Bulletin of the American Physical Society
Joint Meeting of the Four Corners and Texas Sections of the American Physical Society
Volume 61, Number 15
Friday–Saturday, October 21–22, 2016; Las Cruces, New Mexico
Session H4: Computational Physics II |
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Chair: Gus Hart, Brigham Young University Room: Meeting Room 5&6 |
Saturday, October 22, 2016 9:36AM - 10:00AM |
H4.00001: Ab Initio Density Functional Studies of Nanoscale Materials Invited Speaker: Igor Vasiliev Nanomaterials exhibit a variety of unusual physical characteristics resulting from the interplay of quantum confinement and electronic correlations. The unique properties of nanomaterials make them attractive candidates for a broad range of applications in electronics, photonics, photovoltaics, chemistry, catalysis, and materials engineering. The properties of nanoscale structures can be tailored for specific applications by surface modification, chemical functionalization, and doping. My talk will focus on ab initio static and time-dependent density functional calculations for predicting the electronic, optical, magnetic, and transport characteristics of nanoscale materials. The flexibility of the density functional computational approach will be illustrated by its application to several different types of nanostructures, including doped and functionalized carbon nanotubes, graphene, surface-passivated semiconductor nanocrystals, and multiferroic perovskite thin films. [Preview Abstract] |
Saturday, October 22, 2016 10:00AM - 10:12AM |
H4.00002: First-Principle Study of the La$_{0.67}$Sr$_{0.33}$MnO$_{3}$/PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$(001) Interface Mahmoud Hammouri, Edwin Fohtung, Ross Harder, Valeria Lauter, Igor Vasiliev Multiferroic heterostructures composed of thin layers of ferromagnetic and ferroelectric perovskites have attracted considerable attention in recent years. We apply \textit{ab initio} computational methods based on density functional theory to study the characteristics of the magnetoelectric coupling at the (001) interface between La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ (LSMO) and PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$(PZT). The calculations are carried out using the Quantum ESPRESSO electronic structure code combined with Vanderbilt ultrasoft pseudopotentials. Our study shows that the interfacial interaction between LSMO and PZT and the polarization of PZT have a strong influence on the distribution of magnetization within the LSMO layer. A significant change in the magnetization of the LSMO layer adjacent to PZT is observed after reversal of the direction of polarization of PZT. [Preview Abstract] |
Saturday, October 22, 2016 10:12AM - 10:24AM |
H4.00003: Strain-Rate Dependence of Material Strength: Large Scale Atomistic Simulations of Defective Cu and Ta Crystals Jayalath Abeywardhana, Timothy Germann, Ramon Ravelo Large$-$Scale molecular dynamics (MD) simulations are used to model shock wave (SW) and quasi$-$isentropic compression (QIC) in defective copper and tantalum crystals. The atomic interactions were modeled employing embedded$-$atom method (EAM) potentials. Quasi$-$isentropic uniaxial compression is achieved by incorporating a strain rate function in the positions and velocities equations of motion. In this new formalism the change in internal energy is exactly equal to the work done in compression. We examined the deformation mechanisms and material strength for strain rates in the $10^{8}-10^{12} s^{-1}$ range for both Cu and Ta defective crystals. Near and far$-$field X$-$ray diffraction simulations were also performed to infer the required resolution for resolving defect densities. [Preview Abstract] |
Saturday, October 22, 2016 10:24AM - 10:36AM |
H4.00004: A Numerical algorithm for finding pressure induced Acoustic phonon instabilities in crystals Oscar Guerrero In this talk, I will present a fast-efficient computational Pseudo code to find acoustic phonon instabilities in crystals [1]. The code use the stress-strain relations to numerical find sound velocities in terms of second-order elastic constants for various wave modes in uniaxial and hydrostatically compressed crystals for a given wave direction $k$ [2,3]. I use Tantalum as the testing material and the results are compared with Molecular dynamics simulations using the EAM potential formalism [4,5], DFT calculations via VASP and via a stability of the lattice as a function of uniaxial compression by determining the phonon-dispersion relation over the entire BZ [6] . All three methods predict that the lattice first goes unstable at 25{\%} of uniaxial compression along the \textless 100\textgreater direction. The implementation of the code is discussed using the free open source software OCTAVE and the symbolic program MATHEMATICA\\ \\ 1. Guerrero, ETD Collection for University of Texas, El Paso. Paper AAI1477789. \\ 2. Eur. Phys. J. B \textbf{5}, 7-13 (1998) \\ 3. Nature 418.6895 (2002): 307-310 \\ 4. Phys. Rev. B \textbf{88}, 134101 \\ 5. Guerrero et al Journal of Materials Science and Engineering B, 2013. \textbf{3}(3): p. 153-160 6. Phys. Rev. B \textbf{8} [Preview Abstract] |
Saturday, October 22, 2016 10:36AM - 10:48AM |
H4.00005: Fast Method for the Prediction of Intercrystalline Water Molecule Orientation in Ionic Crystals Seyedayat Ghazisaeed, Boris Kiefer Water has previously been reported to affect electronic properties especially of molecules in solution. Thus, water in crystalline materials may be suitable for affecting the physical and chemical properties of the host material as well. However, the crystallographic description of hydrogen in crystalline materials is challenging since it scatters x-ray's poorly and neutron diffraction experiments require significantly larger samples. Therefore in many cases, the location of the hydrogen atoms remains unknown and cannot be obtained directly from crystallographic databases. Here we present a mathematically robust method for the prediction of hydrogen positions of intercrystalline water inside ionic inorganic crystals. We will show that the net torque provides a fast and simple method to establish the orientation of water molecules that avoids the conditional convergence problem of coulomb sums in ionic crystals. We will present and discuss the results of this method for several test materials and compare to available neutron data and \textit{ab-initio} calculations. The successful determination of water molecule orientation is a necessary first step to address the question if and under what circumstances intercrystalline water can provide host materials with new functionalities. [Preview Abstract] |
Saturday, October 22, 2016 10:48AM - 11:00AM |
H4.00006: Temporally Iterative Spatial Refinement: How to Predict the Unpredictable William Black, David Neilsen, Hyun Lim, Eric Hirschmann In nonlinear fluid dynamics, sharp small-scale features can spontaneously develop from smooth fluid flows, which poses a challenge for computational simulations. Adaptive Wavelet Multiresolution Representation (AWMR) techniques use wavelets to adapt the computational grid to the features of a solution, thereby decreasing computational time. We use AWMR to solve the relativistic fluid equations. However, the formation of especially strong shocks can lead to solutions with unacceptable levels of noise. Temporally Iterative Spatial Refinement (TISR) iterates the fluid update in time, adding resolution when these features appear, thereby anticipating the resolution required for the emerging shock. The TISR method thus provides a robust method for accurately computing developing shocks, and can be used in a wide variety of adaptive numerical methods. [Preview Abstract] |
Saturday, October 22, 2016 11:00AM - 11:12AM |
H4.00007: Algebraic search for cooperative tilt patterns in networks of interconnected polyhedra Tyler Averett, Branton Campbell, Thomas Whittle, Siegbert Schmid, Christopher Howard Crystalline solids consisting of networks of interconnected rigid molecules are ubiquitous amongst functional materials, having a wide range of applications. In many cases, the important properties of these materials are sensitive to the tilting of individual rigid units. However, the shared atoms that connect the rigid units together impose severe constraints, so that any tilting must be cooperative throughout the entire network. A variety of methods have been developed for determining which tilt patterns are allowed and which are not for a given material, each having a limited scope. We will present a purely algebraic approach, based on group representation theory, which exhaustively classifies the allowed tilt modes for any network of rigid units. [Preview Abstract] |
Saturday, October 22, 2016 11:12AM - 11:24AM |
H4.00008: Beyond Phase Transitions: an Algorithmic Approach to Flocking Behavior Garett Brown, Manuel Berrondo The emergent behavior of certain collective systems such as starling murmurations reveals coherent behavior arising from the simple, individual interactions of its entities. Using a two-dimensional algorithmic model, we can show that self-driven particles (boids) group together and display emergent flocking characteristics. The model is based on the ideas of consensus and frustration as well as the dynamic interplay between global and local phase transitions. The frustration is a perturbation that drives the boids beyond the simple phase transitions and towards chaotic behavior while the consensus is a topological averaging, that balances the frustration. The results are interpreted in terms of global and local order parameters, and correlation functions. They are presented along with animations created using Wolfram Mathematica. [Preview Abstract] |
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