Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X7: General Theory |
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Sponsoring Units: DCOMP Chair: Jimmy-Xuan Shen, University of California, Santa Barbara Room: 266 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X7.00001: Plasma sheath model in the presence of field-induced electron emission. Jiba Dahal, Venkattraman Ayyaswamy Microplasmas have become an active area of research during the last two decades with several applications including nanomaterial synthesis, electronics, lighting, biomedicine, and metamaterials for controlling electromagnetic waves. The advances in micro/nanofabrication and the further miniaturization of plasma devices have contributed to the increasing role of new physical mechanisms that were previously neglected. Electric field-induced emission of electrons is one such mechanism that is gaining significance particularly with the discovery of novel electrodes that demonstrate excellent field emission properties. These field emitted electrons and their interaction with microdischarges has shown to affect both pre-breakdown and post-breakdown regimes of operation. The current work focuses on the development of self-consistent sheath model that includes the effects of field-induced electron emission. Sheath models presented earlier accounts for other emission mechanisms such as thermionic and secondary electron emission, the strong influence of electric field on electron emission is shown to lead to unique interplay. The results obtained from the sheath model for various parameters including current-voltage characteristics, and ion/electron number density are validated with PIC-MCC results. [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X7.00002: Automatic Renormalization of Local Tensor Networks Adam Jermyn Tensor networks have gained significant interest recently as efficient means for numerically representing and manipulating quantum states and as a way to represent classical partition functions. Progress has been made numerically analyzing these networks, but existing methods remain restricted to systems which either have just one extensive dimension or are infinite and periodic. Here we provide a new efficient method for numerically renormalizing finite local tensor networks in multiple dimensions with no symmetry or periodicity constraints. This method performs well on irregular lattices and with disordered bond energies. We demonstrate the efficacy of this method on large systems with thousands of tensors, reproduce free energy curves of classical spin systems, and investigate higher-order and long-range correlation functions. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X7.00003: Holographic Dynamics from Multiscale Entanglement Renormalization Ansatz Vasilios Passias, Victor Chua, Apoorv Tiwari, Shinsei Ryu The Multiscale Entanglement Renormalization Ansatz (MERA) is a tensor network based variational ansatz that can capture many of the key physical properties of strongly correlated ground states. MERA also shares many deep relationships with the AdS/CFT (gauge-gravity) correspondence by realizing a UV complete holographic duality within the tensor network framework. Motivated by this, we use the MERA tensor network as analysis tool to study the real-time evolution of a periodic 1D paramagnetic phase transverse Ising model in its low energy excited state sector. This network realizes a 'holographic transform' between boundary (spin) and bulk (qubit) degrees of freedom when studying changes in the former and its effects on the latter and vice-versa, despite the boundary theory being in a non-CFT limit. In this limit, we demonstrate a 'dictionary' between the bulk and boundary that realizes many features of the holographic correspondence. We also find a stable holographic quasiparticle, named the 'hologron', and numerically study its bulk 2D Hamiltonian, energy spectrum and dynamics within the MERA network. Holographically, single bulk localized hologrons are dual to extended (non-local) modifications of the ground state in physical (boundary) space. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X7.00004: Geometro dynamics of electrons in deforming crystals Liang Dong, Qian Niu We develop a theory of electronic properties in time-dependent deforming crystals up to the first order of inhomogeneity. The difficulty with inhomogeneous system is overcome by wave-packet method, where the semi-classical equations of motion are derived including Berry phase effect. Under the local approximation, the inhomogeneous crystal is mapped to a bundle of locally periodic lattices and their connection which forms a non-Euclidean space-time for the electrons. The corresponding lattice covariant derivative takes the place of partial derivative. We discuss its physical meaning in lab frame with two examples, adiabatic current due to inhomogeneity and equivalent post-Newtonian gravity at band bottom. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X7.00005: Analysis Deformation Dynamics in Solids Setare Sadeqi, Sanichiro Yoshida This paper describes a Finite Element Analysis (FEA) that we conduct to model a recent field theory of deformation and fracture of solids. This field theory postulates that solids under external load always have local regions where deformation dynamics obeys linear elasticity. Requesting local symmetry in the linear elasticity by introducing a gauge field, the theory derives a set of field equations that describe deformation dynamics for all regimes, i.e., elastic, plastic and fracture regimes. The FEA model solves these field equations under various scenarios. In this presentation, we report our recent FEA in which we simulate deformation of a plate specimen under a tensile load. The right end of the specimen is subject to gradual pull. Simulation results are discussed for several parameters including displacement in the tensile direction, the quantity corresponding to the charge of symmetry, and material's resistive force defined by the field theory. For some of these parameters, simulation results are compared with experiment. Our preliminary results are promising indicating some quantitative agreement with experiment.~~ [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X7.00006: Generalizing the Geometric Approach to Pressure Calculation Daniel W. Sinkovits Pressure is the derivative of the free energy with respect to the change in volume. In a molecular dynamics simulation, there are different ways to define the infinitesimal change in volume, each corresponding to a different deformation field. The instantaneous pressure will vary depending on the choice of deformation field, but the time-average pressure should be the same for all choices. What varies is the spatial weighting of the pressure calculation. W. K. den Otter {\it et al.} [1] introduced this geometric approach to pressure calculation in 2001, but their thermodynamic derivation did not address time-varying deformation fields. I will use a mechanical derivation of pressure to demonstrate how to properly include a time-varying deformation field. This enables a unified understanding of the relationship between the atomic and molecular formulations of pressure. [1] W. K. den Otter, M. Kr\"ohn, and J. H. R. Clarke, Phys. Rev. E {\bf 65}, 016704 (2001). [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X7.00007: Essential factor for DFT predictions of relative energies in FePc: A Diffusion Monte Carlo study Tom Ichibha, Zhufeng Hou, Kenta Hongo, Ryo Maezono The ground state of the isolated iron phthalocyanine (FePc) with $D_{4h}$ symmetry is still unclear because the previous DFT works gave different predictions owing to their XC functionals. We applied CASSCF+DMC to evaluate the relative energies of the electric configurations reliably without XC functionals. $A_{2g}$ configuration is predicted as the ground state, which is, we found, able to be justified by the recent spectroscopy for FePc gas. Furthermore, we considered why the DFTs give the different predictions. We compared several DFTs with DMC, which include different percentages of exact exchange, and we revealed short-range exchange is essential for the prediction. We identified the discrepancies being due to the assumptions made in the superposition model, which has also been employed in literature to explain the ground state of FePc molecule. Our orbital analysis shows that the assumptions are too simple to describe the proper stabilizing mechanism explained by the orbital shapes: Oversimplified symmetry assumptions as well as the ignorance of outer ligand structures cannot capture the stabilization [destabilization] of $b_{2g}$ [$e_g$] orbitals, those actually realizes $A_{2g}$ as the most stable state. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X7.00008: Ba$^{\mathrm{2+}}$-Xe$_{\mathrm{n}}$ Clustering and Subsequent Mobility in $^{\mathrm{136}}$Xe Gas Edan Bainglass, Benjamin Jones, David Nygren, Muhammad Huda The possibility of clustering between Ba$^{\mathrm{2+}}$ and $^{\mathrm{136}}$Xe gas has been investigated as part of a neutrinoless double beta decay (0$\nu \beta \beta )$ detection experiment. The success of the experiment depends in part on the ability of Ba$^{\mathrm{2+}}$ to drift along an imposed electric field towards a detector. The question of clustering was raised due to the highly ionized Ba$^{\mathrm{2+}}$ daughter isotope and its potential of inducing dipoles in the surrounding $^{\mathrm{136}}$Xe gas. Such clustering would alter the mass and effective charge of the particle, thus changing the dynamics of the experiment. Density Functional Theory was employed in producing the potential energy surface for Ba$^{\mathrm{2+}}$-Xe dimer. The Ba$^{\mathrm{2+}}$ ion was modeled by a modified Gaussian basis set to account for the high ionization. Utilizing the modified basis set, clustering for BaXe$_{\mathrm{n}}$ (n$=$2-10) was investigated at 300K and was found to have the highest stability at BaXe$_{\mathrm{3}}$. A Monte Carlo simulation was developed to obtain the drift velocity and derive the mobility coefficient K$_{\mathrm{0}}$ as a function of operating perimeters. Results and algorithms will be presented. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X7.00009: Critical nonequilibrium relaxation in cluster algorithms using the Binder ratio and its application to bond-diluted Ising models Yoshihiko Nonomura, Yusuke Tomita Recently we showed that the critical nonequilibrium relaxation in cluster algorithms is widely described by the stretched-exponential relaxation [1-3]. Explicitly, the absolute value of magnetization at the critical temperature $T_{\rm c}$ behaves as $\langle |m| \rangle \sim \exp (+c_{m}t^{\sigma}$) from the perfectly-disordered state. In the present talk we apply this scheme to the bond-diluted Ising models and show that the exponent $\sigma$ increases continuously and monotonously as the bond density $p$ decreases. Although na\"ive fitting of physical quantities becomes difficult as $p$ approaches the percolation threshold $p_{\rm c}$, we find that the Binder ratio has no such a problem even in the vicinity of $p_{\rm c}$. While the Binder ratio is almost independent of system sizes at $T_{\rm c}$ both at the onset of relaxation and near equilibrium, the exponent $\sigma$ can be estimated accurately by an empirical logarithmic scaling for the size dependence in the intermediate simulation-time region. \smallskip \par \noindent [1] Y.~Nonomura, J.\ Phys.\ Soc.\ Jpn.\ {\bf 83}, 113001 (2014). [2] Y.~Nonomura and Y.~Tomita, Phys.\ Rev.\ E {\bf 92}, 062121 (2015). [3] Y.~Nonomura and Y.~Tomita, Phys.\ Rev.\ E {\bf 93}, 012101 (2016). [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X7.00010: Efficient gradient-based Monte Carlo simulation of materials: Applications to amorphous Si and Fe and Ni clusters Dil Limbu, Parthapratim Biswas We present a simple and efficient Monte-Carlo (MC) simulation of Iron (Fe) and Nickel (Ni) clusters with N$=$5-100 and amorphous Silicon (a-Si) starting from a random configuration. Using Sutton-Chen and Finnis-Sinclair potentials for Ni (in fcc lattice) and Fe (in bcc lattice), and Stillinger-Weber potential for a-Si, respectively, the total energy of the system is optimized by employing MC moves that include both the stochastic nature of MC simulations and the gradient of the potential function. For both iron and nickel clusters, the energy of the configurations is found to be very close to the values listed in the Cambridge Cluster Database, whereas the maximum force on each cluster is found to be much lower than the corresponding value obtained from the optimized structural configurations reported in the database. An extension of the method to model the amorphous state of Si is presented and the results are compared with experimental data and those obtained from other simulation methods. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X7.00011: Population Annealing Monte Carlo for Frustrated Systems Christopher Amey, Jonathan Machta Population annealing is a sequential Monte Carlo algorithm that efficiently simulates equilibrium systems with rough free energy landscapes such as spin glasses and glassy fluids. A large population of configurations is initially thermalized at high temperature and then cooled to low temperature according to an annealing schedule. The population is kept in thermal equilibrium at every annealing step via resampling configurations according to their Boltzmann weights. Population annealing is comparable to parallel tempering in terms of efficiency, but has several distinct and useful features. In this talk I will give an introduction to population annealing and present recent progress in understanding its equilibration properties and optimizing it for spin glasses. Results from large-scale population annealing simulations for the Ising spin glass in 3D and 4D will be presented. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X7.00012: Equilibrium properties in the thermodynamic limit from small-sized molecular dynamics simulations Robinson Cortes-Huerto, Kurt Kremer, Raffaello Potestio We present an accurate and efficient method to obtain equilibrium thermodynamic properties of bulk systems from small-sized molecular dynamics simulations by introducing finite size effects into integral equations of statistical mechanics. We validate the method by calculating thermodynamic properties of prototypical complex mixtures such as the activity coefficients of aqueous urea mixtures and the Kirkwood-Buff integrals of Lennard-Jones fluids. Moreover, our results demonstrate how to identify simulation conditions under which computer simulations reach the thermodynamic limit. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X7.00013: Enhancement of Electrical Conductivity in Multicomponent Nanocomposites. Xiaojuan Ni, Chao Hui, Ninghai Su, Feng Liu To date, very limited theoretical or numerical analyses have been carried out to understand the electrical percolation properties in multicomponent nanocomposite systems. In this work, a disk-stick percolation model was developed to investigate the electrical percolation behavior of an electrically insulating matrix reinforced with one-dimensional (1D) and two-dimensional (2D) conductors via Monte Carlo simulation. The effective electrical conductivity was evaluated through Kirchhoff's current law by transforming it into an equivalent resistor network. The percolation threshold, equivalent resistance and conductivity were obtained from the distribution of nodal voltages by solving a system of linear equations with Gaussian elimination method. The effects of size, aspect ratio, relative concentration and contact patterns of 1D/2D inclusions on conductivity performance were examined. Our model is able to predict the electrical percolation threshold and evaluate the conductivity for hybrid systems with multiple components. The results suggest that carbon-based nanocomposites can have a high potential for applications where favorable electrical properties and low specific weight are required. We acknowledge the financial support from DOE-BES (No. DE-FG02-04ER46148). [Preview Abstract] |
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