Bulletin of the American Physical Society
2015 Annual Meeting of the Far West Section of the APS
Thursday–Saturday, October 29–31, 2015; Long Beach, California
Session F3: Condensed Matter I |
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Chair: Hendrik Ohldag, SLAC, Stanford University Room: CBA-139 |
Friday, October 30, 2015 2:00PM - 2:12PM |
F3.00001: Field-induced dynamical properties of the $XXZ$ model on a honeycomb lattice Pavel Maksimov We present a comprehensive $1/S$ study of the field-induced dynamical properties of the nearest-neighbor $XXZ$ antiferromagnet on a honeycomb lattice using the formalism of the nonlinear spin-wave theory developed for this model. External magnetic field controls spin frustration in the system and induces non-collinearity of the spin structure, which is essential for the two-magnon decay processes. Our results include an intriguing field-evolution of the regions of the Brillouin zone where decays of spin excitations are prominent, a thorough analysis of the singularities in the magnon spectra due to coupling to the two-magnon continuum, the asymptotic behavior of the decay rates near high-symmetry points, and inelastic neutron-scattering spin-spin structure factor obtained in the leading $1/S$ order. [Preview Abstract] |
Friday, October 30, 2015 2:12PM - 2:24PM |
F3.00002: DFT study of CO Oxidation by Molten Carbonate Susan Njoki, Changyong Qin, Godwin Mbamalu In the past years, molten carbonate (MC) has been adopted to solid oxide fuel cells (SOFCs) as booster for the oxygen reduction process in the cathode. The reaction pathway through CO42- as reaction intermediate has been revealed by DFT modelling. Simply, CO42- is a carbonate ion with atomic oxygen attached and it has very strong oxidizing capacity, implying a possible novel catalyst for various oxidization reactions at high temperatures. In this study, we have investigated the CO oxidation by MC using DFT methods. Two reaction intermediates, C2O42- and C2O52-, are formed by CO with CO32- and CO42-, respectively. The activation energy is calculated to be 10.0 kcal/mol through a Langmuir-Hinshelwood mechanism. In addition, the interactions between Nitroxides (NOx) and MC were examined because NOx has been proven as a poisoning gas for many industrial catalysts. This study confirms that NO will react with CO42- to form NO2, and NO2 will then leave MC. No poisoning was found by the DFT modelling, which implies that MC can be potentially used for CO oxidation in the exhaust gas treatment of combustion engines. [Preview Abstract] |
Friday, October 30, 2015 2:24PM - 2:36PM |
F3.00003: Probing the Structure of Density Functional Theory with Density Matrix Renormalization Group Thomas E. Baker, Li Li, Kieron Burke, Steven R. White Density Functional Theory (DFT) is a mathematically exact method for solving quantum system efficiently but that requires approximations. Making these approximations may exclude features seen in experiment or provide inadequate estimates. We may use Density Matrix Renormalization Group (DMRG) in one dimension to find exact DFT quantities [1,2] as a benchmark to test new functionals and to explore computational proof of principles [3,4]. [1] E.M. Stoudenmire, et.~al., {\it Phys.~Rev.~Lett.} {\bf 109}, 056402 (2012) [2] Thomas E.~Baker, et.~al., {\it Phys.~Rev.~B} {\bf 91}, 235141 (2015) [3] Lucas O. Wagner, et.~al., {\it Phys.~Rev.~Lett.} {\bf 111}, 093003 (2013) [4] Lucas O. Wagner, et.~al., {\it Phys.~Rev.~B} {\bf 90}, 045109 (2014) [Preview Abstract] |
Friday, October 30, 2015 2:36PM - 2:48PM |
F3.00004: Deriving the Ionic Electrostatic Potential of Metallic Nanowires Joshua Reinheimer, Jerome Buerki Using the Nanoscale Free Electron Model (NFEM), the electrostatic potential of metallic ionic nanowires is derived. Considering a hard wall potential to confine electrons in the wire, wires of a cylindrical cross section are considered first and their ionic electrostatic potential is calculated. After analyzing the properties of the cylindrical case, the same derivations and calculations will be completed to consider wires of an arbitrary elliptical cross section. Due to the intractable nature of the electrostatic potential functions, numerical methods are employed to help find the potential values [Preview Abstract] |
Friday, October 30, 2015 2:48PM - 3:00PM |
F3.00005: A FLEX-based model for normal state quasiparticle properties of strontium ruthenate John Deisz We utilize a weak-coupling approximation, FLEX, to analyze quasiparticle properties of strontium ruthenate. Utilizing first-principles-derived parameters for the band structure, spin-orbit interaction and Coulomb and exchange interactions, we quantitatively reproduce the band-structure renormalization and linewidths observed in photoemission. However, the electronic specific heat coefficient is underestimated by a factor of two and superconducting Tc is overestimated by 30 K. Analysis of of the band and momentum-resolved self-energy reveals an essentially momentum-independent, Fermi-liquid-like self-energy with the exception of the $\gamma$ band along the $\Gamma$-M cut for which a strongly momentum-dependent, non-Fermi-liquid behavior is observed. [Preview Abstract] |
Friday, October 30, 2015 3:00PM - 3:12PM |
F3.00006: Towards Incorporating More Realistic Effects in Variational Monte Carlo studies of the Fractional Quantum Hall Effect in Graphene Yonas Getachew, Michael Peterson A two-dimensional electron system exposed to a strong perpendicular magnetic field (10 to 30 T) and cooled to very low temperatures (less than 1K) forms a new state of matter that exhibits the fractional quantum Hall effect (FQHE). This phenomenon has been observed in graphene, a naturally occurring two-dimensional electron system. Electrons in graphene have spin and valley degrees of freedom and the physics (e.g., spin and valley polarizations) has remained mysterious. Any accurate model of the FQHE in graphene needs to account for Landau level mixing, a model for which has been formulated in terms of Haldane pseudopotentials using the planar geometry and includes the emergence of three-body interactions between the electrons, in addition to renormalizing the two-body interactions. We discuss a real space formalism that can be used in variational Monte Carlo studies. The method involves an ansatz potential with adjustable parameters such that its pseudopotentials match those found through other methods. We discuss extensions of this formalism to the finite-sized spherical geometry as well as developing a formalism to handle the three-body interactions. [Preview Abstract] |
Friday, October 30, 2015 3:12PM - 3:24PM |
F3.00007: Odd-Triplet Superconductivity in SmCo/Py Exchange Spring Based Josephson Junctions Samuel Hedges, Jiyeong Gu In a Josephson junction, a superconducting wave function can leak into a non-superconducting layer. If this non-superconducting layer is a ferromagnet with a nonhomogeneous magnetization, the odd-triplet component of the superconducting wave function can be generated at the superconductor/ferromagnet interface. Samarium-Cobalt(SmCo)/Permalloy(Py) exchange springs are used to generate a nonhomogeneous magnetic layer in Niobium(Nb)-based Josephson junctions. In these junctions, the critical current and I-V curves are measured as a function of nonhomogeneity of the magnetic layer to search for direct evidence of the odd-triplet component. To understand how these properties vary, a single-domain junction is preferable, such that the supercurrent through the junction is uniform. The lack of a clear Fraunhofer pattern measured for the junctions produced indicates that smaller junction sizes are required to generate a uniform supercurrent. While the critical current and I-V curves were observed to vary with the nonhomogeneity of the magnetic layers, further measurements are required to indicate whether the observed behavior is consistent with the theory of odd-triplet superconductivity. [Preview Abstract] |
Friday, October 30, 2015 3:24PM - 3:36PM |
F3.00008: Motion of a ball rolling on a turntable Benjamin Carter I reconsider the classical problem of a ball rolling on a perfectly rough, horizontal turntable. Previous treatments dating back to the 19th century assumed that the turntable has fixed angular velocity, in which case the ball's orbits turn out to be circular. I assume instead that the turntable has a finite moment of inertia, so that its angular velocity varies as it exchanges angular momentum with the ball. Numerical results support a conjecture of mine, which is that the ball's orbits are conic sections whose foci are collinear with the center of rotation. If my conjecture is true, then the equations of motion can be solved exactly. [Preview Abstract] |
Friday, October 30, 2015 3:36PM - 3:48PM |
F3.00009: Effects of a rotating magnetization on electron correlations in the ballistic regime Josephson Junction Luis Leal, Andreas Bill The pair correlations of electrons leaking from a superconductor into a magnetic material are modified by the local magnetization. We investigate the singlet triplet mixing of these correlations resulting from a domain wall in a ferromagnet. We model our system in the clean limit using a tight-binding Hamiltonian and solve the Bogoliubov-- de Gennes equations to determine the Go'kov functions of the system. We present first results for three different proximity systems: an antiferromagnet, a homogeneous ferromagnet, and a helical magnet with variable twist; all sandwiched between two superconductors. The goal of the study is to revisit how pair correlations are affected by different magnetization configurations, and compare the results in the clean and the diffusive regimes. [Preview Abstract] |
Friday, October 30, 2015 3:48PM - 4:00PM |
F3.00010: Submonolayer Island Nucleation and Growth for Subdiffusive Random Walkers Mikhael Semaan, Jacques Amar Classical nucleation theory has long been used to model submonolayer growth, and kinetic Monte Carlo (KMC) simulations have provided accurate checks of the theory. However, classical nucleation theory deals only with ordinary diffusion on the surface. At least two other types of diffusion exist: superdiffusion, whereby a random walker travels in the same direction over a `persistence length' -- whose distribution is given by a power law distribution -- before changing direction; and subdiffusion, whereby a random walker waits some time $\tau $ -- where $\tau $ also follows a power law distribution -- before taking its next step. While subdiffusive walkers are notably present in many physical scenarios, including Xerography, disordered glasses, and systems with many surface defects which create a large distribution of hopping potentials, the effects of subdiffusion have never been studied in terms of surface growth and nucleation. In this project, the island density scaling exponent $\chi $ is investigated for subdiffusive systems using KMC simulations. Excellent agreement is found between the simulation results and a recently obtained analytical prediction (J. G. Amar, unpublished) based on a self-consistent rate equation approach. [Preview Abstract] |
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