Bulletin of the American Physical Society
85th Annual Meeting of the APS Southeastern Section
Volume 63, Number 19
Thursday–Saturday, November 8–10, 2018; Holiday Inn at World’s Fair Park, Knoxville, Tennessee
Session G01: Condensed Matter V |
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Chair: Hanno Weitering, University of Tennessee, Knoxville Room: Holiday Inn Knoxville Downtown Summit |
Friday, November 9, 2018 2:00PM - 2:12PM |
G01.00001: The Electron Green's Function for the t-t'-J Model in 1,2 … ∞ Dimensions Peizhi Mai, Steven White, Sriram Shastry By combining the extremely correlated Fermi liquid (ECFL) theory and the density matrix renormalization group (DMRG) method, we present an overview of the electron propagator of the t-t'-J model in any dimension for various parameters of the model and electron densities. In 1-d the DMRG method gives essentially exact numerical results, which are shown to compare quite well with the ECFL method. In particular, the momentum dependence can be checked against the exact DMRG result. We use a closed set of equations from ECFL theory that are valid in all dimensions. Their application to infinite dimension has been checked against dynamical mean field theory earlier. These equations have also been applied to recently 2-d. The present study fills the remaining gap, thus giving an overview of the solution in all dimensions. In 1-d at low energies, we find clear signatures of spin-charge separation in both calculations. At higher energies, the spectral functions resemble those in higher dimensions. |
Friday, November 9, 2018 2:12PM - 2:24PM |
G01.00002: Electron localization in two-dimensional extended Hubbard model Hanna Terletska Using Dynamical Cluster Approximation we study phase transitions in 2D extended Hubbard model. We show that non-local inter-site electron-electron interactions in this model lead to emergence of charge ordered phase. We study the properties of such charge ordered phase under different control parameters, including temperature, strength of local interactions and doping. We also, investigate the effect of the non-local interactions on the Mott transition in 2D, and demonstrate that they stabilize the transition to a finite U value. |
Friday, November 9, 2018 2:24PM - 2:36PM |
G01.00003: Using Neural Networks in Determinant Quantum Monte Carlo to study the Holstein Model. Philip M. Dee, Shaozhi Li, Ehsan Khatami, Steven S. Johnston Machine learning techniques have recently occupied the focus of many investigators in computational many-body physics. In particular, some practitioners of quantum Monte-Carlo have considered the efficacy of various "Self-Learning'' techniques which aim to reduce CPU runtime associated with updates and autocorrelation. In this talk, I will discuss our group's efforts to use artificial neural networks (NN) within determinant quantum Monte-Carlo (DQMC) to improve the scaling of CPU runtime with typical system parameters. This work has focused primarily on the singleband Holstein model, which is, perhaps, the simplest model for studying electron-phonon coupling in many body systems. We have explored both fully connected and convolutional NN and used them to study the metallic and insulating phases of the Holstein model. Looking forward, NN-DQMC is well suited for studying not only the Holstein model but extensions thereof. |
Friday, November 9, 2018 2:36PM - 2:48PM |
G01.00004: Phonon Anharmonicity in Single Crystalline SnSe Fengjiao Liu Case, Prakash Parajuli, Rahul Rao, Sriparna Bhattacharya, Ramakrishna Podila, Jian He, Benji Maruyama, Apparao M. Rao Understanding the temperature dependent behavior of phonons that affect the thermal transport properties is crucial for developing efficient thermoelectric materials. Although there are experimental and theoretical evidences for anharmonicity in SnSe, its effect on the entire phonon band structure, which is responsible for the thermal transport, has not been investigated. Here we performed a combined temperature dependent polarized Raman spectroscopic and heat capacity study on fully dense single crystalline SnSe, which revealed that the anharmonicity is driven by soft optical modes in the b-c plane. These modes exhibited strong high temperature broadening, indicating ultrashort phonon lifetimes and high scattering rates. Analysis of the temperature dependent Raman peak frequencies and linewidths revealed phonon decay to be dominated by a three-phonon scattering process. Consistent with this observation, the analysis of our temperature dependent heat capacity data also revealed the presence of strong anharmonicity in SnSe. The anharmonic coefficients calculated from the Raman and heat capacity measurements are in excellent agreement with each other. This study provides a deeper understanding on the role of phonon-phonon scattering and anharmonicity in SnSe. |
Friday, November 9, 2018 2:48PM - 3:00PM |
G01.00005: Analytical model of nanowire geometric diodes Jeremy D Low, Jimmy Custer, James Cahoon Nanowire geometric diodes have unique properties which allow them to function as long wavelength energy harvesters and ultra-high speed signal processors. Geometric diodes operate through symmetry breaking on a scale comparable to the mean-free-path length of charge carriers. Through dopant encoded vapor-liquid-solid growth, followed by wet chemical etching, nanowires can be experimentally fabricated into “sawtooth nanowires”. Sawtooth nanowires exhibit diodie-like behavior (current at an applied voltage is directionally asymmetric) due to the geometric diode effect. Sawtooth geometries are defined by three parameters, length, inner diameter, and outer diameter, and all impact the overall resistance, I-V characteristics, and frequency dependence. We aim to predict the device’s asymmetry dependence on these parameters through a model which calculates the ratio of possible paths ballistic electrons can travel through the wire. The trends predicted by the model are strongly backed by experimental data. We show that the strongest factor controlling asymmetry is the sawtooth angle, a combined metric of neck diameter and sawtooth length. We can use this model to optimize the design of geometric diodes in order to tailor their properties for specific applications. |
Friday, November 9, 2018 3:00PM - 3:12PM |
G01.00006: Modeling Gaseous Diffusion in Nanopores Fenner E. Colson, Douglas A. Barlow A phenomenological theory for gaseous diffusion within nanopores is presented where the flux is given in terms of a probability distribution for scattering path lengths. Using a simple molecular dynamics simulation a model distribution is developed and a Fickian diffusion coefficient determined. This is done for various simple gases within a membrane pore. Diffusion coefficients are computed in each case over a selected temperature range and compared with experimental values when possible. The diffusion is explained in terms of Knudsen and surface effects. This theory is also compared with other prominent theories from the literature used to describe this phenomenon. |
Friday, November 9, 2018 3:12PM - 3:24PM |
G01.00007: Modeling Solid-Liquid Interfaces Using Next Generation Quantum Molecular Dynamics Kevin G Kleiner, Aparna Nair-Kanneganti, Christian Francisco Andres Negre, Ivana Gonzales, Anders Niklasson Fuel cells provide electric current based on oxidation and reduction, and the cathode's oxygen reduction reaction can be catalyzed at the solid-liquid interface of a metal-nitrogen doped graphene sheet and water. However, the precise electron transfers and molecular roles are unknown. While ab-initio density functional theory provides ground state electronic properties and reaction energetics, quantum molecular dynamics can reveal important time-dependent ensemble properties. Initial relaxation of the system's charge distribution is carried out through the density functional tight binding approach, and the required relaxation iterations and computing times are compared for different atomic charge update schemes. With the system starting in equilibrium, the nuclear and electronic degrees of freedom are updated using the extended Lagrangian Born-Oppenheimer MD formulation, and the total system energy is tracked to ensure conservation. Characteristic peaks in system energy, temperature, and charge distribution describe the possible occurrence of a reaction when the oxygen molecule breaks into anion radicals. |
Friday, November 9, 2018 3:24PM - 3:36PM |
G01.00008: Simulating the Insulator-Metal Transition of Vanadium Dioxide Steven B Hancock, Yohannes Abate, David P Landau Transition metal oxides such as vanadium dioxide exhibit very interesting behavior at their insulator-metal-transition point. We design and motivate a novel Monte Carlo method to simulate the percolative behavior seen at the phase boundary. Additionally, we procure and analyze experimental data to verify our method. |
Friday, November 9, 2018 3:36PM - 3:48PM |
G01.00009: Computational Modeling of Crystal Structures, Mechanical Properties, and Vibrational Spectra of Superhard B-C and B-N systems Wei-Chih Chen, Cheng-Chien Chen Superhard materials with a Vickers hardness larger than ~40 GPa have a wide range of industrial applications such as cutting tools and protective coatings. Superhard boron-carbon (B-C) and boron-nitride (B-N) composites are especially important because of their superior high-temperature performance as compared to diamond and their low reactivity with ferrous metals. Here we employ the powerful evolutionary algorithm as implemented efficiently in the USPEX software to predict the crystal structures of superhard B-C and B-N composites. The mechanical properties, electronic structures, phonon and Raman spectra of these predicted structures are computed accordingly also from first principles using density functional theory. Comparison of our calculations and available experimental data will also be discussed. |
Friday, November 9, 2018 3:48PM - 4:00PM |
G01.00010: General Solutions to the Leah Hamiltonian and the Imani Periodic Functions Ronald E. Mickens, 'Kale Oyedeji The Leah dynamic system corresponds to a one-dimensional classical oscillator for which the nonlinear force is proportional to the one-third power of the spring extension beyond its equilibrium position. Its energy function, the Leah Hamiltonian, therefore has a potential energy term proportional to the four-thirds power of the extension. The mathematical solutions to these two equations are called, respectively, the Leah and Imani functions. We demonstrate, by means of an explicit construction, that the Imani functions can be calculated. However, only general mathematical properties may be determined for the Leah functions. It should be noted that both sets of functions have the same general features as the standard cosine and sine functions. |
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