Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session L35: General Contributed: Theory and Simulations of Materials in Extreme Conditions |
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Sponsoring Units: DCMP DCOMP Chair: Jeffrey McMahon, Washington State University Room: 298 |
Wednesday, March 15, 2017 11:15AM - 11:27AM |
L35.00001: On the Possibility of Metastable Metallic Hydrogen Jeffrey McMahon Metallic hydrogen, high- and room-temperature superconductivity, and controlled nuclear fusion have been singled out as the top three problems in physics. All of these involve hydrogen or its isotopes. Solid metallic hydrogen has recently been created in the laboratory at high pressure. Some of the key questions now concern whether this phase will be metastable at lower (and ambient) pressures, and, if so, what its properties are. In this presentation, the theoretical possibility of metastable metallic hydrogen will be discussed. Recent results from quantum Monte Carlo and density-functional theory calculations will be presented. These include the suitability of the latter for studying metallic hydrogen, the zero-temperature (ground- and metastable-state) phase diagram of the atomic (metallic) phase, the dynamical stabilities of the lattices, and, if there is time, their superconducting properties. [Preview Abstract] |
Wednesday, March 15, 2017 11:27AM - 11:39AM |
L35.00002: Non-Born-Oppenheimer diffusion Monte Carlo calculations of solid molecular and atomic hydrogen Yubo Yang, Norm Tubman, David Ceperley We present dynamic-lattice (non-Born-Oppenheimer) diffusion Monte Carlo (DMC) calculations on zero-temperature candidate structures of solid hydrogen at several pressures from 200 GPa to 500 GPa. Four molecular (Cmca-4, Cmca-12, C2/c and P6(1)22) and one atomic (I4(1)/amd) structures are investigated. The candidate structures are roughly separated by 10 mev/proton in enthalpy, making small energy contributions important for the accurate prediction of phase boundaries. In dynamic-lattice DMC, we treat both electrons and protons as quantum particles. This incorporates anharmonic lattice vibrations at the DMC level as well as nonadiabatic coupling between the electron and proton motions. We use an analytic expression for the electron-proton Jastrow to remove the electronic single-particle orbital cusps and allow the electronic wave function to dynamically follow the protons. [Preview Abstract] |
Wednesday, March 15, 2017 11:39AM - 11:51AM |
L35.00003: Effects of pressure on the magnetic properties of FeO: A diffusion Monte Carlo study Joshua Townsend, Luke Shulenburger, Thomas Mattsson, Ken Esler, Ronald Cohen While simple in terms of structure and composition, both experimental and computational investigations have demonstrated that FeO has a rich phase diagram of structural phase transformations, electronic spin transitions, insulator-metal transitions, and magnetic ordering transitions, due to the open-shell occupation of the Fe 3d electrons. We investigated the magnetic and electronic structures of FeO under ambient and high pressure conditions using diffusion Quantum Monte Carlo (QMC) within the fixed-node approximation. QMC techniques are especially well suited to the study of strongly correlated systems because they explicitly include correlation into the ground-state wave function. Here we report on the effects of the choice of trial wave function on the ambient pressure lattice distortion due to AFM ordering, as well as the equation of state, spin collapse, and metal-insulator transitions. [Preview Abstract] |
Wednesday, March 15, 2017 11:51AM - 12:03PM |
L35.00004: High pressure superconducting phase of Sulfur and Phosphorus Gianni Profeta The recent discovery of very high superconducting critical temperature in high pressure phases of SH$_3$ and PH$_3$ opens new perspectives on the research of superconducting materials under high pressure and poses important and stringent conditions on the theoretical predictions of crystal structures and superconducting critical temperatures (Tc). In the same high pressure runs for SH$_3$ and PH$_3$, the superconducting phase diagram of both elemental sulfur and phosphorus have been re-examined, revealing peculiar features which calls for a complete theoretical explanation. In sulfur a discontinuity of Tc as a function of the pressure (P) is observed, while the Tc(P) curve in phosphorus strongly depends on the initial experimental conditions. In this talk, by means of first principles superconducting DFT, we predict the superconducting phase diagram of elemental sulfur and phosphorus under high pressure with unprecedented agreement with available experimental results. We discovered that the discontinuity of Tc(P) in sulfur arises from peculiar and unexpected multiband effects, while the presence of metastable phases explain the puzzling Tc(P) curves in phosphorus. [Preview Abstract] |
Wednesday, March 15, 2017 12:03PM - 12:15PM |
L35.00005: Ab Initio Investigations of High-Pressure Melting of Dense Lithium Raymond Clay, Miguel Morales, Stanimir Bonev Lithium at ambient conditions is the simplest alkali metal and exhibits textbook nearly-free electron behavior. As the density is increased, however, significant core/valence overlap leads to surprisingly complex chemistry. We have systematically investigated the phase diagram of lithium at pressures ranging between two and six million atmospheres. Through a combination of density functional theory based path-integral and classical molecular dynamics simulations, we have investigated the impact of both nuclear quantum effects and anharmonicity on the melting line and solid phase boundaries. We also investigate how the inclusion of nuclear quantum effects and approximations in the treatment of electronic exchange-correlation impact the robustness of previous predictions of tetrahedral clustering in dense liquid Li. Sandia National Laboratories is a multi-mission laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 15, 2017 12:15PM - 12:27PM |
L35.00006: Extended First-principles molecular dynamics toward high temperature Wei Kang, Shen Zhang, Ping Zhang, X.T. He An extended first-principles molecular dynamics method (ext-FPMD)\footnote{S. Zhang, W. Kang, P. Zhang, and X. T. He, Physics of Plasmas \textbf{23}, 042707 (2016)}, which incorporates an analytical formula of the contribution of high energy electrons into the Kohn-Sham-Mermin scheme, is proposed to significantly improve the efficiency of finite-temperature density functional theory for hot dense plasmas while still maintaining the numerical accuracy. The new method eliminates the explosively growing computational costs at high temperature up to thousands of electron volts, and naturally returns to the original Kohn-Sham-Mermin scheme when the temperature is close to zero. It thus consistently deals with materials from 0K to about 2000eV, which is important to the application of inertial confinement fusion. Compared with other first-principles methods for dense plasmas, the newly devised method also keeps the information of electronic structures at high temperature, which gives it an edge in the future study of transport properties. [Preview Abstract] |
Wednesday, March 15, 2017 12:27PM - 12:39PM |
L35.00007: Density-to-Potential Inversions to Guide Development of Exchange-Correlation Approximations at Finite Temperature Daniel Jensen, Adam Wasserman, Andrew Baczewski The construction of approximations to the exchange-correlation potential for warm dense matter (WDM) is a topic of significant recent interest. In this work, we study the inverse problem of Kohn-Sham (KS) DFT as a means of guiding functional design at zero temperature and in WDM. Whereas the forward problem solves the KS equations to produce a density from a specified exchange-correlation potential, the inverse problem seeks to construct the exchange-correlation potential from specified densities. These two problems require different computational methods and convergence criteria despite sharing the same mathematical equations. We present two new inversion methods based on constrained variational and PDE-constrained optimization methods. We adapt these methods to finite temperature calculations to reveal the exchange-correlation potential's temperature dependence in WDM-relevant conditions. The different inversion methods presented are applied to both non-interacting and interacting model systems for comparison. [Preview Abstract] |
Wednesday, March 15, 2017 12:39PM - 12:51PM |
L35.00008: Iron spin crossover in the NAL phase and ferromagnesite Han Hsu Spin crossover (SCO) in iron-bearing minerals has attracted tremendous attention, as SCO leads to anomalous changes of the physical properties of these minerals. The local density approximation $+$ self-consistent Hubbard $U$ (LDA$+U_{sc})$ method, with the $U$ parameters computed self-consistently, has elucidated SCO in many lower-mantle minerals. In this talk, two recent LDA$+U_{sc}$ studies of SCO in earth minerals are presented: the new hexagonal aluminous (NAL) phase [1] and (Mg,Fe)CO$_{3}$ ferromagnesite [2]. The former is considered as a main aluminum host in the subducted basalt, and the latter is believed to be the major carbon carrier in the Earth's lower mantle and play a key role in the deep carbon cycle. For both minerals, the abrupt change of iron quadrupole splitting and the volume/elastic anomalies accompanying the SCO obtained in our calculations are in great agreement with experiments. Our calculations also suggest that the spin transition pressure $P_{T}$ in the NAL phase is not very sensitive to temperature, due to its three nearly degenerate low-spin (LS) states, in contrast with (Mg,Fe)O ferropericlase and (Mg,Fe)CO$_{3}$ ferropericlase, in which $P_{T}$ significantly increases with temperature. [1] H. Hsu, submitted. [2] H. Hsu and S.-C. Huang, Phys. Rev. B \textbf{94}, 060404(R) (2016). [Preview Abstract] |
Wednesday, March 15, 2017 12:51PM - 1:03PM |
L35.00009: Formation of a Quasi 2D-layer of Protons in Hydroxides at High Pressure Romain DUPUIS, Jorge Dolado, Jose Surga, Magali Benoit, Andres Ayuela In this work, we found that a remarkable quasi 2D-layer of protons is formed in hydroxides at high pressure[1]. Among numerous fields, hydroxides are used in chemistry, in the industry (glass, cements) and in geosciences (water retainer, mantle crust). We investigated the dynamical properties of protons considering key nuclear quantum effects. For that, we used the Path Integral Molecular Dynamics method that has become a reference[2, 3]. An archetype system (Ca(OH)$_{2}$) was considered. We found that a 2D-layer of protons is formed at high pressure. A new mechanism, consisting of quantum rotations of protons, controls the diffusion of protons in hydroxides. [1](Submitted to PRL) R. Dupuis, J. Dolado, J. Surga, M. Benoit, A. Ayuela [2]O. Marsalek, C. Pei-Yang, R. Dupuis, M. Benoit, M. M\'eheut, Z. Bacic and M. Tuckerman, {\em J. of Chemical Theory and Computation}, v.~10, no.~4, pp.~1440--1453, 2014. [3]J. Cao and G.~A. Voth, {\em J. Chem. Phys.}, v.~100, no.~7, pp.~5106--5117, 1994. [Preview Abstract] |
Wednesday, March 15, 2017 1:03PM - 1:15PM |
L35.00010: Finite Temperature Effects of the Pressure-Driven Quantum Hall to Nematic Phase Transition in the Second Landau Level Katherine Schreiber, Nodar Samkharadze, Geoffrey Gardner, Michael Manfra, Rudro Biswas, Gabor Csathy The second Landau level of a two-dimensional electron gas is known to exhibit a rich variety of electronic phases which are close in energy. Within the second Landau level we have recently reported a pressure-driven quantum phase transition from a fractional quantum Hall state to an electronic nematic, or stripe, phase occurring at the Landau level filling factor 5/2. This phase transition is special as it is one of the very few known examples of a transition between a topologically ordered phase and a traditional Landau phase. We now report on the temperature dependence of the fractional quantum Hall state and the nematic phase at filling factor 5/2 as they evolve with pressure. In particular, we trace the energy gap of the fractional quantum Hall state and the onset temperature of the nematic state. From these measurements, we obtain a phase diagram of competing phases with topological and nematic orders. [Preview Abstract] |
Wednesday, March 15, 2017 1:15PM - 1:27PM |
L35.00011: Fluid Fe${_{(1-x)}}$H${_x}$ under extreme conditions Alexandra Seclaman, Hugh F. Wilson, Ronald E. Cohen We study the fluid Fe-H binary system using first principles molecular dynamics (FPMD) and a new FPMD-based method, CATS, in order to compute efficiently and accurately the equation of state of Fe-H fluids up to 5 TPa and 30,000K. We constructed GRBV-type LDA pseudopotentials for Fe and H with small rcuts in order to avoid pseudo-core overlap. In the liquid Fe regime we find good agreement with previous works, up to the pressures where data is available. In the high density regime of pure H we also find good agreement with previous results. Previous work has been focused on low Fe concentrations in metallic liquid H. We extend previous studies by investigating several intermediate Fe${_{(1-x)}}$H${_x}$ liquid compositions, as well as metallic liquid H and Fe. Preliminary results indicate extreme compositional pressure effects under isothermic and isochoric conditions, 3.9 TPa difference between Fe and H at 20,000K. Thermal pressure effects are comparatively small, 0.12-0.15 TPa per 10,000K for H and Fe, respectively. Equations of state will be presented and fluid immiscibility will be discussed. [Preview Abstract] |
Wednesday, March 15, 2017 1:27PM - 1:39PM |
L35.00012: Multiscale modeling of electron-ion dynamics in silicon under particle radiation Andre Schleife, Cheng-Wei Lee, Khalid Hattar, Remi Dingreville, Stephen Foiles Effects of fast ions impacting solids are important, in order to quantitatively understand radiation damage, ion beam modification (e.g.\ helium microscopy), or ion implantation. The interaction of a fast ion with kinetic energies in the MeV range and a target involves both electron-ion as well as ion-ion collisions. Ehrenfest molecular dynamics, based on real-time propagation of time-dependent Kohn-Sham equations, provides highly accurate insight into early stages of the defect-formation process and, in particular, into electronic stopping. Thanks to the excellent scalability of our plane-wave implementation, we are capable to perform these parameter-free simulations for supercells with hundreds of atoms using high-performance computing. However, the cost of Ehrenfest dynamics is prohibitively high for entire radiation-cascade development both regarding length- and time scales. Results from TDDFT for silicon targets are used to provide much more accurate information on velocity- and position-dependence of electronic stopping that we transfer into classical molecular dynamics. This results in a multi-scale simulation framework capable of studying crystalline semiconductors such as Si, GaP, or InP. [Preview Abstract] |
Wednesday, March 15, 2017 1:39PM - 1:51PM |
L35.00013: Microstructural properties of hydrogenated amorphous silicon: A first-principles study Durga Paudel, Parthapratim Biswas, Raymond Att-Fynn, David Drabold, Stephen Elliott We present a new approach to simulate complex amorphous materials with an emphasis on structural, electronic, and optical properties of hydrogenated amorphous silicon. The microstructural properties of hydrogen distribution are addressed by simulating very large models using a method that combines classical metadynamics simulations with density-functional calculations. The shape, size and distribution of microvoids and their number density are studied and compared with the same from the small-angle X-ray scattering, hydrogen- and helium-effusion measurements, and neutron diffraction studies on {\it a}-Si:H. Our results suggest that the density of microvoids is of the order of 7-8 $\times 10^{18}$ cm$^{-3}$ for device-quality models with 8-10 at.\,\% H, and it increases to 2-3 $\times 10^{19}$ cm$^{-3}$ for concentration of up to 18 at.\,\%. The geometry of the microvoids has been found to be highly complex with a radius of gyration from 2.8 {\AA} to 4.0 {\AA} for very large models. [Preview Abstract] |
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