a very high level of structural disorder in the K layers. The temperature dependence of the Fe-Se atomic correlation follows that of the Fe-As correlation in LaFeAsO$_{0.89}$F$_{0.11. }$ In K$_{0.8}$Fe$_{1.6+x}$Se$_{2}$, the nearest neighbor Fe-Fe bonds has a lower Einstein temperature and higher structural disorder than in LaFeAsO$_{0.89}$F$_{0.11}$. For higher shells, an enhancement of the second nearest neighbor Fe-Fe interaction is found just below Tc and suggests that correlations between Fe magnetic ion pairs beyond the first neighbor are important in models of magnetic order and superconductivity in these materials. This research is supported by DOE BES Grant DE-FG02-07ER46402 (NJIT) for T.A.T and T.Y. and DOE BES Contract No. DE-AC0298CH10886. (BNL), for S.J.H, G.G, I.K.D, and Q. L. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D22.00008: Interplay between structure and properties in TlFe$_{1.6}$Se$_2$ Andrew May, Huibo Cao, Michael McGuire, Claudia Cantoni, Bryan Chakoumakos, Radu Custelcean, Brian Sales The degree to which Fe vacancies order greatly influences the properties of intercalated FeSe compounds. In this talk, we will consider the relationship between vacancy ordering and the properties of TlFe$_{1.6}$Se$_2$. Unlike the alkali-metal based compounds, such as K$_{0.8}$Fe$_{1.6}$Se$_2$, TlFe$_{1.6}$Se$_2$ does not become superconducting and always forms with the Tl-sites fully occupied. This results in reduced complexity and disorder, allowing the role of Fe vacancies to be probed directly. Common to all of these materials is $\sqrt{5}a \times \sqrt{5}a$ superstructure associated with ordering of the Fe vacancies, which appears coupled to a block-checkerboard antiferromagnetic order. Through subtle changes in composition and processing, the vacancies can order more or less completely, and this results in substantial variations in the magnetic properties of TlFe$_{1.6}$Se$_2$. The electronic behavior is relatively insensitive to these changes, though, as semiconducting behavior is observed in all cases. Finally, high resolution TEM reveals complex local structures, which are markedly different from those observed in K$_{0.8}$Fe$_{1.6}$Se$_2$. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D22.00009: Defect Structures and nonstoichiometry in the K$_{x}$Fe$_{2-y}$Se$_{2}$ by X-ray diffraction. John Claridge, Alexey Ganin, Alec McLennan, John Bacsa, Matthew Rosseinsky, Yasuhiro Takabayashi, Kosmas Prassides The mechanisms of charge carrier density control in the iron pnictide and chalcogenide superconductors are important as small changes in composition produce metal-insulator transitions and generate superconductivity at temperatures of up to 37K in chalcogenides and 55K in pnictides. All of the reported materials are based on a square FeX (X = Se, As) layer built from edge-sharing of FeX$_{4}$ tetrahedra. Insertion of alkali metal cations between FeSe layers affords superconductivity in A$_{x}$Fe$_{2-y}$Se$_{2}$ (A = K, Rb, Cs, Tl: 0.7 $<$ x $<$ 1, 0 $<$ y $<$ 0.5) materials highlites the defect chemistry as the iron charge states close to +2 found in the other Fe-based superconductors require the creation of considerable defect concentrations on either or both iron and alkali metal sites. Ordering of the tetrahedral site vacancies in two crystals of refined compositions K$_{0.93(1)}$Fe$_{1.52(1)}$Se$_{2}$ and K$_{0.862(3)}$Fe$_{1.563(4)}$Se$_{2}$ produces a fivefold expansion of the parent ThCr$_{2}$Si$_{2}$ unit cell in the ab plane which can accommodate 20{\%} vacancies on a single site within the square FeSe layer. The iron charge state is maintained close to +2 by coupling of the level of alkali metal and iron vacancies, doping mechanisms, which can operate at both average and local structure levels will be discussed. These structures will also be considered in terms of their local ordering and with relation to other defect chalcogenide layer structures and possible phase segregation. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D22.00010: Local structural disorder and superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$ Hyejin Ryu, Hechang Lei, A.I. Frenkel, C. Petrovic We report significantly enhanced magnetic moment on K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals after post-annealing and quenching process. In K$_{x}$Fe$_{2-y}$Se$_{2}$ unit cell, there are two Fe sites, Fe1 which has higher symmetry with longer average Fe-Se bond length, and Fe2 which has lower symmetry with shorter average Fe-Se bond length. Temperature dependent X-ray absorption fine structure (XAFS) analysis results on quenched and as grown K$_{x}$Fe$_{2-y}$Se$_{2}$ materials show that quenched K$_{x}$Fe$_{2-y}$Se$_{2}$ materials have increased average Fe-Se bond length and decreased static disorder. This result indicates that occupancy of Fe1 sites increased after post-annealing and quenching process. This result provides clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D22.00011: Novel synthesis method of K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystal Toshinori Ozaki, Hiroyuki Takeya, Satoshi Demura, Keita Deguchi, Yasuna Kawasaki, Hiroyuki Okazaki, Hiroshi Hara, Takahide Yamaguchi, Hiroaki Kumakura, Yoshihiko Takano The discovery of superconductivity in K$_{x}$Fe$_{2-y}$Se$_{2}$ with $\sim $T$_{c}$ 31 K has triggered a great interest in the field of iron-based superconductors [1]. K$_{x}$Fe$_{2-y}$Se$_{2}$ superconductor has several practical advantages of relatively high $T_{c}$, high upper critical field (H$_{c2})$ and less toxicity compared to FeAs-based superconductors. However, the procedure for producing K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystal is complicated and time-consuming: At first, the FeSe precursor was prepared, and then the single crystals of K$_{x}$Fe$_{2-y}$Se$_{2}$ were grown by the self-flux method. The simplification of the synthesis is really important for applications. We present a novel synthesis method of K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystal, which is very simple and quick. A superconducting transition of this sample appeared at T $\sim $31.6 K. After quenching the sample, the calcurated volume fraction from dc magnetic susceptibility was dramatically increased, consistent with the previous report [2]. We will also report the details of the synthesis, transport properties and microstructures of the samples. 1)J.Guo, \textit{et al}, Phys. Rev. B \textbf{82}, 180520 (2010). 2)H. Lei, \textit{at el}, arXiv: 1109.0534. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D22.00012: Ordering driven superconductivity in an alkaline iron selenide Xiao-jia Chen, Qing-Zhen Huang, Jian-Xin Zhu, Minghu Fang, Wei Bao, Jian-Bo Zhang, Ling-Yun Tang, Jinfu Shu, Viktor Struzhkin, Russell Hemley, Ho-kwang Mao Combining the resistivity, synchrotron x-ray diffraction, and neutron diffraction measurements, we investigated the evolution of the structural, magnetic, and superconducting properties with pressure up to 37 GPa of superconducting Rb$_{0.347}$Tl$_{0.347}$Fe$_{1.752}$Se$_{2}$. Extended phase diagram of temperatures and pressure were established for this material. The results show the superconductivity is in reality driven by ordering. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D22.00013: Pressure-tuned superconductivity of iron chalcogenides Liling Sun, Xiao-Jia Chen, Jing Guo, Peiwen Gao, Hangdong Wang, Minghu Fang, Xiaolong Chen, Genfu Chen, Qi Wu, Dachun Gu, Chao Zhang, Xiaoli Dong, Xi Dai, Ho-kwang Mao, Zhongxian Zhao In this talk, we present our recent progress in effect of pressure on superconductivity of newly discovered iron chalcogenide superconductors. We show that the either positive or negative pressure can tune superconductivity of this new kind of superconductors. Superconductivity with higher superconducting transition temperature \textit{Tc }can reemerge after elimination of the initial superconducting phase upon compression. We find that the maximum $T$c of the reemerging superconducting phase is as high as 48.7 K for K$_{0.8}$Fe$_{1.70}$Se$_{2}$ and 48 K for Tl$_{0.6}$Rb$_{0.4}$Fe$_{1.67}$Se$_{2}$, setting a new $T$c record for chalcogenide superconductors. The presence of the second superconducting phase is proposed to be related to pressure-induced quantum criticality. Our findings open up the potential route for the exploration of high-$T$c superconductivity in iron-based and other superconductors. [Preview Abstract] |
Session D23: Metals Theory Oxides Interfaces
Sponsoring Units: DCMPChair: Khorgolkhuu Odbadrakh, Oak Ridge National Laboratory
Room: 255
Monday, February 27, 2012 2:30PM - 2:42PM |
D23.00001: Transferable tight-binding description of the Fe-C interaction Nicholas Hatcher, Georg K.H. Madsen, Ralf Drautz A coherent transferable tight-binding (TB) parameterization including magnetism has yet to be developed for the Fe-C interaction. Although interatomic potentials have been obtained for this system, recent findings show that the results from these potentials are inconsistent with DFT calculations and do not give an accurate portrayal of chemical bonding in the system. Using dual DFT grid and LCAO calculations within GPAW, we obtain one electron wave functions expanded in a multiple-$\zeta$ LCAO basis. This is then down-folded onto an optimal minimal basis, giving a continuous and transferable description of Fe-C bonding. By constructing a TB energy functional using these bond integrals and a parameterized interatomic repulsion, we show how an accurate description of the energy hierarchy of interstitial carbon in Fe-structures can be achieved. Furthermore, we use the model to calculate elastic properties and energies of a variety of Fe-carbides, defects, and carbon diffusion paths. This simple model based on physical insights may be used to study systems containing thousands of atoms. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D23.00002: Empirical potentials from a combined density functional theory / genetic algorithm approach: Illustration for Xe in UO2 Alexander Thompson, Bryce Meredig, Chris Wolverton We have developed a new empirical potential for xenon in uranium dioxide with existing UO2 potentials that achieves accurate xenon defect energetics. This potential was fit to several snapshots of DFT+U molecular dynamics of a single defect configuration using the genetic algorithm code Iterative Potential Refinement (IPR). In IPR, the forces, stresses, and energies from DFT calculations are used to parameterize empirical potentials. Several random sets of parameters are uses to compare against DFT and the genetic algorithm minimizes the error of the parameters with respect to the DFT results. We compare this potential and other xenon potentials to DFT+U using a large set of defect calculations of xenon incorporated into sites with high, intermediate, low strain. Despite only being fit to a single configuration, our new empirical potential gives the very good agreement with DFT+U across a range of xenon incorporation sites and vastly outperforms existing xenon potentials. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D23.00003: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 3:06PM - 3:18PM |
D23.00004: The dielectric response of single-crystal UO$_{2}$ probed by terahertz time-domain spectroscopy Yong An, Trevor Tyson, Steven Conradson We measured the complex dielectric function, \textit{$\varepsilon =\varepsilon $}$_{r}$\textit{+i$\varepsilon $}$_{i}$, of single-crystal UO$_{2}$ in the temperature (T) range of 5-500 K by terahertz (THz) time-domain spectroscopy to study its dielectric response. A critical temperature point of 60 K is found for the measured temperature dependence of \textit{$\varepsilon $}$_{r}(T)$. The dispersion of \textit{$\varepsilon (\omega )$} in the THz range follows the trend of the damped resonant mode of the transverse optical phonon at \textit{$\omega $}$_{TO}$=8.4 THz. Examining the Lyddane-Sachs-Teller relationship reveals mode-softening of \textit{$\omega $}$_{TO}$ in the temperature range of 60-120 K. We then performed optical-pump THz-probe measurements to study the dynamic dielectric response of UO$_{2}$ following photoexcitation. We observe a small pump-induced change in \textit{$\varepsilon $} that lasts for more than one millisecond, indicating heavy electron behavior of photoexcited 5f electrons resulting from strong electron-lattice interaction. The slow relaxation is attributed to the diffusion of optical phonons generated by photoexcited 5f electrons in the UO$_{2}$ crystal. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D23.00005: Magnetocaloric Effect of NiFeCoCrPdx High Entropy Alloys Dustin Belyea, C. Bauer, M. Lucas, J. Horwath, E. Michel, Casey W. Miller FeCoCrNi is one of many ``high entropy alloys'' (HEAs), which are multicomponent alloys with high entropy of mixing. These materials often have high hardness, and resistance to wear and corrosion, making them attractive for applications. Here, we report on the magnetic entropy change and magnetocaloric effect of the FeCoCrNiPdx system. The addition of Pd to FeCoCrNi has been shown to enable the critical temperature to be tuned from 130 K for x=0 to 500 K for x=2. Isothermal magnetization measurements were made on samples with x ranging from 0 to 0.50 as functions of temperature. The magnetic entropy change ($\Delta $S) was calculated using the thermodynamic Maxwell Relation. We find that Pd additions tune the peak $\Delta $S temperature from 130 to 300K, while modestly increasing the peak $\Delta $S magnitude. Interestingly, the addition of Pd leads to an almost doubling of the relative cooling power (RCP), as well as a notable change in the critical behavior of the material. The RCP of $\sim $35 J/kg for a 1T field change puts the Pd-containing HEAs in competition with other magnetocaloric materials in the 100-200 K operating range. This alloy system's combination of durability and a tunable Curie temperature without appreciable change in cooling power may make this system interesting for magnetocaloric applications. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D23.00006: Magnetic and crystallographic properties of Cr1-xFexGe Yuen Yiu, Nirmal Ghimire, David Mandrus, Stephen Nagler, Michael McGuire According to previously published bulk measurements, Cr1-xFexGe exhibits a quantum critical point at x=0.75, where it turns from a paramagnet (for x<0.75) into a ferromagnet (for x>0.75). Cr1-xFexGe is a simple cubic B20 (FeSi) crystal. The endpoints of the alloy are binary compounds that have been studied to some degree. FeGe, the better known of the two, is a spiral ferromagnet similar to MnSi. However, less is known for CrGe, which is thought to be a weakly ferromagnetic paramagnet with bulk properties that may be explained by the paramagnon theory. We report new neutron scattering results on Cr1-xFexGe for x=0.6, 0.7, 0.75, 0.8. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D23.00007: An {\it ab-initio} multiscale method to investigate chemical embrittlement of metals Georg Schusteritsch, Thomas K\"uhne, Efthimios Kaxiras We present a multiscale method, coupling a small region treated by first-principles quantum mechanics, to a large classical atomistics region. Our method is based on total energy arguments that are applicable to metals. We employ Kohn-Sham (KS) Density Functional Theory (DFT) in the region of interest and couple this to the classical Embedded Atom Method (EAM). Results for the chemical embrittlement of metals due to segregated impurities at grain boundaries (GB) are presented. We study the average interplanar strain surrounding the GB and use this as a measure of the atomic relaxation. We apply our method to study the chemical embrittlement of Cu by Bi and Pb impurities and compare this to the effect of Ag impurities, which are known to segregate to the GB but not embrittle Cu. We find that Bi and Pb weaken and hence embrittle the Cu GB. In contrast Ag impurities at the GB plane increase cohesion. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D23.00008: Molecular dynamics study of the thermopower of Ag, Au, and Pt nanocontacts F. Pauly, J.K. Viljas, M. B\"urkle, M. Dreher, P. Nielaba, J.C. Cuevas Using molecular dynamics simulations of many junction stretching processes combined with tight-binding-based electronic structure and transport calculations, we analyze the thermopower of silver (Ag), gold (Au), and platinum (Pt) atomic contacts. In all cases we observe that the thermopower vanishes on average within the standard deviation and that its fluctuations increase for a decreasing minimum cross section of the junctions. However, we find a suppression of the fluctuations of the thermopower for the s-valent metals Ag and Au, when the conductance originates from a single, perfectly transmitting channel. Essential features of the experimental results for Au, Ag, and copper (Cu) of Ludoph and van Ruitenbeek [Phys. Rev. B 59, 12290 (1999)], as yet unaddressed by atomistic studies, can hence be explained by considering the atomic and electronic structure at the disordered narrowest constriction of the contacts. For the multivalent metal Pt our calculations predict the fluctuations of the thermopower to be larger by one order of magnitude as compared to Ag and Au, and suppressions of the fluctuations as a function of the conductance are absent. Main features of our results are explained in terms of an extended single-level model. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D23.00009: First-Principles Study of Hydrogen Permeation in Palladium-Gold Alloys Angelo Bongiorno, Shucheng Xu Density functional theory and lattice model calculations are combined to study the permeability of hydrogen in Pd lightly alloyed with Au. This study shows that small amounts of Au substitutions in Pd lead to, respectively, an increase and decrease of the diffusivity and solubility of hydrogen in the alloy. The competition between these two phenomena depends on temperature and can yield dilute PdAu membranes with a hydrogen permeability higher than pure Pd. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D23.00010: Hydrogen Embrittlement in Zirconium: a Quasi-Continuum Density Functional Theory Study Q. Peng The hydrogen embrittlement in Zirconium becomes a very important and emergent issue for academia, industry and policy makers as a result of the Japan nuclear accident. The hydride formation, diffusion and embrittlement in zircolay will impact dramatically on the development of advanced nuclear energy systems, the life time extension of the current nuclear fleet and dry storage of spent nuclear fuel. Quasi-Continuum Density Functional Theory (QCDFT) is a powerful concurrent multiscale method based entirely on density functional theory (DFT) and allows quantum simulations of materials properties of a large system with billions of atoms. Using QCDFT modeling, we found that the presents of hydrogen at the cracktip of zirconium, both on crack surface and in-bulk, will form zirconium hydrides and embrittle the system. The concentration of hydrogen and orientation of crack plays important roles in such embrittlement. The mechanism of hydrogen embrittlement under various loading conditions will be discussed. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D23.00011: Effects of disorder on the T-dependent bandstructure of purple bronze Li$_2$Mo$_{12}$O$_{34}$ Thomas Jarlborg, Piotr Chudzinski, Thierry Giamarchi The band structures of ordered and disordered Li$_2$Mo$_{12}$O$_{34}$ are calculated by use of ab-initio DFT-LMTO method. The unusual band dispersion in the z-direction obtained in previous band calculations is confirmed for the ordered structure, and the overall band structure agree reasonably with existing photoemission data. The T-dependent band broadening is calculated from configurations with thermal disorder of the atomic positions within the unit cell. The band structure shows important band broadening of the two bands at the Fermi energy. The bands are particularly sensitive to in-plane movements of Mo sites. Already disorder due to zero-point motion makes a band broadening of the order of 20 meV and creates a sizable band overlap. The effect of Li vacancies on the two bands is relatively small. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D23.00012: Metamagnetic transition self-propelled by the spin injection Alexander Zyuzin, A.Yu. Zyuzin We study metamagnetic phase transition of itinerant electrons controlled by the spin injection mechanism. The current flow between a ferromagnetic metal and a metamagnetic metal produces the non-equilibrium shift of chemical potential for spin-up and spin-down electrons. This shift acts as an effective magnetic field driving the metamagnetic transition between low and high magnetization states of the metamagnet in the vicinity to the contact with the ferromagnet. We show that high magnetization state of the metamagnet self propels into the bulk of the metamagnet and the length of this state has threshold dependence on the electrical current. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D23.00013: Generalized Statistical Thermodyanmics Applied to Small Material Systems Robert Cammarata When characterizing the behavior of small material systems, surface effects can strongly influence the thermodynamic behavior and need to be taken into account in a complete thermal physics analysis. Although there have been a variety of approached proposed to incorporate surface effects, they are often restricted to certain types of systems (e.g., those involving incompressible phases) and often invoke thermodynamics parameters that are often not well-defined for the surface. It is proposed that a generalized statistical mechanics based on the concept of thermodynamic availability (exergy) can be formulated from which the surface properties and their influence on system behavior can be naturally and rigorously obtained. This availability-based statistical thermodynamics will be presented and its use illustrated in a treatment of nucleation during crystallization. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D23.00014: $d + i d$ superconductivity on the honeycomb bilayer Milica Milovanovic, Jaksa Vucicevic, Darko Tanaskovic We demonstrate that for interlayer attractive interactions on bilayer honeycomb lattice with large interlayer hopping, a time reversal symmetry breaking $d$-wave topological superconductor is a dominant phase. We find that small momentum order parameter expansion has $d_{x^2-y^2} + i d_{xy}$ symmetry around both Dirac points and discuss a possible relevance of this state for experiments on graphene bilayer. [Preview Abstract] |
Session D24: Bionanotechnology and Nanostructured Sensors
Sponsoring Units: FIAPChair: Jayant Kumar, University of Massachusetts Lowell
Room: 256
Monday, February 27, 2012 2:30PM - 2:42PM |
D24.00001: Zirconium tungstate/epoxy resin nanocomposites with negative coefficient of thermal expansion for all-dielectric cryogenic temperature sensors Erich See, Vladimir Kochergin, Lauren Neely, Madrakhim Zayetnikov, Gianluigi Ciovati, Hans Robinson The $\alpha $-phase of zirconium tungstate (ZrW$_{2}$O$_{8})$ has the remarkable property that its coefficient of thermal expansion (CTE) is negative over its entire range of thermal stability (0-1050K), and through this range it has a nearly constant negative CTE. When ZrW$_{2}$O$_{8}$ nanoparticles are mixed into a polymer resin, the resulting composite has a reduced CTE when compared with that of the pure polymer. However, previous research on such composites has occurred only near room temperature. We show that at cryogenic temperatures, it is possible to make ZrW$_{2}$O$_{8}$/resin nanocomposites with negative CTE. By coating a fiber-optic Bragg grating with such a composite, we were able to create an all-optical temperature sensor without the use of metals, which would be of particular use in superconducting RF cavities. The sensor has sensitivity down to at least 2 K, six times lower than previous fiber-optic temperature sensors. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D24.00002: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 2:54PM - 3:06PM |
D24.00003: Strong two-photon-fluorescence from semiconducting polymer nanoparticles for high contrast imaging of cancerous cells Soumitra Satapathi, Anoop Pal, Lian Li, Suresh Gadde, Dhimiter Bello, Lynne Samuelson, Jayant Kumar Strong two-photon-induced fluorescence was observed from a series of novel fluorescent semiconducting polymer nanoparticles using femtosecond laser pulses at 800 nm. The conjugated polymer nanoparticles were fabricated by a simple technique known as the ``mini emulsion'' technique. The quadratic dependence of the two-photon-fluorescence was confirmed by varying the laser intensity. The two-photon-absorption cross- sections of the nanoparticles were determined in aqueous dispersions by comparing with that of Rhodamine 6G. The deep penetration of the near-infrared laser together with large absorption cross-section demonstrated here, renders these fluorescent polymer nanoparticles as ideal candidates for high contrast in vivo fluorescent imaging of certain tumor cells. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D24.00004: Surface Modified Gadolinium Phosphate Nanoparticles as MRI Contrast Agents Matthieu F. Dumont, Celine Baligand, Elisabeth S. Knowles, Mark W. Meisel, Glenn A. Walter, Daniel R. Talham Nanoparticles of GdPO$_{4}$H$_{2}$O were synthesized in a water/oil microemulsion using IGEPAL CO-520 as surfactant resulting in 50 nm to 100 nm particles that are dispersible and stable in water. Using surface modification chemistry previously established for zirconium phosphonate surfaces,\footnote {J. Monot et al., \textit{J. Am. Chem. Soc.} 130 (2008) 6243.} the particles are directly modified with 5'-phosphate terminated oligonucleotides, and the specific interaction of the divalent phosphate with Gd$^{3+}$ sites at the surface is demonstrated. The ability of the modified nanoparticles to act as MRI contrast agents was determined by performing MR relaxivity measurements at 14 T. Solutions of nanopure water, Feridex{\textregistered} and Omniscan{\textregistered} (FDA cleared contrast agents) in 0.25{\%} agarose were used for comparison and control purposes. MRI data confirm that GdPO$_{4}$H$_{2}$O nanoparticles have relaxivities (r$_{1}$,r$_{2})$ comparable to commercially available contrast agents.\footnote {H. Hifumi et al., \textit{J. Am. Chem. Soc. }128 (2006) 15090.} In addition, biofunctionalization of the surface of the nanoparticles does not prevent their function as MRI contrast agents. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D24.00005: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 3:30PM - 3:42PM |
D24.00006: Surface Plasmon Resonance Imaging of the Enzymatic Degradation of Cellulose Microfibrils Kyle Reiter, Adam Raegen, Anthony Clarke, Jacek Lipkowski, John Dutcher As the largest component of biomass on Earth, cellulose represents a significant potential energy reservoir. Enzymatic hydrolysis of cellulose into fermentable sugars, an integral step in the production of biofuel, is a challenging problem on an industrial scale. More efficient conversion processes may be developed by an increased understanding of the action of the cellulolytic enzymes involved in cellulose degradation. We have used our recently developed quantitative, angle-scanning surface plasmon resonance imaging (SPRi) device to study the degradation of cellulose microfibrils upon exposure to cellulosic enzymes. In particular, we have studied the action of individual enzymes, and combinations of enzymes, from the \textit{Hypocrea Jecorina }cellulase system on heterogeneous, industrially-relevant cellulose substrates. This has allowed us to define a characteristic time of action for the enzymes for different degrees of surface coverage of the cellulose microfibrils. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D24.00007: Non-destructive method for measuring the Ca/P ratio in human bone Alexander Slepko, Alexander Demkov Hydroxyapatite (HA) [Ca$_{10}$(PO$_{4})_{6}$(OH)$_{2}$] is the main mineral constituent in human bone. It crystallizes in hexagonal and monoclinic phase, which are very similar in structure and properties. A critical measure for healthy bones is the Ca/P ratio which in turn affects the dielectric constant of the mineral constituent. The dielectric constant of HA varies between 5 and 20 depending on the Ca/P ratio in the sample [J. Mater. Sci.: Mater. Med. \textbf{21}, 399]. We suggest exploiting this large span in the dielectric constant in a non-destructive method to measure the Ca/P ratio in bone by optical spectroscopy. Using density functional theory we calculate the long-range corrected phonon dispersion. We find that only modes around 330 cm$^{-1}$ are strongly affected by the dielectric constant. The shifts in frequency can be up to 20 cm$^{-1}$ as you span the range of the dielectric constant. Thus, by measuring the optical shift and comparing with calibrated samples it is possible to draw conclusions on the Ca/P ratio in the mineral. Importantly, we find the same modes in both the monoclinic and hexagonal phases to be sensitive to changes in the dielectric constant. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D24.00008: Surface Functionalized Nanocoax Biosensors Binod Rizal, Michelle Archibald, Gregory McMahon, Natasha Erdman, Stephen Shepard, Michael J. Burns, Thomas C. Chiles, Michael J. Naughton We have adapted the nanocoax array architecture for high sensitivity, all-electronic chemical and biological sensing. We previously demonstrated ppb concentration level detection sensitivity to volatile organic compounds in dry air using the nanocoax array with nanoporous coax annuli [1]. Here, we report progress toward modifying/functionalizing the coax metal surfaces to enable specific binding of target molecules (e.g. proteins, toxins, pathogenic organisms), followed by electronic interrogation via capacitance/impedance spectroscopy. As a proxy for target molecules, and in order to confirm the ability to selectively functionalize desired surfaces in our nanopillar / nanocoax geometry, we have selectively attached strepavidin-functionalized core-shell CdSe/ZnS quantum dots to gold nanopillars. Next steps will include substituting antibodies for the quantum dots, and measuring the capacitance and impedance response to the introduction of protein (PSA , CA-125, etc.) in serum. Ref. [1]: H.Z. Zhao, B. Rizal, G. McMahon, H. Wang, P. Dhakal, T. Kirkpatrick, Z.F. Ren, T.C. Chiles, D. Cai and M.J. Naughton (submitted). [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D24.00009: Magnetic wire trap arrays for biomarker-based molecular detection Gregory Vieira, Kalpesh Mahajan, Gang Ruan, Jessica Winter, R. Sooryakumar Submicrometer-scale magnetic devices built on chip-based platforms have recently been shown to present opportunities for new particle trapping and manipulation technologies. Meanwhile, advances in nanoparticle fabrication allow for the building of custom-made particles with precise control of their size, composition, and other properties such as magnetism, fluorescence, and surface biomarker characteristics. In particular, carefully tailored surface biomarkers facilitate precise binding to targeted molecules, self-actuated construction of hybrid structures, and fluorescence-based detection schemes. Based on these progresses, we present an on-chip detection mechanism for molecules with known surface markers. Hybrid nanostructures consisting of micelle nanoparticles, fluorescent quantum dots, and superparamagnetic iron oxide nanoparticles are used to detect proteins or DNA molecules. The target is detected by the magnetic and fluorescent functionalities of the composite nanostructure, whereas in the absence of the target these signals are not present. Underlying this approach is the simultaneous manipulation via ferromagnetic zigzag nanowire arrays and imaging via quantum dot excitation. This chip-based detection technique could provide a powerful, low cost tool for ultrasensitive molecule detection with ramifications in healthcare diagnostics and small-scale chemical synthesis. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D24.00010: Strategies to enhance the bioavailability of curcumin: a potential antitumor drug Abhishek Kumar, Joshna Chittigori, Lian Li, Lynne Samuelson, Daniel Sandman, Jayant Kumar Curcumin is a polyphenol which has elicited considerable interest for its antioxidant and anti tumor properties. Although curcumin may be used as potential therapeutic drug, it is very sparingly soluble in water which makes it less bioavailable under physiological conditions. We report two approaches to make curcumin more bioavailable. The first approach involves fabricating colloidal dispersions of curcumin in the range of tens of nanometers. The second approach involves functionalization of curcumin with polyethylene glycol (PEG) to render it water dispersible or soluble. Since curcumin is a fluorescent molecule as well as a potential drug, its interactions with cells have been investigated using one and two photon confocal fluorescence imaging. We have also observed strong interaction between curcumin and metal ions, which may have physiological implications. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D24.00011: Comparison anti-bacterial effect of silver/polystyrene nanocomposites on gram negative and positive bacteria Akhtarolmolook Kazemi, Maryam Raftari, Sajjad Tollabimazraehno, Mohammad Mahdavi, Azam Irajizad Silver nanoparticles/polystyrene nanocomposites were prepared via casting the solution of polystyrene in a mixture of carbon tetrachloride and acetone containing silver nanoparticles. Colloidal silver nanoparticles in acetone were synthesized by pulsed laser ablation (PLA) of pure bulk silver. Casting the colloidal silver nanoparticles in a solution of polystyrene results in a yellowish transparent polymeric sheet. TEM images show rather spherical nanoparticles with mean diameter of 5 nm. Ag/PS nanocomposites were characterized by UV-VIS spectroscopy. In this study, we also investigated the antimicrobial activity of silver nanocomposites against Escherichia coli (E.coli) and Staphylococcus aureus (S.aureus) as a model for Gram negative and Gram positive bacteria. Antibacterial tests were performed against E. coli and S.aureus, on LB agar plates containing different amount of nanoparticles. Our results showed at all these concentrations, the nanoparticles caused a growth delay of E. coli, increasing the concentration of nanoparticles increased this growth delay. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D24.00012: Luminescence of Rare-Earth-Doped Nanoparticles with Aromatic Linker Molecules Tess Senty, Mohita Yalamanchi, Yanwei Zhang, Anya Leach, Mohindar Seehra, Xiaodong Shi, Alan Bristow Rare-earth-doped vanadate glasses retain their luminescence when formed as shells around magnetic cores [1]. This property has prompted speculation that composite magneto-photoluminescent (CMPL) structures can be used in biological applications. For example, CMPL nanoparticles can be magnetically tuned to separate cells, proteins and nucleic acids [2]. A crucial step in realizing this goal is to attach organic linkers (between the rare-earth-doped shell and bio-probes), which do not affect the luminescence. We demonstrate with IR spectroscopy that Eu:YVO$_{4}$ nanoparticles treated with benzoic acid, 3-nitro 4-chloro-benzoic acid and 3,4-dimethoxy benzoic acid all result in the modification of the surface states, replacing the native metal-hydroxyl bond with a longer chain aromatic linker, which can be later functionalized. Photoluminescence spectra under UV-excitation show that the dominant $^{5}$D$_{0} \quad \to $ $^{7}$F$_{2}$ transition at $\sim $620 nm is unaffected by the chemical treatment. The result provides a platform to facilitate the attachment of bio-probes to Eu:YVO$_{4}$ nanoparticles and related CMPL nanostructures with Fe$_{2}$O$_{4}$ cores. \newline [1] N. B. McDowell et al, J. Appl. Phys. 107, 09B327 (2010). \newline [2] T. R. Sathe et al, Anal. Chem. 78, 5627 (2006). [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D24.00013: Light-powered nanoparticle -- MEMS hybrid Evgeniya Moiseeva, Tomas Lucas, Guandong Zhang, Andre Gobin, Cindy Harnett This work presents a light-actuated microelectromechanical (MEMS) structure with bistable elements and its applications in cellular-scale actuation. These devices are built using a metal/oxide bilayer with a stress mismatch. In the hybrid design that uses gold nanoparticles for localized heating, a nanoparticle coating is patterned onto the selected part of the device by Parylene micro-stenciling before releasing the structures from the planar substrate. These gold nanoparticles are attractive for use in $\mu $TAS and biotechnology due to their biocompability and inertness, the ability to conjugate to the surface via thiol chemistry or electrostatic interaction, and especially their strong tunable absorption in the near infrared region. Integration of the near infrared (IR)-resonant gold nanoparticles with bistable MEMS structures creates light-driven hybrid actuators that quickly respond to narrow-band IR light. The movement of the MEMS device is achieved through controlled thermal expansion, with actuation speed in the millisecond range. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D24.00014: Motion observation and SPR measurements of kinesin motility on microtubules A. Sikora, D. Oliveira, K. Kim, A.L. Liao, M. Umetsu, T. Adschiri, W. Hwang, W. Teizer Motor proteins convert chemical energy directly into mechanical work with high efficiency ($\sim $50{\%}). One of these proteins, kinesin, is used in the cell to transport organelles. It ``walks'' along biopolymer tracks called microtubules and, depending on the type, can reach speeds of a few micrometers per second. Kinesin can carry intracellular cargo over long distances against several piconewtons of loads and is barely limited by the cargo size. Motion of streptavidin-coated quantum dots carried by kinesin on microtubules will be presented. We have expressed biotinylated Kinesin-1 using \textit{Escherichia coli}. Attachment to quantum dots was performed using the strong binding affinity between streptavidin and biotin. Microtubules, labeled with rhodamine, allow visualization by fluorescence microscopy. The measured speed of our kinesin fits well with results found in the literature. Surface Plasmon Resonance (SPR) measurements allow the identification and strength evaluation of bonding. Using this technique, we will present results on the binding between our expressed kinesin and microtubule. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D24.00015: Construction of Quantum Dot Micro/Nano-track as a Seedbed for Kinesin Motor Proteins K. Kim, A. L. Liao, A. Sikora, D. Oliveira, M. Umetsu, T. Adschiri, W. Hwang, W. Teizer Kinesin is a motor protein engaged in motion on microtubules. It is involved in various subcellular processes such as cell division and transportation of intracellular cargo. The system of (1) kinesin, (2) \textit{in-vitro} polymerized microtubules and (3) specifically functionalized quantum dots has been employed to realize kinesin motility \textit{in-vitro}. Selective attachment of the kinesin motor protein to a surface is an important prerequisite for the development of artificial bio-engineered devices utilizing these dynamic processes in \textit{in-vitro} systems. We will elucidate the feasibility of using a micro/nano-track paved with streptavidin coated quantum dots as fluorescently traceable linkers for biotinylated kinesins. This will be based on results from fluorescence microscopy observations of recent motility assays and quantum dot distributions observed on structured PMMA (polymethyl-methacrylate) surfaces patterned by electron beam lithography. [Preview Abstract] |
Session D25: Focus Session: Matter at Extreme Conditions - New Experimental Capabilities
Sponsoring Units: DCOMP GSCCM DMPChair: Thomas Sewell, University of Missouri
Room: 257A
Monday, February 27, 2012 2:30PM - 2:42PM |
D25.00001: Ultrafast, high resolution, phase contrast imaging of shock response with synchrotron radiation: opportunities and challenges S.N. Luo, B.J. Jensen, D.E. Hooks, K.J. Ramos, J.D. Yeager, K. Kwiatkowski, T. Shimada, D.A. Fredenburg, K. Fezzaa Designing materials that function at dynamic extremes and predicting dynamic materials response require experimental investigations of their time, rate and microstructure dependences. Key to such experiments are {\it in situ}, in-volume, temporally and spatially resolved measurements (e.g., x-ray imaging and diffraction). Here we report ultrafast ($<$100 ps), high resolution ($\sim$3 $\mu$m), dynamic phase contrast imaging (PCI) measurements during high strain-rate loading (100 ns scale). A gas gun was installed at 32ID beamline of the Advanced Photon Source for dynamic loading, and dynamic PCI measurements were performed with a single x-ray pulse on representative materials/processes, including cylinder impact and penetration, large-cell foam compaction, cerium jet formation and granular material compression. We present overall experimental scheme and opportunities for dynamic materials research as seen from the preliminary results, as well as challenges both for photon sources and detectors. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D25.00002: Probing Matter at an Atomic Unit of Pressure using convergent compression James Hawreliak, Damian Swift, Jon Eggert, Dave Braun, Steve Rothman, Gilbert Collins Geometric confinement significantly increases the shock pressure as a spherically-converging shock approaches the central focus. Inertial confinement fusion is one area where this technique enables the ~100 MBar ablation pressure to multiply to the several-GBar pressure required for fusion. We are using x-ray radiography of a spherically convergent shock wave in a solid sphere to explore material equations of state at pressures which exceed the atomic unit of pressure (Eh/a0 = 300MBar); the energy density of a hydrogen atom. Measuring materials properties above this pressure will breach yet another significant barrier in our quest to understand extreme states, and will open a completely new realm where the atomic nature of matter is very strongly perturbed. We will discuss initial experiments preformed on the Omega laser facility and plans for future experiments on the NIF. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D25.00003: High-resolution phase contrast imaging of brittle failure during impact loading Kyle Ramos, Brian Jensen, Luo Shengnian, Daniel Hooks, John Yeager, Kris Kwiatkowski, Tsutomu Shimada, Kamel Fezzaa Heterogeneous processes involved in brittle failure necessitate in situ, spatially resolved observation. An impact capability has recently been developed in which synchrotron phase contrast imaging (PCI), at the 32-ID beamline of the Advanced Photon Source, can be used to resolve crack interfaces during dynamic deformation. The imaging is both fast and high-resolution as images with approximately 3 micrometer resolution are obtained from single x-ray pulses ($<$100 ps duration). Experiments have been performed to investigate questions regarding velocimetry interpretation, the effect of stress states, and whether cracking can occur under uniaxial compression. Uniaxial compression and tension in planar impact configurations and cylindrical impact penetration has been used to vary stress states and observe failure. PCI and velocimetry results from these experiments will be presented for a range of brittle materials spanning glasses and ceramics. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D25.00004: Deformation and material dynamics under ultrafast compression Invited Speaker: Michael Armstrong For decades, dynamic compression experiments have been used to determine the equation of state of materials, and examine material deformation at high strain rates. Within the last 15 years, ultrafast optical methods have been used to characterize deformation at strain rates in excess of 10$^{10}$/s. Recently such experiments have found broad consistency with empirical laws formulated at orders of magnitude lower strain rates, but have also discovered intriguing phenomena on short time scales, such as elastic stress orders of magnitude beyond the yield strength. These experiments explore the ultimate limits of material relaxation via deformation, and the results suggest exciting possibilities for practical and scientific application of ultrafast compression, including nonequilibrium material synthesis, determination of the equation of state with a small scale experiment, and the investigation of ultrahigh density with a table top laser. Here we will talk about our experiments on the ultrafast deformation of metals, including aluminum and iron, and the ultrafast compression of deuterium. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D25.00005: High-resolution phase contrast imaging of jet formation in shocked cerium to examine material strength Brian Jensen, Sheng Luo, Frank Cherne, Guy Dimonte, Guillermo Terrones, Daniel Hooks, Kyle Ramos, John Yeager, Kris Kwiatkowski, Tsutomu Shimada, Kamel Fezzaa Understanding the dynamic properties of metals has been a long-standing scientific challenge. Experiments are needed to locate phase boundaries, to obtain equation-of-state data within those boundaries, and to examine properties such as material strength in the relevant phases. Efforts have been underway in recent years to examine the multiphase equation-of-state for cerium largely because of its complex phase diagram that exists at relatively moderate pressures and temperatures. To date, experiments have been performed to determine the Hugoniot, the shock-melt transition, and to examine the low-pressure phases through the critical point. In the current work, we present novel data that uses ultrafast, high-resolution phase contrast imaging (PCI) to examine jet-formation in cerium for impact stresses that span the alpha-phase up to the melt boundary. These experiments were performed using a recently developed capability at the 32-ID beamline of the Advanced Photon Source that couples the PCI method with an impact system to obtain real-time, spatially resolved images during dynamic compression. Experimental results will be presented and compared with recent efforts that use more traditional shock-release and double-shock loading to examine strength. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D25.00006: High Resolution Diagnostics for Simultaneous Measurements of Velocity and Density in Shock-Driven Instabilities Ricardo Mejia-Alvarez, Sridhar Balasubramanian, Greg Orlicz, Kathy Prestridge The interaction between a shock wave and the interface between two fluids of different density might induce macroscopic mixing of the fluids. It is generally accepted that baroclinic vorticity, resulting from misalignments between the density gradient across the interface and the pressure gradient of the shock wave, impels this macroscopic mixing. So far, the Extreme Fluids Team at Los Alamos National Laboratory has conducted the only detailed studies of the structure of the developing instability. These studies encompass simultaneous measurements of velocity and density via combined Particle Image Velocimetry (PIV) and Planar Laser Induced Fluorescence (PLIF). Using this approach, the above mentioned Team has conducted extensive studies over a varicose curtain of heavy gas (SF$_6$). Since a curtain implies two succeeding interfaces, a new Vertical Shock Tube (VST) was developed for simultaneous characterization of velocity and density fields of single-interface shock-driven flows. This talk is intended to present some of the results obtained for double-interface shock driven flows, as well as describing the characteristics, challenges, and range of possibilities of the laser diagnostics incorporated in the VST. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D25.00007: LCLS: A new frontier for time-resolved in situ lattice measurements in materials at extreme conditions Despina Milathianaki, Damian Swift, James Hawreliak, Richard Lee While laser-based pump-probe experiments have provided significant insights into the processes induced by dynamic loading (e.g. shock-induced phase transitions, elastic-plastic response), the material behavior at the lattice level has been extremely difficult to unveil due to the temporal and signal-to-noise limitations of laser-based x-ray techniques. Here we present recent dynamic x-ray diffraction measurements on the shock-induced behavior of polycrystalline Mg in the 10-40 GPa regime, followed by future experiments in the field of dynamic high pressure science using the free electron laser at LCLS as an ultrafast x-ray probe. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D25.00008: Progress towards Single Shot Spectroscopic Techniques for Time-Resolved Measurements in the Diamond Anvil Cell Douglas Allen Dalton, R. Stewart McWilliams, M.F. Mahmood, Alexander F. Goncharov We will discuss how we are bridging the gap between static diamond anvil cell and dynamic shock experiments using various spectroscopic techniques which utilize nonlinear optics. Using pulsed laser techniques, we can achieve extreme temperatures while probing optical and chemical changes on fast time scales. Recent developments incorporating broadband spectroscopy into the laser heated diamond anvil cell have indicated that probing phase transitions while measuring temperature is possible [1]. Various methods for incorporating nonlinear vibrational spectroscopy (such as CARS) into the diamond anvil cell will be discussed. The application of these optical diagnostics to pulsed laser heating and table-top shock experiments [2] will be presented. \\[4pt] [1] R.S. McWilliams et al., in preparation. \\[0pt] [2] M.R. Armstrong et al., J. Appl. Phys., \textbf{108}, 023511, (2010). [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D25.00009: Phase diagram of shock and ramp-compressed tin Amy Lazicki, Jonathon Eggert, Ryan Rygg, Damian Swift, James McNaney, Gilbert Collins We will present powder x-ray diffraction results on laser-ramp-compressed solid tin up to 600 GPa, and discuss new methods for detecting the melting transition. Tin has a complex phase diagram with multiple observed and predicted high pressure phases and a moderate melting temperature, making it an ideal subject for a fundamental study of material properties using new techniques. Ramp compression in the solid allows access to extremely dense condensed phases and in the liquid the possibility for dynamically freezing molten tin. With newly developed x-ray diffraction methods we examine crystal structure, strength and texture in the dynamically compressed phases, and explore the possibility of a new method for mapping out melting curves. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D25.00010: Ultrafast Shock Interrogation of Hydrogen Peroxide/Water Mixtures: Thermochemical Predictions of Shock Condition Chemistry Joseph Zaug, Michael Armstrong, Sorin Bastea, Jeffrey Carter, I.-F. William Kuo, Jonathan Crowhurst, Christian Grant Hydrogen peroxide is a powerful oxidizer and its concentrated aqueous solutions exhibit very high reactivity, even sustaining detonation under strong enough confinement. Due to its simple composition and basic expected decomposition kinetics hydrogen peroxide is very suitable for studying the interplay of high pressures, temperatures and reactivity and their effect on the equation of state, particularly at the boundary between detonating and non-detonating behavior. To this end we performed speed of sound and picosecond time resolved shock measurements on solutions of hydrogen peroxide of concentrations from 30 to 90 percent, and analyzed the results in terms of common assumptions of chemical equilibrium in reactive fluid mixtures. Experimental shock states were achieved up to a maximum pressure of 20 GPa with corresponding shock velocities of 6-7 km/sec. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D25.00011: Simulation of Forward and Inverse X-ray Scattering From Shocked Materials John Barber, Quinn Marksteiner, Cris Barnes The next generation of high-intensity, coherent light sources should generate sufficient brilliance to perform in-situ coherent x-ray diffraction imaging (CXDI) of shocked materials. In this work, we present beginning-to-end simulations of this process. This includes the calculation of the partially-coherent intensity profiles of self-amplified stimulated emission (SASE) x-ray free electron lasers (XFELs), as well as the use of simulated, shocked molecular-dynamics-based samples to predict the evolution of the resulting diffraction patterns. In addition, we will explore the corresponding inverse problem by performing iterative phase retrieval to generate reconstructed images of the simulated sample. The development of these methods in the context of materials under extreme conditions should provide crucial insights into the design and capabilities of shocked in-situ imaging experiments. [Preview Abstract] |
Session D26: Focus Session: Physics of Energy Storage Materials - Catalysis and H2 Storage
Sponsoring Units: DCOMP DMPChair: Juergen Eckert, University of California, Santa Barbara
Room: 257B
Monday, February 27, 2012 2:30PM - 3:06PM |
D26.00001: Design Principles for Oxygen Reduction and Evolution on Oxide Catalysts Invited Speaker: Yang Shao-Horn Driven by growing concerns about global warming and the depletion of petroleum resources, developing renewable energy production and storage technologies represent one of the major scientific challenges of the 21$^{st}$ century. A critical element in pursuit of this quest is the discovery of efficient and cost-effective catalysts used in solar fuel production via electrochemical energy conversion processes such as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), both of which are central to the efficiencies of direct{\-}solar and electrolytic water-splitting devices, fuel cells, and metal-air batteries. Although the Sabatier's principle provides a qualitative argument in tuning catalytic activity by varying the bond strength between catalyst surface and reactant/product (neither too strong nor too weak leading to the maximum activity at moderate bond strength), it has no predictive power to find catalysts with enhanced activity. Identifying a ``design principle'' that links catalyst properties to the catalytic activity is critical to accelerate the search for highly active catalysts based on abundant elements, and minimize the use of precious metals. Here we establish a molecular principle that governs the activities of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for oxide catalysts, where the activities primarily correlate to the $\sigma $* orbital (``e$_{g}$'') occupation of surface transition{\-}metal cations established by systematic examination of more than ten to fifteen transition{\-}metal oxides. The intrinsic ORR and OER activities exhibit a volcano-shaped dependence on the e$_{g}$ occupancy and the activities peak at an e$_{g}$ occupancy close to unity. Our findings reflect the critical influence of the $\sigma $* orbital on the energetics of surface reaction intermediates on surface transition metal ions such as the O$_{2}^{2-}$/OH$^{-}$ displacement and the OH$^{-}$ regeneration, and thus highlight the importance of surface oxide electronic structure in controlling catalytic activities. Using the established molecular principle, we further demonstrate that an alkaline earth cobalt oxide with a chemical formula of Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3{\-}\delta }$ (BSCF), catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than the state-of-the-art iridium oxide catalyst in basic solutions. \\[4pt] [1] J. Suntivich, H.A. Gasteiger, N. Yabuuchi, H. Nakanishi, J. B. Goodenough and Y. Shao-Horn, Design Principles for Oxygen Reduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal-Air Batteries, Nature Chemistry, \underline {3}, 546--550 (2011).\\[0pt] [2] Jin Suntivich, Kevin J. May, Hubert A. Gasteiger, John B. Goodenough and Yang Shao-Horn, A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles, ScienceExpress, Science DOI: 10.1126/science.1212858, (2011). [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D26.00002: Modeling of hydrogen evolution reaction on the surface of GaInP$_{2}$ Woon Ih Choi, Brandon Wood, Eric Schwegler, Tadashi Ogitsu GaInP$_{2}$ is promising candidate material for hydrogen production using sunlight. It reduces solvated proton into hydrogen molecule using light-induced excited electrons in the photoelectrochemical cell. However, it is challenging to model hydrogen evolution reaction (HER) using first-principles molecular dynamics. Instead, we use Anderson-Newns model and generalized solvent coordinate in Marcus-Hush theory to describe adiabatic free energy surface of HER. Model parameters are fitted from the DFT calculations. We model Volmer-Heyrovsky reaction path on the surfaces of CuPt phase of GaInP$_{2}$. We also discuss effects of surface oxide and catalyst atoms that exist on top of bare surfaces in experimental circumstances. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D26.00003: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 3:30PM - 3:42PM |
D26.00004: In situ Raman Evidence for Reversible Room-Temperature Hydrogenation in Pt-doped Carbons Xiaoming Liu, Youjian Tang, Enshi Xu, Thomas Fitzgibbons, Huan-Hsiung Tseng, Ming-Sheng Yu, Cheng-Si Tsao, John Badding, Vincent Crespi, Angela Lueking Atomic hydrogen spillover was investigated by in situ Raman spectroscopy and density functional theory. In the presence of Pt nanoparticles, modes related to Basal plain hydrogenation were observed for activated carbon and graphene, respectively, during Raman measurements in 100 bar H2. The modes were absent when Pt, carbon, or H2 were omitted from the experiment. Substituting H2 with D2 led to the expected isotopic shift for a hydrogen-dominated vibrational mode. The mode disappeared and reappeared over several cycles of exposure to H2 or He at room temperature, consistent with room-temperature reversibility of H chemisorbed to the activated carbon in the presence of Pt nanoparticles. Reversibility apparently arises from a facilitated transition of H from a chemisorbed state to a more mobile physisorbed state, followed by recombination and release. Reversibility for Pt/graphene was much less pronounced, suggesting that structural heterogeneities in the sample carbon support and/or catalyst-carbon contact are important factors. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D26.00005: Hydrogen storage on calcium coated boron (hetero-)fullerenes: A DFT study Suleyman Er, Geert Brocks, Gilles A. de Wijs Using density functional theory (DFT), we investigate hydrogen storage properties of calcium-coated molecular systems of B$_{80}$ boron fullerene, C$_{48}$B$_{12}$ boron-doped heterofullerenes, and well-known C$_{60}$ fullerene. Here, we consider the most common and low-lying isomers of B$_{80}$ and C$_{48}$B$_{12}$. We find that the Ca-coated molecules have the following properties: (\emph{1}) The binding of metal atoms to B$_{80}$ or to C$_{48}$B$_{12}$ molecules is much stronger than their binding to a C$_{60}$ molecule. (\emph{2}) B$_{80}$ and C$_{48}$B$_{12}$ have larger electron affinities than their carbon only counterpart, and accordingly discharge the surface Ca atoms more efficiently. (\emph{3}) B$_{80}$ molecule, however, shows structural deformations upon reacting with Ca atoms. (\emph{4}) C$_{48}$B$_{12}$Ca$_{6}$, however, is stable at elevated temperatures. C$_{48}$B$_{12}$ has well-exposed, positively charged Ca atoms on its surface, and binds up to six hydrogen molecules per metal center with hydrogen binding energies of 0.17-0.14 eV/H$_{2}$, that are suitable for ambient temperature hydrogen storage. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D26.00006: Role of nano in catalysis: Pd catalyzed H desorption from MgH$_{2}$ Weiyu Xie, Damien West, Yiyang Sun, Shengbai Zhang Magnesium hydride (MgH$_{2})$ is promising for on-board hydrogen (H) storage with the major hurdle being the slow desorption kinetics. H desorption from ball-milled MgH$_{2}$ peaks at two slightly different temperatures, which further split in the presence of palladium catalyst. It has been experimentally demonstrated that nanostructuring can eliminate the high temperature peak. However, the effect of nanostructuring cannot be explained by thermodynamic destabilization due to quantum size effect. Our first-principles calculation reveals that there exist two reaction pathways for H desorption from MgH$_{2}$. One involves H vacancy (SV) diffusion at surface, while the other one involves H atom diffusion in bulk. The SV pathway self-terminates as dehydrogenation eventually eliminates the exposed MgH$_{2}$ region. Therefore, it is size-sensitive and fully functions only when the surface-to-bulk ratio is large, which is available only in nanostructures. Our calculation further shows that the SV pathway significantly lowers the desorption barrier, because it decouples the H transport process with the surface liftoff process and benefits from a fact that diffusion of vacancies at surface can have significantly lower barrier than that in bulk. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D26.00007: Particle-size dependence of the activation energy for decomposition of lithium amide Chris Van de Walle, Khang Hoang, Anderson Janotti Lithium amide (LiNH$_{2})$ is a promising material for reversible hydrogen storage, yet atomistic mechanisms behind the dehydrogenation process are unknown. The activation energy for LiNH$_{2}$ decomposition has been observed to strongly vary with ball milling, suggesting a dependence of the thermodynamics and kinetics of the decomposition on the particle size. We have examined these mechanisms based on first-principles calculations for native point defects and defect complexes in LiNH$_{2}$. We propose that the decomposition of LiNH$_{2}$ into lithium imide (Li$_{2}$NH) and ammonia (NH$_{3})$ occurs through two competing mechanisms, one involving the formation of native defects in the interior of the material and the other at the surface. As a result, the prevailing mechanism and hence the activation energy depend on the surface-to-volume ratio, or the specific surface area, which changes with the particle size. We explain the observed variations of activation energy, and address the role played by LiH in the dehydrogenation of (LiNH$_{2}$+LiH) mixtures. The relationship between the structure of hydrogen-related defects and the end products in the decomposition reaction can be extended to other complex hydrides. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D26.00008: Deliquescence of NaBH$_4$ computed from density functional theory Ping Li, Wissam Al-Saidi, Karl Johnson Complex hydrides are promising hydrogen storage materials and have received significant attention due to their high hydrogen-capacity. The hydrolysis reaction of NaBH$_4$ releases hydrogen with both fast kinetics and high extent of reaction under technical conditions by using steam deliquescence of NaBH4. This catalyst-free reaction has many advantages over traditional catalytic aqueous phase hydrolysis. The first step in the reaction is deliquescence, i.e. adsorption of water onto NaBH$_4$ surface and then formation of a liquid layer of a concentrated NaBH$_4$ solution, which is quickly followed by hydrogen generation. We have used periodic plane wave density functional theory to compute the energetics and dynamics of the initial stages of deliquescence on the (001) surface of NaBH$_4$. Comparison of results from standard generalized gradient approximation functionals with a dispersion-corrected density functional show that dispersion forces are important for adsorption. We used DFT molecular dynamics to assess the elementary steps in the deliquescence process. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D26.00009: Electronic Structure and Molecular Dynamics Calculations for KBH$_{4}$ Dimitrios Papaconstantopoulos, Andrew Shabaev, Khang Hoang, Michael Mehl, Nicholas Kioussis In the search for hydrogen storage materials, alkali borohydrides MBH$_{4}$ (M=Li, Na, K) are especially interesting because of their light weight and the high number of hydrogen atoms per metal atom. Electronic structure calculations can give insights into the properties of these complex hydrides and provide understanding of the structural properties and of the bonding of hydrogen. We have performed first-principles density-functional theory (DFT) and tight-binding (TB) calculations for KBH$_{4}$ in both the high temperature (HT) and low temperature (LT) phases to understand its electronic and structural properties. Our DFT calculations were carried out using the VASP code. The results were then used as a database to develop a tight-binding Hamiltonian using the NRL-TB method. This approach allowed for computationally efficient calculations of phonon frequencies and elastic constants using the static module of the NRL-TB, and also using the molecular dynamics module to calculate mean-square displacements and formation energies of hydrogen vacancies. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D26.00010: Dehydrogenation of LiBH$_4$ nanoclusters: A first-principles study Ebrahim Hazrati, Filipe Vasconcelos, Geert Brocks, Robert de Groot, Gilles de Wijs Recent experimental studies\footnote{A.F. Gross et al. J. Phys. Chem. C 112, 5651, 2008}$^,$\footnote{X. Liu et al. Chem. Mater. 23, 1331, 2011} show faster desorption kinetics, improved reversibility and more favorable thermodynamics for confined LiBH$_4$ nanoparticles than the bulk. Using density functional theory calculations, we first discuss the geometries and energetics of LiBH$_4$, LiH, LiB, Li and B clusters. Secondly, we study the effects of particle size on the decomposition pathway of LiBH$_4$ clusters. Our calculations show that only very small clusters of LiBH$_4$ (up to 12 formula units) are significantly destabilized relative to the bulk. High stability of small clusters of LiBH$_4$ originates from the fact that surface energies are very low for bulk LiBH$_4$. (100), (010), (101) and (011) surfaces are almost degenerate with surface energies of 0.113, 0.102, 0.115 and 0.097~J/m$^2$, respectively. Clusters of LiH, LiB, Li and B are more strongly destabilized than the LiBH$_4$ clusters upon decreasing the cluster size. We show that, in contrast to the bulk, destabilized clusters of LiBH$_4$ decompose to (LiB)$_n$ clusters. Finally, we present some of our preliminary NMR chemical shift results for different LiBH$_4$ surface terminations. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D26.00011: First-principles Modeling of Diffusion during Hydrogenation of LiBH$_4$ Chao Yu, Vidvuds Ozolins LiBH$_4$ has been studied extensively because of its high volumetric and gravimetric hydrogen content. However, experiments show that hydrogen release is very slow at temperatures up to 300 C, which severely limits applications in mobile storage. Using density-functional theory calculations, we systematically study bulk diffusion of defects during solid-state hydrogenation reactions. The defect concentration and concentration gradients are calculated for a variety of defects, including charged vacancies and interstitials. We find that low concentration gradients limit the rate of hydrogen desorption. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D26.00012: Bulk diffusion of defects in LiAlH4/Li3AlH6 Biljana Rolih, Vidvuds Ozolins From various experimental studies on decomposition of hydrogen storage materials it has been proposed that bulk diffusion of metal species may be the rate limiting step in hydrogen storage reactions. A recently developed theoretical model uses density functional theory to study the underlying processes involved in bulk diffusion. To date this model has been applied to study dehydrogenation of NaAlH4. However further study of alkaline and alkali earth metals merits attention, particularly LiAlH4 with a high gravimetric hydrogen density, accessible at moderate temperatures. This study uses density functional theory to obtain concentration gradients and diffusivities of native charged and neutral defects in Li3AlH6 and LiAlH4. The flux of each defect is obtained and thus the activation energy for each defect. Our results show that diffusion of metal species is a possible rate limiting process in the system. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D26.00013: First-Principles Study of the Li-Na-Ca-N-H System: Compound Structures and Hydrogen-Storage Properties Pattanasak Teeratchanan, Fei Zhou, Kyle Michel, Vidvuds Ozolins Mixed-metal amides and imides are being widely investigated as potential hydrogen storage materials. Using a combination of first-principle DFT calculations, grand-canonical linear programming, and prototype electrostatic ground state (PEGS) approaches, we predict hydrogen storage reactions in the Li-Na-Ca-N-H system. The enthalpies, entropies, static, zero-point, and T\textgreater{}0K vibrational energies of known compounds together with our predictions of some incomplete experimental crystal structures are presented. [Preview Abstract] |
Session D27: Invited Session: Materials for Energy Applications
Sponsoring Units: DMP GERAChair: Robert Nemanich, Arizona State University
Room: 258AB
Monday, February 27, 2012 2:30PM - 3:06PM |
D27.00001: The DOE SunShot Initiative: Science and Technology to enable Solar Electricity at Grid Parity Invited Speaker: Ramamoorthy Ramesh The SunShot Initiative's mission is to develop solar energy technologies through a collaborative national push to make solar Photovoltaic (PV) and Concentrated Solar Power (CSP) energy technologies cost-competitive with fossil fuel based energy by reducing the cost of solar energy systems by $\sim $ 75 percent before 2020. Reducing the total installed cost for utility-scale solar electricity to roughly 6 cents per kilowatt hour (1{\$}/Watt) without subsidies will result in rapid, large-scale adoption of solar electricity across the United States and the world. Achieving this goal will require significant reductions and technological innovations in all PV system components, namely modules, power electronics, and balance of systems (BOS), which includes all other components and costs required for a fully installed system including permitting and inspection costs. This investment will re-establish American technological and market leadership, improve the nation's energy security, strengthen U.S. economic competitiveness and catalyze domestic economic growth in the global clean energy race. SunShot is a cooperative program across DOE, involving the Office of Science, the Office of Energy Efficiency and Renewable Energy and ARPA-E. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D27.00002: Integrating the multifunction necessary for electrochemical energy storage into energy- and size-scalable ultraporous nanoarchitectures Invited Speaker: Debra Rolison Designing high performance energy-storage devices that combine nanometric feature size with well-wired transport paths requires an architectural perspective. We chose carbon aerogel-like nanofoam papers as attractive plug-and-play electrode substrates because of such desirable properties as high specific surface area, electronic conductivity, and through-connected pore structure. Achieving this blend of desirable properties requires an optimal balance of critical architectural features: (1) open, 3D interconnected macropores sized at 100 to 300 nm (a difficult-to-obtain size range in porous carbons) and (2) pore walls of a size that reduce dead weight and volume (preferably ca. 20-nm wall thickness for 100- to 300-nm voids), yet retain mechanical strength and flexibility without compromising electronic conductivity (preferably ca. 20 S/cm). Carbon nanofoam papers provide a low cost and scalable nanocomposite that exists within this ``Goldilocks zone'' of desirable properties and which has catalyzed breakthroughs in our work with asymmetric electrochemical capacitors, air cathodes for metal/air batteries, lithium-ion batteries, 3D batteries, and semifuel cells. New charge-storage or catalytic functionality is imparted to internal carbon walls simply by transporting reactants within the 3D macroporous. Self-limiting modification strategies allow us to incorporate conformal, nanoscopic ``paints'' of metal (Mn, Ti, Ru, Fe) or polymer (redox-active or electron insulating) or to specifically adsorb metal nanoparticles (Pt, Au, Pd, Ag) throughout the macroscopic thickness (0.07 to 0.3 mm) of carbon nanofoam papers as dictated by the requirements of a specific end application. For instance, modification with 10-nm MnO$x$ increases the mass-, geometric-, and volume-normalized capacitance (2- to 10-fold) relative to the native carbon nanofoam without significantly altering its high-rate character and provides a structure that can be used in an asymmetric electrochemical capacitor or used to an air cathode in a Zn/air cell to electrocatalyze oxygen reduction and provide pulse power. Our redesigned carbon nanofoam offers a versatile design platform for much-needed advances in a broad range of multifunctional energy storage and conversion. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D27.00003: Bio-inspired Approaches to Solar Energy Conversion Invited Speaker: Michael Wasielewski Natural photosynthesis is carried out by organized assemblies of photofunctional tetrapyrrole chromophores and catalysts within proteins that provide specifically tailored environments to optimize solar energy conversion. Artificial photosynthetic systems for practical solar fuels production must collect light energy, separate charge, and transport charge to catalytic sites where multi-electron redox processes will occur. The primary goal of our research in this field is to understand the fundamental principles needed to develop integrated artificial photosynthetic systems. These principles include how to promote and control: 1) energy capture, charge separation, and long-range directional energy and charge transport, 2) coupling of separated charges to multi-electron catalysts for fuel formation, and 3) supramolecular self-assembly for scalable, low-cost processing from the nanoscale to the macroscale. The central scientific challenge is to develop small, functional building blocks, having a minimum number of covalent linkages, which also have the appropriate molecular recognition properties to facilitate self-assembly of complete, \textit{functional} artificial photosynthetic systems. This lecture will describe our use of ultrafast optical spectroscopy and time-resolved EPR spectroscopy to understand charge transport in self-assembled structures for artificial photosynthesis. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D27.00004: Quantum-well and quantum-dot structures for high-efficiency photovoltaics Invited Speaker: Edward Yu Quantum-well and quantum-dot semiconductor heterostructures offer a variety of opportunities for achieving photovoltaic power conversion efficiencies in excess of the Shockley-Queisser limit for single-homojunction solar cells. However, realization of such efficiencies is likely to require a combination of very high quality epitaxial growth or nanostructure synthesis to minimize carrier trapping and recombination, detailed understanding and analysis of optical absorption and nonequilibrium carrier transport processes, and light trapping to enable efficient optical absorption in very thin device layers. We discuss work in which GaAs/InGaAs/InAs semiconductor quantum-well and quantum-dot solar cells are realized in designs that enable efficient collection of photogenerated carriers from quantum-wells and dots, and combined with subwavelength-scale metal and dielectric structures that enable incident photons to be scattered into guided optical modes within a thin-film device, thereby enabling increased absorption efficiency in very thin device layers. Several aspects of this work will be addressed. Measurement of electric-field-dependent photocurrent response enables design of structures in which photogenerated carriers are collected efficiently from quantum-well or quantum-dot structures in the intrinsic region of a p-i-n junction solar cell. Processing to remove epitaxially grown device layers from their original growth substrate enables metal and dielectric nanostructures to be designed and integrated with the semiconductor epitaxial layer structures to scatter incident photons into strongly guided optical modes within the semiconductor. Finally, detailed analysis of quantum-well and quantum-dot optical absorption as well as optical mode structure within the device enables optimization of the absorption and mode profiles to achieve maximum power conversion efficiency. Both computational and experimental results derived from these approaches will be described. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D27.00005: Combined conversion of heat and light with Photon Enhanced Thermionic Emission for solar energy harvesting Invited Speaker: Nicholas Melosh Recently a new mechanism for solar energy harvesting based on photon-enhanced thermionic emission (PETE) was proposed. This two-step process uses photons to excite carriers into the conduction band, followed by thermionic emission from the conduction band, and electron collection at a low-workfunction anode. This process effectively combines both heat and light, and its efficiency was calculated to exceed ideal single junction photovoltaics since it harvests some of the heat that is normally lost within PV devices. Experimental measurements demonstrated this mechanism in GaN materials, yet the quantum efficiency was very low. Here we discuss the loss mechanisms in the PETE process, and several approaches to overcome them. In particular we focus on surface recombination effects and absorption losses, and demonstrate a heterostructured device that increases the quantum efficiency by two orders of magnitude. Prospects for combined-cycle devices incorporating a PETE converter as a topping cycle on conventional thermal cycles are also analyzed. [Preview Abstract] |
Session D28: Focus Session: Dopants and Defects in Semiconductors - Oxides and Interfaces
Sponsoring Units: DMPChair: Michael Stavola, Lehigh University
Room: 258C
Monday, February 27, 2012 2:30PM - 3:06PM |
D28.00001: Interdiffusion, Unintentional Doping and Electronic Reconstruction at Polar/nonpolar Oxide Interfaces Invited Speaker: Scott Chambers The observation of conductivity at the interface of insulating polar and non-polar perovskites (general form ABO$_{3})$ has sparked considerable interest worldwide, with much of the work to date being focused on the LaAlO$_{3}$/SrTiO$_{3}$(001) heterojunction. Many attribute the interface conductivity to an electronic reconstruction alleviating the polar discontinuity via a film-to-interface charge transfer. However, the possibility of dopant- and/or defect-mediated conductivity cannot be ruled out, especially when the interfaces are not atomically abrupt. Electronic reconstruction requires an electric field within the film to facilitate the charge transfer process. However, x-ray photoemission spectroscopy studies reveal little or no electric field in the LAO film either above or below the critical thickness for conductivity, calling into question the validity of the electronic reconstruction model. In order to gain deeper insight into the electronic properties, it is worthwhile to manipulate the interface by changing the B-site cation in the polar perovskite. There are no low-lying $d$--derived bands in LaAlO$_{3}$ Therefore, if conductivity occurs, it ought to be driven by either the wholesale transfer of charge from the LaAlO$_{3}$ O2$p$-derived band into the STO (i.e. electronic reconstruction), or unintentional doping and/or defect creation. By replacing Al with a transition metal cation, we inject a new degree of freedom into the band structure -- partially occupied $d $orbitals -- and, thus, enable other mechanisms of charge redistribution. We have explored this concept by placing Cr(III) at the B-site and describe the electronic properties of epitaxial LaCrO$_{3}$/STO(001) heterojunctions. To minimize defect creation, the films were deposited using molecular beam epitaxy (MBE), in which the incoming atom energies are very low ($<$0.1 eV). Core-level and valence-band x-ray photoemission spectra measured for MBE-grown LaCrO$_{3}$/SrTiO$_{3}$(001) yield band offsets and potential gradients within the LaCrO$_{3}$ sufficient to trigger an electronic reconstruction to alleviate the polarity mismatch. Yet, the interface is insulating. Based on first principles calculations, we attribute this unexpected result to interfacial cation mixing combined with charge redistribution within CrO$_{2}$ layers, enabled by low-lying $d$ states within LaCrO$_{3}$, which suppresses an electronic reconstruction. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D28.00002: Calculation of charge transition levels of oxygen vacancies in rutile TiO$_2$ using the GW method Andrei Malashevich, Manish Jain, Steven G. Louie Titanium dioxide (TiO$_2$) is a semiconductor displaying photovoltaic and photocatalytic properties and is widely used in numerous technological applications, such as solar cells, hydrolysis of water, photocatalysis, etc. Defects are important for optical properties of TiO$_2$, e.g, they play a crucial role in photocatalytic reactions. Gaining a deep understanding of the influence of intrinsic defects, such as vacancies and interstitials, on the properties of this material is highly desirable. In spite of many theoretical and experimental investigations of defects in TiO$_2$, controversies still remain. In this work, the charge transition levels of oxygen vacancies are studied in the rutile form of TiO$_2$. For this purpose the GW method is employed for the calculation of the band gap and the position of the defect levels in the gap. The effects of lattice relaxations for systems with various charge states are taken into account. This is done with the help of hybrid functionals, since LDA or PBE mean fields do not describe defect levels properly, which may result in incorrect lattice relaxations. To account for spurious effects due to periodic images of the charged defects, appropriate electrostatic correction techniques are used. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D28.00003: Strain effect on diffusion properties of oxygen vacancies in bulk and subsurface of rutile TiO2 Dajun Shu, Zhaowu Wang, Mu Wang The influences of external strain on diffusion properties of the bulk and subsurface oxygen vacancy (OV) in rutile TiO$_2$ are systematically studied using first-principle calculations. For OVs in bulk, we find that tensile (compressive) strain applied in the [001] direction or isotropically applied in the equivalent [110] and [1$\bar 1$0] directions reduces (increases) the energy barriers of diffusion. Anisotropic strain applied in [110] and [1$\bar 1$0] increases the energy barriers of diffusion in the two directions. Meanwhile it results in anisotropic diffusion behaviors. Between [110] and [1$\bar 1$0], the bulk OV prefers to diffuse along the one in which more compressive or less tensile strain is applied. From subsurface to surface, the most energetically favorable OV pathway is along the [110] rows terminated with the surface bridging oxygen atoms. The diffusion barrier of the OV in the first trilayer is much lower than that of a bulk OV. External in-plane tensile strain can further reduce the energy barrier of the subsurface OV diffusion, and thus help to improve the diffusion of OVs from bulk to surface. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D28.00004: Vibrational spectroscopy of O-H and O-D centers in TiO$_{2}$ Figen Bekisli, Michael Stavola, W. Beall Fowler While the vibrational properties of O-H centers in TiO$_{2}$ have been studied for many years, recent experiments suggest a new picture of their behavior. In the 1970s, Bates and Perkins found a single, sharp, strongly polarized O-H line in TiO$_{2}$, and similarly for O-D and O-T [1]. On the contrary, recent studies by Herklotz \textit{et al.} [2] find three closely spaced O-H (O-D) lines that were assigned to two different charge states of an O-H (O-D) shallow donor. We have in our possession the very TiO$_{2}$ samples studied many years ago by Bates and Perkins. We have introduced H and D into these samples and also into TiO$_{2}$ samples obtained recently. High-resolution vibrational spectroscopy performed as a function of temperature at Lehigh provides new insight into the different vibrational properties seen for O-H in TiO$_{2}$ in the 1970's and in recent studies by Herklotz \textit{et al.} [1] J.B. Bates and R.A. Perkins, Phys. Rev. B \textbf{16}, 3713 (1977). [2] F. Herklotz \textit{et al.}, Phys. Rev. B \textbf{83}, 235202 (2011).. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D28.00005: Intrinsic Spin-Orbit Effects in Strontium Titanate Cuneyt Sahin, Giovanni Vignale, Michael E. Flatt\'e We have calculated spin relaxation times via the Elliott-Yafet mechanism for strontium titanate as a function of temperature. The approach uses a low-energy effective spin-orbit Hamiltonian constructed from a tight-binding model with atomic spin-orbit interactions. The intrinsic spin Hall conductivity for strontium titanate has also been calculated from the same low-energy Hamiltonian using a Berry's phase technique. Modifications to the spin relaxation and spin Hall conductivity from elastic strain at an interface will also be described. This work was supported by an ARO MURI. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D28.00006: Prediction of a ``half semiconductor'' for spintronics in non-compensated n-p codoped TiO$_{2}$ Wenguang Zhu, Xing-Qiu Chen, Hanno Weitering, Zhenyu Zhang, G. M. Stocks Based on hybrid density functional calculations, we predict that by doping non-compensated Cr-N pairs a normal wide-band-gap semiconductor TiO$_{2}$ can be altered to a ``half semiconductor'', in which both the top and the valence band and the bottom of the conduction band are fully spin-polarized and exclusively contributed by the same spin component. The underlying formation mechanism of such an unusual band structure is revealed via detailed electronic structure analysis. The magnetic property of the material will also be discussed in this talk. Such a ``half semiconductor'' material may provide a new twist to generate and manipulate spin currents for spintronics. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D28.00007: Charged and neutral oxygen vacancies at MgO surfaces under realistic temperature and pressure conditions Norina A. Richter, Sergey V. Levchenko, Matthias Scheffler Surface O vacancies (F-centers) can strongly influence catalytic properties of MgO and metal clusters supported on MgO, but the experimental determination of their concentration at catalytic conditions is difficult. We employ density-functional theory and the \textit{ab initio} atomistic thermodynamics approach to determine concentration and charge states of F-centers at (111) and flat and stepped (100) surfaces of MgO at realistic (\textit{T, p}) conditions. Slab models and the virtual-crystal approximation [1] are used to model charged defects at surfaces. We find a strong dependence of F$^{+}$ and F$^{2+}$ formation energy on the exchange-correlation (XC) functional. Varying the amount of screening and fraction of exact exchange within the HSE functional, we find a linear correlation between defect formation energies and calculated valence-band width of the host material, in line with recent results for bulk systems [2]. Using this correlation and extrapolating to experimental band width, we conclude that only F$^{2+}$ centers can be present in significant concentrations at the (100) terraces at realistic conditions. --- [1] L. Vegard, Z. Phys. \textbf{5}, 17 (1921); M. Scheffler, Physica \textbf{146B}, 176 (1987); [2] R. Ramprasad \textit{et al.}, subm. to Phys. Rev. Lett. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D28.00008: Pressure dependence of the large polaron transport in anatase $TiO_{2}$ single crystals Ja\'{c}im Ja\'{c}imovi\'{c}, Cristian Vaju, Helmuth Berger, Arnaud Magrez, Viktor Cerovski, Radomir \v{Z}iki\'{c}, Richard Ga\'{a}l, L\'{a}szl\'{o} Forr\'{o} Anatase is a $TiO_{2}$ polymorph which is a 3.2 eV gap semiconductor interesting for several applications, including catalysis, photocatalysis, and, especially, dye-sensitized solar cells. Surprisingly, transparent single crystals of anatase grown in our laboratory show a metallic resistivity above 60 K which origin is a shallow donor level created by oxygen vacancies. The high value of the resistivity and its $T^{3} $ temperature dependence are the result of the polaronic nature of the charge carriers which is supported by the Seebeck coefficient (S). The application of hydrostatic pressure fails to close the donor level and to extend the conducting state to the entire temperature range. Instead, we have found a non-monotonic variation of the low temperature activation energy with applied pressure which is ascribed to the change of polaron's mobility. Thermo-electric power exhibits an unconventional temperature and pressure dependence shedding an additional light on the conductivity mechanism in this compound. The pressure dependence of S is governed by the transport of the large entropy associated with the polaron formation. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D28.00009: Revisiting the mechanism of photocatalytic activities in N-doped TiO$_{2}$ Naoto Umezawa, Jinhua Ye Photocatalysis possesses a great potential for environmental remediation and fuel production [1]. Nitrogen doped TiO$_{2}$ is a well-known visible-light sensitive photocatalyst where deep impurity states associated with substitutional nitrogen at oxygen sites (N$_{O})$ are believed to be the source of the red shift in photo-absorption edge. However, such a deep level should trap hole carriers, degrading oxidation process. The contradiction between the deep N$_{O}$ level and rather a high oxidation power of N-doped TiO$_{2}$ has been an unsolved puzzle. Here, we propose a convincing mechanism which successfully solves the riddle. N$_{O}$ strongly binds with a titanium atom at an interstitial site, forming a defect-impurity band, which consists of bonding and anti-bonding states of nitrogen $p$ and titanium $d$ and narrows the band gap. Such a newly formed band, which is connected to the valence band maximum of the host TiO$_{2}$, becomes the migration path of photo-induced hole carriers, assisting carrier transfer to the surface. This clearly explains the photocatalytic activity of N-doped TiO$_{2}$ both for the visible-light absorption and the oxidation reaction. [1] Hua Tong, Shuxin Ouyang, Yingpu Bi, Naoto Umezawa, Mitsutake Oshikiri, and Jinhua Ye, Advanced Materials DOI: 10.1002/adma.20110275 [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D28.00010: Femtosecond laser doped and nanostructured TiO$_{2}$ for photocatalysis Katherine Phillips, Elizabeth Landis, Cynthia Friend, Eric Mazur We present a novel method for femtosecond-laser doping of titanium dioxide (TiO$_{2})$ for above bandgap absorptance by irradiating titanium metal in the presence of oxygen and dopants. With a bandgap of 3.2 eV for the anatase crystalline phase, TiO$_{2}$ most strongly absorbs in the UV range ($\lambda <$ 387 nm). However, doping with metals and nitrogen has been shown to create intermediate states in the bandgap. Using femtosecond laser doping techniques on titanium in a gaseous environment, we produce laser-induced periodic surface structures. Altering the gas composition and pressure does not change the surface morphology, but it does impact the chemical composition of the surface. We present compositional data from x-ray photoelectron and Raman spectroscopy and structural data from scanning electron microscopy. Our research presents an innovative approach using laser scanning techniques to alter the structure of TiO$_{2}$ and generate a new material for visible-light photocatalysis that has the potential for watersplitting. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D28.00011: First-principles study of interstitial hydrogen in yttria-stabilized zirconia. Apostolos Marinopoulos Hydrogen is a common impurity in oxides and has been known to exhibit a dual behavior: either by being a dopant or alternatively an amphoteric impurity with the transition (pinning) level, E(+/-), lying inside the gap [1]. By means of calculations based on density-functional theory (DFT) and a hybrid-functional scheme (Heyd-Scuseria-Ernzerhof) we have studied the incorporation of hydrogen in yttria-stabilized zirconia. Equilibrium sites and formation energies were determined and the role of intrinsic oxygen vacancies needed to stabilize the cubic phase of the oxide was particularly examined. Whereas, in its positively-charged state, $H^{+}$, hydrogen was found exclusively to form a dative-type bond with O ions, the neutral paramagnetic $H^{0}$ displayed a coexistence with deep interstitial configurations with minimal lattice relaxation of the host lattice. A number of atomic-level mechanisms and migration paths were explored in order to understand this site interplay and the dynamics of neutral $H^{0}$ in a way that is consistent with the existing experimental data. [1]~C.G. Van de Walle and J. Neugebauer, Nature {\bf 423}, 626 (2003). [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D28.00012: Fundamental limits on optical transparency of transparent conducting oxides: free-carrier absorption in SnO$_2$ Hartwin Peelaers, Emmanouil Kioupakis, Chris G. Van de Walle Transparent conducting oxides (TCOs) are a technologically important class of materials used as transparent contacts in optoelectronic devices, such as flat-panel displays, touch screens, solar cells, and light-emitting diodes. These applications are possible because the TCOs combine high electrical conductivity with transparency to visible light. However, the large concentration of free electrons introduces a source of absorption that forms a fundamental limit to the transparency. We evaluated the importance of phonon-assisted free-carrier absorption in SnO$_2$ completely from first principles. We also provide insight into the mechanisms that govern absorption in different wavelength regimes. Our results show that the absorption is weak in the visible, but it increases by as much as a factor of 5 in the UV. For the infrared region, we show that the absorption increases with the wavelength, and that this increase is proportional to the third power of the wavelength. We also explain this third power dependency. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D28.00013: Self-trapping of holes in p-type oxides: Theory for small polarons in MnO Haowei Peng, Stephan Lany Employing the $p$-$d$ repulsion to increase the valence band dispersion and the energy of the VBM is an important design principle for p-type oxides, as manifested in prototypical $p$-type oxides like Cu$_2$O or CuAlO$_2$ which show a strong Cu-$d$/O-$p$ interaction. An alternative opportunity to realize this design principle occurs for Mn(+II) compounds, where the $p$-$d$ orbital interaction occurs dominantly in the fully occupied $d^5$ majority spin direction of Mn. However, the ability of Mn to change the oxidation state from +II to +III can lead to a small polaron mechanism for hole transport which hinders p-type conductivity. This work addresses the trends of hole self-trapping for MnO between octahedral (rock-salt structure) and tetrahedral coordination (zinc-blende structure). We employ an on-site hole-state potential so to satisfy the generalized Koopmans condition. This approach avoids the well-known difficulty of density-functional calculations to describe correctly the localization of polaronic states, and allows to quantitatively predict the self-trapping energies. We find that the tetrahedrally coordinated Mn is less susceptible to hole self-trapping than the octahedrally coordinated Mn. [Preview Abstract] |
Session D29: Focus Session: Qubits in Diamond II
Sponsoring Units: GQIChair: David Awschalom, UCSB
Room: 259A
Monday, February 27, 2012 2:30PM - 3:06PM |
D29.00001: Quantum information and quantum sensing with NV diamond Invited Speaker: Philip Hemmer |
Monday, February 27, 2012 3:06PM - 3:18PM |
D29.00002: Control of hybrid quantum registers in diamond Tim Hugo Taminiau, Toeno van der Sar, Machiel S. Blok, Gijs de Lange, Hannes Bernien, Wolfgang Pfaff, Lucio Robledo, Ronald Hanson Nitrogen Vacancy (NV) centers in diamond provide a robust quantum register consisting of the NV electron spin, the nuclear spin of the associated nitrogen atom and nearby nuclear spins of carbon impurities ($^{13}$C). The constituents of this hybrid system each have different properties that make them ideal for different roles. For example, the electron spin is readily polarized and read-out optically using spin-dependent fluorescence, whereas long coherence times make the nuclear spins ideal quantum memories. However, because all these systems evolve and decohere on very different time scales, it is challenging to control the register and to protect it from decoherence due to the surrounding environment at the same time. Here we discuss our recent progress in initializing, controlling and reading out all the spins in such few-qubit NV quantum registers. To this end, we combine dynamical decoupling of the electron spin with control of all the nuclear spins, and explore different initialization schemes. These few qubit registers might then be used to implement quantum search algorithms and error correction protocols. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D29.00003: Quantum interference of single photons from two remote Nitrogen-Vacancy centers in diamond Alexander Kubanek, Alp Sipahigil, Emre Togan, Michael Goldman, Yiwen Chu, Nathalie de Leon, Alexander Zibrov, Mikhail Lukin The interference of two identical photons impinging on a beam splitter leads to perfect photon coalescence where both photons leave through the same output port. This fundamental effect, known as Hong-Ou-Mandel (HOM) interference [1], can be used to characterize the properties of quantum emitters with high accuracy. This is a particularly useful tool for quantum emitters embedded in a solid state matrix because their internal properties, unlike those of atoms in free space, can vary substantially from emitter to emitter due to interactions with the environment. Here, we demonstrate HOM interference of photons emitted from two single Nitrogen-Vacancy (NV) centers in diamond that are spatially separated by 2 meters. The frequencies of the photons are controlled by tuning individual optical transitions of associated NVs via a DC electric field. The indistinguishability of the photons paves the way for entanglement generation between remote solid state qubits. [1] C. K. Hong, Z.Y. Ou, and L. Mandel, Phys. Rev. Lett. 59, 2044 (1987). [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D29.00004: Coherent excitation dynamics of single spins in diamond A.L. Falk, G.D. Fuchs, D.D. Awschalom, V.V. Dobrovitski The spin dynamics of nitrogen vacancy (NV) centers in diamond during non-resonant optical excitation are fundamentally important for both optical initialization and fluorescence-based spin read-out. While such processes rely on the preservation of the longitudinal spin projection, an NV center's lack of orbital coherence at room temperature might suggest that its quantum phase would be destroyed during excitation. We address this question using Ramsey experiments, quantum process tomography and theoretical modeling and establish limits on the coherence loss of an NV center during optical excitation [1]. By treating the excitation and spin precession as a quantum process, we measure a process fidelity of $F=0.87\pm0.03$, which includes excited state dephasing during measurement. Extrapolation to the moment of optical excitation yields $F\approx0.95$. These results provide a new understanding of NV centers' spin coherence during optical excitation and are crucial for efforts to use coherent evolution in the excited state for spin control. \\[4pt] [1] G. D. Fuchs, A. L. Falk, V. V. Dobrovitski, and D. D. Awschalom, \emph{submitted} (2011) [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D29.00005: Optimizing the resolution and the sensitivity of a scanning NV magnetometer Sungkun Hong, Patrick Maletinsky, Michael Grinolds, Lan Luan, Birgit Hausmann, Mikhail Lukin, Ronald Walsworth, Marko Loncar, Amir Yacoby A nitrogen-vacancy (NV) center in diamond has been recently considered as an outstanding atomic-scale magnetic field sensor. At the heart of NV based magnetometry is the ability to control the position of an NV within few nanometers to a sample, while preserving its spin coherence and readout fidelity. To this end, we previously developed a fabrication procedure for creating a monolithic scanning diamond nanopillar containing a single NV center. Here we present further optimization of our devices by locating NV centers with nanoscale accuracy as well as improving their magnetic field sensitivty. For locating the NV center, we developed a nanoscale imaging of a NV center in the device via near-field fluorescence quenching, which facilitates post selection of devices with NV centers being closer to the surface. In addition to enhancing photon collection via wave guiding through the nanopillar, we also significantly improved the spin coherence times of our devices via dynamic decoupling. This results in a magnetic field sensitivity of 30 nT/sqrt(Hz), unprecedented for scanning NV centers. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D29.00006: High Dynamic Range Magnetometry with a Single Spin in Diamond N.M. Nusran, M. Ummal Momeen, M.V. Gurudev Dutt Detection of the weak magnetic fields associated with nanometer sized volumes of spins could allow for non-invasive, element-specific probing of a variety of important physical and biological systems. Averaging out random noise which is the commonly used standard measurement strategy (SM) in most nano-sensors, will at best lead to a field variance that is inversely proportional to the total averaging time. Further, there exists a trade-off between the field sensitivity and the dynamic range in the SM. In this work, we demonstrate an alternative approach for accurate magnetic sensing, using novel phase estimation algorithms (PEA), implemented on a single electronic spin associated with the nitrogen-vacancy (NV) defect center in diamond. The field variance in our approach scales down faster than the SM. The trade-off between the field sensitivity and the dynamic range no longer exists in this approach. Our results show an improvement of $\sim 6.25 dB$ in the field sensitivity compared to the SM, over a large field sensing range ($ \sim \pm 0.3 mT$). Besides their direct impact on applications in demonstrated nanoscale magnetic sensing and imaging, this may also open the way for application of other quantum feedback and control techniques to magnetometry. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D29.00007: Room temperature solid-state quantum bit with second-long memory Georg Kucsko, Peter Maurer, Christian Latta, David Hunger, Liang Jiang, Fernando Pastawski, Norman Yao, Steven Bennet, Daniel Twitchen, Ignacio Cirac, Mikhail Lukin Realization of stable quantum bits (qubits) that can be prepared and measured with high fidelity and that are capable of storing quantum information for long times exceeding seconds is an outstanding challenge in quantum science and engineering. Here we report on the realization of such a stable quantum bit using an individual $^{13}$C nuclear spin within an isotopically purified diamond crystal at room temperature. Using an electronic spin associated with a nearby Nitrogen Vacancy color center, we demonstrate high fidelity initialization and readout of a single $^{13}$C qubit. Quantum memory lifetime exceeding one second is obtained by using dissipative optical decoupling from the electronic degree of freedom and applying a sequence of radio-frequency pulses to suppress effects from the dipole-dipole interactions of the $^{13}$C spin-bath. Techniques to further extend the quantum memory lifetime as well as the potential applications are also discussed. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D29.00008: Mechanism for optical initialization of spin in NV$^{-}$ center in diamond Sangkook Choi, Manish Jain, Steven G. Louie Optical initialization of the negatively charged nitrogen-vacancy (NV$^{-})$ center in diamond, the experimental manipulation of its degenerate mixed ground state into an un-entangled spin state through optical means, makes it one of the best candidates for realization of individually addressable spins in the solid state for quantum computing and other studies under ambient conditions. However, its exact mechanism is still not clear. Based on exact diagonalization of a many-electron Hamiltonian with parameters derived from ab initio GW calculations, the present study elucidates the electronic structure of the NV$^{- }$center and puts forward a concrete optical initialization mechanism. We calculated the ordering and energy surfaces of the low-energy many-body states and the relaxation processes of photo-excitation responsible for the optical initialization. Intersystem crossings are shown to be essential. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D29.00009: Long-range quantum gates using dipolar crystals Hendrik Weimer, Norman Yao, Chris Laumann, Mikhail Lukin We propose to use the magnetic dipole interaction in high density arrays of nitrogen-vacancy centers to enable long-range quantum logic between distant spin qubits. In our approach, an effective interaction between remote qubits is achieved by adiabatically following the ground state of the dipolar chain across a quantum phase transition [1]. We demonstrate that the proposed quantum gate is particularly robust against disorder and enables coherent coupling between qubits on distances that are compatible with sub-wavelength addressing techniques.\\[1em] [1] H. Weimer et al., arXiv:1009.1003 (2011). [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D29.00010: Magnetic imaging of a single electron spin using a scanning NV magnetometer under ambient conditions Michael Grinolds, Sungkun Hong, Patrick Maletinsky, Lan Luan, Ronald Walsworth, Mikhail Lukin, Amir Yacoby It has long been an outstanding challenge to develop a magnetometer capable of detecting individual spins under ambient conditions. Nitrogen-vacancy (NV) centers are an attractive candidate for such a sensor, as even at room temperature their spins can be initialized and read out optically, have long coherence times, and are localized on atomic lengthscales. Here we present measurements using a scannable NV center to magnetically image the dipole field of a single electron spin. For a target spin, we chose to use an additional NV spin as it can be initialized and driven independently from other proximal spins, allowing us to perform dynamical decoupling schemes on both sensing and target NV spins. Magnetic images are taken under ambient conditions and are achieved through optimizing NV magnetic sensitivity ($<$60 nT/sqrt(Hz)) as well as minimizing the distance between the NV center and our target spin ($<$50nm). [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D29.00011: Fabrication of thin diamond membranes for quantum information processing Igor Aharonovich, Jonathan Lee, Andrew Magyar, Evelyn Hu Coupling of nano-photonic devices to color centers in diamond offers exceptional opportunities to enhance our understanding of light-matter interactions. The formation of thin single crystal diamond membranes containing such centers, is an important prerequisite for the fabrication of diamond based devices. However, there are challenges in forming such membranes in ways that do not compromise the quality of the cavities or the optical properties of the emitters. Here we report the formation of optically active diamond membranes and the subsequent fabrication of optical cavities. In our approach, 1.7 $\mu $m thick diamond membranes were generated by forming a sacrificial layer using ion implantation, followed by thermal annealing. These membranes then served as templates for the epitaxial overgrowth of $\sim $ 300 nm of diamond using CVD. Remarkably, the regrown films reveal the presence of optically active defects which were \textit{not} present in the template, such as silicon-vacancy (SiV) or nitrogen vacancy centers. Microdisk cavities were then formed from the regrown single crystal diamond membranes. Whispering gallery modes (WGMs) with quality factors of $\sim $ 3000 were measured from the diamond cavities. Spectral overlap of WGMs with the zero phonon line of SiV centers was observed and lifetime reduction of the coupled emitter -- cavity system was measured. The demonstration of coupling between diamond emitters and a single crystal diamond cavity is a crucial step towards diamond integrated nano-photonic networks. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D29.00012: Noises of spin baths for qubits in diamond Zhihui Wang, Anirban Das, Daniel Lidar, Susumu Takahashi Nitrogen-vacancy (NV) center in diamond is a promising qubit candidate for quantum information processing and high precision magnetometry and is an excellent platform for studying quantum spin dynamics [1,2]. Overcoming spin decoherence of NV centers is critical to the applications. Coupling to spin baths of paramagnetic impurities and nuclei is a major decoherence source for NV centers. Therefore, recent theoretical and experimental efforts have aimed at suppressing the bath noises. In this presentation, we will discuss effects of the spin baths on the qubits at different regimes including high magnetic fields where the degree of the electron spin polarization is almost complete [3]. We will also discuss dynamical decoupling sequences to investigate spin bath noises. \\[4pt] [1] R. Hanson et al., \textit{Science.} \textbf{320}, 352 (2008). \\[0pt] [2] G. de Lange et al., \textit{Science}. \textbf{330}, 60 (2010) \\[0pt] [3] S. Takahashi et al., \textit{Phys. Rev. Lett.}\textbf{101}, 047601 (2008) [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D29.00013: Spectroscopy of composite solid-state spin environments for improved metrology with spin ensembles Nir Bar-Gill, Linh Pham, Chinmay Belthangady, David LeSage, Paola Cappellaro, Jeronimo Maze, Mikhail Lukin, Amir Yacoby, Ronald Walsworth For precision coherent measurements with ensembles of quantum spins the relevant Figure-of-Merit (FOM) is the product of spin density and coherence lifetime, which is generally limited by the dynamics of spin coupling to the environment. Significant effort has been invested in understanding the causes of decoherence in a diverse range of spin systems in order to increase the FOM and improve measurement sensitivity. Here, we apply a coherent spectroscopic technique to characterize the dynamics of a composite solid-state spin environment consisting of Nitrogen-Vacancy (NV) color centers in room temperature diamond coupled to baths of electronic spin (N) and nuclear spin (13C) impurities. For diamond samples with a wide range of NV densities and impurity spin concentrations we employ a dynamical decoupling technique to minimize coupling to the environment, and find similar values for the FOM, which is three orders of magnitude larger than previously achieved in any room-temperature solid-state spin system, and thus should enable greatly improved precision spin metrology. We also identify a suppression of electronic spin bath dynamics in the presence of a nuclear spin bath of sufficient nuclear spin concentration. This suppression could inform efforts to engineer samples with even larger FOM for solid-state spin ensemble metrology and collective quantum information processing. [Preview Abstract] |
Session D30: Focus Session: Quantum Information for Quantum Foundations - Entanglement and Causal Structure
Sponsoring Units: GQIChair: Howard Barnum, University of New Mexico
Room: 259B
Monday, February 27, 2012 2:30PM - 2:42PM |
D30.00001: Quantum measurement bounds beyond uncertainty relations Seth Lloyd, Vittorio Giovannetti, Lorenzo Maccone Quantum measurements are limited by bounds such as the Heisenberg uncertainty relations which limit the accuracy of measuring a quantity via the standard deviation of the conjugate one. This talk shows that the accuracy of measuring a quantity such as phase or time is limited by the expectation value of the conjugate quantity. This result proves the long-standing conjecture -- recently challenged -- that the ultimate phase-precision limit in interferometry is lower bounded by the inverse of the total number of photons employed in the estimation process. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D30.00002: Quantum sharability: Entanglement is less monogamous than you think Peter Johnson, Benjamin Schumacher, Lorenza Viola One of the essential features of entanglement is monogamy. However, is it well known that there are entangled states which are not monogamous. A generalization of the monogamy property is provided by the concept of sharability. For bipartite states, we say a state (or relationship) is $1$-$n$ sharable if a subsystem can simultaneously share this relationship with $n$ other subsystems. Any state with a limited sharing capacity is entangled, and the strictness of the limit corresponds to how entangled the state is. We present the most interesting findings of sharability of bipartite qubit and qudit states and describe an application to quantum teleportation. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D30.00003: New entanglement properties in systems of higher-spin particles Jay Lawrence, Mario Gaeta, Andrei Klimov We describe a new entanglement property of four-qutrit states that is inaccessable to any number of qubits. In such states, every particle is equally entangled with all others, as in GHZ states, but the entanglement is more robust than that of any four-qubit state. We describe the entanglement properties of related generalized graph states of three-state and five-state particles, and show how these suggest that new entanglement properties will emerge more generally for systems of p+1 particles, each having p states, where p is a prime number. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D30.00004: Optimal Probabilistic Simulation of Quantum Channels from the Future to the Past Dina Genkina, Giulio Chiribella, Lucien Hardy We introduce the study of probabilistic protocols that simulate quantum channels transforming input states in the future into output states in the past. The maximum probability for such a simulation is set by causality and, we claim, depends on the amount and type (classical or quantum) of information the given channel can transmit. In particular, we focus on probabilistic teleportation with multiple copies of input and output. We show that as the number of input copies increases, the maximum probability of successful teleportation increases, a feature that is impossible in classical physics. As the number of input copies tends to infinity, the teleportation probability converges to the maximum probability for the simulation of an ideal classical channel from the future to the past. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D30.00005: The asymmetry properties of pure quantum states Iman Marvian, Robert Spekkens The \emph{asymmetry properties} of a state relative to some symmetry group specify how and to what extent the given symmetry is broken by the state. Characterizing these is found to be surprisingly useful for addressing a very common problem: to determine what follows from a system's dynamics (possibly open) having that symmetry. We demonstrate and exploit the fact that the asymmetry properties of a state can be understood in terms of information-theoretic concepts. We show that for a pure state $\psi$ and a symmetry group $G$, they are completely specified by the characteristic function of the state, defined as $\chi_{\psi}(g)\equiv \langle \psi|U(g)|\psi\rangle$ where $g\in G$ and $U$ is the unitary representation of interest. Based on this observation, we study several important problems about the interconversion of pure states under symmetric dynamics such as determining the conditions for reversible transformations, deterministic irreversible transformations and asymptotic transformations. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D30.00006: Equilibration of Global Observables to Microcanoical Measure for Complex Systems Joseph Emerson, Cozmin Ududec In the last few years we have seen significant progress in understanding how the canonical features of quantum statistical mechanics can be derived rigorously from an exact treatment of the underlying quantum mechanical system. Here we consider a related problem: we determine sufficient conditions under which one can derive an effective equilibration to the microcanonical ensemble for the measurement statistics of the global observables of a closed system. Our central assumption is that the unitary time-evolution operator of the system is sufficiently complex when expressed in the eigenbasis of the observable of interest that its eigenstates can be modeled by a typical unitary chosen from the Haar measure (or Circular Unitary Ensemble). This assumption is well-motivated from numerical studies in the field of quantum chaos where this property has been observed for simple model systems whose classical counterparts are globally chaotic. Further, we discuss the time scale on which equilibration occurs in the context of two models for the eigenvalues of the dynamical system. We argue that our results are a natural consequence of an epistemic view of pure quantum states, but may be surpising or even controversial for adherents of other interpretational perspectives. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D30.00007: Coarse-graining makes it hard to see micro-macro entanglement Sadegh Raeisi, Pavel Sekatski, Christoph Simon Observing quantum effects such as superpositions and entanglement in macroscopic systems requires not only a system that is well protected against environmental decoherence, but also sufficient measurement precision. Motivated by recent experiments, we study the effects of coarse-graining in photon number measurements on the observability of micro-macro entanglement that is created by greatly amplifying one photon from an entangled pair. We compare the results obtained for a unitary quantum cloner, which generates micro-macro entanglement, and for a measure-and-prepare cloner, which produces a separable micro-macro state. We show that the distance between the probability distributions of results for the two cloners approaches zero for a fixed moderate amount of coarse-graining. Proving the presence of micro-macro entanglement therefore becomes progressively harder as the system size increases. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D30.00008: On the quantification of resourcefulness in quantum information Yuval Sanders, Ben Fortescue, Gilad Gour Quantum information processing tasks cannot be performed for free; several types of informational resource must be consumed. Such resources are often expensive: entanglement, for example, is quite difficult to distribute between distant parties. Efficient consumption of informational resources is therefore desirable for practical quantum information processing. Determination of the efficiency of a given protocol requires some method of quantifying the resources present before and after the implementation of a protocol. In the prototypical case of entanglement, one or several entanglement monotones are often used to determine the entanglement cost of a protocol. The mathematical definition of an entanglement monotone has undergone multiple revisions since its formal introduction by Vidal. Currently, a real-valued function of quantum states is considered an entanglement monotone if the function is monotonically decreasing under application of channels that can be enacted using only local operations with classical communication. In this lecture, I argue that even this simple definition is unnecessarily restrictive for fully characterising the entanglement of a state and propose a more general scheme of relative quantifiers of entanglement. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D30.00009: Scalar Fields via Causal Tapestries William Sulis Causal tapestries provide a framework for implementing an explicit Process Theory approach to quantum foundations which models information flow within a physical system. We consider event-transition tapestry pairs. An event tapestry ${\rm O}$ is a 4-tuple (L, K, M, \underline {I}$_{p })$ where K is an index set of cardinality $\kappa $, M = $M$ x F($M)$ x D x P($M')$ a mathematical structure with $M$ a causal space, F($M)$ a function space, D a descriptor space, P($M')$ either a Lie algebra or tangent space on a manifold $M'$, \underline {I}$_{p }$ an event tapestry. L consists of elements of the form [$n$]$<$\textit{$\alpha $}$>${\{}$G${\}}, $n$ in K, \textit{$\alpha $} in M and $G$ an acyclic directed graph whose vertex set is a subset of L$_{p}$ Likewise, a transition tapestry $\Pi $ is a 4-tuple (L', K', M', \underline {I'}$_{p })$ where M' = $M'$ x F($M')$ x D' x P'($M)$. The dynamic generates a consistent succession of ${\rm O}-\Pi $ pairs by means of a game based on the technique of forcing used in logic to generate models. This dynamic has previously been shown to be compatible with Lorentz invariance. An application of this approach to model scalar fields is presented in which each informon is associated with a function of the form $_{ }$ f($\pi $k$_{1 }$/$\sigma _{1 }$,{\ldots},$\pi $k$_{N }$/$\sigma _{N}$ )sin ( $\sigma _{1 }$t$_{1 }$--$\pi $k$_{1 })$/ ( $\sigma _{1 }$t$_{1 }$--$\pi $k$_{1 })$ {\ldots}.sin ( $\sigma _{N }$t$_{N }$--$\pi $k$_{N })$/ ( $\sigma _{N }$t$_{N }$--$\pi $k$_{N })$ and the WSK interpolation theorem is used to generate the resulting scalar field on the causal manifold. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D30.00010: Topology of Quantum Discord Nga Nguyen, Robert Joynt Quantum discord is arguably a more sensitive measure of quantum correlations than quantum entanglement, and may be able to serve as a resource for quantum computation. All quantum correlations are subject to destruction by external noise. The route by which this destruction takes place depends on the shape of the hypersurface of zero discord in the space of generalized Bloch vectors. In the case of 2 qubits, we show that, except at the origin, this hypersurface is a 9-dimensional manifold with boundary embedded in a 15-dimensional background space. This is done by computing the tangent vectors explicitly and verifying that there are no self-intersections. We discuss the implications for the time evolution of discord in physical models, which contrasts sharply with the evolution of entanglement. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D30.00011: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 4:42PM - 4:54PM |
D30.00012: Quantumness versus entanglement in quantum measurements Gerardo Adesso, Marco Piani We analyze a hierarchy of quantumness measures for composite systems, defined in terms of the entanglement necessarily created between systems and apparata during local measurements. We prove that the quantumness so defined is always greater than intra-systems entanglement, establishing a firm ordering relation between different non-classical features of correlations. We analyze qualitatively and quantitatively the flow of correlations in iterated measurements, showing that quantumness and entanglement can never decrease along von Neumann chains. Our results provide a comprehensive framework to understand and quantify general signatures of quantumness in multipartite states, and prove how useful the broader study of the quantumness of correlations can be to shed light on issues in quantum information processing, in the quantum theory of measurement and in quantum foundations. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D30.00013: Operational interpretation of the G-asymmetry for Abelian groups Michael Skotiniotis, Gilad Gour In a reference frame alignment protocol the sender, Alice, prepares a quantum system in a state $ket{\psi}$, that serves as a token of her reference frame, and sends this system to a receiver, Bob, who performs a measurement and learns about the reference frame. We derive the state and measurement that maximize the accessible information in a reference frame alignment protocol. We show that in the limit where a large number of systems are sent, the accessible information per copy equals the Holevo bound. The latter was shown to be equal to the relative entropy of frameness, or $G$-asymmetry, of the state $ket{\psi}$, a measure of resourcefulness analogous to the relative entropy of entanglement. We show that for a reference frame alignment protocol, associated with a finite abelian group, $Z_N$, or the continuous group $U(1)$, associated with the important case of photon number super-selection, the rate of accessible information is quantified by the linearized, regularized $G$-asymmetry. Our result provides an information theoretic operational interpretation for the $G$-asymmetry that has been thus far lacking. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D30.00014: Considerations of Closed Systems that Measure Particles Michael Steiner, Ronald Rendell The Measurement Problem has been of fundamental concern since the discovery of Schrodinger's equation. We have been developing a framework for which this problem can be considered under the assumption that the detector is a closed system. Considerations that such systems satisfy will be presented. Questions of whether or not such a framework is possible for which the various considerations can be met will be presented. Related existing work in the literature will be presented. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D30.00015: Is entanglement signaling really impossible? Jack Sarfatti Quantum entanglement cannot be used as a communication channel without an auxiliary light speed limited classical key to unlock the message at the receiver? Hermitian observables guarantee orthogonal sender base states that erase any nonlocal influence of the sender settings on the detection probabilities at the receiver. However, this is no longer true when the entangled whole has different macro-quantum coherent Glauber sender states. Glauber states are non-orthogonal eigenstates of the non-Hermitian photon destruction operator. The Born probability interpretation breaks down because of ``phase rigidity'' (P.W. Anderson's ``More is different''). This is a new regime that is to orthodox quantum theory what general relativity is to special relativity. Antony Valentini has argued that the breakdown of the Born probability rule entails ``signal non locality'' (aka entanglement signals). The space-time interval between the sending and the receiving irreversible measurements is irrelevant depending only on the free will of the local observers. That is, this is a pre-metrical topological information effect. There is asymmetry between the sending and the receiving. Therefore, there is no ambiguity between active (retro) cause and passive effect. [Preview Abstract] |
Session D31: Focus Session: Topological Insulators: Synthesis & Characterization - Thin Films
Sponsoring Units: DMPChair: Anthony Richardella, Pennsylvania State University
Room: 260
Monday, February 27, 2012 2:30PM - 2:42PM |
D31.00001: Epitaxial growth of high quality Bi$_{2}$Se$_{3}$ thin films on CdS Xufeng Kou, Liang He, Faxian Xiu, Murong Lang, Yong Wang, Alexei Federov, Xinxin Yu, Jin Zou, Kang Wang We report the experiment of high quality epitaxial growth of Bi$_{2}$Se$_{3}$ thin films on lattice-matched hexagonal CdS (0001) substrates using a solid source molecular-beam epitaxy system. Layer-by-layer growth of single crystal Bi2Se3 has been observed from the first quintuple layer with larger surface triangular terraces. The improved film quality facilitates the characterization of surface states during magneto-transport measurements, such as high Hall mobility of $\sim $6000 cm2/V?s, a distinct Shubnikov-de Haas (SdH) oscillations and weak anti-localization cusp in the magnetic field dependent longitudinal resistance. These characteristics of Bi$_{2}$Se$_{3}$ thin films promise a variety of potential applications in ultra-fast, low-power dissipation devices. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D31.00002: Fabrication of High Quality Topological Insulator Thin Films and Heterostructures Li Zhang, Robert Hammond, Merav Dolev, Mac Beasley, Aharon Kapitulnik In this talk, I will present a method of fabrication high quality topological insulator thin films and heterostructures with ferromagnet materials using MBE with a RF Selenium cracker cell and pulsed laser deposition. I will also show some preliminary results on the physical properties of those films, including topography, crystal structure and transport properties. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D31.00003: MBE growth and transport of the topologically tunable (Bi$_{1-x}$ In $_{x})_{2}$Se$_{3}$ system Matthew Brahlek, Namrata Bansal, Nikesh Koirala, Suyang Xu, Zahid Hasan, Seongshik Oh A current challenge in the field of topological insulators (TI) is identifying a clear transport signal of the surface conduction. The structural similarity between Bi$_{2}$Se$_{3}$ and In$_{2}$Se$_{3}$ allowed us to combine the two to obtain (Bi$_{1-x}$ In $_{x})_{2}$Se$_{3}$; Bi$_{2}$Se$_{3}$ has inverted bands, and thus is a non-trivial insulator. In$_{2}$Se$_{3}$ has no inverted bands and is therefore a trivial band insulator with energy gap 1.3-1.9eV. The mixing ratio x can be thought of as a knob to switch the system from a trivial to a non-trivial state. I will briefly discuss our scheme for producing atomically smooth molecular beam epitaxial grown thin films. I will also discuss our work on transport in the TI-to-non TI regime, and the metal to insulator regime, and compare these results with angle resolved photo emission spectroscopy data. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D31.00004: Hybrid Physical-Chemical Vapor Deposition of Bi$_{2}$Se$_{3}$ Thin films on Sapphire Joseph Brom, Yue Ke, Renzhong Du, Jarod Gagnon, Qi Li, Joan Redwing High quality thin films of topological insulators continue to garner much interest. We report on the growth of highly-oriented thin films of Bi$_{2}$Se$_{3}$ on c-plane sapphire using hybrid physical-chemical vapor deposition (HPCVD). The HPCVD process utilizes the thermal decomposition of trimethyl bismuth (TMBi) and evaporation of elemental selenium in a hydrogen ambient to deposit Bi$_{2}$Se$_{3}$. Growth parameters including TMBi flow rate and decomposition temperature and selenium evaporation temperature were optimized, effectively changing the Bi:Se ratio, to produce high quality films. Glancing angle x- ray diffraction measurements revealed that the films were c-axis oriented on sapphire. Trigonal crystal planes were observed in atomic force microscopy images with an RMS surface roughness of 1.24 nm over an area of 2$\mu $mx2$\mu $m. Variable temperature Hall effect measurements were also carried out on films that were nominally 50-70 nm thick. Over the temperature range from 300K down to 4.2K, the carrier concentration remained constant at approximately 6x10$^{18 }$cm$^{-3}$ while the mobility increased from 480 cm$^{2}$/Vs to 900 cm$^{2}$/Vs. These results demonstrate that the HPCVD technique can be used to deposit Bi$_{2}$Se$_{3}$ films with structural and electrical properties comparable to films produced by molecular beam epitaxy. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D31.00005: Pulsed Laser Deposition of Epitaxial Topological Insulator Thin Films: Bi$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{2}$Se Shixiong Zhang, Li Yan, Jingbo Qi, Mujin Zhuo, Yongqiang Wang, Rohit P. Prasankumar, Quanxi Jia, S. Tom Picraux While high quality epitaxial thin films of topological insulators have been achieved by molecular beam epitaxy, there has been little progress using other thin film growth techniques. Here, we report the growth of high quality epitaxial Bi$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{2}$Se thin films on silicon (111) and YSZ (111) substrates by pulsed laser deposition (PLD). Systematic structural characterization of the films using x-ray diffraction and transmission electron microscopy has demonstrated that a low laser pulse rate is the key to achieving high quality epitaxial films. Rutherford backscattering spectrometry measurements suggest that the film composition is strongly influenced by the growth temperature and background gas pressure. The electrical transport properties of the films grown at the optimal conditions will also be discussed. Since PLD is an excellent tool to grow a variety of functional oxides, including multiferroics, magnetic semiconductors and high temperature superconductors, the growth of epitaxial topological insulator thin films by the same technique paves the way to synthesize multi-layered heterostructures of the above materials and search for novel physics arising from the resulting interfacial couplings. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D31.00006: Growth and properties of half-Heusler DyPdBi Brian Mulcahy, Mao Zheng, Carolyn Kan, James Eckstein Some half-Heusler phases have been predicted by Chadov, \textit{et al}, to exhibit topological electronic structure. In addition to providing an exciting new topological system, the breadth of the elemental parameter space for this system opens the door for investigation of the interplay between many novel physical states with the topological system. We have grown thin films of one of these phases, the cubic half-Heusler material DyPdBi, using carefully flux matched molecular beam epitaxy. Crystalline quality was monitored via \textit{in situ} RHEED and verified by \textit{ex situ} x-ray diffraction measurements. Transport measurements indicate the emergence of interesting correlated behavior at low temperature. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D31.00007: Vacancy Reduction, Structural and Electronic Studies of Epitaxial Films of Topological Insulators F. Katmis, V. Fatemi, H. Steinberg, L. Orona, P. Wei, P. Jarillo-Herrero, J. Moodera We have developed methods for controlling the carrier concentration via a vacancy concentration reduction procedure in the MBE grown epitaxial topological insulator (TI) thin film on various substrates to reach the intrinsic features of TI. Our single crystalline TI thin films allowed us to systematically investigate the nature of coherent transport in this system. For structural characterization of TI thin films, various non-distractive methods, such as x-ray and electron based diffraction techniques, were used as a local probe to understand the long-, short-range atomic ordering and also lattice site occupation. Besides the improved electronic properties of the layers, as grown crystalline films density increased by 20{\%} due to controlled vacancy reduction, determined by in-situ x-ray diffraction. Furthermore, correlation of vacancies and Se ion migration was observed to be the likely reason for lowering the carrier concentration. Our study also shows the dependence of carrier mobility and the vacancy concentration which has been optimized. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D31.00008: Growth and in-situ ultra-high resolution ARPES studies of the Bi-Te family of topological insulators James Lee, Felix Schmitt, Rob Moore, Inna Vishik, Ming Yi, Z.X. Shen Topological insulators have received intense focus in the condensed matter community due to their academic and technical potential. The Quantum Anomalous Hall state is an example of the exotic physics that could have a major industry impact if it can be realized and controlled. While the topologically protected states live at interfaces between insulators of two topological classes, investigations of the underlying electronic structure via angle resolved photoemission spectroscopy requires pristine surfaces. Here we present results from in situ ultra-high resolution laser ARPES investigations at low temperatures of the doped Bi-Te family of topological insulator thin films grown via molecular beam epitaxy. Electronic structure evolution as a function of dopant, dopant level and thickness will be presented and compared to theoretical predictions. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D31.00009: Fabrication of Bismuth Selenide Topological Insulating Samples Lucas Orona, Hadar Steinberg, Valla Fatemi, Ferhat Katmis, Jagadeesh Moodera, Pablo Jarillo-Herrero In this talk, I will discuss fabrication of nanometric topological insulator Bi$_{2}$Se$_{3}$ devices. Our group uses two fabrication methodologies: Epitaxial thin films and single crystal exfoliation. I will discuss the benefits and drawbacks of each methodology. I will also address the effects on device performance by various steps of the fabrication process. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D31.00010: Interfacing 2D and 3D Topological Insulators: Bi(111) Bilayer on Bi$_2$Te$_3$ Toru Hirahara, Gustav Bihlmayer, Yusuke Sakamoto, Manabu Yamada, Hidetoshi Miyazaki, Shin-ichi Kimura, Stefan Bl\"ugel, Shuji Hasegawa Topological insulators (TI) are insulating materials but have metallic edge states that carry spin currents and are robust against nonmagnetic impurities [1]. While there have been a large number of reports on three-dimensional (3D) TI, only few works have been done in terms of two-dimensional (2D) TI. In the present paper, we report the successful formation of bilayer Bi, which was theoretically predicted to be a 2D TI [2]. We deposited bilayer Bi on a 3D TI $\mathrm{Bi_2Te_3}$, which the lattice mismatch is very small. From angle-resolved photoemission spectroscopy measurements and {\it ab initio} calculations, the electronic structure of the system can be understood as an overlap of the band dispersions of bilayer Bi and $\mathrm{Bi_2Te_3}$. Our results show that the Dirac cone is actually robust against nonmagnetic perturbations and imply a unique situation where the topologically protected one- and two-dimensional edge states are coexisting at the surface [3]. \\[0pt] [1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. {\bf 82}, 3045 (2010).\\[0pt] [2] S. Murakami, Phys. Rev. Lett. {\bf 97}, 236805 (2006).\\[0pt] [3] T. Hirahara {\it et al.,} Phys. Rev. Lett. {\bf 107}, 166801 (2011). [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D31.00011: Fabrication of Bi$_{2}$Te$_{3}$ Nanodots by Droplet Epitaxy on GaAs substrates Zhaoquan Zeng, Chen Li, Dongsheng Fan, Yusuke Hirono, Timothy Morgan, Xian Hu, Jian Wang, Meenakshi Singh, Zhiming Wang, Shui-Qing Yu, Aqiang Guo, Gregory Salamo Bi$_{2}$Te$_{3}$, as a three-dimensional topological insulator, causes wide attention. Here, we report the fabrication of Bi$_{2}$Te$_{3}$ nanodots on GaAs substrate by droplet epitaxy using molecular beam epitaxy (MBE). Reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS), and Raman measurement revealed the existence of Bi$_{2}$Te$_{3}$ nanodots. Several approaches were developed to control the size and density of as-grown Bi$_{2}$Te$_{3}$ nanodots. Temperature and density dependent magneto-transport measurements were investigated. This may provide a platform for the interaction investigation among topological insulators, semiconductors, ferromagnets and superconductors. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D31.00012: AFM, Raman and electrical transport studies of topological insulating materials subjected to argon plasma etching Isaac Childres, Jifa Tian, Ireneusz Miotkowski, Yong Chen Plasma etching is an important tool in nano-device fabrication. We report a study on argon plasma etching of exfoliated flakes of topological insulator materials Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, Sb$_{2}$Te$_{3}$ and Bi$_{2}$Te$_{2}$Se. We present data from atomic force microscopy (AFM), Raman spectroscopy and low-temperature magneto-transport measurements. The thickness of our samples measured by AFM is observed to decrease approximately linearly with plasma exposure time. We extract an etching rate for each type of material. For the initial increase in plasma exposure time, we observe increasing intensity and width of the characteristic E$_{g}$$^{2}$ Raman peak with little change in peak position. The width of this peak for etched flakes becomes larger than those of unetched samples of the same thickness. Additionally, we find that even moderate etching can significantly reduce the conductivity and hall mobility. These results indicate disorder is generated by plasma etching and impedes both phonon and carrier transport. Our findings are valuable for understanding the effects of argon plasma etching on topological insulator materials and using irradiation as a potential method to introduce controlled disorder in such materials. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D31.00013: Tuning the spatial location of topological surface states via proximity effects Guangfen Wu, Yan Sun, Hua Chen, Xiaoguang Li, Ping Cui, Jinlan Wang, Xingqiu Chen, Zhenyu Zhang In order to exploit promising applications of topological insulators in quantum computing, spintronics, and catalysis, one prerequisite is to gain effective manipulation of the spatial distribution of the topological surface states (TSS). We use first-principles calculations to investigate the interfacial proximity effects on the TSS for hybrid systems consisting of semiconducting thin films with different bandgaps, spin-orbital coupling (SOC) strengths,~and lattice mismatches grown on the TI substrate of Bi$_{2}$Se$_{3}$. Our results show that the spatial location of the robust TSS can be tuned by the interplay of the effects associated with the SOC strength and the band gap size of the semiconductor. Potential experimental confirmations of these strong predictions are also discussed. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D31.00014: Metal-Supported High Crystalline Bi$_{2}$Se$_{3}$ Quintuple Layers Jeong Heum Jeon, Won Jun Jang, Jong Keon Yoon, Sang-youn Weon, Se-jong Kahng Atomically flat thin films of Bi$_{2}$Se$_{3}$ were grown on Au(111) metal substrate using molecular beam epitaxy. Hexagonal atomic structures and quintuple-layer steps were observed at the surfaces of grown films using scanning tunneling microscopy. Multiple sharp peaks from (003) family layers were characterized by X-ray diffraction measurements. The atomic stoichiometry of Bi and Se was considered using X-ray photoemission spectroscopy. Moir\'e patterns were obtained at the surfaces of one quintuple layer films due to lattice mismatch between Bi$_{2}$Se$_{3}$ and Au. Our experiments suggest that Au is a reasonable material for electrodes in Bi$_{2}$Se$_{3}$ devices. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D31.00015: Ab initio study of epitaxial graphene on top of Sb$_{2}$Te$_{3}$ topological insulator Kyung-Hwan Jin, Seung-Hoon Jhi Understanding topological phase as observed in Dirac materials such as graphene and topological insulator (TI) has been a central issue in the field of condensed matter physics. Graphene and TI exhibit unique 2D electronic structures that attract great attention for potential application to spintronic devices. Heterostructures of graphene and TI provide interesting platforms to explore exotic electronic and transport properties of Dirac materials. Electronic structures of graphene in contact with TI were investigated using first-principles methods and tight-binding models. The Dirac cones of graphene on top of TI surface show several interesting features including band-gap opening and band splitting. By fitting first-principles calculations to tight-binding models, we analyzed the origin of the changes in the Dirac cones. We found that both intrinsic and extrinsic spin-orbit couplings are enhanced significantly due to proximity to topological insulator Sb$_{2}$Te$_{3}$ and that graphene turns into quantum spin-Hall phase. Our results suggest that graphene is also useful as a probe of TI surface states. [Preview Abstract] |
Session D32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Lattice Dynamics
Sponsoring Units: DMP DCOMPChair: Ilya Grinberg, University of Pennsylvania
Room: 261
Monday, February 27, 2012 2:30PM - 3:06PM |
D32.00001: Dynamical couplings in ferroelectrics and multiferroics Invited Speaker: Dawei Wang First-principle-based molecular dynamics simulations are developed and used to investigate dynamical phenomena in the $\textrm{Pb(Zr,Ti)}\textrm{O}_{3}$ (PZT) ferroelectric solid solutions and in the multiferroic BiFeO$_3$ system\footnote{D. Wang, J. Weerasinghe,and L. Bellaiche, Phys. Rev. B {\bf 83},020301(R) (2011).}$^,$\footnote{D. Wang, E. Buixaderas, J. \'I\~niguez, J. Weerasinghe,H. Wang, and L. Bellaiche,Phys. Rev. Lett. {\bf 107}, 175502 (2011). }. Several interesting effects are reported, including: (1) the existence of \emph{two} $E$ modes in PZT in the 50--75\quad cm$^{-1}$ range for temperatures smaller than $\simeq 200$\quad K when the system is in its $R3c$ phase. Such existence originates from a {\it linear} coupling between ferroelectric (FE) motions and tiltings of oxygen octahedra; (2) a Fermi resonance (FR) emerging from a {\it nonlinear} coupling between FE distortions and oxygen octahedra tilts. This FR manifests itself as the doubling of a nominally-single FE mode in a purely FE phase, when the resonant frequency of the FE mode is close to the first overtone of the oxygen octahedra tiltings; and (3) the prediction of an electromagnon peak, that results from specific interactions between magnetic dipoles, FE motions and oxygen octahedra tiltings, in BiFeO$_3$. Some of these results have been confirmed by recent Raman scattering experiments, and analytical models are also developed to better understand such effects.\footnote{This work is done in collaboration with L. Bellaiche (University of Arkansas), J. Hlinka (Institute of Physics, Czech Republic), E. Buxiaderas (Institute of Physics, Czech Republic), J. \'I\~niguez (Institut de Ci\`encia de Materials de Barcelona, Spain) and J. Weerasinghe (University of Arkansas)} [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D32.00002: A-site ion vibrational modes in ABO3 polar perovskites Invited Speaker: Jiri Hlinka It is well known that Pb(B'B'')O3 family of complex perovskites contains materials with very useful dielectric, piezoelectric and ferroelectric properties. The overall electromechanical response of these system is believed to be related to their characteristic micro- to nanoscale structural correlations (chemical clusters, polar nanoregions or nanotwins). At the level of the ABO3 unit cell, the remarkable polarizability of these materials originates from lead-ion displacements, and the anharmonic motion of these ions, loosely linked to the core oxygen octahedra network, largely dominates in the low-frequency phonon modes in the system. Therefore, the investigation of the low-frequency phonon modes in these systems is of a great importance. The aim of this contribution is to report recent experimental studies of such low-frequency vibrations by inelastic X-ray, neutron and light scattering techniques, in materials like lead zirconate titanate, lead magnesium niobium titanate or lead titanate. In particular, attention will be payed to our recent inelastic X-ray scattering studies of PZT lattice dynamics. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D32.00003: Fully-atomistic first-princples approach to the temperature-dependent lattice-dynamical properties of perovskite oxides Mathias Ljungberg, Jacek Wojdel, Jorge Iniguez We present a methodology for the parametrization of effective Hamiltonians for ferroelectric perovskites that provides a fully atomistic description of the materials. The parameters in our effective models are obtained from first-principles calculations; no experimental input is used. The potential energy surface is represented by a truncated Taylor expansion around the ideal cubic perovskite structure, and is, up to second order, exactly equivalent to the first-principles computed one. Higher order terms are determined by fitting to the first-principles energetics and phonon dispersion relations of the relevant low-symmetry phases. By treating all structural degrees of freedom explicitly, our models allow us to investigate the competition between the soft modes that drive the ferroelectric phase transitions in these materials and other, secondary modes. We will show that such an interaction is of significant importance in the prototypical ferroelectric compound PbTiO3; our method's ability to capture it constitutes a considerable improvement upon earlier approaches. We will also report on our current work to extend this methodology to treat inhomogeneous materials, as e.g. superlattices. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D32.00004: Lattice dynamics of cubic PbTiO$_3$ Izumi Tomeno, Jaime A. Fernandez-Baca, Karol J. Marty, Yorihiko Tsunoda, Kunihiko Oka The lattice dynamics of cubic PbTiO$_3$ has been investigated using inelastic neutron scattering. We found four kinds of soft modes in cubic PbTiO$_3$: (1) the TO modes toward the $\Gamma$ point, (2) the TA $\Lambda_3$ mode toward the $R$ point, (3) the TA $\Lambda_3$ mode around the midpoint (1/4,1/4,1/4), and (4) the TA branches in the entire range. Moreover, the TO $\Sigma_4$ branch becomes flat away from the zone center. The steep dispersion of the TO modes toward $\Gamma$ is isotropic and confined to the region $\xi<0.2$. The temperature dependence of the $\Gamma_{15}$ mode up to 1173 K is explained by a combination of the Lydanne-Sachs-Taller relation and the Curie-Weiss law. In contrast, the TA $\Lambda_3$ modes at the midpoint and $R$ point are weakly temperature dependent. The coexistence of the soft $\Gamma_{15}$ and $R_{25}$ modes is in agreement with the predicted phonon instability. The midpoint softening suggests the tendency toward forming a fourfold periodicity along the [1,1,1] direction. The energy of the TO $\Delta_5$ branch for cubic PbTiO$_3$ is considerably higher than that for Pb(Zn$_{1/3}$Nb$_{2/3}$)O$_3$. This indicates that the TO modes are dominated by the B-site atom motion. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D32.00005: BaTiO$_{3}$ nanocrystals studied by Raman spectroscopy A.K. Farrar, T. Engman, D.A. Tenne, S. Adireddy, G. Caruntu Variable-temperature Raman spectroscopy study of BaTiO$_{3}$ nanocrystals of varied sizes (from 8 to 20 nm) will be presented. Highly uniform cube-shaped BaTiO$_{3}$ nanocrystals have been prepared by solvothermal synthesis at temperatures below 140\r{ }C and characterized by x-ray diffraction and transmission electron microscopy. Raman spectra (measured with ultraviolet and visible excitation) show that all nanocrystals studied are ferroelectric; nanocrystal size effect on the Curie temperature has been investigated. Temperature evolution of Raman spectra (10--600K) demonstrates that the ferroelectric phase of the nanocrystals is different from the bulk BaTiO$_{3}$. The transitions from tetragonal to orthorhombic and from orthorhombic to rhombohedral phases, which are characteristic for bulk BaTiO$_{3}$, have not been observed in the nanocrystals; the ferroelectric phase in the nanocrystals is the same in the entire temperature range below $T_{c}$, and is different from any of the bulk phases. The observed behavior may be explained by complex polarization patterns theoretically predicted for zero-dimensional ferroelectrics. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D32.00006: Raman spectra and lattice dynamics of disordered complex perovskite BaMg$_{1/3}$Ta$_{2/3}$O$_3$ Severian Gvasaliya, Dan H\"uvonen, Sergey Lushnikov, Elena Popova, Tatiyana Shaplygina, Andrey Zheludev In relaxor ferroelectrics the chemical and the displacive ionic disorders coexist and may cause a relaxation of the selection rules for Raman scattering. We performed a Raman scattering study of BaMg$_{1/3}$Ta$_{2/3}$O$_3$ (BMT), which is chemically disordered cubic perovskite showing no evidences for displacive disorder. Polarized Raman spectra from a single crystal of BMT were collected in the temperature range of 5 $-$ 550 K. We are going to discuss the symmetry assignments of the observed modes and their temperature evolution. Simplified shell-model for the lattice dynamics of BMT will be presented. The results for BMT will be compared to the well-known observations for the Raman spectra from related relaxor ferroelectrics PbMg$_{1/3}$Ta$_{2/3}$O$_3$ and PbMg$_{1/3}$Nb$_{2/3}$O$_3$. In particular, the lowest Raman line observed in BMT is at $\sim$110 cm$^{-1}$, whereas the doublet line in PbMg$_{1/3}$Ta$_{2/3}$O$_3$ is observed around 50 cm$^{-1}$. Also, we found out that the width of well-isolated A$_{1g}$ line of BMT is approximately two times narrower than that observed in relaxors. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D32.00007: Frequency shift of Raman modes due to an applied electric field and domain inversion in LiNbO3 Greg Stone, Brian Knorr, Venkatraman Gopalan, Volkmar Dierolf We report changes in the frequency of several Raman modes in congruent and near-stoichiometric lithium niobate that are observed under the application of external applied electric fields parallel to the ferroelectric axis and after domain inversion. A comparison of the direction of the frequency shifts due to an applied electric field and domain inversion reveals that after forward poling there is a reduction of the internal field that is dependent on the intrinsic defect concentration present in the crystal. Upon back poling, the internal field returns to its original state. A further inspection of the Raman peaks reveals that the magnitude of the frequency shift is not consistent between an applied electric field and domain inversion. This indicates that the change in the local internal field introduced by domain inversion is not limited to the ferroelectric axis, but the fields orthogonal to the ferroelectric axis change. Work supported by the grants: NSF-DMR 0602986 and 1008075 [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D32.00008: Infrared phonon anomaly and magnetic excitations in single-crystal Cu$_{3}$Bi(SeO$_{3})_{2}$O$_{2}$Cl Kevin H. Miller, C. Martin, X. Xi, H. Berger, G.L. Carr, D.B. Tanner Infrared reflection as a function of temperature has been measured on the anisotropic single-crystal Cu$_{3}$Bi(SeO$_{3})_{2}$O$_{2}$Cl. The complex dielectric function and optical properties along all three crystal axes of the orthorhombic cell were obtained via Kramers-Kronig analysis and by fits to a Drude-Lorentz model. Below 110 K drastic anomalies in the phonon spectrum (e.g., new modes and splitting of existing modes) are observed along all three crystal axes. Transmission in the terahertz region as a function of temperature has revealed magnetic excitations originating below the ferromagnetic ordering temperature, T$_{c}$=24 K. The origin of the excitations in the magnetic state will be discussed in terms of their polarization and externally-applied magnetic field dependence. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D32.00009: Hydrogen diffusion and tunneling in KTaO$_{3}$ from first principles Hazem Abu Farsakh, Chris G. Van de Walle The high proton conductivity in many perovskites has attracted interest for potential applications, such as in fuel cells. A promising approach to increase their conductivity at lower temperatures involves enhancing quantum-mechanical tunneling. In order to determine the role of tunneling for H in KTaO$_{3}$ we employ first principles calculations for H interstitial atoms. We identify H binding sites and diffusion channels and calculate the associated activation energies. In addition, we analyze the effect of lattice relaxations on the diffusion barriers. Through calculating the 3D potential energy surface of H, we determine accurate H tunneling rates by numerically solving Schr\"{o}dinger's equation for H in the 3D potential energy surface. Finally, we examine lattice vibrations and analyze their role in assisting proton tunneling. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D32.00010: Scaling of Flat Band Potential and Dielectric Constant as a Function of Ta Concentration in Ta-TiO2 Epitaxial Films Y.L. Zhao, A. Roy Barman, S. Dhar, A. Annadi, M. Motapothula, J.H. Wang, H.B. Su, M. Breese, T. Venkatesan, Q. Wang Electrochemical impedance spectroscopy measurements of pulsed laser deposited single crystal anatase TiO2 thin films with various concentrations of Ta substituting for Ti were carried out. UV-visible measurements show a systematic increase of the bandgap with Ta incorporation. Corresponding Mott-Schottky plot was applied to obtain a continuous shift of the flat band potential with increasing free charge carrier concentration. This was verified theoretically by ab initio calculation which shows that extra Ta d-electrons occupy Ti t2g orbital with increasing Ta concentration, thereby pushing up the Fermi level. The Mott-Schottky results were consistent when compared with Hall effect and temperature dependent resistivity measurements. From the measured deviation of carrier densities from Hall and Mott-Schottky measurements we have estimated the static dielectric constant of the TiO2 as a function of Ta incorporation. We are able to estimate the shifts of both the conduction and valence bands from these measurements. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D32.00011: Raman spectroscopic studies of Ti$_{1-x}$Ta$_{x}$O$_{2}$ alloy thin films S. Saha, A. RoyBarman, C.B. Tay, T. Sarkar, Y. Zhao, S. Tripathy, S. Dhar, - Ariando, S.J. Chua, T. Venkatesan Anatase Ti$_{1-x}$Ta$_{x}$O$_{2}$ thin films have been of interest not only because of the recently found defect originated room temperature ferromagnetism, but also because of the wide possibilities of its application as transparent conducting oxide in flat panel displays, light emitting diodes and solar cells. The incorporation of a foreign element in a host oxide crystal has conventionally been referred to as doping. However, recently we have experimentally shown that even with as less as 1{\%} Ta incorporation in TiO$_{2}$, a totally new alloy system is formed. Here we present a Raman and x-ray diffraction study of anatase Ti$_{1-x}$Ta$_{x}$O$_{2}$ thin films grown on (100) LaAlO$_{3}$ substrate by PLD to understand the crystal structure and defects in the Ta-incorporated TiO$_{2}$ thin films. We find that as Ta is incorporated in the TiO$_{2}$ lattice the out-of-plane phonons undergo red-shift while the in-plane phonon undergoes a blue-shift, suggesting an expansion of the TiO$_{2}$ lattice along the out-of-plane direction with a concomitant in-plane contraction. [Preview Abstract] |
Session D33: Physics of Batteries and Fuel Cells
Sponsoring Units: GERAChair: Matthew Panzer, Tufts University
Room: 106
Monday, February 27, 2012 2:30PM - 2:42PM |
D33.00001: Effect of Iron-based Impurities on the performance of nanostructured C-LiFePO$_{4}$ cathode materials for Li ion Batteries P. Vaishnava, A. Dixit, K. Bazzi, M.B. Sahana, C. Sudakar, M. Nazri, V. Naik, V.K. Garg, A.C. Oliveira, G.A. Nazri, R. Naik We report synthesis of pure and C-LiFePO$_{4}$ nanoparticles in 20-30 nm size by sol-gel method. Three samples of C-LiFePO$_{4}$ were prepared by mixing 0.25M, 0.50M, and 1M lauric acid in the precursor solutions for carbon coating the particles. The samples were~characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), IR spectroscopy, SQUID magnetometery, Raman spectroscopy, and Fe$^{57}$ M\"{o}ssbauer spectroscopy. All the samples were thoroughly investigated for their electrochemical properties. The XRD measurements showed all the samples are single phase materials with no impurity phase. However, we identified at least three residual non crystalline impurity phases simultaneously using Fe$^{57 }$M\"{o}ssbauer spectroscopy, XPS, and the magnetic measurements. The elemental chemical states for Fe 2p, P 2p, and O 1s are analyzed using XPS for LiFePO$_{4}$ and compared with those of C-LiFePO$_{4}$ materials. SQUID magnetometery measurements suggest an antiferromagnetic transition $\sim $50 K in both pure LiFePO$_{4 }$and C-LiFePO$_{4}$ samples. The role of various phases, such as FeP, Fe$_{2}$P, $\alpha $-Fe and Fe$_{2}$O$_{3 }$identified and analyzed by Fe$^{57}$ M\"{o}ssbauer spectroscopy and XPS, will be discussed in relationship to the electrochemical properties of the cathode materials. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D33.00002: Role of Ce and In doping in the performance of LiFePO$_{4}$ cathode material for Li ion Batteries Balaji Mandal, Mariam Nazri, Prem P. Vaishnava, Vaman M. Naik, Gholam A. Nazri, Ratna Naik Recently, the olivine LiFePO$_{4}$ has attracted attention as a promising cathode material for Li ion batteries. However, its poor electronic conductivity is a major challenge for its industrial applications. Different approaches have been taken to address this problem. Here, we report a method of improving its conductivity by doping In and Ce ions at the Fe site. We prepared the samples by sol-gel method followed by annealing at 650 \r{ }C in Ar (95{\%}) +H2(5{\%}) atmosphere for 5 hrs. XRD and Raman spectroscopy confirm that the olivine structure remains unchanged upon doping with In and Ce up to 5 wt{\%}. XRD analysis shows the values of the lattice parameters increase with doping as the ionic radii of Ce and In ions are larger than that of the Fe$^{2+}$ ion. This observation also suggests that both Ce and In ions replace Fe ions and not the Li ions in the material. Upon doping, ionic conductivity was found to increase from 10$^{-9}$ to 10$^{-4}$ Ohm$^{-1}$cm$^{-1}$. Interestingly, Ce doped LiFePO$_{4}$ showed a higher conductivity than In doped LiFePO$_{4}$. SEM measurements show a bigger grain size of $\sim $300-500 nm in doped LiFePO$_{4}$ which decreased to $\sim $50 nm when the materials were synthesized using 0.25M lauric acid as a precursor. The electrochemical characteristics of the doped LiFePO4 along with conductivity and Raman data will be presented. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D33.00003: Formation of small polarons in Li$_{2}$O$_{2}$ and implications for Li-air batteries Joongoo Kang, Yoon-Seok Jung, Su-Huai Wei, Anne Dillon Lithium-air batteries (LABs) have recently been revitalized as a promising electrical energy storage system due to their exceptionally high theoretical energy density. However, its usage is limited by poor rate capability and large polarization in the cell voltage due primarily to the formation of Li$_{2}$O$_{2}$ in the air cathode. Here, using hybrid density functional theory, we found that the formation of small polarons in Li$_{2}$O$_{2}$ is the origin that limits the electron transport in Li$_{2}$O$_{2}$. Consequently, the low electron mobility contributes to the hysteresis in cell voltage and limits the power density of the LABs. We suggest that similar behavior should exist in other peroxides, and p-type doping in Li$_{2}$O$_{2}$ could significantly improve the performance of LABs at high current densities. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D33.00004: The Li-induced Conversion Reaction of Ultra-Thin FeF2 Films Ryan Thorpe, Sylvie Rangan, Robert Bartynski, Ozgur Celik, Leszek Wielunski Iron (II) fluoride has recently gained interest as a possible cathode material in Li-ion conversion batteries. Conversion materials like this could potentially store 2-3 times more energy than conventional intercalation battery materials by utilizing the full range of charge states in their constituent transition metal ions, i.e., Fe$^{(2+)}$F$_{2}$ + 2Li$^{+}$ + 2e$^{-}$ $\rightarrow$ Fe$^{0}$ + 2LiF. Using surface science techniques, we are able to observe this reaction at the FeF$_{2}$-Li interface. We have grown 5nm films of high-purity polycrystalline FeF$_{2}$ in ultra-high vacuum and deposited atomic Li on the surface to simulate the conversion reaction in the absence of external contaminants. Using UV photoemission (UPS), x-ray photoemission (XPS), and inverse photoemission (IPE) spectroscopies, we have measured the composition and charge states of these materials. XPS of the FeF$_{2}$ sample after various Li exposures indicate a direct conversion from Fe$^{2+}$ to Fe$^{0}$, with no intermediary phases. The growth of a Fermi edge in UPS and IPE also indicates the formation of metallic Fe, while peaks characteristic of LiF can be seen in UPS after sufficient Li exposure. These results are consistent with those of recent TEM measurements on real electrochemical cells. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D33.00005: Electrochemical Performance of Lithium Iron Phosphate Doped with Tungsten Hanu Arava, Andrew Trenchard, Gan Liang, Hui Fang Due to its high thermal stability, low cost and high theoretical charge capacity, LiFePO4 has emerged as one of the most promising cathode materials for large-scale lithium ion batteries. In this work, we systematically investigated the effect on structure and electrochemical properties brought by W doping on Fe site of LiFePO$_{4}$. LiFe$_{1-x}$W$_{x}$PO$_{4}$ (x= 0. 0.01, 0.02, 0.03) samples with and without carbon coating were prepared by using solid-state reaction. The phase and structure of as prepared powders were characterized by X-ray diffraction and scanning electron microscope. Cycling charge and discharge measurement at various C-rates and cyclic voltammetry were employed to reveal the electrochemical properties. Results showed that carbon coating dramatically improved the capacity at fast C-rate. 2 at.{\%} W doping was observed to have the highest charge capacity with 143 mAh/g at 0.1C and a 109 mAh/g for 1C. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D33.00006: Model conversion reaction: Li reactivity on iron oxifluorides Sylvie Rangan, Ryan Thorpe, Ozgur Celik, Robert Bartynski Iron fluorides have gained interest as choice materials for conversion reaction-based batteries. Owing to their large band gaps and their ability to store up to three electrons per formula unit, batteries using these materials operate at high voltages and high energy densities. However the large band gap inhibits charge conduction and thus impedes efficient charging and discharging cycles. Two paths have been taken to overcome this limitation: 1) The use of nanoparticles embedded into a conducting carbon matrix improves both ionic and electronic conduction; and 2) The use of iron oxifluorides, which are characterized by a slightly reduced energy gap that facilitates electronic conduction. It is the latter point that that is central to this study as, curiously, relatively little is known about the electronic structure of iron oxifluorides and their interaction with Li, a key aspect of a storage cell's electrochemistry. Model conversion reactions have been studied by evaporating Li on FeF$_{2}$ and FeF$_{x}$O$_{y}$ samples. Using X-ray and UV photoemission as well as inverse photoemission spectroscopies, the occupied states and the unoccupied electronic states of the resulting samples have been probed. Transmission electron microscopy has been used in parallel to investigate phase analysis. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D33.00007: Shape and volume changes in Lithiated Silicon anodes for batteries Ekin Cubuk, Wei Wang, Efthimios Kaxiras Silicon is one of the most promising materials for use as an anode for Lithium-ion batteries due to its capacity to hold a large number of Li atoms (up to 4.4 Li per Si atom). One of the biggest challenges in using Silicon as anode material is mechanical failures due to large volume expansion during the lithiation process. Recently detailed experiments have been reported on the dependence of volume change on the crystal orientation, especially for nano-scale structures. We investigate the microscopic mechanisms for shape changes during lithiation of Si by comparing the reaction mechanisms of Li atoms on different surfaces of crystalline Si using first-principles calculations. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D33.00008: Effect of carbon support on catalytic efficiency and durability in fuel cells Cecile Malardier-Jugroot, Michael Groves, Deborah Durbin, Manish Jugroot New nanomaterials already play a key role in several emerging technologies. For instance, in fuel cell technology, catalytic efficiency can be greatly enhanced due to the high surface area of nanomaterials. Improving the durability and efficiency of a platinum catalyst is an important step in increasing its utility when incorporated as the anode or cathode of a proton exchange membrane fuel cell (PEMFC). The authors have shown using Density Functional Theory methods [1] that doping the carbon support of the Pt catalyst can increase the durability and efficiency of the catalyst. This paper will present the effect of doping of the carbon support on the complete reaction path of the Oxygen Reduction Reaction using \textit{ab initio} structural methods as well as a complete \textit{ab initio} molecular dynamics characterization of the reaction. In addition, the electronic structure of the carbon support was shown to improve the metal/CO interaction for the development of a membrane to prevent catalyst poisoning [2]. The paper will also emphasize the effect of the solvent, which is experimentally shown to be crucial. [1] M. Groves, A. Chan, C. Malardier-Jugroot and M. Jugroot, \textit{Chem. Phys. Letters}, $481$(4-6), 214-219, 2009 [2] D. Durbin and C. Malardier-Jugroot, \textit{J. Phys. Chem. C}, $115$ (3), 808--815, 2011 [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D33.00009: Probing the Reversibility Limit of Lithium Ion Transport in Solid Film Batteries Howard Wang, Liwei Huang, Kaikun Yang, Gregory Downing, Alec Talin, Paul Naney, Jason Zhang, Jon Owejan, Jeffrey Gagliardo, Jeanette Owejan Time-resolved neutron depth profiling (TR-NDP) has been used to measure the lithium distribution in electrode layers of thin film batteries during charge/discharge cycles. TR-NDP data demonstrate quantitatively that ionic transport in electrodes follows the electric current in the external circuits under normal charge/discharge conditions whereas deviates upon sudden structural changes. The reversibility limit of ionic transport has been quantified to indicate the onset of battery failure. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D33.00010: Energy Storage and Generation from Thermopower Waves Joel Abrahamson, Sayalee Mahajan, Wonjoon Choi, Nicole Schonenbach, Jungsik Park, Michael Walsh, Jared Forman, Jae-Hee Han, Kourosh Kalantar-zadeh, Michael Strano We have demonstrated through simulation and experiment that the nonlinear coupling between an exothermic chemical reaction in a fuel and a nanowire or nanotube with large axial heat conduction accelerates the thermal reaction wave along the nano-conduit. The thermal conduit rapidly transports energy to unreacted fuel regions, and the reaction wave induces a concomitant thermopower wave of high power density, producing electrical current in the same direction. At up to 14 W/g, this can be substantially larger than the power density offered by current micro-scale power sources (e.g. fuel cells, batteries) and even about seven times greater than that of commercial Li-ion batteries. MEMS devices and wireless sensor networks would benefit from such high power density sources to enable functions such as communications and acceleration hampered by present power sources. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D33.00011: In-situ TEM observations of the nanoscale electrochemistry in a Li-ion cell Jianyu Huang The lithiation-delithiation processes of anode materials in lithium ion batteries were observed by in-situ electron microscopy. The lithiation-delithiation was strongly materials, size, and orientation dependent. Upon charging of SnO$_{2}$ nanowires, we observed high density of dislocations in the reaction front, while in charging of ZnO nanowires, we observed discrete cracks in the reaction front. In charging Si nanowires, we found the volume expansion was highly anisotropic, resulting in a dumbbell-shaped cross-section and cracking, eventually splitting the single nanowire into sub-wires. Carbon coating not only increases rate performance but also alters the lithiation-induced strain of SnO$_{2}$ nanowires. The radial expansion of the coated nanowires was completely suppressed. The lithiation process of individual Si nanoparticles was strongly size-dependent, i.e., there exists a critical particle size with a diameter of $\sim $ 150 nm, below which the particles neither cracked nor fractured upon lithiation, above which the particles first formed cracks and then fractured. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D33.00012: Gasoline-powered series hybrid cars cause lower life cycle carbon emissions than battery cars Christoph Meinrenken, Klaus S. Lackner Battery cars powered by grid electricity promise reduced life cycle green house gas (GHG) emissions from the automotive sector. Such scenarios usually point to the much higher emissions from conventional, internal combustion engine cars. However, today's commercially available series hybrid technology achieves the well known efficiency gains in electric drivetrains (regenerative breaking, lack of gearbox) even if the electricity is generated onboard, from conventional fuels. Here, we analyze life cycle GHG emissions for commercially available, state-of the-art plug-in battery cars (e.g. Nissan Leaf) and those of commercially available series hybrid cars (e.g., GM Volt, at same size and performance). Crucially, we find that series hybrid cars driven on (fossil) gasoline cause fewer emissions (126g CO2eq per km) than battery cars driven on current US grid electricity (142g CO2eq per km). We attribute this novel finding to the significant incremental emissions from plug-in battery cars due to losses during grid transmission and battery dis-/charging, and manufacturing larger batteries. We discuss crucial implications for strategic policy decisions towards a low carbon automotive sector as well as relative land intensity when powering cars by biofuel vs. bioelectricity. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D33.00013: Design and Characterization of a Microcombustor for Thermophotovoltaic Devices Abbey Licht, Michael Motola-Barnes, Meth Bandara, Han Chen, Tom Vandervelde While batteries are currently used to provide power to devices in remote areas, their low energy density (.5MJ/kg) increases carrier weight, limiting the range of applications. Microcombustor systems, on the other hand, rely on hydrocarbon fuels which have a much greater energy density (40MJ/kg) and generate the same power at a fraction of the weight. The microcombustor when paired with a high-efficiency energy extracting device, such as a thermophotovoltaic cell, presents a complete micro-power system. This work describes the fabrication and characterization of a catalytic microcombustor designed specifically to optimize the power production of a thermophotovoltaic cell. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D33.00014: Reverse electrowetting -- a new approach to high-power harvesting of mechanical energy Tom Krupenkin, J. Ashley Taylor, Supone Manakasettharn Over the last decade electrical batteries have emerged as a critical bottleneck in portable electronics development. High-power mechanical energy harvesting can potentially provide a valuable alternative to the use of batteries, but until now, its adoption has been hampered by the lack of an efficient mechanical-to-electrical energy conversion technology. In this talk a novel mechanical-to-electrical energy conversion method is discussed. The method is based on reverse electrowetting (REWOD) -- a novel microfluidic phenomenon. Electrical energy generation is achieved through the interaction of arrays of moving microscopic liquid droplets with novel nanometer-thick multilayer dielectric films. Advantages of this process include the production of high power densities, up to 1 KW sq. m; the ability to directly utilize a very broad range of mechanical forces and displacements; and the ability to directly output a broad range of currents and voltages, from several volts to tens of volts. We hope that the REWOD-based energy harvesting can provide a novel technology platform for a broad range of new electronic products and enable reduction of cost, pollution, and other problems associated with the wide-spread battery use. [Preview Abstract] |
Session D34: Focus Session: Nano I -- Clusters
Sponsoring Units: DCPChair: Shiv Khanna, Virginia Commonwealth University and Gabor Somorjai, UC Berkeley
Room: 107A
Monday, February 27, 2012 2:30PM - 3:06PM |
D34.00001: Cluster Structure and Reactions: Gaining Insights into Catalytic Processes Invited Speaker: A. Welford Castleman To many researchers outside the field of cluster science it comes as a surprise that much can be learned of its relevance to catalysis, even restricting the discussion to ionized systems. The objective of this talk is to present how fundamental insights into reaction mechanisms can be gained through employing alternative approaches that complement rather than supersede more conventional methods in the field of catalysis. In view of the well acknowledged role of defect centers in effecting reactivity, and the preponderance of recent papers presenting evidence of the importance of charged sites, the desire to conduct repetitive experiments is clear. Presented herein are approaches using clusters to accomplish this in order to unravel fundamental catalytic reaction mechanisms, and to use identified superatoms and the concepts of element mimics to tailor catalysts with desired functionality. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D34.00002: Photoelectron Angular Distributions of Transition Metal Dioxide Anions - a joint experimental and theoretical study Ivan Iordanov, Dasitha Gunaratne, Christopher Harmon, Jorge Sofo, A.W. Castleman, Jr Angular-resolved photoelectron spectroscopy (PES) studies of the MO$_{2}$- (M=Ti, Zr, Hf, Co, Rh) clusters are presented for the first time along with theoretical calculations of their properties. We confirm previously reported non-angular PES results for the vertical detachment energies (VDE), vibrational energies and geometric structures of these clusters and further explore the effect of the 'lanthanide contraction' on the MO$_{2}$- clusters by comparing the electronic spectra of 4d and 5d transition metal dioxides. Angular-resolved PES provides the angular momentum contributions to the HOMO of these clusters and we use theoretical calculations to examine the HOMO and compare to our experimental results. First-principles calculations are done using both density functional theory (DFT) and the coupled-cluster, singles, doubles and triples (CCSD(T)) methods. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D34.00003: Stability and Spectroscopic Properties of Singly and Doubly Charged Anions Swayamprabha Behera, Puru Jena Considerable interest currently exists in expanding the pool of hetero-atomic negative ions which have electron affinities much higher than that of halogen atoms. These molecules, called superhalogens, usually consist of a metal atom (M) at the core surrounded by halogen atoms (X) and are represented by the formula MX$_{n+1}$ where n is the maximal valence of the metal atom. We have studied the possibility if pseudohalogens such as CN, which mimic the chemistry of halogen atoms, can be used as building blocks of new superhalogens. Using density functional theory, Moller-Plessett perturbation theory and coupled clusters methods we have studied systematically the structure and spectroscopic properties of M(CN)$_{n }$systems (M=Na, Mg, Al; n=1-3 for Na, 1-4 for Mg, and 1-5 for Al) and compared the results to that of corresponding MCl$_{n}$ clusters. We find that there is a significant difference between these two systems. This is because pseudohalogens have a tendency to dimerize and hence, we find that for these clusters the values of adiabatic detachment energy and the electron affinity may not be the same. Also, we have studied the dianions of M(CN)$_{n}$ and MCl$_{n}$ complexes to determine the critical size required for their stability. We show that CN moieties stabilize a dianion better than halogen atoms do due to the increase in the phase space over which added electrons are delocalized. This could play an important role in interpreting future experimental data on M(CN)$_{n}$ complexes. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D34.00004: Structure and stability of the M$_{8-n}$N$_n$C$_{12}$ (M=Ti, Zr; N=Sc, Y and n=1,2,3) Met-Cars as building blocks of cluster-assembled materials Cuneyt Berkdemir, Shi-Bo Cheng, A. Welford Castleman Jr., Jorge O. Sofo Clusters can be used as building blocks for new materials. However, in order to form a bulk material with clusters, they should be chemically stable. This stability can be characterized by a closed-shell electronic configuration having a large HOMO-LUMO gap. Met-Cars, metal-carbon species composed of early transition metals bonded to carbon, are stable but very reactive. We propose a method to lower their reactivity by metal atom substitution with lower atomic number atoms. We report DFT results on M$_{8-n}$N$_n$C$_{12}$ (M = Ti, Zr; N = Sc, Y, and n=1,2,3) Met-Cars in the neutral, cationic and anionic charge states. Our results show that the isoelectronic M$_6$N$_2$C$_{12}$, M$_5$N$_3$C$_{12}^-$ and M$_7$N$_1$C$_{12}^+$ Met-Cars have closed-shell electronic configurations and larger HOMO-LUMO gaps (1.0-1.7 eV) than that of the M$_8$C$_{12}$. The intercluster interaction between two isolated neutral M$_6$N$_2$C$_{12}$ Met-Cars is relatively weak compared to the M$_8$C$_{12}$ dimers. Due to the weak interaction of the isolated neutral Met-Cars, their unique properties would be retained during assembly. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D34.00005: Interplay of geometric and electronic structure in metalloid gallium clusters Dmitri Schebarchov, Nicola Gaston Over the last two decades, the so-called ``renaissance of main group chemistry'' has led to significant advances in the synthesis, isolation and characterization of metalloid gallium clusters. What distinguishes these from other metalloid species (e.g. ligand-protected gold, silver, palladium, etc.) is their structural diversity, with the existence of four different Ga$_{22}$ frameworks being a particularly striking example. To gain more insight into this polymorphism, we carried out electronic structure calculations using density functional theory. Our calculations verify that two of the ligand-protected Ga$_{22}$ isomers can to some degree be viewed as superatom complexes - their respective metalloid cores are more or less close-packed, roughly spherical, and exhibit a well-defined electronic shell structure with a completely filled outer-most shell. The other two frameworks contain a slightly distorted icosahedral Ga$_{12}$ core without a central atom - an unusual arrangement for metals - and the underlying electronic structure is more complex. This talk will serve as a summary of our calculations and illustrate the interplay of geometric and electronic structure in metalloid gallium clusters. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D34.00006: Cluster Structure Selection Based on High Vertical Electron Affinity: The Case of TiO$_{2}$ Clusters Noa Marom, Minjung Kim, James Chelikowsky We study the structure and electronic properties of (TiO$_{2})_{2-10}$ clusters using basin hopping based on density functional theory (DFT), combined with many-body perturbation theory in the GW approximation. We show that in photoemission experiments performed on anions the isomers with the high electron affinity are selectively observed rather than those with the lowest energy. These isomers possess a highly reactive Ti$^{3+}$ site. The selectivity for highly reactive clusters may be exploited for applications in catalysis. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D34.00007: Growing Ag Clusters in Superfluid He Droplets Luis F. Gomez, Evgeny Loginov, Naihao Chiang, Avik Halder, Nicholas Guggemos, Vitaly Kresin, Andrey F. Vilesov Here we report on the growth and study of Ag clusters, ranging in size from 10 to 10$^{7}$ atoms, in a beam of superfluid He droplets. The droplets were used to capture Ag atoms from a hot oven, which then recombined in the interior of the droplet at sub-Kelvin temperature. The structure of the obtained Ag clusters was studied in situ via laser spectroscopy of their plasmon resonance. Furthermore, the clusters were surface-deposited and studied via transmission electron microscopy. The images have provided for a measure of the cluster flux and size distribution, which is in good agreement with an estimate based on the energy balance of Ag cluster growth in He droplets. The images also reveal an astounding change in shape of the deposited clusters to elongated and track-shaped with increased droplet size. This is ascribed to the formation of vortices within the He droplets whose cores are traced by the Ag atoms and clusters. The possible formation mechanism of the vortices and their stability will also be discussed. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D34.00008: Atomic and Electronic Structure of Ag$_{n}$ ( n $\le $ 13 ) Clusters and their Reactivity with O$_{2 }$ Gabriel U. Gamboa, Arthur C. Reber, Shiv N. Khanna First principles theoretical studies on the atomic structure, stability, and electronic structure of neutral and anionic Ag$_{n}$ clusters have been carried out within a gradient corrected density functional approach. It is shown that the clusters are marked by planar or layered structures. For most clusters, the ground state of anions has lowest spin multiplicity. To examine the reactivity of clusters, containing even number of electrons, with O$_{2}$, we calculated the spin excitation energy representing the energy required to excite the cluster to the triplet configuration. It is shown that several of these even electron anionic species and in particular Ag$_{13}^{-}$ have high spin excitation energy indicating that they should be inert towards reactivity with oxygen. The theoretical predictions are shown to be in agreement with preliminary experimental data on the reactivity of anionic species. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D34.00009: On the Reactivity of Al$_{13}$I$_{n}^{-}$ and Al$_{14}$I$_{n}^{-}$ Clusters with Methanol M.B. Abreu, A.C. Reber, S.N. Khanna Al$_{13}$ and Al$_{14}$ cluster anions act as halogen or alkaline earth superatoms respectively when bound by I atoms. Al$_{13}$I$_{2}^{-}$ and Al$_{14}$I$_{3}^{-}$ have enhanced resistance to oxidation by oxygen because of the clusters' closed electronic shells, however the reactivity of aluminum clusters with methanol depends on the presence of complementary active sites. We have examined the reactivity of Al$_{13}$I$_{n}^{-}$ and Al$_{14}$I$_{m}^{-}$ with methanol to identify if the presence of electronegative Iodine may induce active sites on the cluster. The presence of a single Iodine atom on Al$_{13}^{-}$ is insufficient to activate the cluster, however two adjacent ligands induce an active site and makes the cluster highly reactive. The Al$_{14}$I$_{m}^{-}$ clusters are found to be reactive with methanol highlighting the importance of geometric shell closures in ligand protected clusters. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D34.00010: Bonding and Electronic Structure of Cluster Assemblies with Metal Carbonyls Meichun Qian, Arthur Reber, Shiv Khanna, Sukhendu Mandal, Hector Saavedra, Paul Weiss, Ayusman Sen Understanding the factors controlling the band gap energies of cluster-assembled materials is an important step towards creating nano-assemblies with tailored properties. To this end, we have investigated the band gap energies of cluster assemblies involving arsenic clusters bound to metal carbonyl charge-transfer complexes, [As$_{7}$M(CO)$_{3}$]$^{3-}$ M = Cr, Mo, W. The binding of a single charge-transfer complex is shown to have a small effect on the band gap energy as the arsenic lone pair orbital and metal carbonyl orbitals are closely aligned in energy, resulting in a gap similar to the original cluster. The band gap energy is also found to be insensitive to the architecture of the assembled material. In the case where two charge-transfer complexes are bound to the cluster, the bottom of the conduction band is shown to be localized on a solvent molecule bound to the metal carbonyl. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D34.00011: Stability and Shell Magnetism in Transition Metal Doped Calcium clusters Victor M. Medel-Juarez, J. Ulises Reveles, S.N. Khanna, V. Chauhan, P. Sen Clusters of many metallic elements are known to exhibit enhanced stability at valence electron counts 2, 8, 18, 20, 34, 40{\ldots} that can be understood within a simple spherical confined nearly free electron gas model. In this work we show a magnetic species whose stability is rationalized on a modification of the above shell sequence through deformations of the spherical geometry and through enhanced exchange splitting of the electronic shells via impurity atoms with large atomic orbital exchange splitting. Through first principles theoretical studies of the electronic structure and stability of TMCa$_{8}$ (TM= Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) clusters we identify a stable magnetic FeCa$_{8}$ cluster of 24 valence electrons distributed into a closed 1S2 1P6 1D10 2S2 shell sequence of 20 paired electrons and with 4 electrons occupying the majority 2D$_{xy}$2D$_{x -y}^{2 2}$, 2D$_{xz}$ and 2D$_{yz}$ levels while the unfilled 2D$_{z}^{2}$ level is separated by a large energy gap of 0.61 eV arising from atomic deformation. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D34.00012: On the stability and oxidation of Pdn (n=1-7) clusters on rutile TiO2(110) S. Vincent Ong, Shiv Khanna First principles theoretical studies of the atomic and electronic structure of Pd$_n$ (n=1-7) clusters supported on a TiO$_2$(110) surface, and O$_2$ activation by such clusters, have been carried out within a gradient corrected density functional approach. It is shown that the supported Pd$_n$ cluster geometries are driven by competing effects including intra-cluster interactions favoring compact geometries and cluster support interactions that favor geometries that flatten out in the TiO$_2$(110) surface channel. When exposed to O$_2$, a single Pd atom only activates the O-O bond while all other clusters energetically favor a broken O-O bond. The differing behavior of the Pd atom is proposed to originate from the minimal amount of charge transferred from Pd to O$_2$ and its spin excitation energy. For Pd$_n$O$_2$ (n=2-7), it is shown that while the first O is adsorbed on the Pd$_n$ cluster, the second O occupies a site above a lattice Ti site at the Pd-Ti interface and is indicative of spill over O atoms. The theoretical finding are compared with recent experiments on the structure and oxidation of CO by supported clusters in the presence of O$_2$. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D34.00013: Strengthening of Au-Au bonds in small gold clusters by adsorbing noble gases Luca M. Ghiringhelli, Sergey Levchenko, Matthias Scheffler In state-of-the-art experiments for the vibrational spectra of metal clusters in the gas phase, photodissociation spectroscopy is performed on clusters complexed with noble gas (RG) atoms, where a RG atom is usually expected to form a weak van der Waals bond. By employing DFT (PBE functional with selected comparisons to PBE0, and to MP2 and CCSD(T) calculations), we surprisingly find a partially covalent bond of {\em neutral} dimers with RG. For RG = Ar, Kr, Xe one or two RG atoms can bind in a linear molecule with Au$_2$. While both Hirschfeld and Mulliken analyses show a small electron transfer from the RG to Au$_2$, the Au-Au bond {\em shortens} and the Au-Au stretch frequency increases. This is inconsistent with the expected effect of electron transfer to the antibonding orbital of the dimer. Electron-density ($n$) differences between the bonded systems and the isolated fragments show an accumulation of $n$ between RG and the neighboring Au atom, and between the gold atoms. The analysis of the projected density of states reveals that, although only non-bonding orbital interactions and no charge transfer occurs between RG and Au$_2$, the $d$-electrons of Au$_2$ are redistributed due to the interaction with RG in such a way that the Au-Au $\sigma_s$ bond is strengthened. [Preview Abstract] |
Session D35: Focus Session: DFT II: Molecular Conductance; Charge Transfer
Sponsoring Units: DCPChair: Arindam Chakraborty, Syracuse University
Room: 107B
Monday, February 27, 2012 2:30PM - 3:06PM |
D35.00001: On the derivative discontinuity in molecular junctions Invited Speaker: Charles Stafford Both the wave and particle aspects of the electron play essential roles in transport through single-molecule junctions. The wave character is implicit both in the Landauer formula used to understand nanoscale transport and in the very chemical bonds holding the junction together, while the particle aspect is manifested in phenomena such as Coulomb blockade and shot noise. The dominant computational paradigm for transport in single-molecule junctions involves local or semilocal approximations to density functional theory combined with nonequilibrium Green's functions. This approach does exceptionally well at describing the wave aspect of the electron, but fails to describe the particle aspect---due to the omission of the derivative discontinuity in the exchange-correlation potential that arises in the limit of vanishing lead-molecule coupling. To understand the role of the derivative discontinuity in molecular junctions, we investigated the transport and occupancy of a simple Anderson model of a molecular junction. We showed\footnote{Justin P. Bergfield, Zhenfei Liu, Kieron Burke, Charles A. Stafford, arXiv:1106.3104v2} that the exact single-particle Kohn-Sham potential of density functional theory reproduces the linear-response transport of the Anderson model exactly, despite the lack of a Kondo peak in its spectral function. Using Bethe ansatz techniques, we calculated this potential exactly for all coupling strengths, including the cross-over from mean-field behavior to charge quantization caused by the derivative discontinuity. The implications of our results for more complex molecular junctions will be discussed. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D35.00002: A density functional that works for transport through Anderson junction Zhenfei Liu, Justin Bergfield, Kieron Burke, Charles Stafford Transport through an Anderson junction can be exactly described by density functional theory, at zero temperature and in the linear response regime. Using Bethe ansatz, we calculate the exact Kohn-Sham potential delivering the exact transmission. We propose a simple parametrization for the Kohn-Sham potential, using a known exact condition. Our parametrization faithfully reproduces numerical results, including the gradual development of the derivative discontinuity that is essential in describing Coulomb blockade correctly. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D35.00003: Non-steady state in quantum transport Elham Khosravi, Stefan Kurth, Gianluca Stefanucci, E.K.U. Gross The standard approach to quantum transport combines the Landauer-Buettiker (LB) formalism with ground-state density functional theory (DFT). The basic assumption of this approach is that a steady state is achieved after turning on a DC bias. Here we show that this assumption is ``not'' valid in general and and give examples for which no steady state develop within several adiabatic (time-local) approximations as well as in non-interacting systems. In these cases a time-dependent description of transport is essential. For the non-interacting case, the presence of bound states in a biased system is shown analytically and numerically to lead to persistent, localized current oscillations which can be much larger than the steady part of the current (PCCP. 11, 4535(2009)). For the interacting case, the discontinuity of the exchange-correlation potential of DFT in the context of electron transport for an interacting nanojunction attached to biased leads, gives rise to a dynamical state characterized by correlation-induced current oscillations in the Coulomb-blockade regime (PRL. 104, 236801(2010)). In addition, for multistable systems, the time-dependent approach describes if and how a solution can be reached through time evolution. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D35.00004: DFT methods for conjugated materials: From benchmarks to functionals John Sears, Jean-Luc Bredas From a theoretical standpoint, many of the problems of interest in the study of pi-conjugated materials for organic electronics applications pose a particular challenge for many modern density functional theory methods. Systematic errors have been observed, for instance, in the description of charge-transfer excitations at donor/acceptor interfaces, in linear and non-linear polarizabilites, as well as in the geometric and electronic properties of conjugated polymers [1,2]. We will discuss recent results in our lab aimed at: (i) understanding the sources of error for some of these problems; (ii) addressing these errors using tuned long-range corrected functionals; and (iii) using these results to guide the development of state-of-the-art methodologies in a new open-source DFT code. \\[4pt] [1] J. S. Sears, T. Korzdorfer, C. R. Zhang, and J. L. Bredas, J. Chem. Phys. 135 151103 (2011)\\[0pt] [2] T. Korzdorfer, J. S. Sears, C. Sutton, and J. L. Bredas, J. Chem. Phys., accepted. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D35.00005: Fundamental and excitation gaps in molecules of relevance for organic photovoltaics from an optimally tuned range-separated hybrid functional Sivan Refaely-Abramson, Roi Baer, Leeor Kronik The fundamental and optical gaps of relevant molecular systems are of primary importance for organic-based photovoltaics. Unfortunately, whereas optical gaps are accessible with time-dependent density functional theory (DFT), the highest -occupied - lowest-unoccupied eigenvalue gaps resulting from DFT calculations with semi-local or hybrid functionals routinely and severely underestimate the fundamental gaps of gas-phase organic molecules. Here, we show that a range-separated hybrid functional, optimally tuned so as to obey Koopmans' theorem, provides fundamental gaps that are very close to benchmark results obtained from many-body perturbation theory in the GW approximation. We then show that using this functional does not compromise the possibility of obtaining reliable optical gaps from time-dependent DFT. We therefore suggest optimally-tuned range-separated hybrid functionals as a practical and accurate tool for DFT-based predictions of photovoltaically relevant and other molecular systems. For more details, see S. Refaely-Abramson, R. Baer, L. Kronik, Phys. Rev. B 84, 075144 (2011). [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D35.00006: (1) Modeling emissive charge-transfer states in solution phase functionalized silsesquioxanes. (2) On symmetry hidden charge transfer states: Lessons for the design of functionals Barry D. Dunietz, Shaohui Zheng, Heidi Phillips, Eitan Geva Range separate hybrid functionals are used to study the charge transfer processes in chromophores attached to silsesquioxanes molecular species. We investigate the experimentally observed red shifting of the emission spectra in comparison to the spectral shift for the individual ligand. Solvent effects are accounted for via a combination of constrained density functional theory and the polarizable continuum (PCM) model. We quantitatively reproduce the experimental red shift and identify the emissive state as a ligand-to-ligand, rather than a ligand-to-silsesquioxane, charge-transfer state. We also find that the enhanced red-shift cannot be explained without accounting for solvation effects and demonstrate the importance of using a range-separated hybrid functional, as opposed to more traditional functionals such as B3LYP, to obtain reliable predictions regarding the emissive state. If time allows we will also discuss the limitations and successes of RSH functionals in treating the case of charge transfer processes that are hidden by system symmetry. We use related models to draw insight for designing improved functionals. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D35.00007: Density Functional Methods and Electronic Processes in Organic Materials Qin Wu When modeling the fundamental processes in organic electronic materials, ab initio calculations play an important role because they provide an independent source of information. It is thus critical to use accurate and reliable ab initio methods. In this talk, we will share our experience in using density functional methods to study charge generation and transport in some organic systems. These include prototypical polythiophene and polyfluorene, as well as some newly synthesized conjugated molecules. They all have strong dispersion forces and strong electron-vibration coupling; both are well-known difficult effects for density functional methods to capture accurately. We will describe our effort in exploring ways to do meaningful calculations. Close collaborations with experimental work will also be emphasized. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D35.00008: Modelling Charge Transfer Reactions and Excitations with Subsystem DFT Michele Pavanello, Johannes Neugebauer The subsystem formulation of DFT known as Frozen Density Embedding (FDE) offers an excellent platform for studying charge transfer reactions in solvated systems, such as biosystems. We present the necessary theory developments for the calculation of the electronic couplings as well as the charge transfer excitations from FDE derived densities. We present preliminary calculations on DNA oligomers radical cations that include donor-bridge, donor-bridge-acceptor, and fully solvated systems. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D35.00009: Ab initio molecular dynamics study of the electric double-layer capacitance at solution-electrode interfaces Yasunobu Ando, Yoshihiro Gohda, Shinji Tsuneyuki Electric double-layer (EDL) is known as an important stage for electrochemical reactions at electrode-liquid interfaces. It has also attracted growing interest for its applications to electronic devices, called EDL capacitors (supercapacitors) and EDL transistors. In efficient development of each device, predicting the EDL capacitance in light of the material features is required. However, Helmholtz capacitance, the part of the EDL capacitance depending on the microscopic structure, has still not been estimated theoretically. Therefore, to evaluate that, we calculated the structure of solution-electrode interfaces by using ab initio molecular dynamics with effective screening medium method. As a result, we made it possible to estimate the Helmholtz capacitance taking the effect of the molecular orientation of the water and the electronic polarization in the water molecules due to the electric field into account. Apparent dielectric constant of the water near the interface can also be calculated. Interestingly, the results reveal that the existence of a first layer of the water molecules near the electrode determines the distance of closest approach of hydrated ions. Moreover, the estimated dielectric constant of the first layer differs from that predicted by the classical theory. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D35.00010: Assessing the fitness of various exchange-correlation functionals for TD-DFT studies of charge-transfer excitations in organic dyes Pratibha Dev, Saurabh Agrawal, Niall English Dye Sensitized Solar Cells (DSSCs) are a possible alternative to the more expensive silicon-based cells. Theoretical research in this field has highlighted some of the issues with time-dependent density functional theory (TD-DFT) that is widely used to study electronic excitations of matter. The situation is complicated by the fact that several classes of approximations to the exchange correlation functional can be employed, however, not one of these strictly outperforms the others in its description of charge-transfer excitations. In this work, UV-Vis spectra are calculated using TD-DFT for several organic dyes -- alizarin, squaraine, 4-(N, N-dimethylamino) benzonitrile, polyene-linker dyes and triphenylamine-donor dyes. We studied the dyes within three approximations (PBE, B3LYP and CAM-B3LYP) to the exchange-correlation functional. In the dyes considered here, a correlation exists between the functional performance and the spatial overlap of the states involved in the excitations. This overlap can be quantified to provide a good guideline for choosing the right functional when studying intramolecular charge transfer in dyes. It will be an invaluable tool when studying these molecules within more challenging systems, such as dye-titania complexes in DSSCs. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D35.00011: Electronic density functional theory in the grand canonical ensemble, electrochemistry, and the underpotential deposition of Cu/Pt(111) Ravishankar Sundararaman, Kendra Weaver, Tomas Arias The study of electrochemical systems within electronic density functional theory requires the handling of non-neutral electronic systems in the plane-wave basis in order to accurately describe charged metallic surfaces; this can be accomplished in joint density functional theory by adding an electrolyte with Debye screening \footnote{K. L. Weaver and T. A. Arias (under preparation)}. This capability opens up the opportunity to work in the grand canonical ensemble at fixed chemical potential $\mu$ for the electrons, which corresponds directly to the experimental setting in electrochemistry. We present efficient techniques for electronic density functional calculations at fixed $\mu$, and demonstrate the improvement in predictive power over conventional neutral calculations using the underpotential deposition of Cu/Pt(111) as an example: for the first time, we calculate absolute voltages for electrochemical processes in excellent agreement with experiment, instead of voltage shifts alone. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D35.00012: First-Principles Studies of Structure and Energy Level Alignment of Thiophene Assemblies on Methyl-Terminated Si(111) Min Yu, Peter Doak, Jeffrey Neaton Adsorption of organic molecules on semiconductor photocatalysts has attracted significant attention for energy conversion applications. In this work, we use density functional theory and many-body perturbation theory within the GW approximation to study the geometry, binding energetics, and energy level alignment of a model ligand, thiophene (C4H4S), chemisorbed via a C-Si bond on methyl-terminated silicon(111) substrates. We quantify the impact of coverage, interface dipoles, hybridization, and polarization effects on level alignment. For sufficiently weakly-coupled frontier orbitals, we explore the extent to which the self-energy change upon adsorption relative to the gas phase is dominated by nonlocal electrostatic polarization effects [1]. The implications of our results for other thiophene-related ligands, and future spectroscopic experiments, will be thoroughly discussed. We acknowledge DOE for support through JCAP, and NERSC for computational resources.\\[4pt] [1] J. B. Neaton, M. S. Hybertsen and S. G. Louie, Phys. Rev. Lett. 97, 216405 (2006). [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D35.00013: Electronic level alignment at a metal-molecule interface from a short-range hybrid functional Ariel Biller, Isaac Tamblyn, Jeffery B. Neaton, Leeor Kronik Hybrid functionals often exhibit a marked improvement over semi-local functionals in the description of the electronic structure of organic materials. Because short-range hybrid functionals, notably the Heyd-Scuseria-Ernzerhof (HSE) functional, can also describe the electronic structure of metals reasonably well, it is interesting to examine to which extent they can correctly describe the electronic structure at metal-organic interfaces. Here, we address this question by comparing HSE calculations with many-body perturbation theory calculations in the GW approximation, or with experimental photoemission data, for two prototypical systems: benzene on graphite and benzene diamine on gold. For both cases, we find that while HSE yields results that are somewhat closer to experiment than those of semi-local functionals, the HSE prediction is still lacking quantitatively by $\sim$1 eV. We show that this quantitative failure arises because HSE does not correctly capture the fundamental gap of the organic, or its renormalization by the metal. These discrepancies are traced back to missing long-range exchange and correlation components, an explanation which applies to any conventional or short-range hybrid functional. [Preview Abstract] |
Session D36: Focus Session: Environment I: Aerosols and Aqueous Solutions
Sponsoring Units: DCPChair: Don Baer, Pacific Northwest Research Laboratory and Ilja Siepmann, University of Minnesota
Room: 107C
Monday, February 27, 2012 2:30PM - 3:06PM |
D36.00001: Uncovering the formation mechanism of atmospheric nanoparticles Invited Speaker: Hanna Vehkamaki Atmospheric aerosol affect human health, visibility and radiation budget of the Earth. The current estimate is that 20-80\% of aerosols particles are formed in the atmosphere by condensable gases. Experimental and theoretical data indicates that the formation of new particles in the atmosphere in most cases very likely involves sulphuric acid assisted with some base molecules. The role of ions in atmospheric particle formation is has been widely discussed during recent years. The diameter of the forming clusters is 1-2nm, falling between the smallest size where brute force quantum mechanical treatment is possible, and macroscopic size where bulk thermodynamics in applicable. Recent experiments at the CLOUD chamber in CERN have provided molecular-level information on the charged fraction of the nucleating clusters, but the theoretical framework needed to convert this into information on neutral clusters is still lacking. We have used a cost-effective multi-step computational chemistry method involving automated configurational sampling, density functional theory geometry optimizations and coupled-cluster energy calculations, to study the stability of charged and neutral sulfuric acid clusters containing ammonia and dimethylamine. Combined with a cluster dynamics model ACDC, we are able to replicate the formation rates observed in the CLOUD chamber, as well as match observed formation rates in Hyyti\"al\"a Smear II station in Finland. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D36.00002: Molecular dynamics of binary and ternary nanodroplets with a miscibility gap Gerald Wilemski, Fawaz Hrahsheh The structure of nanodroplets plays an important role in many natural processes including particle nucleation and aerosol formation in the atmosphere. Among other factors, chemical miscibility and surface tension strongly affect the structure of multicomponent nanodroplets at low temperature. Here, we investigate the structure of water/nonane and water/butanol/nonane nanodroplets using molecular dynamics (MD). Our MD results confirm our theoretical predictions of nonspherical nanodroplet (Russian-Doll) structures at low temperatures using density functional and lattice Monte Carlo techniques. We systematically study the variation of the droplet structure with temperature and with butanol concentration. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:54PM |
D36.00003: Organization at the Air-Aqueous Interface by Heterodyne-detected Phase-Sensitive Sum Frequency Spectroscopy Invited Speaker: Heather Allen Water and ions organize at the air -- aqueous interface and the ion distributions within this region give rise to interfacial electric double layers. Geochemical solid-aqueous and atmospheric aerosol relevant air-aqueous interfaces were studied using vibrational sum frequency generation (VSFG), and in some cases, heterodyne-detected VSFG spectroscopy. Solid-aqueous and air-liquid interfaces were also investigated using total internal reflection Raman spectroscopy, infrared reflection absorption spectroscopy (IRRAS), and Brewster angle microscopy (BAM), respectively. Here, we show results from aqueous solutions containing salts such as sulfates and chlorides where surface adsorption and electric field direction reversal was observed. Of the salts studied at the air-aqueous interface with heterodyne-detected VSFG, the magnitude of the electric field in the surface extending to the subsurface regions decreases in the order: (NH$_{4})_{2}$SO$_{4} \quad >$ Na$_{2}$SO$_{4} \quad >$ Na$_{2}$CO$_{3 }\ge $ CaCl$_{2} \quad >$ NaCl; the electric field is opposite in direction for the sulfates and carbonate relative to the chloride salts. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D36.00004: First-principles study of the infrared spectrum of the ice Ih (0001) surface Huang Patrick, T. Anh Pham, Eric Schwegler, Giulia Galli Ice particles catalyze a number of processes relevant to atmospheric and environmental chemistry, and the elucidation of these reactions require knowledge of the ice surface structure. Although it is well known that the structure of bulk ice-Ih is proton disordered, the understanding of the microscopic structure of the ice surface is still limited. Recent theoretical studies $^{1,2}$ suggest that the basal (0001) surface of ice Ih is significantly more proton ordered than the bulk. In this work we compute infrared (IR) spectra of several ordered and disordered models of the (0001) surface of ice, and investigate the sensitivity of these spectra to the microscopic details of the surface structure. In particular we discuss possible signatures of disorder in the computed spectra. [1] D. Pan et al., Phys. Rev. Lett. 101, 155703 (2008); [2] V. Buch et al., Proc. Natl. Acad. Sci. U.S.A. 105, 5969 (2008) [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:42PM |
D36.00005: 1st principle simulations of ions in water solutions: Bond structure and chemistry in the hydration shells of highly charged ions Invited Speaker: John Weare Methods of direct simulation (Monte Carlo and molecular dynamics) have provided new insights into the structure and dynamics of electrolyte solutions. However, these methods are limited by the difficulty of developing reliable ion-solvent and solvent-solvent potential interactions in the highly perturbed hydration region. To model the interactions in this region methods of simulation that are based on the direct on the fly solution to the electronic Schr\"{o}dinger equation (ab-initio molecular dynamics, AIMD) are being developed. However, 1st principle methods have their own problems because the solution to the electronic structure problem is intractable unless rather uncontrolled approximations are made (e.g. density functional theory, DFT) and there is high computational cost to the solution to the Schr\"{o}dinger equation. To test the accuracy of AIMD methods we have directly simulated the XAFS spectra for a series of transition metal ions Ca$^{2+}$, Cr$^{3+}$, Mn$^{2+}$, Fe$^{3+}$, Co$^{2+}$, Ni$^{2+}$, Cu$^{2+}$, and Zn$^{2+}$. Despite DFT's well know deficiencies, the agreement between the calculated XAFS spectra and the data is almost quantitative for these test ions. This agreement supports the extension of the interpretation well beyond that of the usual XAFS analysis to include higher-order multiple scattering signals in the XAFS spectra, which provide a rigorous probe of the first shell distances and disorders. Less well resolved features of the spectra can still be analyzed and are related to 2nd shell structure. The combination of XAFS measurements and the parameter free AIMD method leads to new insights into the hydration structure of these ions. While strictly local DFT +gga provides excellent agreement with data, the addition of exact exchange seems to provide slightly better structural agreement. The computational complexity of these calculations requires the development of simulation tools that scale to high processor number on massively parallel supercomputers. Our present algorithm scales to nearly 100,000 processors. However, even with high scaling the time to solution is very long. We are also developing and testing new methods to improve the performance of simulation and new sampling methods that more efficiently explore phase space and can reach longer time frames. Results of calculation of the hydration structure and dynamics of highly charges ions and free energy calculations of ion association will be presented. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D36.00006: Reactions of Solvated Electrons Initiated by Sodium Atom Ionization at the Vacuum-Liquid Interface Justin Wiens, William Alexander, Gilbert Nathanson, Timothy Minton Solvated electrons are powerful reagents in the liquid phase that break chemical bonds and create new reactive species, including hydrogen atoms. Electrons and hydrogen atoms born near the surface, however, behave differently than those created within the liquid. We explore this behavior by exposing liquid glycerol to a beam of sodium atoms. The Na atoms ionize in the surface region, generating electrons that react with deuterated glycerol, C$_{3}$D$_{5}$(OD)$_{3}$, to produce D atoms, D$_{2}$, D$_{2}$O, and glycerol fragments. Surprisingly, 40{\%} of the D atoms desorb into vacuum before attacking C-D bonds to produce D$_{2}$. These D atoms must traverse the interfacial region before desorbing, demonstrating that Na ionization prepares reactive species that reside momentarily at the surface and often escape before reacting with the solvent. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D36.00007: Heterogeneous Photochemistry and Optical Properties of Mineral Dust Aerosol Invited Speaker: Vicki Grassian It is now widely recognized that heterogeneous reactions of mineral dust aerosol with trace atmospheric gases impact the chemical balance of the atmosphere and the physicochemical properties of these particles. Field studies using single particle analysis, have now shown that the chemistry is mineralogy specific and follows the trends expected from laboratory studies. These laboratory studies, which were initiated over a decade ago, have focused on the nighttime chemistry of mineral dust aerosol which is really only ``half'' the story. This talk will focus on two aspects of solar light interaction with mineral dust aerosol. First, the heterogeneous photochemistry of adsorbed chromophores (e.g. nitrate ion) and light absorbing components of mineral dust (iron oxides and titanium dioxide) is discussed. These heterogeneous photochemical reactions are poorly understood and laboratory studies to better quantify these reactions in order to determine the impact on the chemical balance of the atmosphere are needed, as will be discussed. Second, the optical properties of mineral dust aerosol measured by extinction infrared spectroscopy and visible light scattering show that shape effects are extremely important for mineral dust aerosol. [Preview Abstract] |
Session D37: SPS Undergraduate Research II
Sponsoring Units: SPSChair: Gary White, SPS/AIP
Room: 108
Monday, February 27, 2012 2:30PM - 2:42PM |
D37.00001: Low Temperature Resistive Switching Behavior in a Manganite Christopher Salvo, Melinda Lopez, Stephen Tsui The development of new nonvolatile memory devices remains an important field of consumer electronics. A possible candidate is bipolar resistive switching, a method by which the resistance of a material changes when a voltage is applied. Although there is a great deal of research on this topic, not much has been done at low temperatures. In this work, we compare the room temperature and low temperature behaviors of switching in a manganite thin film. The data indicates that the switching is suppressed upon cooling to cryogenic temperatures, and the presence of crystalline charge traps is tied to the physical mechanism. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D37.00002: Atomic Steps on Thermally Annealed Oxide Substrates Icon Mazzaccari, Rafiya Javed, Amlan Biswas Atomically smooth perovskite oxide substrates are necessary for high quality thin-film growth. We have optimized the annealing conditions for the substrates, LaAlO$_{3}$, SrTiO$_{3}$ and NdGaO$_{3}$, which are commonly used substrates for growing thin-film perovskite oxides. The optimal annealing temperatures were between 950 $^{o}$C$^{ }$and 1050 $^{o}$C and the annealing time for each sample was varied by approximately 20{\%}. Atomic force microscopy was used to compare the surfaces of the annealed substrates with the unannealed ones. These images capture the terrace steps that occur due to the annealing process, and we have confirmed that the heights of the steps are approximately one unit cell (0.4 nm). Currently, we are investigating methods for determining the site termination of the substrates and their effects on thin-film growth. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D37.00003: Field-Induced Phase Transitions in the S=1/2 Quasi-1D Antiferromagnet BaCo$_{2}$V$_{2}$O$_{8}$ Giovanni Franco-Rivera, N.A. Fortune, S.T. Hannahs, H.D. Zhou, H. Tsujii, Y. Takano We have performed magnetocaloric effect and specific heat measurements in the S=1/2 quasi-one-dimensional Ising-like antiferromagnet BaCo$_{2}$V$_{2}$O$_{8}$ in magnetic fields up to 24 T at temperatures down to 0.3 K. The magnetocaloric effect measurements were made in order to investigate the field-direction dependence of a magnetic transition near 3.7 T. The specific heat and magnetocaloric effect measurements were employed to determine the H-T phase diagram when the field is applied parallel to the $c$ axis of the sample. We have found a new antiferromagnetically ordered phase between 19.7 T and 23.1 T, most likely a spin-flop phase. At low fields, our results agree with the known N\'{e}el ordered phase boundary. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D37.00004: Magnetism in Transition Element Doped In$_{2}$O$_{3}$ Dilute Magnetic Semiconductors M. Langhoff, E. Nahlik, Y. Kolekar, P. Kahol, K. Ghosh There is currently a tremendous research effort in the area of dilute magnetic semiconductors (DMS). It is proposed that a DMS exhibiting ferromagnetic properties at room temperature could be used in a new class of devices termed spintronics. Whereas standard electronics work on the principle of manipulation of charge properties of an electron, spintronics work on controlling electron spin. Indium oxide is a wide band gap semiconductor with unique optical and electrical properties. Defect concentrations such as transition metal doping and oxygen vacancies in In$_{2}$O$_{3}$ can tune the electrical/magnetic behavior from ferromagnetic metal-like to ferromagnetic semiconducting to paramagnetic insulating. Bulk materials of magnetic element (Fe, Co and Cr) doped In$_{2}$O$_{3 }$have been made using a standard solid state reaction method. Structural and magnetic properties have been measured using standard techniques. XRD analysis confirmed single phase In$_{2}$O$_{3 }$with no impurity phases due to addition of magnetic elements. Magnetization as a function of applied magnetic field and temperature were collected on all the samples using a SQUID magnetometer. Detailed structural and magnetic properties will be presented in this talk. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D37.00005: Magnetic Properties of Ordered Nanoporous Nickel Films Jiabin Liu, Han-Chang Yang, X.M. Cheng Nanoporous materials have been of great interest for applications such as biosensors and energy storage. Magnetic properties and the magnetization reversal mechanism of nanoporous magnetic materials remain to be fully understood. In this work, we report the fabrication and magnetic properties of ordered nanoporous nickel (Ni) films. The fabrication involved the following steps: self-assembly of monodispersed polystyrene spheres, electrochemical deposition of desired materials, and sphere removal by a dissolution process. Scanning electron microscopy (SEM) images confirmed the highly ordered three-dimensional hexagonal closed pack structures of the Ni films. We characterized magnetic properties of the three-dimensional nanoporous Ni films using vibrating sample magnetometer (VSM). Magnetic hysteresis loops and first-order reversal curves (FORCs) were measured on the nanoporous Ni films of 200 nm pore size with different thicknesses. Analysis on hysteresis loops and FORC distributions shed light on the reversal mechanism of magnetization and magnetostatic interactions of ordered three-dimensional porous structures. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D37.00006: Electrical characterization of CVD graphene Yarely D\'avila, Nicholas Pinto, Zhengtang Luo, Alan Johnson Jr. Graphene is a one atom thick carbon sheet that can be obtained via exfoliation of graphite or via chemical vapor deposition (CVD). By using a very simple shadow masking technique, gold contact pads were evaporated over the graphene thereby eliminating chemical etching that is required when using photolithography and often leads to sample contamination. CVD graphene was electrically characterized in a FET configuration under different experimental conditions that include UV exposure, gas sensing and temperature. Our measurements yielded a carrier mobility of up to 3000 cm$^{2}$/V-s for some devices. Exposure to UV dopes graphene in a controlled manner. The doping level could be maintained indefinitely in vacuum or could be completely reversed by slight heating in air without loss of device performance. The FET's were also tested at different temperatures with little change in the transconductance response. Exposure to ammonia gas $n$-doped graphene while exposure to NO$_{2}$ $p$-doped it. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D37.00007: Electronic Structure Properties of Graphene/Boron Nitride Layered Systems Max Petulante, Nam Le, Lilia Woods We explore the properties of systems composed of two or three layers of graphene and hexagonal boron nitride (hBN) using the Vienna Ab-Initio Simulation Package (VASP), a software package for performing first principles simulations based on density functional theory (DFT). Particular attention is given to the contribution of inter-layer dispersion interactions, which are modeled within VASP by the ``DFT-D2'' method of Grimme. We obtain the binding and van der Waals energies, and inter-layer separations for the most stable stacking configurations of each of the following systems: hBN/graphene, graphene/hBN/graphene, hBN/graphene/hBN, hBN/hBN/graphene, and graphene/graphene/hBN. We observe that the addition of hBN layers to graphene structures induces a band gap, ranging from 0.024 eV, for the graphene/hBN/graphene arrangement, to 0.16 eV, for the hBN/graphene/hBN arrangement. These results, specifically band gaps on the same order as those of silicon and germanium, indicate that graphene/hBN layered structures may have applications in electronics. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D37.00008: Magnetoresistance of Strained Rare Earth Manganese Oxide Thin Films: Effect of Post-Deposition Thermal Treatment Tyler Goehringer, Parul Srivastava, E. Kevin Tanyi, David Schaefer, Rajeswari Kolagani We have studied the effects of thermal treatment (annealing) on the properties of thin films of manganese oxide materials La$_{0.67}$Ca $_{0.33}$ MnO$_{3 }$and La$_{0.67}$ Ba$_{.0.33}$MnO$_{3}$, which are known to exhibit colossal magnetoresistance (CMR) and an insulator to metal transition . These materials$_{ }$show properties which are desirable for applications such as magnetic sensors, bolometric infrared sensors, or field effect devices. Two properties of interest, magnetoresistance (MR) and insulator metal transition are especially sensitive to strain because of the relationship between the properties and the symmetry of the crystal. The properties are also sensitive to the oxygen stoichiometry of the film due to changes in the valence state of the manganese which determines the charge carrier density. Recent results from our laboratory have shown a large magnetoresistance in strained films grown on lattice mismatched substrates. The MR continues to increase with decreasing temperature in these films. In order to understand the contribution of strain to the observed MR behavior, in we employ `thermal annealing' to allow for relaxation of strain. Annealing also serves to achieve the optimum oxygen stoichiometry. We will present the results of our annealing studies and their implications for understanding the MR in strained films. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D37.00009: Grain Boundaries In Thin Film Organic Semiconductors Cortney Bougher, Shawn Huston, Eitan Lees, Jeremy Ward, Abdul Obaid, Marsha Loth, John Anthony, Oana Jurchescu, Brad Conrad We utilize conductive atomic force microscopy (C-AFM) and tunneling atomic force microscopy (TUNA) to characterize dynamics of electronic transport across fluorinated triethylsilylethynyl anthradithiophene (diF-TES ADT) grain boundaries. The crystallization of diF-TES ADT grown on SiO$_{2}$ will be discussed and related to comparable molecules. The resulting voltage drop between individual crystals as a function of dopants will be discussed in terms of charge transport models and compared to current device work. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D37.00010: Formation of Interfacial Carbide Layers in Multilayer Ti/DLC Thin Films Jasen Scaramazza, Zachary Buck, Tyler Donato, Brittany Curran, C.A. Lunk, S.E. Lofland, J.D. Hettinger Titanium (Ti)/Diamond-like-carbon (DLC) and Chromium (Cr)/Carbon (C) multilayer films were prepared on c-axis oriented single crystal sapphire (Al$_{2}$O$_{3})$ substrates using magnetron sputtering. Interfacial properties of the films were analyzed using x-ray reflectivity and scanning electron microscopy. When DLC is sputtered on a layer of Ti, an interfacial layer of titanium carbide (TiC) forms which is reported for the first time. Energy provided by the substrate bias necessary to facilitate DLC sp3 bond formation is suspected of allowing TiC to synthesize in a thin layer before DLC forms. It was also found that DLC has difficulty forming on Cr. These results are relevant to biomedical applications where DLC is applied as a low friction/wear film that can be formed on the surface of implants composed mainly of titanium. Further investigation into the medical and tribological effects of TiC interfacial layers is suggested. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D37.00011: Properties of Cr$_{2}$AlC MAX phase thin films prepared by reactive magnetron sputtering Zachary Buck, Tyler Donato, Christopher Rotella, Carl Lunk, S.E. Lofland, J.D. Hettinger M$_{n+ 1}$AX$_{n}$ (MAX) phases, where $n$ is 1, 2, and 3, M is an early transition metal, A is an A-group element, and X is either C or N, are ternary carbides with unique properties such as low density, easy machinability, and good oxidation resistance. The MAX phase Cr$_{2}$AlC is of particular interest for industrial applications to its excellent high-temperature oxidation resistance and relatively low synthesis temperature. We prepared Cr$_{2}$AlC thin films on c-axis oriented single crystal Al$_{2}$O$_{3}$, glassy carbon and Si thermal oxide substrates using reactive magnetron sputtering as precursor materials for carbide-derived carbon (CDC) films for ``on-chip'' supercapacitors. Film deposition was optimized using elemental composition data obtained by WDXRF. Optimized films were characterized using XRD and scanning electron microscopy. It was found that textured Cr$_{2}$AlC films only form when the composition was Al-rich allowing the formation of a Cr$_{5}$Al$_{8}$ interfacial layer. As film composition was optimized, the interfacial layer did not form but the XRD peaks associated with the Cr$_{2}$AlC also decreased in magnitude. Extremely high-textured films were grown when a thin buffer layer of CrAl$_{2}$ was deposited on the substrate before depositing the Cr$_{2}$AlC films. This result suggests that Cr$_{2}$AlC films may not be ideal for CDC applications since the films may ``lift-off'' during conversion due to the existence of the naturally occurring buffer-layer. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D37.00012: Fluid-like conducting regions in solid manganite thin films Rafiya Javed, Hyoungjeen Jeen, Amlan Biswas Hole-doped manganese oxides (manganites) exhibit a fluid-phase separated state in which the conducting and ferromagnetic phase (FMM) behaves like a fluid embedded in an insulating background phase [1]. While the manganite remains a solid, its local electrical properties behave as a fluid. The fluid behavior of the conducting regions can be tested by observing the effect of an electric field on the shape of the FMM regions. This change in shape is expected to result in anisotropic resistivity. This property was tested by designing microstructures of the manganite film, which simplify the experiment by localizing the electric field to a region the size of the FMM regions. We are testing for possible magnetic anisotropy since the conducting regions are also ferromagnetic. We will also discuss the size effect on the magnetic properties. ~ [1] Dhakal et. al., Phys. Rev. B 75, 092404 (2007) [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D37.00013: The intrinsic coupling of polarization with terahertz pulses in a ferroelectric nanowire Kimberly Schultz, Ryan Herchig, Kevin McCash, Inna Ponomareva We use first-principles-based molecular dynamics simulations to study the interaction of a terahertz (THz) radiation with polarization in a ferroelectric ultrathin nanowire made of a lead zirconate titanate alloy. In our computational experiment, a 12 nm thick nanowire is first annealed down to temperature of 300 K and then subjected to a wide variety of THz pulses which differ in width, strength and frequency. Such nanowire develops an electrical polarization along the nanowire axial direction which couples strongly with incoming THz radiation. The atomistic resolution of our computational experiments allows us to trace the intrinsic polarization response and energy propagation/dissipation mechanisms that occur at the scale of femtoseconds. Our simulations were carried out under MVE and MVT conditions and the results did not vary significantly between the two ensembles. We further explore how the unique features of such response could be utilized in an ultrafast THz nanoswitches. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D37.00014: Magnetic Element Doped Transparent Conducting In$_{2}$O$_{3 }$Thin Films for Spintronic Applications E. Nahlik, A. Laudari, M. Langhoff, K. Ghosh Tremendous research efforts are underway to exploit the property of electron spin in spintronics. Spintronic devices critically depend on the availability of a specific materials system for spin injection, manipulation and detection. Transition metal (Cr, Fe, or Co) doped wide band gap oxide semiconductors possess these properties. ~Indium oxide (In$_{2}$O$_{3})$ is a wide band gap semiconductor with unique optical and electrical properties. Here, we investigate the effect of Cr, Fe, or Co doping on electrical and optical properties of In$_{2}$O$_{3}$ thin films. Thin films have been grown on sapphire and quartz substrates using pulse laser deposition method. Electrical and optical characteristics have been measured using UV-VIS spectroscopy and magneto-transport techniques. Optical transmittance and electrical parameters such as carrier concentration and carrier mobility vary with growth parameters such as growth temperature of the substrate and oxygen pressure of the chamber. These details will be discussed during this presentation. [Preview Abstract] |
Session D39: Quantum Protocols: Tomography, Communication, and Coding
Sponsoring Units: GQIChair: Ivan Deutsch, University of New Mexico
Room: 109B
Monday, February 27, 2012 2:30PM - 2:42PM |
D39.00001: Practical characterization of quantum devices without tomography Olivier Landon-Cardinal, Steven Flammia, Marcus Silva, Yi-Kai Liu, David Poulin Quantum tomography is the main method used to assess the quality of quantum information processing devices, but its complexity presents a major obstacle for the characterization of even moderately large systems. Part of the reason for this complexity is that tomography generates much more information than is usually sought. Taking a more targeted approach, we develop schemes that enable (i) estimating the ?delity of an experiment to a theoretical ideal description, (ii) learning which description within a reduced subset best matches the experimental data. Both these approaches yield a signi?cant reduction in resources compared to tomography. In particular, we show how to estimate the ?delity between a predicted pure state and an arbitrary experimental state using only a constant number of Pauli expectation values selected at random according to an importance-weighting rule. In addition, we propose methods for certifying quantum circuits and learning continuous-time quantum dynamics that are described by local Hamiltonians or Lindbladians. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D39.00002: Quantum tomography with small number of copies: a simple estimator for qubit states Hui Khoon Ng, Berthold-Georg Englert In quantum tomography, one performs repeated measurements on $N$ copies of a given but unknown state and constructs an estimator for the state from the gathered data. A common way of converting the data into an estimator is the maximum-likelihood (ML) method, where the estimator is the state with the largest probability of giving rise to the observed data. ML methods methods work well for large $N$, since the likelihood function for large $N$ is sharply peaked around its maximum. For small $N$, however, there is a significant neighborhood of states around the maximum with nearly equal probability of giving rise to the data. One can then imagine using the likelihood function as a weight to construct an estimator as an average over states. This motivates the introduction of the ``mean estimator,'' also previously discussed for quantum tomography in the spirit of Bayesian estimation by Blume-Kohout [NJP 12, 043034(2010)]. Here, we extend the mean estimator for a classical die problem to an estimator for qubit states, and demonstrate its advantage over ML estimators. We also discuss a way of overcoming the common complaint of rank-deficiency in ML estimators for our estimator. This simple estimator should be useful as a convenient first estimate for any qubit tomography experiment. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D39.00003: Joint quantum tomography of state preparation and measurements using only known quantum operations Marcus da Silva, Jay Gambetta An important problem in quantum information is the complete characterization of quantum devices, which is usually referred to as ``quantum tomography''. Quantum tomography procedures exist for the characterization of quantum states (quantum state tomography), operations (quantum process tomography) and measurements (quantum measurement tomography) --- and each of these procedures can be performed using only product states, local unitary operations and local projective measurements. Here we consider the problem of jointly characterizing both the initial state as well as the measurement observable of a system using only well characterized quantum operations. We find that neither local unitaries nor local completely-positive trace-preserving maps are sufficient for obtaining a complete description of the state preparation and measurement of the system, and describe a scheme that uses non-unital trace-reducing physical maps to obtain such a description. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D39.00004: Single shot quantum state estimation via continuous measurement in a strong back-action regime Robert Cook, Ivan Deutsch Quantum state reconstruction is a fundamental task in quantum information science. The standard approach employs many projective measurements on a series of identically prepared systems in order to collect sufficient statistics of an informationally complete set of observables. An alternative procedure is to reconstruct quantum state by performing weak continuous measurement collectively on an ensemble, while simultaneously applying time varying controls [1]. For known dynamics, the measurement history determines the initial state. In current implementations the shot noise of the probe dominates over projection noise so that measurement-induced backaction is negligible. We generalize this to the regime where quantum backaction can play a significant role, even for small numbers of particles. Using the framework of quantum filtering theory, we model the reconstruction of the state of a qubit through collective spin measurement via the Faraday interaction and magnetic field controls, and develop a maximum-likelihood estimate based on the Fisher information contained in the measurement record. \\[4pt] [1] A. Silberfarb and I. H. Deutsch, ``Quantum-state reconstruction via continuous measurement,'' Phys. Rev. Lett. 95, 030402 (2005). [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D39.00005: Beyond maximum-likelihood estimation Berge Englert When the estimators in quantum state tomography or quantum process tomography are obtained by maximizing the likelihood, which has become the method of choice, a unique result is not obtained if the data are informationally incomplete. By combining maximum-likelihood (ML) estimation with Jaynes's maximum-entropy (ME) principle, a unique estimator can be determined, and this is possible by an efficient iterative algorithm. The resulting estimators, however, can have the familiar deficiencies of maximum-likelihood estimators. Alternative estimation procedures that avoid these drawbacks are wanted. The talk reports on MLME estimation as well as alternative approaches with a Bayesian touch. [References: Phys. Rev. Lett. 107 (2011) 020404; arXiv:1110.1202] [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D39.00006: Experimental Realization of Adaptive Qubit Tomography Dylan H. Mahler, Joshua Combes, Lee A. Rozema, Ardavan Darabi, Chris Ferrie, Aephraim Steinberg, Robin Blume-Kohout In quantum state tomography, an informationally complete set of measurements is made on N identically prepared quantum systems and from these measurements the quantum state can be determined. In the limit as $N \rightarrow \infty$, the estimation of the state converges on the true state. The rate at which this convergence occurs depends on both the state and the measurements used to probe the state. To characterize the quality of a set of measurements the fidelity of the estimation with the true state, averaged over a prior distribution of states, is used as a figure of merit. It is known [1] that for states very close to the surface of the bloch sphere, the average infidelity ($1-F$) goes down with a rate proportional to $\frac{1}{\sqrt{N}}$. It has also been shown that there exists a gap between collective measurement protocols and local measurement protocols, but that local \textit{adaptive} measurement protocols can come close to saturating the collective measurement bound of $\frac{1}{N}$ [2]. Here we present an experimental demonstration of one qubit tomography that achieves a rate of convergence of $\frac{1}{N}$ with only a single adaptive step and local measurements.\\[4pt] [1] Phys. Rev. A 78, 052122 (2008)\\[0pt] [2] Phys. Rev. Lett. 97, 130501 (2006) [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D39.00007: Scalable estimation of computational gate fidelities Easwar Magesan, Jay Gambetta Scalable methods for characterizing the noise affecting a quantum system are of significant interest in theoretical and experimental quantum information theory. Since completely characterizing the noise is exponentially hard in the number of qubits comprising the system, there has been significant effort in developing scalable methods for characterizing particular features of the noise. In particular, ``randomized benchmarking'' has been shown to be a robust and scalable method for estimating the average error rate over the set of quantum computational gates. We propose a new protocol that allows for benchmarking individual quantum gates rather than the average over the entire set. The protocol consists of a mixture of deterministic and random applications of computational gates and is shown to be scalable in the number of qubits comprising the system. The method is robust against state preparation and measurement errors and is valid provided the average variation of the noise over the gate set can be sufficiently bounded. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D39.00008: Quantum controlled paths for perfect discrimination of no-signalling channels Giulio Chribella A no-signalling channel transforming systems in Alice's and Bob's local laboratories is compatible with two different causal structures: one where Alice's output can be sent to Bob's input and another where Bob's output can be sent to Alice's input. I show that a quantum superposition of circuits operating within these two causal structures enables the perfect discrimination between two no-signalling channels that could not be perfectly distinguished by any ordinary circuit [1]. Such a quantum superposition can be in principle achieved by introducing a qubit that controls the path followed by quantum systems, routing them to different ports of the given no-signalling channel. \\[4pt] [1] G. Chiribella, \emph{Perfect discrimination of no-signalling channels via quantum superposition of causal structures}, arXiv:1109.5154. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D39.00009: Experimental characterization of coherent dynamics in a spin chain Chandrasekhar Ramanathan, James Lee, Paola Cappellaro, Lorenza Viola, David Cory We experimentally characterize the coherent room-temperature magnetization dynamics of a spin chain evolving under an effective double-quantum Hamiltonian. Our results indicate that a localized magnetic moment travels down the chain with a group velocity of $6.04\pm0.38$ $\mu$m/s, corresponding to coherent transport over $N\approx 26$ spins on the timescale of the experiment. We also characterize the influence of the ends of the chains on the magnetization dynamics. Our results are in excellent agreement with a nearest-neighbor-coupled analytical model that predicts that the dynamics are restricted to a Liouville space that only grows quadratically with the number of spins. This suggests that the long-range couplings present in the experimental system only cause a slow leakage out of the subspace. As the double-quantum Hamiltonian is related to the standard one-dimensional XX Hamiltonian by a similarity transform, our results can be directly extended to XX quantum spin chains, which have been extensively studied in the context of both quantum magnetism and quantum information processing [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D39.00010: Experimental Monte Carlo Quantum Process Certification Lars Steffen, Arkady Fedorov, Matthias Baur, Marcus Palmer da Silva, Andreas Wallraff Experimental implementations of quantum information processing have now reached a state, at which quantum process tomography starts to become impractical, since the number of experimental settings as well as the computational cost of the post processing required to extract the process matrix from the measurements scales exponentially with the number of qubits in the system. In order to determine the fidelity of an implemented process relative to the ideal one, a more practical approach called Monte Carlo quantum process certification was proposed in Ref.~[1]. Here we present an experimental implementation of this scheme in a circuit quantum electrodynamics setup. Our system is realized with three superconducting transmon qubits coupled to a coplanar microwave resonator which is used for the joint-readout of the qubit states. We demonstrate an implementation of Monte Carlo quantum process certification and determine the fidelity of different two- and three-qubit gates such as \textsc{cphase}-, \textsc{cnot}-, \textsc{2cphase}- and Toffoli-gates. The obtained results are compared with the values obtained from conventional process tomography and the errors of the obtained fidelities are determined. \\[4pt] [1] M.~P.~da~Silva, O.~Landon-Cardinal and D.~Poulin, arXiv:1104.3835(2011) [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D39.00011: Quantum filtering one bit at a time Jason Ralph, Neil Oxtoby We consider the purification of a quantum state using the information obtained from a continuous measurement record, where the classical measurement record is digitized to a single bit per measurement after the measurements have been made. Analysis indicates that efficient and reliable state purification is achievable for one- and two-qubit systems. We also consider quantum feedback control based on the discrete one-bit measurement sequences. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D39.00012: New results in fault-tolerant quantum computing Gerald Gilbert, Yaakov Weinstein We compare the accuracy of two methods used to construct a logical zero state appropriate for the [7,1,3] CSS quantum error correction code in a non-equiprobable Pauli operator error environment. The first method is to apply error correction, via syndrome measurement, on seven physical qubits all in the state zero, using four-qubit Shor states to implement the syndrome measurements. The second method is to directly implement the [7,1,3] encoding gate sequence. We find surprising results that show that even at the most basic level there is still much to be learned about achieving fault tolerance. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D39.00013: On the Use of Shor States for the [7,1,3] Quantum Error Correcting Code Yaakov Weinstein, Sidney Buchbinder We explore the effect of Shor state construction methods on logical state encoding and quantum error correction for the [7,1,3] Calderbank-Shor-Steane quantum error correction code in a nonequiprobable error environment. We determine the optimum number of verification steps to be used in Shor state construction and whether Shor states without verification are usable for practical quantum computation. These results are compared to the same processes of encoding and error correction where Shor states are not used. We demonstrate that the construction of logical zero states with no first order error terms may not require the complete edifice of quantum fault tolerance. With respect to error correction, we show for a particular initial state that error correction using a single qubit for syndrome measurement yields a similar output state accuracy to error correction using Shor states as syndrome qubits. In addition, we demonstrate that error correction with Shor states has an inherent sensitivity to bit-flip errors. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D39.00014: Quantum codes with low weight stabilizers Alexey A. Kovalev, Ilya Dumer, Leonid P. Pryadko We study quantum cyclic stabilizer codes whose stabilizer can be always defined by one or two stabilizer generators. Our main goal is to construct low-weight stabilizer generators that can yield quantum codes with high code rate and simple error correction. To do so, we apply the classical quaternary representation of stabilizer codes and extend our recent study of one-generator cyclic codes [1]. For any stabilizer generator of weight four or five, we formulate a necessary and sufficient condition for its commutativity. We then proceed with a design of additive cyclic codes with such generators. In some cases, we also extend our commutativity condition and code design to generators of weight six. In particular, quantum cyclic codes with stabilizers of weight four are mapped to the generalized toric codes. Here we also extend the notion of toric codes using a translationally invariant generator and periodic boundary conditions on a two dimensional lattice. Some of our numerically constructed codes can be redefined by means of Code Word Stabilized (CWS) representation [1] as quantum versions of repetition codes. We particularly concentrate on codes with a fixed nonzero rate for which the minimum distance asymptotically grows as the blocklength grows.\\[4pt] [1] arXiv:1108.5490v1 [Preview Abstract] |
Session D40: Focus Session: Systems Biology and Biochemical Networks I
Sponsoring Units: DBIO GSNPChair: Jin Wang, SUNY Stony Brook
Room: 156A
Monday, February 27, 2012 2:30PM - 3:06PM |
D40.00001: Optimal Information Processing in Biochemical Networks Invited Speaker: Chris Wiggins A variety of experimental results over the past decades provide examples of near-optimal information processing in biological networks, including in biochemical and transcriptional regulatory networks. Computing information-theoretic quantities requires first choosing or computing the joint probability distribution describing multiple nodes in such a network --- for example, representing the probability distribution of finding an integer copy number of each of two interacting reactants or gene products while respecting the `intrinsic' small copy number noise constraining information transmission at the scale of the cell. I'll given an overview of some recent analytic and numerical work facilitating calculation of such joint distributions and the associated information, which in turn makes possible numerical optimization of information flow in models of noisy regulatory and biochemical networks. Illustrating cases include quantification of form-function relations, ideal design of regulatory cascades, and response to oscillatory driving. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D40.00002: Adaptation at the output of the chemotaxis signaling pathway Junhua Yuan, Richard Branch, Basarab Hosu, Howard Berg The chemotaxis signaling pathway allows bacterial cells to sense and respond to changes in concentrations of chemical attractants or repellents. In E. coli, the machinery required for bacterial chemotaxis includes two large membrane-embedded multiprotein complexes, one that processes input (receptor clusters) and the other that generates output (flagellar motors). These complexes are coupled by diffusion of a small phosphorylated cytoplasmic protein, CheY-P, which binds to the flagellar motors, increasing the probability that they spin clockwise. Receptor output (the steady-state concentration of CheY-P) varies from cell to cell. However, the motor is ultrasensitive, with a narrow [CheY-P] operating range. How might the receptor output and motor input be matched? By combining various techniques such as FRET, single-motor TIRF, and single-motor bead assay, we showed that the motor shifts its operating range to match the receptor output by changing its composition. The number of FliM subunits in the C-ring increases in response to a decrement in the concentration of CheY-P, increasing motor sensitivity. Such adaptive remodeling is likely to be a common feature in the operation of many molecular machines. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D40.00003: The energy cost of accurate adaptation in networks with incoherent type-1 feed-forward loop Ganhui Lan, Yuhai Tu The incoherent type-1 feed forward loop (I1-FFL) is a common regulatory motif in many biochemical networks, some of which are responsible for accurate sensory adaptation. In this work, we analyze the sensitivity and adaptation function of the I1-FFL type enzymatic reaction networks. We show that detailed balance is broken in I1-FFL and continuous energy dissipation is needed to improve both the sensitivity and the adaptation accuracy of the network. Our study revealed a relation between the performance improvement and the energy dissipation rate. We find that this energy-assisted improvement is bounded (limited) by intrinsic properties of the molecular reaction system. The performance-energy relation in I1-FFL is similar to the recently obtained Energy-Speed-Accuracy relation in networks with negative-feedback-loop, another key motif for accurate sensory adaptation. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D40.00004: Noise places constraints on eukaryotic gradient sensing and chemotaxis Bo Hu, Danny Fuller, William Loomis, Wen Chen, Wouter-Jan Rappel, Herbert Levine Chemotaxis is characterized by the directional cell movement following external chemical gradients. It plays a crucial role in a variety of biological processes including neuronal development, wound healing and cancer metastasis. Ultimately, the accuracy of gradient sensing is limited by the fluctuations of signaling components, e.g. the stochastic receptor occupancy on cell surface. We use concepts and techniques from statistical physics, estimation theory, and information theory to quantify the stochastic and nonlinear information processing in eukaryotic chemotaxis. We mainly address the following questions: (1) What are the physical limits of eukaryotic spatial gradient sensing? (2) How to characterize the movements of chemotactic cells? (3) How much gradient information can be reliably gained by a chemotactic cell? By answering those questions, we expect to derive new insights for general biological signal processing systems. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D40.00005: Process-driven inference of biological network structure: feasibility, minimality, and multiplicity Invited Speaker: Chen Zeng For a given dynamic process, identifying the putative interaction networks to achieve it is the inference problem. In this talk, we address the computational complexity of inference problem in the context of Boolean networks under dominant inhibition condition. The first is a proof that the feasibility problem (is there a network that explains the dynamics?) can be solved in polynomial-time. Second, while the minimality problem (what is the smallest network that explains the dynamics?) is shown to be NP-hard, a simple polynomial-time heuristic is shown to produce near-minimal solutions, as demonstrated by simulation. Third, the theoretical framework also leads to a fast polynomial-time heuristic to estimate the number of network solutions with reasonable accuracy. We will apply these approaches to two simplified Boolean network models for the cell cycle process of budding yeast (Li 2004) and fission yeast (Davidich 2008). Our results demonstrate that each of these networks contains a giant backbone motif spanning all the network nodes that provides the desired main functionality, while the remaining edges in the network form smaller motifs whose role is to confer stability properties rather than provide function. Moreover, we show that the bioprocesses of these two cell cycle models differ considerably from a typically generated process and are intrinsically cascade-like. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D40.00006: Communication channels between membrane bound proteins James Sethna, Benjamin Machta, Sarah Veatch Much of what might be called biological computation takes place on the plasma membrane, a 2D liquid composed of a diverse soup of lipids and embedded proteins. Motivated by the recent discovery that these membranes seem to be tuned close to a 2D liquid-liquid critical point, we set out to understand the different channels through which membrane bound proteins can communicate. Diffusing proteins can carry out reactions like phosphorylation when they come in contact with each other. Near criticality, proteins can also exert long-ranged critical Casimir forces on one another by coupling to the local composition order parameter. By modulating the growth and breakdown of the rigid cytoskeleton, they can direct forces on even more distant regions. In addition, proteins can control the release and production of second messengers that diffuse either through the bulk, or in the plane of the membrane itself. By making simple models for these processes we bound functional measures for them as communication channels. These include information theoretic measures of bandwidth, as well as physical measures of energetic efficiency and speed. Our results will likely shed light on the functional role of clustering and other collective behaviors often seen in experiments. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D40.00007: Maximum entropy principle for predicting response to multiple-drug exposure in bacteria and human cancer cells Kevin Wood, Satoshi Nishida, Eduardo Sontag, Philippe Cluzel Drugs are commonly used in combinations larger than two for treating infectious disease. However, it is generally impossible to infer the net effect of a multi-drug combination on cell growth directly from the effects of individual drugs. We combined experiments with maximum entropy methods to develop a mechanism-independent framework for calculating the response of both bacteria and human cancer cells to a large variety of drug combinations comprised of anti-microbial or anti-cancer drugs. We experimentally show that the cellular responses to drug pairs are sufficient to infer the effects of larger drug combinations in gram negative bacteria, \textit{Escherichia coli}, gram positive bacteria, \textit{Staphylococcus aureus}, and also human breast cancer and melanoma cell lines. Remarkably, the accurate predictions of this framework suggest that the multi-drug response obeys statistical rather than chemical laws for combinations larger than two. Consequently, these findings offer a new strategy for the rational design of therapies using large drug combinations. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D40.00008: Robust regulation of oscillatory Min-protein patterns Jacob Halatek, Erwin Frey Robust spatial patterning was crucial just from the beginning of cellular evolution, and is key to the development of multicellular organisms. In E. Coli, the oscillatory pole-to-pole dynamics of MinCDE proteins functionality prevent improper cell divisions apart from midcell. Min-oscillations are characterized by the remarkable robustness with which spatial patterns dynamically adapt to variations of cell geometry. Moreover, adaption, and therefore proper cell division, is independent of temperature. These observations raise fundamental questions about the underlying core mechanisms, and about the role of spatial cues. With a conceptually novel and universal approach to cellular geometries, we introduce a robust model based on experimental data, consistently explaining the mechanisms underlying pole-to-pole, striped and circular patterns, as well as the observed temperature-dependence. Contrary to prior conjectures, the model predicts that MinD and cardiolipin domains are not colocalized. The key mechanisms are transient sequestration of MinE, and highly canalized transfer of MinD between polar zones. MinD channeling enhances midcell localization and facilitates stripe formation, revealing the potential optimization process from which robust Min-oscillations originally arose. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D40.00009: Mathematical Analysis of Biomolecular Network Reveals Connections Between Diseases Guanyu Wang Connections between cancer and metabolic diseases may consist in the complex network of interactions among a common set of biomolecules. By applying singularity and bifurcation analysis, the phenotypes constrained by the AKT signaling pathway are identified and mapped onto the parameter space, which include cancer and certain metabolic diseases. By considering physiologic properties (sensitivity, robustness and adaptivity) the AKT pathway must possess in order to efficiently sense growth factors and nutrients, the region of normal responses is located. The analysis illuminates the parameter space and reveals system-level mechanisms in regulating biological functions (cell growth, survival, proliferation and metabolism) and how their deregulation may lead to the development of diseases. The analytical expressions summarize the synergistic interactions among many molecules, which provides valuable insights into therapeutic interventions. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D40.00010: Modeling the mammalian circadian clock Craig Jolley, Hiroki Ueda In biology, important processes often depend on a temporal schedule. The 24-hour periodicity of solar illumination caused by the earth's rotation has consequences for environmental factors such as temperature and humidity as well as ecological factors such as the presence of food, predators, or potential mates. As a result, many organisms have evolved to develop a circadian clock that allows them to anticipate these environmental changes in the absence of direct temporal cues. In recent years, extensive efforts have been made to deconstruct the biological clockwork from various organisms, develop mathematical models of circadian function, and construct synthetic analogues to test our understanding. My present work has two major foci. First, we have used regulatory principles revealed by recent experimental work to construct a model of the core genetic oscillator of the mammalian circadian system that captures key system-level behaviors. Second, we are exploring the possibility of a post-translational phosphorylation-based oscillator that is coupled to the core oscillator, conferring enhanced robustness and stability on the complete system. A simple model of this post-translational oscillator reveals key design constraints that must be satisfied by any such oscillator. [Preview Abstract] |
Session D42: Focus Session: Physics of Cancer I -- Evolution and Resistance
Sponsoring Units: DBIOChair: Robert Austin, Princeton University
Room: 156C
Monday, February 27, 2012 2:30PM - 3:06PM |
D42.00001: Drug Resistance and the Kinetics of Metastatic Cancer Invited Speaker: Krastan B. Blagoev Most metastatic cancers after initial response to current drug therapies develop resistance to the treatment. We present cancer data and a theory that explains the observed kinetics of tumor growth in cancer patients and using a stochastic model based on this theory we relate the kinetics of tumor growth to Kaplan-Meyer survival curves. The theory points to the tumor growth rate as the most important parameter determining the outcome of a drug treatment. The overall tumor growth or decay rate is a reflection of the balance between cell division, senescence and apoptosis and we propose that the deviation of the decay rate from exponential is a measure of the emergence of drug resistance. In clinical trials the progression free survival, the overall survival, and the shape of the Kaplan-Meyer plots are determined by the tumor growth rate probability distribution among the patients in the trial. How drug treatments modify this distribution will also be described. At the end of the talk we will discuss the connection between the theory described here and the age related cancer mortality rates in the United States. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D42.00002: Lattice Boltzmann simulations of lymphatic pumping Christian Kunert, Tim P. Padera, Lance L. Munn Lymphatic flow plays an important role in the progress of many diseases, including lymphedema and metastasis. However lymphatic pumping and flow is poorly understood. Here, we present a computer model that is based on biological observations of lymphatic pumping. Fluid flow is simulated by a D2Q9 lattice Boltzmann model. The boundary of the vessels moves according to shear-induced nitric oxide production, and wall motion transfers momentum to the fluid to induce flow. Because the model only includes local properties, it can be highly parallelized. In our case we utilize graphic processors (GPU) to achieve high performance computation. We show that the model provides stable pumping over a wide range of parameter values, with optimum flow achieved in the biological ranges. Furthermore, we investigate the efficiency by analyzing the flow rate and pumping frequency in order to compare the behavior of the model with existing in vivo data. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D42.00003: Mechanical and Structural Changes of the Pericellular Coat during Cell Proliferation Louis T. McLane, Anna Granqvist, Patrick Chang, Anthony Kramer, Jennifer E. Curtis The organization of a hyaluronan and proteoglycan-rich pericellular coat at the cell surface has been shown to facilitate cell migration and mitosis. These several microns thick, swollen grafted polymer matrices are directly correlated with efficient proliferation, migration, and in extreme cases have been associated with the metastatic spread of cancer. For example, hyaluronan synthesis is enormously increased when oncogenic viruses transform fibroblasts and elevated levels of hyaluronan are associated with the hyperproliferative and malignant phenotypes in melanoma and various carcinomas. Studies on cancer cell lines have shown that overproduction of hyaluronan and excess proteoglycan enhances their anchorage independent growth, tumorigenicity and metastatic potential. It has long been suspected that the mechanical and structural changes associated with enhanced pericellular matrix are in part responsible for these effects. Here we present measurements of pericellular coat mechanics and structure, investigating how it changes with cell cycle as well as increased or decreased proteoglycan content. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D42.00004: Can low energy electrons lead to strand breaks in DNA under realistic conditions? Jorge Kohanoff, Maeve Smyth It is widely accepted that low energy electrons produced by ionizing radiation play an important role in causing DNA damage [B. Bouda\"iffa et al., Science {\bf 287}, 1658 (2000)]. Understanding the behaviour of DNA components in the condensed phase due to such electrons is a fundamental step towards modelling this damage within a realistic environment. Here we present a computational study of the effect of low energy electrons in condensed phase models of solvated DNA fragments. First we show that excess electrons, which are initially found delocalized, quickly localize around the nucleobases within a 15 fs time scale. Energies and time scales are comparable for each of the bases [M. Smyth and J. Kohanoff, Phys. Rev. Lett. {\bf 106}, 238108 (2011)]. The phosphodiester bond C$_{3'}$-O$_{3'}$, which under normal conditions is very stable, weakens significantly upon electron attachment both, in the gas and in the condensed phase. Computation of free energy profiles show that barriers for bond cleavage are in the region of 5-8 kcal/mol for all the solvated nucleotides, thus suggesting that this kind of event is quite likely at ambient temperature. This supports the notion that ionizing radiation can actually lead to DNA strand breaks in the physiological environment. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D42.00005: Emergence of fractal behavior and other changes of cell surface during malignant transformation: AFM study of human cervical epithelial cells Maxim Dokukin, Nataliia Guz, Craig Woodworth, Igor Sokolov Fractal behavior, self-similarity when zooming in or out, is frequently found in natural patterns emerged from chaos or any far from equilibrium systems. While expected and observed for tissues, the emergence of fractal behavior associated with malignant transformations was not observed at the level of single cell. Here report on the appearance of fractal behavior when normal human cervical epithelial cells become malignant. This was found by analyzing the adhesion maps imaged with AFM working in HarmoniX mode. Normal and malignant (a mix of cancerous and precancerous) cells were enzymatic only extracted from cervical tissue of healthy individuals and cancer patients, respectively. A surprising 100{\%} discrimination of malignant and normal cells was observed. Although we previously reported differences in surface (brush) layer of cancer cells, the unambiguous quantitative divergence of the fractal behavior of the adhesion maps is a surprise (in particular, when compared to no difference found in the regular AFM images). The nature of the observed difference in the adhesion behavior will be discussed. These results may suggest that the fractal dimensionality can be treated as a new potential ``physicomarker'' for detection of individual cervical cancer cells. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D42.00006: Improved survival in rats with glioma using MRI-guided focused ultrasound and microbubbles to disrupt the blood-brain barrier and deliver Doxil Muna Aryal, Yong Zhi Zhang, Natalia Vykhodtseva, Juyoung Park, Chanikarn Power, Nathan McDannold Blood-brain-barrier (BBB) limits the transportation of most neuropeptides, proteins (enzymes, antibodies), chemotherapeutic agents, and genes that have therapeutic potential for the treatment of brain diseases. Different methods have been used to overcome this limitation, but they are invasive, non-targeted, or require the development of new drugs. We have developed a method that uses MRI-guided focused ultrasound (FUS) combined with circulating microbubbles to temporarily open BBB in and around brain tumors to deliver chemotherapy agents. Here, we tested whether this noninvasive technique could enhance the effectiveness of a chemotherapy agent (Doxil). Using 690 kHz FUS transducer and microbubble (Definity), we induced BBB disruption in intracranially-implanted 9L glioma tumors in rat's brain in three weekly sessions. Animals who received BBB disruption and Doxil had a median survival time of 34.5 days, which was significantly longer than that found in control animals which is 16, 18.5, 21 days who received no treatment, BBB disruption only and Doxil only respectively This work demonstrates that FUS technique has promise in overcoming barriers to drug delivery, which are particularly stark in the brain due to the BBB. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D42.00007: Insights into protein -- DNA interactions, stability and allosteric communications: A computational study of MutS-DNA recognition complexes Lacramioara Negureanu, Freddie Salsbury DNA mismatch repair proteins (MMR) maintain genetic stability by recognizing and repairing mismatched bases and insertion/deletion loops mistakenly incorporated during DNA replication, and initiate cellular response to certain types of DNA damage. The most abundant MMR mismatch-binding factor in eukaryotes, MutS, recognizes and initiates the repair of base-base mismatches and small insertion/deletions. We performed molecular dynamics simulations on mismatched and damaged MutS-DNA complexes. A comprehensive DNA binding site analysis of relevant conformations shows that MutS proteins recognize the mismatched and platinum cross-linked DNA substrates in significantly different modes. Distinctive conformational changes associated with MutS binding to mismatched and damaged DNA have been identified and they provide insight into the involvement of MMR proteins in DNA-repair and DNA-damage pathways. Stability and allosteric interactions at the heterodimer interface associated with the mismatch and damage recognition step allow for prediction of key residues in MMR cancer-causing mutations. A rigorous hydrogen bonding analysis for ADP molecules at the ATPase binding sites is also presented. A large number of known MMR cancer causing mutations among the residues were found. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D42.00008: Glycoprotein Mucin Molecular Brush on Cancer Cells and its Correlation with Resistance Against Drug Delivery Xin Wang, Aalok Shah, Robert Campbell, Kai-tak Wan Uptake of cytotoxic drugs by typical tumor cells is limited by the dense dendritic network of oligosaccharide mucin chains that forms a mechanical barrier. Atomic force microscopy is used to directly measure the force needed to pierce the mucin layer to reach the cell surface. Measurements are analyzed by deGennes' steric reputation theory. Multi-drug resistant ovarian tumor cells shows significantly larger penetration load compared to the wide type. A pool of pancreatic, lung, colorectal, and breast cells are also characterized. The chemotherapeutic agent, benzyl-$\alpha $-GalNac, for inhibiting glycosylation is shown to be effective in reducing the mechanical barrier. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D42.00009: Bcl-2 apoptosis proteins, mitochondrial membrane curvature, and cancer Ghee Hwee Lai, Nathan Schmidt, Lori Sanders, Abhijit Mishra, Gerard Wong, Olena Ivashyna, Eric Christenson, Paul Schlesinger, Kiyotaka Akabori, Christian Santangelo Critical interactions between Bcl-2 family proteins permeabilize the outer mitochondrial membrane, a common decision point early in the intrinsic apoptotic pathway that irreversibly commits the cell to death. However, a unified picture integrating the essential non-passive role of lipid membranes with the contested dynamics of Bcl-2 regulation remains unresolved. Correlating results between synchrotron x-ray diffraction and microscopy in cell-free assays, we report activation of pro-apoptotic Bax induces strong pure negative Gaussian membrane curvature topologically necessary for pore formation and membrane remodeling events. Strikingly, Bcl-xL suppresses not only Bax-induced pore formation, but also membrane remodeling by disparate systems including cell penetrating, antimicrobial or viral fusion peptides, and bacterial toxin, none of which have BH3 allosteric domains to mediate direct binding. We propose a parallel mode of Bcl-2 pore regulation in which Bax and Bcl-xL induce antagonistic and mutually interacting Gaussian membrane curvatures. The universal nature of curvature-mediated interactions allows synergy with direct binding mechanisms, and potentially accounts for the Bcl-2 family modulation of mitochondrial fission/fusion dynamics. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D42.00010: Drift, draft, waves, and ratcheting: evolutionary models of cancer progression Christopher McFarland, Kirill Korolev, Leonid Mirny Recent evidence has found that many mutations are detrimental to tumor growth in cancer. These mutations, called \textit{passenger mutations,} are distinguished from the better known cancer-causing \textit{driver mutations}, which increase a tumor cell's proliferative capacity. How passenger mutations arise, despite selection against them, is not well understood nor fully explained by existing theory. Here, we extend several population genetics models to explain their accumulation in a precancerous population of cells and compare our findings against stochastic simulations. We find that passenger mutations alter and can avert progression to cancer. The probability of cancer depended heavily, and non-monotonically, on both the deleteriousness and rate of mutations. Counter-intuitively, high mutation rates decrease the likelihood of cancer---a finding recently corroborated by clinical data[1]. Our models suggest that therapies exploiting passenger mutations can avert cancer and may be more effective than targeting driver mutations. \begin{enumerate} \item Birkbak, N.J., et al., \textit{Paradoxical relationship between chromosomal instability and survival outcome in cancer.} Cancer Res, 2011. \textbf{71}(10): p. 3447-52. \end{enumerate} [Preview Abstract] |
Session D43: Invited Session: Recent Advances in the Physics of Fractures
Sponsoring Units: DCMP GSNPChair: James Sethna, Cornell University
Room: 157AB
Monday, February 27, 2012 2:30PM - 3:06PM |
D43.00001: Failure of molecules, bones, and the Earth itself Invited Speaker: Sinan Keten Materials fail by recurring rupture and shearing of interatomic bonds at microscopic, molecular scales, leading to disintegration of matter at macroscale, and a loss of function. In this talk, the state-of-the-art of investigations on failure mechanisms in materials will be presented, in particular focusing on atomistic origin of deformation and fracture, and the relationships between molecular mechanics and macroscale behavior. Simple examples of fracture phenomena are used to illustrate the significance and impact of material failure on our daily lives. Based on case studies, mechanisms of failure of a wide range of materials are discussed, ranging from tectonic plates to rupture of single molecules, and an explanation on how atomistic simulation can be used to complement experimental studies and theory to provide a novel viewpoint in the analysis of complex systems is provided. Biological protein materials are used to illustrate how extraordinary properties are achieved through the utilization of intricate structures where the interplay of weak and strong chemical bonds, size and confinement effects, and hierarchical features play a fundamental role. This leads to a discussion of how even the most robust biological material systems fail, leading to diseases that arise from structural and mechanical alterations at molecular, cellular, and tissue levels. New research directions in the field of materials failure and materials science are discussed and the impact of improving the current understanding of materials failure for applications in nanotechnology, biotechnology, medicine as well as the built environment. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D43.00002: Fast fracture in slow motion: Dynamic fracture and the effect of near-tip elastic nonlinearities in brittle gels Invited Speaker: Jay Fineberg We present recent results of fracture experiments in poly-acrylamide gels [1]. These gels are soft polymers in which the characteristic sound speeds are on the order of a few meters/sec - thereby slowing down fracture dynamics by 3 orders of magnitude. We first show that the dynamics of rapid cracks are universal; the fracture of gels exhibits characteristic features that are identical with those seen in ``classic'' materials such as glass. These include: \begin{itemize} \item Excellent quantitative agreement with the two different equations of motion for single dynamic cracks predicted by Linear Elastic Fracture Mechanics (LEFM) -- each for different classes of loading conditions. \item The same branching instabilities, localized waves confined to the crack front, and the characteristic structure formed on the resulting fracture surface as observed in ``standard'' amorphous brittle materials, such as soda-lime glass. \end{itemize} We utilize the ``slow motion'' inherent in the fracture of gels to experimentally and theoretically investigate the structure of the deformation fields that surround the tip of highly dynamic cracks. We find that: \begin{itemize} \item The singular fields predicted by LEFM change their structure due to nonlinear elastic effects that dominate the near-tip region [3]. \item This non-linear elastic region provides a quantitative explanation for the oscillatory instability of cracks [2,4] as their speed approaches the Rayleigh wave speed. \end{itemize} These results provide a quantitative first-principles description of how elastic nonlinearity influences the rapid dynamics of a crack. \\[4pt] [1] A. Livne, G. Cohen, and J. Fineberg, Physical Rev. Lett. \textbf{94}, 224301 (2005); T. Goldman, A. Livne, and J. Fineberg, Physical Rev. Lett. \textbf{104}, 11430 (2010).\\[0pt] [2] A. Livne, O. Ben-David, and J. Fineberg, Phys. Rev. Lett.,\textbf{98}, 124301 (2007).\\[0pt] [3] A. Livne, E. Bouchbinder, and J. Fineberg, Phys. Rev. Lett. \textbf{101}, 264301 (2008);. E. Bouchbinder, A. Livne, and J. Fineberg, Phys. Rev. Lett. \textbf{101}, 264302 (2008); A. Livne, E. Bouchbinder, I. Svetlizky, and J. Fineberg, Science \textbf{327}, 1359 (2010).\\[0pt] [4] E. Bouchbinder, Phys. Rev. Lett. \textbf{103}, 164301 (2009). [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D43.00003: Shapes formed by interacting cracks Invited Speaker: Karen Daniels Brittle failure through multiple cracks occurs in a wide variety of contexts, from microscopic failures in dental enamel and cleaved silicon to geological faults and planetary ice crusts. In each of these situations, with complicated stress geometries and different microscopic mechanisms, pairwise interactions between approaching cracks nonetheless produce characteristically curved fracture paths. We investigate the origins of this widely observed ``en passant'' crack pattern by fracturing a rectangular slab which is notched on each long side and subjected to quasi-static uniaxial strain from the short side. The two cracks propagate along approximately straight paths until they pass each other, after which they curve and release a lens-shaped fragment. We find that, for materials with diverse mechanical properties, each curve has an approximately square-root shape, and that the length of each fragment is twice its width. We are able to explain the origins of this universal shape with a simple geometrical model. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D43.00004: Fracture Statistics: Universality vs. Nucleation Invited Speaker: Ashivni Shekhawat We reexamine several common assumptions about fracture strength, utilizing large-scale simulations of a fuse network model and applying both renormalization-group and nucleation theory methods. Statistical distributions of fracture strengths are believed to be universal and material independent. The universal Weibull and Gumbel distributions emerge as a consequence of the ``weakest-link hypothesis'' and have been studied in the classical theory of extreme value statistics. These distributions are also the fixed points of a renormalization group (RG) flow. However, the engineering community often ignores the Gumbel distribution and uses the Weibull form almost exclusively to fit experimental data. Further, such fits are often extrapolated beyond the available data to estimate the probability of rare events in a variety of applications ranging from structural reliability to insurance pricing. Our recent studies of the random fuse network model raises doubts about most of these practices. We find that the emergent distribution of fracture strengths is the Gumbel distribution. However, the extremely slow convergence to the universal Gumbel form renders it unusable at least in this case. On the other hand, we show that a non-universal distribution derived by using a Griffiths type nucleation theory (due to Duxbury et al.) converges rapidly even for moderate system sizes. We find that while extrapolating the RG based universal Gumbel distribution is perilous and gives wildly incorrect predictions, the nucleation based non-universal results can be extrapolated with confidence. It is entertaining that fracture provides wonderful examples of the statistical mechanics tools developed to study both continuous as well as abrupt phase transitions. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D43.00005: Avalanches in crack front propagation Invited Speaker: Stefano Zapperi We study avalanches in a model for a planar crack propagating in a disordered medium. Due to long-range interactions, avalanches are formed by a set of spatially disconnected local clusters, the sizes of which are distributed according to a power law. We derive a scaling relation between the local cluster and the global avalanche distributions. For length scales above a crossover length proportional to the Larkin length, the aspect ratio of the local clusters scales with the roughness exponent of the line model. For smaller lengthscales we observe multiscaling in the crack line correlations. Our analysis provides an explanation for experimental results on planar crack avalanches in Plexiglas plates, but the results are applicable also to other systems with long-range interactions such as Barkhausen avalanches in ferromagnetic thin films. [Preview Abstract] |
Session D44: Invited Session: Topological Quantum Computing with Majorana Fermions
Sponsoring Units: GQI DCMPChair: Gil Refael, California Institute of Technology
Room: 157C
Monday, February 27, 2012 2:30PM - 3:06PM |
D44.00001: Non-abelian anyons and topological quantum information processing in 1D wire networks Invited Speaker: Jason Alicea Topological quantum computation provides an elegant solution to decoherence, circumventing this infamous problem at the hardware level. The most basic requirement in this approach is the ability to stabilize and manipulate particles exhibiting non-Abelian exchange statistics -- Majorana fermions being the simplest example. Curiously, Majorana fermions have been predicted to arise both in 2D systems, where non-Abelian statistics is well established, and in 1D, where exchange statistics of any type is ill-defined. An important question then arises: do Majorana fermions in 1D hold the same technological promise as their 2D counterparts? In this talk I will answer this question in the affirmative, describing how one can indeed manipulate and harness the non-Abelian statistics of Majoranas in a remarkably simple fashion using networks formed by quantum wires or topological insulator edges. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D44.00002: Realization and detection of Majorana modes at generic spin-orbit coupled semiconductor/s-wave superconductor interfaces Invited Speaker: Jay Sau Majorana fermions are hitherto unobserved particles that have been theoretically predicted to have non-Abelian statistics that may be used for Topological Quantum Computation. For over a decade the only candidate systems for observing Majorana fermions were the non-Abelian $\nu=5/2$ fractional quantum Hall state and chiral p-wave superconductors. More recently, motivated by developments in the area of topological insulators it was realized that a more general class of topological superconductors, some of which may be as simple as the interface of InAs and Al, should support such excitations. This talk will start by explaining why superconductors are a natural host for Majorana fermions. Following this, it will be argued that Majorana fermions should exist in generic semiconductor/superconductor interfaces, both in 1D and 2D, the crucial ingredients being s-wave superconductivity, spin-orbit coupling, and Zeeman splitting. Such Majorana fermions at the end of a nanowire appear as a magnetic-field tunable zero-bias peak in the STM spectrum with quantized conductance. Following this experimental challenges \textit{en route} to realizing Majorana fermions in these structures such as disorder and the required tuning of the chemical potential of the semiconductor, will be discussed. Finally, we will conclude by showing how the spin-orbit coupled nanowires motivate a class of intrinsically number conserving microscopic models for topological superconductor with end Majorana fermions. The bosonization approach used to study this one-dimensional model, directly connects the Majorana fermions, which are typically described as Bogoliubov quasiparticles in mean-field theory, to an emergent Ising order in the one-dimensional nanowire model. The robustness of the topological degeneracy to weak local perturbations can be explicitly demonstrated. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 4:18PM |
D44.00003: The Search for Majorana Fermions in Semiconductor Nanowires Invited Speaker: Leo Kouwenhoven Majorana Fermions can arise as quasi-particles in specially designed nanoscale, electronic devices. Our approach is to use semiconductor nanowires with strong spin-orbit interaction (InAs or InSb). We induce superconductivity in the nanowires and control the electron density through a nearby gate. Several properties are measured such as the spin-orbit strength (including the dependence on the magnetic field direction), the induced superconducting gap (including magnetic field dependence) and the flow of supercurrents. For the determined experimental values we estimate the temperature scale to be $\sim $2 Kelvin as the transition temperature for the reaching the phase of a topological superconductor. Majorana Fermions should be detectable as special features in the tunneling conductance or in the periodicity of an interferometer setup (SQUID geometry). [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:54PM |
D44.00004: Manipulation of Majorana fermions using superconducting qubits Invited Speaker: Anton Akhmerov Majorana fermions are special particles, predicted to appear in certain superconductors. They are extremely useful for quantum computation, due to the possibility to store quantum information in the degenerate ground state of the system. Moreover braiding Majorana fermions around each other allows to implement certain quantum gates in a fault-tolerant manner. I introduce a scheme of quantum computation with Majorana fermions which relies on interplay of charging and Josephson energy to measure, controllably couple, and braid Majorana fermions. The advantage of this scheme is that it fully relies on control elements usual for superconducting cirquitry and does not require fine tuning on the scale of Fermi wavelength. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:30PM |
D44.00005: Disorder effects in topological quantum wires, and alternative platforms for Majorana Fermion realization Invited Speaker: Piet Brouwer Spinless p-wave superconducting wires can be in a topological phase in which they harbor Majorana bound states at their ends. Although there are no known spinless p-wave superconductors in nature, several routes to the artificial creation of such systems have been proposed. In this talk, I will discuss how non-idealities in some of the proposed routes, such as potential disorder and deviations from a strict one-dimensional limit, affect the topological phase. In particular, I'll discuss how the topological phase can persist at weak disorder or for multichannel wires, although some of the signatures of the presence of Majorana fermions are obscured. [Preview Abstract] |
Session D51: Liquid Crystals: Chromonics and Nematics
Sponsoring Units: DCMP DFDChair: Peter Collings, Swarthmore University
Room: Boston Convention Center 154
Monday, February 27, 2012 2:30PM - 2:42PM |
D51.00001: Lyotropic chromonic liquid crystals in the biphasic region Xuxia Yao, Alejandro Rey, Jung Park, Mohan Srinivasarao Lyotropic chromonic liquid crystals have a wide coexistence temperature range where the isotropic and nematic phases are in equilibrium. Negative tactoids (isotropic droplets in the nematic medium) or positive tactoids (nematic droplets in the isotropic medium) form and grow as the nuclei of the new phases in the biphasic region. We studied the growth of tactoids as a function of temperature, the prolate shape of tactoids as well as their thermal fluctuation, based on which the viscoelastic properties of chromonic liquid crystals were obtained. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D51.00002: Order Parameter measurements of Chromonic Liquid Crystal Benzopurpurin 4B using polarized Raman scattering Karthik Nayani, Jung Ok Park, Mohan Srinivasarao Benzopurpurin 4B (BPP4B), a commonly used textile dye, is known to form chromonic liquid crystal phases in aqueous solutions at fairly low concentrations ($<$0.5 wt{\%}) in comparison with other chromonic liquid crystals. Also the aggregation properties and the structure of the aggregates in aqueous solutions of BPP4B are not well understood. Recently McKitterick et al. reported a study on the aggregation and phase behavior of BPP4B in water.\footnote{C. B. McKitterick, N. L. Erb-Satullo, N. D. LaRacuente, A. J. Dickson, P. J. Collings. \textit{J. Phys. Chem. B}. \textbf{2010}. 114, 1888.} Further understanding of the behavior of BPP4B in aqueous solutions can be gained by studying how the order parameter of its liquid crystalline phase varies with some relevant parameters. Planar monodomains of BPP4B were obtained using a flat rectangular capillary. Thermal evolution of the order parameter of these aligned monodomains was carried out using polarized Raman scattering measurements. Further, the concentration dependence and the effect of salt on the order parameter were studied. The variation of the order parameter with the above parameters was correlated to the structure of the aggregates using the UV-Vis absorption data. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D51.00003: Textural transformations in lyotropic chromonic liquid crystals under confinement Alireza Shams, Xuxia Yao, Alejandro D. Rey, Jung Ok Park, Mohan Srinivasarao Lyotropic chromonic liquid crystals under capillary confinement display textural transformations between planar radial and planar polar modes, in which a +1 disclination branches into two +1/2 lines. The texture transformation is characterized by the nature and kinematics of the branch point, the aperture angle, and the shape of the lines. This work presents and validates a model of these four phenomena, which yield the viscoelastic moduli of these novel mesophases. [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D51.00004: Molecular Aggregation in Disodium Cromoglycate Gautam Singh, D. Agra-Kooijman, P.J. Collings, Satyendra Kumar Details of molecular aggregation in the mesophases of the anti-asthmatic drug disodium cromoglycate (DSCG) have been studied using x-ray synchrotron scattering. The results show two reflections, one at wide angles corresponding to $\pi -\pi $ stacking (3.32 {\AA}) of molecules, and the other at small angles which is perpendicular to the direction of molecular stacking and corresponds to the distance between the molecular aggregates. The latter varies from 35 - 41 {\AA} in the nematic (N) phase and 27 -- 32 {\AA} in the columnar (M) phase. The temperature evolution of the stack height, positional order correlations in the lateral direction, and orientation order parameter were determined in the N, M, and biphasic regions. The structure of the N and M phases and the nature of the molecular aggregation, together with their dependence on temperature and concentration, will be presented. [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D51.00005: Flexopolydispersity of Nematic Liquid Crystals Yue Shi, Dong Chen, Rizwan Mahmood, Noel Clark Flexopolydispersity is the coupling of gradients in the director field \textbf{n}(\textbf{r}) of a nematic phase made from a polydisperse mixture of anisotropic particles in solution to the spatial change in variables describing the local mean particle shape, size, and concentration: In solutions of sufficiently polydisperse plates this coupling can lead to a nematic phase with a ``blue-phase'' like array of +1 defect lines. Such a structure has been observed in the lyotropic nematic phase of solutions of graphene oxide sheets. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D51.00006: Nematic order in toroidal and higher-genus droplets Ekapop Pairam, Jaya Lakshmi, Alberto Fernandez-Nieves We generate toroidal and higher-genus droplets filled with nematic liquid crystal which are stabilized inside viscoelastic fluid with a non-zero yield stress. The work presented here is the preliminary observations of our experiments. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D51.00007: Seeing and Sculpting Nematic Liquid Crystal Textures with the Thom construction Bryan Chen, Gareth Alexander Nematic liquid crystals are the foundation for modern display technology and also exhibit topological defects that can readily be seen under a microscope. Recently, experimentalists have been able to create and control several new families of interesting defect textures, including reconfigurably knotted defect lines around colloids (Ljubljana) and the ``toron,'' a pair of hedgehogs bound together with a ring of double-twist between them (CU Boulder). We apply the Thom construction from algebraic topology to visualize 3 dimensional molecular orientation fields as certain colored surfaces in the sample. These surfaces turn out to be a generalization to 3 dimensions of the dark brushes seen in Schlieren textures of two-dimensional samples of nematics. Manipulations of these surfaces correspond to deformations of the nematic orientation fields, giving a hands-on way to classify liquid crystal textures which is also easily computable from data and robust to noise. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D51.00008: Patterned Liquid Crystal Droplets Jakub Kolacz, Andrew Konya, Julio Avila, Feng Wang, Qi-Huo Wei Geometrical confinement has a significant influence on the structure and properties of liquid crystals. Prior research in this area is mainly on polymer-dispersed liquid crystals and liquid crystals in porous media where the liquid crystal droplets are usually non-uniform in size. Here we use microfabrication techniques to pattern liquid crystals into droplets of different geometrical shapes and sizes, and study the liquid crystal ordering in these liquid crystal droplets. The experimental observations will be compared with simulation results. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D51.00009: The mechanism of controlling liquid crystal surface pretilt angle on plasma beam sputtered films Ru-Pin Pan, Meng-Chiou Huang, Wei-Ta Wu, Cheng-Wei Lai, Hsin-Ying Wu In liquid crystal (LC) devices, the surface alignment is essential. The polyimide (PI) film is commonly used to make LC molecules parallel to the surface. A rubbing process is usually applied to choose a particular direction on the surface. A pretilt angle is also induced, which is useful but usually very small. In previous works, we have found out that the sputtered ion-oxide films can give a homeotropic alignment to LC, i,e, the LC molecules are perpendicular to the surface. In this work, we combine these two effects by sputtering the ion-oxide particles onto the PI coated glasses. By adjusting the sputtering conditions, the LC alignment are controlled. A wide range of pretilt angles have been achieved, while the rubbing process is no longer required. A thorough study by varying the sputtering conditions, such as voltage, current, and time duration, and observing the pretilt angles is carried out. The sputtered surfaces are examined with scanning electron microscope to see the coverage. By considering the charge distribution and electric field within the sputter, a quantitative model is then developed, which explains how the sputtering conditions affect the pretilt angles almost perfectly. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D51.00010: Transient Splitting of Conoscopic Isogyres of a Uniaxial Nematic Young-Ki Kim, Bohdan Senuk, Luana Tortora, Samuel Sprunt, Matthias Lehmann, Oleg D. Lavrentovich The phase identification is often based on conoscopic observations of homeotropic cells: A uniaxial nematic produces a pattern with crossed isogyres, while the biaxial nematic shows a split of isogyres. We demonstrate that the splitting of isogyres occurs even when the material remains in the uniaxial nematic phase. In particular, in the bent core material J35, splitting of isogyres is caused by change of the temperature. The effect is transient and the isogyres return to a uniaxial (crossed) configuration after a certain time that depends on sample thickness, temperature, and rate of temperature change; the time varies from a few seconds to tens of hours. The transient splitting is caused by the temperature-induced material flow that triggers a (uniaxial) director tilt in the cell. The flows and the director tilt are demonstrated by the CARS microscopy and fluorescent confocal polarizing microscopy (FCPM). This transient effect is general and can be observed even in E7 and 5CB. The effect should be considered in textural identifications of potential biaxial nematic materials. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D51.00011: A comparison between ``cybotactic'' groups in bent-core and rod-like nematic liquid crystals Saonti Chakraborty, Nicholas Diorio, Will Carr, James Gleeson, Antal Jakli, Samuel Sprunt It is becoming increasingly clear that short-range smectic-CP order is the basis for some of the unusual macroscopic properties of bent-core nematic (BCN) liquid crystals. By analyzing small angle X-ray diffraction patterns taken on a bent-core and a chemically related calamitic (rod-like) nematic, we have attempted to clarify the nature of the ``cybotactic groups'' (or molecular clusters) contributing to this short-range order in BCNs, and to distinguish their signature from the scattering due to smectic fluctuations normally observed above a nematic to smectic transition. We find that persistent, finite-sized, tilted smectic clusters, with short, temperature-independent correlation lengths, account for the scattering observed from the BCN, while the calamitic material provides a remarkably clear example of temperature-dependent fluctuations in smectic order observed even far above a smectic-C phase. Supported by NSF DMR-0964765 [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D51.00012: Optimizing liquid crystalline properties for bio-sensing at aqueous interfaces Wilder Iglesias, Nicholas L. Abbott, Elizabeth K. Mann, Antal J\'akli Recent studies show that surfactant or phospholipid assemblies can be monitored at interfaces between aqueous solutions and thermotropic liquid crystals. The capability of these liquid crystals to change birefringence with the reordering induced by the decorated surface allows to study and characterize dynamical phenomena happening at the interfaces. In this work we tune the surface anchoring and the viscoelastic properties of the liquid crystal mesogens in order to increase sensitivity and optimize the response to events at the surface. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D51.00013: Competing Ordering Processes at Liquid Crystal Surfaces Laden with Semifluorinated Alkane Molecules Xunda Feng, Ahmed Mourran, Martin Moeller, Christian Bahr Ellipsometric measurements elucidate the interplay between the surface order at the isotropic liquid crystal/air interface and a structural phase transition in a Gibbs film on the same interface. Gibbs films formed by the semifluorinated alkane C18H37--C12F25 exhibit a sharp transition from a dilute state at higher temperatures to a dense state at lower temperatures. The transition temperature can be tuned by controlling the C18H37--C12F25 concentration in the bulk liquid crystal phase. When the transition takes place in the temperature range in which a molecular thin smectic or nematic film exists at the isotropic liquid crystal/air interface, the smectic surface order is destroyed while the nematic surface order is affected by a change of the orientation of the liquid crystal molecules. The ellipsometric data indicate that both behaviors result from a change of the anchoring condition of the liquid crystal molecules in contact with the Gibbs film. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D51.00014: Revealing the inner arrangement of cholesteric liquid crystals confined in polymeric electrospun fibers Giusy Scalia, Eva Enz, Vera La Ferrara, Oronzo Cal\'o, Jan Lagerwall Cholesteric liquid crystals, like other types of LCs, can be confined inside polymeric fibers by coaxial electrospinning. In this way the interesting optical properties of cholesterics could be transferred to very long fibers that can form flexible or rigid mats according to the outer sheath material. Selective reflection was easily detected from polymeric fibers with cholesteric LC core. Despite the uniformity of the external morphology of the fibers, evaluated by SEM, defects in the optical texture could be observed in some locations as well as differences in the wavelength of the reflected light. The reason for such differences needs to be clarified in order to achieve a uniform, controlled optical texture. The understanding was achieved by direct observation of the cross section of the LC-filled fibers by cutting and sectioning the fibers by Focused Ion Beam (FIB). This revealed differences in dimensions of the inner cavity correlating them to the observed wavelengths of the selectively reflected light, but also changes in shape, in some parts with strongly varying width that accounts for the defect lines observed. We could also visualize the effect of flow instability of the jet during spinning, inducing the formation of chains of LC droplets. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D51.00015: Macroscopic torsional strain and induced molecular conformational deracemization Rajratan Basu, Joel Pendery, Rolfe Petschek, Robert Lemieux, Charles Rosenblatt A macroscopic helical twist is imposed on an achiral nematic liquid crystal by controlling the azimuthal alignment directions at the two substrates. On application of an electric field the director rotates in the substrate plane. This electroclinic effect, which requires the presence of chirality, is strongest at the two substrates and increases with increasing imposed twist distortion. We present a simple model involving a tradeoff among bulk elastic energy, surface anchoring energy, and deracemization entropy that suggests the large equilibrium director rotation at the surfaces induces a deracemization of chiral conformations in the molecules, quantitatively consistent with experiment. [Preview Abstract] |
Session D52: General Statistical and Nonlinear Physics and GSNP Student Prize Talks
Sponsoring Units: GSNPChair: Corey O'Hern, Yale University
Room: 153C
Monday, February 27, 2012 2:30PM - 2:42PM |
D52.00001: Dynamic phase transition in the classical anisotropic XY model on a square lattice William Baez, Trinanjan Datta Ginzburg-Landau analysis of the anisotropic XY model in a spatially homogeneous oscillating magnetic field on a square lattice suggests the existence of several dynamical phases - Ising symmetry restoring order (Ising SRO), Ising symmetry breaking order (SBO), XY symmetry restoring order (XY SRO), and XY symmetry breaking order (XY SBO). We investigate the presence of these phases and the dynamic phase transition (DPT) between these phases using classical Monte Carlo simulation techniques. We explore the system for a range of values for the external field amplitude, field frequency, and anisotropy parameter. Utilizing the period-averaged magnetization (in both the x- and y- component) as the dynamic order parameter we confirm the presence of multiple DPT transitions in the model. We also construct the probability density histograms of the dynamic order parameter to validate the existence of the four DPT phases. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D52.00002: Non-universal local critical exponents at a non-equilibrium phase transition Michel Pleimling, Hyunhang Park We study the dynamic phase transition in the two-dimensional semi-infinite kinetic Ising model in an oscillating field. We focus on the critical regime where the competition between the half-period of the oscillating field $t_{1/2}$ and the metastable lifetime $\langle \tau \rangle$ is most pronounced. We focus on layer-dependent quantities, such as the period-averaged magnetization per layer $Q(z)$ and the layer susceptibility $\chi_Q(z)$, and determine surface critical exponents through finite size scaling. We find that the values of these non-equilibrium exponents are non-universal as they depend on the strength of the surface couplings. Results for the three-dimensional model are also briefly discussed and compared to the two-dimensional case. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D52.00003: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 3:06PM - 3:18PM |
D52.00004: Extremal Optimization for p-Spin Models Stefan Falkner, Stefan Boettcher It was shown recently that finding ground states in the 3-spin model on a 2d dimensional triangular lattice poses an NP-hard problem [1]. We use the extremal optimization (EO) heuristic [2] to explore ground state energies and finite-size scaling corrections [3]. EO predicts the thermodynamic ground state energy with high accuracy, based on the observation that finite size corrections appear to decay purely with system size. Just as found in 3-spin models on $r$-regular graphs, there are no noticeable anomalous corrections to these energies. Interestingly, the results are sufficiently accurate to detect alternating patters in the energies when the lattice size $L$ is divisible by 6. Although ground states seem very prolific and might seem easy to obtain with simple greedy algorithms, our tests show significant improvement in the data with EO. \\[4pt] [1] PRE 83 (2011) 046709,\hfil\break [2] PRL 86 (2001) 5211,\hfil\break [3] S. Boettcher and S. Falkner (in preparation). [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D52.00005: A novel Kinetic Monte Carlo algorithm for Non-Equilibrium Simulations Prateek Jha, Vladimir Kuzovkov, Bartosz Grzybowski, Monica Olvera de la Cruz We have developed an off-lattice kinetic Monte Carlo simulation scheme for reaction-diffusion problems in soft matter systems. The definition of transition probabilities in the Monte Carlo scheme are taken identical to the transition rates in a renormalized master equation of the diffusion process and match that of the Glauber dynamics of Ising model. Our scheme provides several advantages over the Brownian dynamics technique for non-equilibrium simulations. Since particle displacements are accepted/rejected in a Monte Carlo fashion as opposed to moving particles following a stochastic equation of motion, nonphysical movements (e.g., violation of a hard core assumption) are not possible (these moves have zero acceptance). Further, the absence of a stochastic ``noise'' term resolves the computational difficulties associated with generating statistically independent trajectories with definitive mean properties. Finally, since the timestep is independent of the magnitude of the interaction forces, much longer time-steps can be employed than Brownian dynamics. We discuss the applications of this scheme for dynamic self-assembly of photo-switchable nanoparticles and dynamical problems in polymeric systems. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D52.00006: How an aggregation process helps us understanding solid fragmentation Vledouts Alexandre, Nicolas Vandenberghe, Emmanuel Villermaux We report on an experiment intended to understand the fragmentation of a ring composed of cohesive solid spheres (magnets in dipolar interaction). At initial time, the ring is forced to expand radially and the spheres separate from each other. Because of the dipolar attractive force between the spheres, their uniform angular distribution is unstable and the spheres aggregate with each other to form fragments. We record the full dynamics of the spheres assembly and we show that the final fragment size distribution is the signature of the aggregation process giving birth to it. In particular, we introduce a Weber number $We$, based on the radial velocity of the ring, the density of the spheres and their magnetization. We find that the final mean fragment size scales like $We^{-1/3}$ and that the standard deviation of the fragments distribution is proportional to it. We will also discuss the relation between our findings and the fragmentation of elastic rings studied by Sir N. Mott. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D52.00007: Thermodynamic limit, quasi-stationary states and the range of pair interactions Andrea Gabrielli, Michael Joyce, Bruno Marcos ``Quasi-stationary'' states are approximately time-independent out of equilibrium states which have been observed in a variety of systems of particles interacting by long-range interactions. We investigate here the conditions of their occurrence for a generic pair interaction $V(r \rightarrow \infty) \sim 1/r^\gamma$ with $\gamma > 0$, in $d>1$ dimensions. We generalize analytic calculations known for gravity in $d=3$ to determine the scaling parametric dependences of their relaxation rates due to two body collisions, and report extensive numerical simulations testing their validity. Our results lead to the conclusion that, for $\gamma < d-1$, the existence of quasi-stationary states is ensured by the large distance behavior of the interaction alone, while for $\gamma > d-1$ it is conditioned on the short distance properties of the interaction, requiring the presence of a sufficiently large soft-core in the interaction potential. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D52.00008: ABSTRACT WITHDRAWN |
Monday, February 27, 2012 4:06PM - 4:18PM |
D52.00009: Numerical study of Potts models with aperiodic modulations: influence on first-order transitions Nilton Branco, Daniel Girardi We perform a numerical study of Potts models on a rectangular lattice with aperiodic interactions along one spatial direction. The number of states $q$ is such that the transition is a first-order one for the uniform model. The Wolff algorithm is employed, for many lattice sizes, allowing for a finite-size scaling analyses to be carried out. Three different self-dual aperiodic sequences are employed, such that the exact critical temperature is known: this leads to precise results for the exponents. We analyze models with $q=6$ and $15$ and show that the Harris-Luck criterion, originally introduced in the study of continuous transitions, is obeyed also for first-order ones. The new universality class that emerges for relevant aperiodic modulations depends on the number of states of the Potts model, as obtained elsewhere for random disorder, and on the aperiodic sequence. We determine the occurrence of log-periodic behavior, as expected for models with aperiodic modulated interactions. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D52.00010: BREAK |
Monday, February 27, 2012 4:30PM - 4:42PM |
D52.00011: Boundaries Matter for Confined Colloidal Glasses Gary Hunter, Kazem V. Edmond, Eric R. Weeks We confine dense colloidal suspensions within emulsion droplets to examine how confinement and properties of the confining medium affect the colloidal glass transition. Samples are imaged via fast confocal microscopy. By observing a wide range of droplet sizes and varying the viscosity of the external continuous phase, we separate finite size and boundary effects on particle motions within the droplet. Suspensions are composed of binary PMMA spheres in organic solvents while the external phases are simple mixtures of water and glycerol. In analogy with molecular super-cooled liquids and thin-film polymers, we find that confinement effects in colloidal systems are not merely functions of the finite size of the system, but are strongly dependent on the viscosity of the confining medium and interactions between particles and the interface of the two phases. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D52.00012: Dynamics near shear-jamming for a dense granular system Jie Ren, Joshua Dijksman, Robert Behringer This talk will present several systematic experimental studies of a two-dimensional, frictional dense granular system subjected to simple shear deformation. The first experiment consists of linear shear for densities smaller than the isotropic jamming point, and examines both the evolution of the average stress and the evolution of force network. These measures reveal three distinguishable regimes of the granular system with increasing shear strain: unjammed, fragile, and shear-jammed regimes. The second experiment uses small amplitude cyclic shear to probe the dynamical response of the states from the first experiment. For fragile or jammed regimes, cyclic shear drives the system through transient states that evolve towards relatively stable forces networks and system-averaged stress. The timescale of the transient increases rapidly as the system moves deeper into the fragile, or shear-jammed regimes. These experiments also involve particle tracking (displacements and rotations) to search for and characterize non-affine motion and spatial heterogeneity. There is a clear increase in particle diffusion with increasing density and shear strain amplitude, even when the system is still unjammed and experiences only minimal stress. When the system is fragile or jammed, the heterogeneity of particle displacements reveals subtle correlations with the force network. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D52.00013: Elasticity of adherent active cells on a compliant substrate Shiladitya Banerjee, Aaron F. Mertz, Eric R. Dufresne, M. Cristina Marchetti We present a continuum mechanical model of rigidity sensing by livings cells adhering to a compliant substrate. The cell or cell colony is modeled as an elastic active gel, adapting recently developed continuum theories of active viscoelastic fluids. The coupling to the substrate enters as a boundary condition that relates the cell's deformation field to local stress gradients. In the presence of activity, the substrate induces spatially inhomogeneous contractile stresses and deformations, with a power law dependence of the total traction forces on cell or colony size. This is in agreement with recent experiments on keratinocyte colonies adhered to fibronectin coated surfaces. In the presence of acto-myosin activity, the substrate also enhances the cell polarization, breaking the cell's front-rear symmetry. Maximal polarization is observed when the substrate stiffness matches that of the cell, in agreement with experiments on stem cells. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D52.00014: Differential geometry of the space of Ising models Benjamin Machta, Ricky Chachra, Mark Transtrum, James Sethna We use information geometry to understand the emergence of simple effective theories, using an Ising model perturbed with terms coupling non-nearest-neighbor spins as an example. The Fisher information is a natural metric of distinguishability for a parameterized space of probability distributions, applicable to models in statistical physics. Near critical points both the metric components (four-point susceptibilities) and the scalar curvature diverge with corresponding critical exponents. However, connections to Renormalization Group (RG) ideas have remained elusive. Here, rather than looking at RG flows of parameters, we consider the reparameterization-invariant flow of the manifold itself. To do this we numerically calculate the metric in the original parameters, taking care to use only information available after coarse-graining. We show that under coarse-graining the metric contracts very anisotropically, leading to a ``sloppy'' spectrum with the metric's Eigenvalues spanning many orders of magnitude. Our results give a qualitative explanation for the success of simple models: most directions in parameter space become fundamentally indistinguishable after coarse-graining. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D52.00015: Seeing and Sculpting Nematic Liquid Crystal Textures with the Thom construction Bryan Chen, Gareth Alexander Nematic liquid crystals are the foundation for modern display technology and also exhibit topological defects that can readily be seen under a microscope. Recently, experimentalists have been able to create and control several new families of interesting defect textures, including reconfigurably knotted defect lines around colloids (Ljubljana) and the ``toron,'' a pair of hedgehogs bound together with a ring of double-twist between them (CU Boulder). We apply the Thom construction from algebraic topology to visualize 3 dimensional molecular orientation fields as certain colored surfaces in the sample. These surfaces turn out to be a generalization to 3 dimensions of the dark brushes seen in Schlieren textures of two-dimensional samples of nematics. Manipulations of these surfaces correspond to deformations of the nematic orientation fields, giving a hands-on way to classify liquid crystal textures which is also easily computable from data and robust to noise. [Preview Abstract] |
Session D53: Focus Session: Jamming -- Nonlinear Acoustics and Vibrational Response
Sponsoring Units: GSNP DFDChair: Bulbul Chakraborty, Brandeis University
Room: 153B
Monday, February 27, 2012 2:30PM - 2:42PM |
D53.00001: Vibrational Modes in Colloidal Crystals Ke Chen, Tim Still, Kevin Aptowicz, Arjun Yodh We investigate vibrational modes in quasi-two-dimensional colloidal crystals using video microscopy and displacement covariance matrix analysis. Debye scaling in the phonon density of states and the dispersion curve for two-dimensional hexagonal crystals are recovered for both mono-layer and double layer colloidal crystals. Using ``soft spots'' analysis, low-frequency quasi-localized phonon modes, which were found to coincide with fragile regions in glasses [1] appear to be spatially correlated with structural defects in colloidal crystals. Thus, ``soft spots'' may be a useful general identifier for defects in both crystalline and amorphous solids. This work is supported by NSF DMR 0804881, MRSEC DMR11-20901, and by NASA NNX08AO0G. \\[4pt] [1] K. Chen et al, Phys. Rev. Lett. 107, 108301 (2011) [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D53.00002: Acoustic measurement of a granular density of state Eli Owens, Karen Daniels Measurements of the vibrational density of states $D(\omega)$ in glasses reveal that an excess number of low-frequency modes is associated with a loss of mechanical rigidity. An excess number of such modes has also been observed in both simulations of idealized granular materials near the jamming point, and in experiments on colloids. We experimentally investigate similar features in a jammed, quasi-two-dimensional granular material. We mimic thermal fluctuations using an electromagnetic driver to inject acoustic white noise, while piezoelectric sensors embedded inside a subset of the particles provide measurements of single-particle velocities. By analogy with conventional thermal techniques, we calculate a $D(\omega)$-like quantity via the spectrum of the velocity autocorrelation function. We measure $D(\omega)$ as a function of the confining pressure and find that the peak in the density of states shifts to higher frequency with system pressure. At low pressure, disordered systems have more low frequency modes than do hexagonally-packed systems. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D53.00003: Elastic weakening of a dense granular medium by acoustic fluidization Xiaoping Jia, Jerome Laurent, Siet Wildenberg, Martin van Hecke Elastic waves propagating through a dense granular pack provide a unique probe of the elastic properties and internal dissipation of the medium [1], and also allow investigating the irreversible rearrangement of the contact network at large vibration amplitude. In this talk, we describe two distinct types of nonlinearity, i.e. hertzian and frictional, at the grain contact by sound amplitude and velocity measurements, respectively, under different confining pressure [2]. Beyond certain wave amplitude, the sound-matter interaction becomes irreversible, leaving the medium in a weakened and slightly compacted state. A slow recovery of the initial elastic modulus is observed after acoustic perturbation, revealing the plastic creep growth of microcontacts. The cross-correlation function of configuration-specific acoustic speckles highlights the relationship between the macroscopic elastic weakening and the local change of the contact networks, induced by strong sound vibration, in the absence of appreciable grain motion. We show that the softening of elastic modulus is much more pronounced with the shear wave (up to 20{\%}) than with the compressional wave (to 10{\%}). \\[4pt] [1] Th. Brunet, X. Jia and P. Mills, Phys. Rev. Lett \textbf{101}, 138001 (2008) \\[0pt] [2] Th. Brunet, X. Jia and P. Johnson, Geophys. Res. Lett \textbf{35}, L19308 (2008); X. Jia, Th. Brunet and J. Laurent, Phys. Rev. E \textbf{00}, 000300(R) (2011) [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:42PM |
D53.00004: Nonharmonicity in vibrated granular solids Invited Speaker: Carl Schreck We have shown that granular packings composed of frictionless particles with repulsive contact interactions are strongly nonharmonic. When infinitesimally perturbed along linear response eigenmodes of the static packing, energy leaks from the original mode of vibration to a continuum of frequencies due solely to contact breaking even when the system is under significant compression. Further, vibrated packings possess well-defined equilibrium positions that are different than those of the unperturbed packing. The vibrational density of states obtained using the displacement matrix and velocity autocorrelation function methods exhibit an increase in the number of low-frequency modes over that obtained from linear response of the static packing. The form of the density of states in vibrated granular packings is reminiscent of the low-frequency behavior of the vibrational density of states in fluid systems. We also investigate the effects of inter-particle friction, dissipation, particle shape, and degree of positional order on the density of states and thermal transport properties in driven granular packings. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D53.00005: Irreversible Incremental Behavior in a Granular Material Luigi La Ragione, Vanessa Magnanimo, James Jenkins, Hernan Makse We test the elasticity of dense, isotropic, compressed aggregates of frictional spheres using cyclic increments of shear and volume strain in a numerical simulation. For both types of increments, we measure irreversibility in relative displacements and contact forces that is stronger for the increments in shear. The strength of the irreversibility increases as the average number of contacts per particle (the coordination number) decreases. This irreversibility may be associated with the opening of contacts in an increment of loading, pointed out in a recent paper of Schreck et al. (PRL, 2011); such contact opening could lead to irreversible rearrangement of the contact network when the increment is relaxed. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D53.00006: Experimental Measurements of Force Propagation in Vibrated Photoelastic Disks Aline Hubard, Mark Shattuck We measure and analyze the propagation forces in vibrated disks under constant pressure with different amplitudes and frequencies. We use photoelastic particles to visualize the stress within each particle using a high-speed video camera. From the images we can extract the time dependent force at each contact to determine how force propagates through the contact network. Using mono-disperse particles we focus on force propagation during the phase transition from an ordered solid-like state to a disordered fluid-like state as we change the vibration amplitude. With bi-disperse particles we compare with the transition to a disordered solid-like state. [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D53.00007: Nonlinear acoustics of glass bead packings at vanishing static pressures Vincent Tournat, Vitalyi Gusev We present here a set of recent results obtained in three experimental configurations: linear and nonlinear acoustic probing of a granular slab at different compacities, surface acoustic waves in granular layers submitted to gravity, resonances of a granular layer with an in-depth elasticity gradient. We succeeded to overcome the experimental issues associated to the dramatic increase of acoustic wave attenuation when the confining pressure diminishes. Interpretations reveal that the manifestations of nonlinear effects (self-demodulation, nonlinear resonance, second harmonic or subharmonic generation...) allow to isolate the different types of nonlinearities involved (Hertz, clapping, stick-slip, hysteresis...). Also, some discrepancies are observed for the extracted linear elastic parameters scaling laws as a function of vanishing pressure (lower than 100 Pa typically) between the different developed experimental configurations and the theoretical predictions. Explanations for these discrepancies are given. We show than under some conditions, it is necessary to take into account the coupling of grain motion with that of the saturating air. Application of these results to the probing of granular layers under destabilization will be presented. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D53.00008: Acoustic Echoes in Model Glasses Justin Burton, Sidney Nagel At low temperatures, glasses and crystals behave in qualitatively different ways. In particular, glasses have a great many more low-energy excitations that have traditionally been explained in terms of a distribution of dilute, two-level quantum states that are created by clusters of particles tunneling between two nearly degenerate ground states. Strong evidence for this model has come from the saturation effects and acoustic echoes [1] observed in these excitations. We show that, in contrast to conventional wisdom, the quasi-localized, strongly anharmonic, normal modes of jammed systems [2] can produce acoustic echoes due to the shift in the mode frequency with increasing amplitude. We observe this both in jammed packings of spherical particles with finite-range, Hertzian repulsions, and in model glasses interacting with a Lennard-Jones potential. In contrast to pulse echoes in two-level systems, a distinguishing feature of these ``anharmonic echoes'' is the appearance of multiple echoes after two excitation pulses, a feature also observed in experiments [1].\\[4pt] [1] B. Golding and J. E. Graebner. Phys. Rev. Lett. \textbf{37}, 852 (1976).\\[0pt] [2] N. Xu, V. Vitelli, A. J. Liu, and S. R. Nagel. Europhys. Lett. \textbf{90}, 56001 (2010). [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D53.00009: Extreme Physics and Rearrangements near Jamming Martin van Hecke, Zorana Zeravcic, Alex Siemens, Johannes Simon, Daniel Geelen Near jamming, linear response becomes irrelevant. I briefly discuss how this gives rise to a range of intrinsically nonlinear, even extreme, phenomena. Moreover, reversibility also breaks down, and I will discuss the nature of rearrangements near jamming. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D53.00010: Formation, Propagation, and Attenuation of Shocks Waves in Jammed Matter Leopoldo Gomez, Vincenzo Vitelli We study the formation and propagation of fully non-linear waves in jammed granular media. Close to the jamming point, an arbitrary initial distortion of the media will induce the formation of non-linear finite amplitude waves. There are two regimes in the evolution of these waves. At early times non-linear interactions dominate the propagation, leading to a temporal evolution strongly dependent on the initial distortion. At long times the propagation is characterized by a new universal regime, dominated by hydrodynamical attenuation. Here the non-linear waves evolve in a self-similar fashion, characterized by a power law attenuation whose exponent is weakly dependent on the initial pressure of the system. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D53.00011: Jamming of soft spheres at finite temperature : a granular experiment Corentin Coulais, Olivier Dauchot, Robert Behringer At large packing fraction, disordered packings of particles with repulsive contact interactions jam into a rigid state where they withstand finite shear stresses before yielding. For frictionless particles and at zero temperature, the jamming transition coincides with the onset of iso-staticity and many geometrical and mechanical properties scale with the distance to the jamming point. What are the vestige of jamming at finite temperature and how jamming impacts the thermodynamics of glasses remain open issues. We address these questions experimentally by investigating the dynamics of both the density field and the force network of an horizontally shaken bi-disperse packing of photo-elastic disks. The average number of contact clearly displays an abrupt transition which we interpret as the jamming transition. Besides, dynamical heterogeneities are observed and their amplitude exhibits a maximum, which, in turn, signs a dynamical transition. We discuss in detail the interplay between these two transitions and how they depend on the particle softness and amplitude of the horizontal vibration. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D53.00012: Density of vibrational modes in partially crystalline granular packings Thibault Bertrand, Carl F. Schreck, Mark Shattuck, Corey S. O'Hern Numerous numerical results have shown that systems of monodisperse frictionless disks crystallize readily and that disordered mechanically stable packings are rarely obtained. We numerically investigate the dependence of the cluster size distribution on system size and quench rate. We also investigate the effect of crystallization on the vibrational response outside the linear response regime. We study changes in the density of vibrational modes due to changes in the average crystallite size and perturbation amplitude in partially crystalline granular packings. In particular we determine how the number of contacts (above the isostatic value) affects anharmonic response in granular packings. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D53.00013: Studying the low-frequency quasilocalized modes in disordered colloidal systems Lei Xu, Peng Tan, Ning Xu, Andrew Schofield In disordered colloidal systems, we experimentally measure the normal modes with covariance matrix method, and clarify the origin of low-frequency quasilocalization at single-particle level. We observe important features from both jamming and glass simulations: there is a plateau in the density of states which is suppressed upon ompression, as predicted by jamming; within the same systems, we also find that the low-frequency quasilocalization originates from the coupling between large vibrations of defective structures and transverse excitations, consistent with recent glass simulation. The coexistence of these features demonstrates an experimental link between jamming and glass. Extensive simulations further show that such structural origin of quasilocalization is universally valid for various temperatures and volume fractions. [Preview Abstract] |
Session D54: Focus Session: Complex and co-evolving networks - Foundations in Complex Networks
Sponsoring Units: GSNPChair: Bruno Goncalves, Indiana University
Room: 152
Monday, February 27, 2012 2:30PM - 2:42PM |
D54.00001: Dynamics of Random Graphs with Bounded Degrees Eli Ben-Naim, Paul Krapivsky We investigate the dynamic formation of regular random graphs. In our model, we pick a pair of nodes at random and connect them with a link if both of their degrees are smaller than $d$. Starting with a set of isolated nodes, we repeat this linking step until a regular random graph, where all nodes have degree $d$, forms. We view this process as a multivariate aggregation process, and formally solve the evolution equations using the Hamilton-Jacobi formalism. We calculate the nontrivial percolation thresholds for the emergence of the giant component when $d\geq 3$. Also, we estimate the number of steps until the giant component spans the entire system and the total number of steps until the regular random graph forms. These quantities are non self-averaging, namely, they fluctuate from realization to realization even in the thermodynamic limit. [Preview Abstract] |
Monday, February 27, 2012 2:42PM - 2:54PM |
D54.00002: Influence of Boundary Conditions on Metastable Lifetimes for The Ising Model on the Hyperbolic Plane Howard L. Richards, Dipendra Sharma Chapagain, James Molchanoff Some corals grow in shapes that resemble 3D models of the hyperbolic plane, since this allows them to have greater area for a given growth radius. Each polyp could be represented by an Ising site, with ``feeding'' = ``up'' and ``retracted'' = ``down''. The mechanisms of metastable decay could be interpreted as how the coral as a whole reacts to changing conditions of food availability or predation. Previous studies have shown that there is a spinodal field for the Ising model on a regular lattice in the hyperbolic plane if it is infinite or has periodic or mean-field boundary conditions. This happens because the size of the boundary grows asymptotically at the same rate as the droplet volume, in clear contrast with droplets in the Euclidean plane. Our simulations show, however, that the spinodal field disappears if more physically relevant open boundary conditions are used instead. [Preview Abstract] |
Monday, February 27, 2012 2:54PM - 3:06PM |
D54.00003: Quantifying the complexity of random Boolean networks Xinwei Gong, Joshua E.S. Socolar We study two measures of the complexity of spatially extended systems in the context of random Boolean networks. A measure defined by Shalizi et al. for cellular automata, based on a criterion for optimal statistical prediction [1], does not distinguish between the spatial inhomogeneity of the ordered phase and the dynamical inhomogeneity of the disordered phase. A modification in which complexities of individual nodes are calculated yields vanishing complexity values for networks in the ordered and critical regimes and for highly disordered networks, peaking somewhere in the disordered regime. Individual nodes with high complexity are the ones that pass the most information from the past to the future, a quantity that depends in a nontrivial way on both its own Boolean function and the location of the node within the network. \\[4pt] [1] C. R. Shalizi, K. L. Shalizi, and R. Haslinger, Phys. Rev. Lett. 93, 118701 (2004). [Preview Abstract] |
Monday, February 27, 2012 3:06PM - 3:18PM |
D54.00004: Synchronization in Noisy Networks with Multiple Time Delays David Hunt, Gyorgy Korniss, Boleslaw Szymanski We expand our previous work of uniform time delays in stochastic, linearly-coupled synchronization problems\footnote{D. Hunt, G. Korniss, B.K. Szymanski, Phys. Rev. Lett. \textbf{105}, 068701 (2010)} by including descriptions of networks with multiple delays. Non-uniform time delays can arise when there are multiple sources of delay, e.g. the time to transmit and the time to process information. In this particular two-delay case\footnote{D. Hunt, G. Korniss, B.K. Szymanski, Phys. Lett. A \textbf{375}, 880 (2011)}, the primary limitation on the network to synchronize without any centralized direction does not come from restrictions in the transmission of a node's state to its neighbors; rather it depends on the ability for each node to process and respond to the information about itself in the context of its local environment. Furthermore, given a network's structure, there are optimal transmission delays for which the network remains synchronizable for longer processing delays. As a result, synchronization is not always improved--and in some cases it can be totally destroyed--by minimizing the transmission delays. For special cases we also study the scaling function that quantifies the synchronization of the system. This shows the limitation of synchronization in a noisy network [Preview Abstract] |
Monday, February 27, 2012 3:18PM - 3:30PM |
D54.00005: Continuous Percolation by Discontinuities Jan Nagler In the very recent article [Science {\bf 333}, 322 (2011)], O. Riordan and L. Warnke, state that (i) {\em any rule based on picking a fixed number of random vertices gives a continuous transition}, and (ii) that therefore {\em explosive percolation is continuous}. It is equally true that certain percolation processes based on picking a fixed number of random vertices are discontinuous. Here we resolve this seeming paradox. We exemplify this by studying an extremal case of a process that is continuous in the sense of Riordan and Warnke but still exhibits infinitely many discontinuous jumps in arbitrary vicinity of the onset of the continuous phase transition. Moreover, we demonstrate analytically that continuity at the phase transition and discontinuity of the percolation process are compatible and generic for certain competitive percolation systems. [Preview Abstract] |
Monday, February 27, 2012 3:30PM - 3:42PM |
D54.00006: Random Transverse Ising Model on Annealed Complex Networks Ginestra Bianconi In order to shed light on critical phenomena on cuprates, here we propose a stylized model capturing the essential characteristics of the superconducting-insulator transition of a highly dynamical, heterogenous granular material: the Disordered Quantum Tranverse Ising Model (DQTIM) on Annealed Complex Network. We show that when the networks encode for high heterogeneity of the expected degrees described by a power law distribution, the critical temperature for the onset of the supercoducting phase diverges to infinity as the power-law exponent $\gamma$ of the expected degree distribution is less than 3, i.e. $\gamma < 3$. Moreover we investigate the case in which the critical state of the electronic background is triggered by an external parameter $g$ that determines an exponential cutoff in the power law expected degree distribution characterized by an exponent $\gamma$. We find that for $g = g_c$ the critical temperature for the supercondutor-insulator transition has a maximum if $\gamma > 3$ and diverges if $\gamma<3$. [Preview Abstract] |
Monday, February 27, 2012 3:42PM - 3:54PM |
D54.00007: Dynamical Instability in Boolean Networks as a Percolation Problem Shane Squires, Michelle Girvan, Edward Ott Boolean networks, a widely used model of gene regulatory networks, exhibit a phase transition between a stable regime, in which small perturbations die out, and an unstable regime, in which small perturbations grow exponentially. We show that this phase transition can be mapped onto a static percolation problem which predicts the critical point and the long-time Hamming distance between perturbed and unperturbed systems. The results, which apply to Boolean networks with a broad class of topologies and update functions, are confirmed by numerical simulations. [Preview Abstract] |
Monday, February 27, 2012 3:54PM - 4:06PM |
D54.00008: Critical Behavior of the Ising Model on Small-world Hanoi Networks Trent Brunson, Stefan Boettcher The addition of small-world bonds on hierarchical lattices changes a typical Ising model ferromagnetic phase transition to one of infinite order, referred to as the inverted-Berezinski-Kosterlitz-Thouless transition. We study this shift in phase behavior on Hanoi networks, which are one-dimensional Ising chains connected by small-world bonds that are self-similar and hierarchical in structure [1]. The phase behavior of the Ising model near T$_{c}$ on Hanoi networks is studied using an exact renormalization group and Monte Carlo techniques. We show that compared to the Migdal-Kadanoff hierarchical lattice, Hanoi networks possess characteristics in their thermodynamic densities that are more physical. These densities are studied in detail and the behavior of their critical exponents near T$_{c}$ is described. By introducing a continuous parameter which regulates the strength of small-world bonds in the Hanoi networks, we begin to uncover the essential small-world properties that dictate this change in phase behavior from second- to infinite-order. \\[4pt] [1] S. Boettcher and C.T. Brunson, Phys. Rev. E, \textbf{83}, 021103 (2011) [Preview Abstract] |
Monday, February 27, 2012 4:06PM - 4:18PM |
D54.00009: Popularity versus similarity in growing networks Dmitri Krioukov, Fragkiskos Papadopoulos, Maksim Kitsak, Mariangeles Serrano, Marian Boguna Preferential attachment is a powerful mechanism explaining the emergence of scaling in growing networks. If new connections are established preferentially to more popular nodes in a network, then the network is scale-free. Here we show that not only popularity but also similarity is a strong force shaping the network structure and dynamics. We develop a framework where new connections, instead of preferring popular nodes, optimize certain trade-offs between popularity and similarity. The framework admits a geometric interpretation, in which preferential attachment emerges from local optimization processes. As opposed to preferential attachment, the optimization framework accurately describes large-scale evolution of technological (Internet), social (web of trust), and biological (E.coli metabolic) networks, predicting the probability of new links in them with a remarkable precision. The developed framework can thus be used for predicting new links in evolving networks, and provides a different perspective on preferential attachment as an emergent phenomenon. [Preview Abstract] |
Monday, February 27, 2012 4:18PM - 4:30PM |
D54.00010: Is it really a small world after all Baruch Barzel, Albert-Laszlo Barabasi One of the most intriguing revelations in the study of complex networks is the ubiquitous appearance of small worlds, that is networks exhibiting a small, and sometimes even ultra-small, average path length. This suggests that these networks feature globally connected dynamics, where all nodes are affected by all other nodes, given the short distance between them. Nevertheless, empirical data on the dynamics of such networks shows that in practice, such global connectedness is rarely observed. To address this gap between the topology and the observed dynamics of networks we developed the network correlation function method, a framework in which we obtain the patterns of influence between nodes in the network. In simple words we complement the topological description of {\it who is connected to whom}, by the dynamical description of {\it who is affected by whom}. Strikingly, using this method, we find that small world topology tends to avoid global dynamics, while non-small worlds could potentially support it. We test our results on a set of networks from various fields, ranging from social to biological networks, and discuss the implications on the dynamical stability of these systems. [Preview Abstract] |
Monday, February 27, 2012 4:30PM - 4:42PM |
D54.00011: Metric Structure of Bipartite Networks Maksim Kitsak, Fragkiskos Papadopoulos, Dmitri Krioukov Many social, biological and technological systems can be conveniently represented as bipartite networks, consisting of two disjoint sets of elements along with edges connecting only elements from different sets. Many of such systems are characterized by high values of bipartite clustering coefficient. We also find that pairs of elements in these bipartite systems tend to have many common neighbors. We present a natural interpretation of these observations. We suggest that elements of the above bipartite systems exist in underlying metric spaces, such that the observed high clustering is a topological reflection of the triangle inequality, the key property of metric space. We propose a simple stochastic mechanism of formation of bipartite networks embedded in metric spaces. We prove that this mechanism is able to reproduce the observed topological properties of bipartite networks. We also discuss the possibility of constructive embedding of real bipartite systems into metric spaces. In my talk I will overview the concept of hidden metric spaces with respect to both unipartite and bipartite networks. I will also discuss existing methods used to infer hidden metric spaces in real networks and possible applications for bipartite networks. [Preview Abstract] |
Monday, February 27, 2012 4:42PM - 4:54PM |
D54.00012: Topological features of a fractal model for complex networks Lin Bo, Hernan Makse We study the construction and topological features of fractal and non-fractal hierarchical tree-like models generated through an inverse-renormalization growth mechanism with various parameters. These complex networks are characterized by scale-free distribution of connections, clustering coefficient, modular structure, degree correlation and a set of fractal dimensions. We compare the results with analytic expressions and show the dependence of topological properties on growing parameters. Networks with different tendency of hub-hub repulsion are produced and classified in terms of degree correlations. Interloops and intraloops are introduced into growing process to test robustness and stability of networks under attack. [Preview Abstract] |
Monday, February 27, 2012 4:54PM - 5:06PM |
D54.00013: Control Centrality and Hierarchical Structure in Complex Networks Yang-Yu Liu, Jean-Jacques Slotine, Albert-Laszlo Barabasi We introduce the concept of control centrality ($C_{\mathrm{c}}$) to quantify the ability of a single node to control a directed weighted network. We map the control centrality into a combinatorial optimization problem. We calculate the distribution of control centrality for several real networks and find that it is mainly determined by the network's degree distribution. We show that the underlying hierarchical structure of a general directed network plays an important role in determining the distribution of control controllability. We rigorously prove that in directed acyclic graphs, i.e. directed networks without loops, a node's control centrality is uniquely determined by its topological position in the underlying hierarchical structure of the network. Our finding on the relation between control centrality and the hierarchical structure inspires us to design an effective random attack strategy against the controllability of malicious networks, without requiring the detailed knowledge of the network structure. We test our random attack strategy on several real networks and find it is indeed effective and even comparable to those targeted attacks which rely on the detailed knowledge of the network structure. [Preview Abstract] |
Monday, February 27, 2012 5:06PM - 5:18PM |
D54.00014: A community detection approach to image segmentation and its phases Dandan Hu, Peter Ronhovde, Zohar Nussinov In this talk, I will discuss ``unsupervised'' image segmentation that relies on phase diagram structure of the community detection method. Specifically, we apply a replica-inference-based community detection method. ``Community detection'' describes the general problem of partitioning a complex system involving many elements into optimally decoupled communities of such elements. In our image segmentation analysis, we invoke a multi-resolution community detection variant to ascertain the overall structure of the image at different resolutions. Information based measures (e.g., the normalized mutual information) are used to determine the significant structures at which ``replicas'' of the systems are strongly correlated. We report on the ``easy'', ``hard'', and ``unsolvable'' phases of the corresponding Potts model at both zero and finite temperatures. The optimal image segmentations are obtained by choosing parameters at the easy phase of the Potts model. The determination of the phase diagram in the analysis of the image segmentation is proved to be highly efficient. We demonstrate in a detailed study of various test cases that our method is fast and accurate and to be especially suited to the detection of camouflaged images. [Preview Abstract] |
Monday, February 27, 2012 5:18PM - 5:30PM |
D54.00015: Ordinary Percolation with Discontinuous Transitions Vijay Singh, Stefan Boettcher We study percolation on hierarchical networks using generating functions and renormalization group techniques. Our exact results show the presence of novel features in these networks including the existence of non-trivial critical points, three distinct regimes in the phase diagram and, most importantly, a discontinuity in the formation of the extensive cluster at a critical point $p_{c}<1$ . At $p_{c}$, the order parameter $P_{\infty}$ describing the probability of any node to be a part of the largest cluster, jumps instantly to a finite value. We present simple examples of small-world networks with various hierarchies of long range bonds, indicating that the presence of discontinuous transitions is generic.\\[4pt] [1] S. Boettcher, V. Singh, and R.M. Ziff. Ordinary Percolation with Discontinuous Transitions. Arxiv preprint arXiv:1110.4288 (2):2 5, 2011. [Preview Abstract] |
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