Bulletin of the American Physical Society
APS March Meeting 2020
Volume 65, Number 1
Monday–Friday, March 2–6, 2020; Denver, Colorado
Session P03: Materials in Extremes: Warm Dense MatterFocus
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Sponsoring Units: GSCCM Chair: Shuai Zhang, University of Rochester Room: 107 |
Wednesday, March 4, 2020 2:30PM - 3:06PM |
P03.00001: Visualization of ultrafast melting with femtosecond electron diffraction Invited Speaker: Mianzhen Mo Understanding the structural dynamics of ultrafast laser-induced melting is important for applications ranging from laser micromachining to warm dense matter experiments.Direct experimental observations of ultrafast melting have previously not been possible because of the small time and length scales involved. However, recent advances in ultrafast-electron-diffraction (UED) techniques [1] have opened up an exciting opportunity to study the transient atomic dynamics with femtosecond temporal resolution. |
Wednesday, March 4, 2020 3:06PM - 3:18PM |
P03.00002: Nuclear Quantum Effects in Two-temperature Hydrogen via Orbital-free DFT Path Integral MD Sam Trickey, Dongdong Kang, Kai Luo, Valentin Karasiev, Keith Runge Among systems at extreme conditions, hydrogen is special: its small nuclear mass implicates significant nuclear quantum effects (NQEs). An increasingly important set of extreme-condition systems has electrons driven to temperature Te far from the ion temperature Ti. For such two-temperature hydrogen, we report path integral molecular dynamics calculations driven by state-of-the-art orbital-free DFT calculations for the electrons. When the ratio of the ionic thermal de Broglie wavelength to their mean distance is larger than about 0.35, the ionic radial distribution function is strongly affected by NQEs. Moreover, NQEs induce a substantial increase in both the ionic and electronic pressures. |
Wednesday, March 4, 2020 3:18PM - 3:30PM |
P03.00003: A new capability for large-scale linear scaling Kohn Sham DFT calculations for materials at high temperatures. Sebastien Hamel, Mandy Bethkenhagen, John Pask, Phanish Suryanarayana, Babak Sadigh We developed a new capability for the accurate and efficient quantum simulation of material properties across an extreme range of densities, pressures and temperatures. This code, SQDFT, enables the use of full Kohn-Sham quantum molecular dynamics from the condensed matter regime, through the warm dense matter regime and into the plasma regime, well beyond the previous state-of-the-art which was restricted to temperatures below approximately 100 000 Kelvins. We demonstrate this new capability by calculating the Hugoniot curve of different materials up to millions of degrees Kelvin and Gigabar of pressure and investigating the structural and transport properties of the materials under these extreme conditions. We also present a performance study of our SQDFT code on various supercomputing platforms. SQDFT scales linearly with system size which allows us to run full Kohn-Sham QMD simulations with several thousand of atoms. |
Wednesday, March 4, 2020 3:30PM - 3:42PM |
P03.00004: Atom-in-jellium equations of state and melt curves in the white dwarf regime Damian Swift, Thomas Lockard, Sebastien Hamel, Christine J Wu, Lorin Benedict, Philip A Sterne, Heather Whitley Atom-in-jellium calculations of the electron states, and perturbative calculations of the Einstein frequency, were used to construct equations of state (EOS) from around 10−5 to 107 g/cm3 and 10−4 to 106 eV for elements relevant to white dwarf (WD) stars. This is the widest range reported for self-consistent electronic shell structure calculations. Elements of the same ratio of atomic weight to atomic number were predicted to asymptote to the same T = 0 isotherm. A generalized Lindemann criterion based on the amplitude of the jellium oscillations, previously used to extrapolate melt curves for metals, was found to reproduce previous thermodynamic studies of the melt curve of the one component plasma with a choice of vibration amplitude consistent with low pressure results. For elements for which low pressure melting satisfies the same amplitude criterion, such as Al, this melt model thus gives a likely estimate of the melt curve over the full range of normal electronic matter; for the other elements, it provides a useful constraint on the melt locus. |
Wednesday, March 4, 2020 3:42PM - 3:54PM |
P03.00005: First Principles Calculations of the Equation of State of MgSiO3 in the Gigabar Regime Felipe Gonzalez, Francois Soubiran, Henry Peterson, Burkhard Militzer The equation of state (EOS) of silicates at extreme conditions is of significant interest in planetary science and high-pressure physics. Using path integral Monte Carlo (PIMC) and density functional theory molecular dynamics (DFT-MD) simulations, we study the properties of MgSiO3 enstatite in the regime of warm dense matter. We generate a consistent equation of state for MgSiO3 over a wide range of temperature and density conditions, ranging from 104 to 108 K and 0.1- to 20-fold the ambient density. An analysis of the structural and thermodynamic properties of the liquid is provided as L and K shell electrons are ionized with increasing temperature and pressure. The shock Hugoniot curve, that we derived from our EOS, is in very good agreement with the experimental data available for megabar pressures. In the gigabar regime, shock experiments are predicted to reach maximal compression. We analyze the shape of the Hugoniot curve under these conditions and demonstrated that the shock compression is controlled by the ionization of L and K shell electrons of the three nuclei. |
Wednesday, March 4, 2020 3:54PM - 4:06PM |
P03.00006: Ionization Potential Depression in Dense Plasmas Predicted with Ab Initio Methods Burkhard Militzer, Maximilian Böhme, Gerard Massacrier, Jan Vorberger, Francois Soubiran The degree of ionization is an important quantity that directly affects the equation of state of plasmas as well as their optical and transport properties. At high density, the electronic states are affected by the interaction with nearby particles. In energy space, bound states are pushed closer to the continuous spectrum of free-particle states. These changes are typically encapsulated in the terms of ionization potential depression (IPD). An accurate description of IPD becomes important when spectroscopic measurements are employed to determine the thermodynamic state of plasmas. In their seminal work, Stewart and Pyatt (SP) have developed an analytical model to describe the IPD. However, in Physical Review E 97 (2018) 063207, diverging trends between SP predictions and ab initio results were reported for hot, dense aluminum. Here, we report results from a comprehensive set of density functional molecular dynamics simulations that were designed to study the IPD of different materials under various thermodynamic conditions. We analyze the strengths and weaknesses of SP models and discuss possible reasons why they may become unreliable at high density. |
Wednesday, March 4, 2020 4:06PM - 4:18PM |
P03.00007: High Temperature Density Functional Theory Calculations Patrick Hollebon, Travis Sjostrom Kohn-Sham density functional theory calculations are widely used as an accurate ab-initio approach to solving the many-body problem in the warm dense matter regime, encompassing conditions approximately ranging from 0.1 to 50 times solid density and temperatures from 0.5 to 100 eV. Nonetheless computational costs can quickly become prohibitive with rising temperature due to the growing number of partially occupied states involved. This is particularly pertinent for accurate molecular dynamics simulation that require large box sizes and timescales to reach convergence. For these reasons, orbital-free methods are an attractive alternative for temperatures beyond approximately 10 eV, however the increased computational efficiency of simple kinetic energy functionals comes at the cost of physical accuracy. Here we present a hybrid approach in which the Kohn-Sham equation is solved for the low-lying states only, whilst an orbital-free method is applied to describe the remaining electrons. Our hybrid methods are used to perform equation of state calculations in the warm dense matter regime which we then benchmark against conventional Kohn-Sham calculations. |
Wednesday, March 4, 2020 4:18PM - 4:30PM |
P03.00008: Electron-thermal contribution to the equation of state model based on two-temperature quantum molecular dynamics simulations Lin Yang, John Emrich Klepeis, Philip A Sterne, Heather D Whitley, Shuai Zhang We present results using two-temperature quantum molecular dynamics (QMD) simulations to identify the electron-thermal contributions to the equation of state (EOS) and make comparison with the Purgatorio-based model. The two-temperature approach treats the electron and the ion temperature independently in a QMD simulation. We applied the method to some low-Z materials (B, BN, and B4C) and a high-Z transition-metal oxide at temperatures ranging from room temperature to 106 K and 1 to 5 times compression in densities. Our results suggest that the electron-thermal contribution begins to dominate around 1.2x105 K (10 eV) and is material as well as density dependent. For each material, we identify the temperature-density regime where the electron-thermal contribution is independent of ionic configurations. We illustrate our findings by comparing electron pressure and specific heat with Purgatorio-based model. |
Wednesday, March 4, 2020 4:30PM - 4:42PM |
P03.00009: Atom-in-jellium equations of state for high Z elements Tom Lockard, Lorin Benedict, Christine J Wu, Philip A Sterne, Damian Swift We have previously reported atom-in-jellium (AJ) predictions of the equation of state (EOS) and melt locus of low- and mid-Z elements. Here we report equivalent results for a selection of high Z elements that are also under experimental investigation at high energy density facilities. In comparison with previous EOS models for these substances, based on Thomas-Fermi theory, the shock Hugoniot exhibits structures corresponding to the ionization of successive electron shells. Proportionately, the difference from TF theory is less than was found for lower Z elements, though it can still amount to a factor ~2 in pressure as each ionization occurs. The broad trend of the Hugoniot is however closer to TF predictions than was observed at lower Z. The cold curve and ambient isochore deduced from AJ calculations is generally quite close to the corresponding TF calculation. |
Wednesday, March 4, 2020 4:42PM - 4:54PM |
P03.00010: Multiscale Simulation Method for Plasma Flows Abdourahmane Diaw, Jeff Haack, Mike Mckerns Mckerns, Robert Pavel In high-energy density flows, a number of dynamical and structural data is needed to be collected at the microscale using first principles calculations. Experimental results and their analysis, on the other hand, are determined by measurements at the macro-scale in space over long scales in time. Thus, one major disparity that is currently inhibiting progress in this area is the extrapolation of microscale information into macroscopically relevant scales. Inertial confinement fusion experiments, for example, are fundamentally multi-physics in nature; understanding the connection between experimental observables, and the underlying microphysics, is needed to fully assess capsule performance. In this work, we use uncertainty quantification driven learning algorithms coupled with directed sampling to automate the learning the learning of a multiscale model that is valid for both atomistic processes and continuum phenomena. The multiscale framework couples a molecular dynamics with a Boltzmann kinetic model to describe mesoscale phenomena, and which is tuned by minimizing the model error. We will discuss the code written to build a valid multiscale model relevant to high-energy density experiments, and how that tool may be leveraged to facilitate the learning of plasma models. |
Wednesday, March 4, 2020 4:54PM - 5:06PM |
P03.00011: Spin-orbit coupling in first-principles optical and X-ray spectroscopy spectra under WDM conditions Nils Brouwer, Vanina Recoules, Marc Torrent The prediction and analysis of optical and X-Ray spectra of materials under WDM conditions is of great scientific interest. The Kubo-Greenwood (KG) formula associated with the Projector Augmented-Wave (PAW) approach [1] has proven its efficiency to compute these spectra and has been implemented in a few DFT codes, including the ABINIT code [2]. One important aspect when treating optical and X-Ray spectroscopy, especially for heavier elements, is the spin-orbit coupling (SOC). |
Wednesday, March 4, 2020 5:06PM - 5:18PM |
P03.00012: Quasi-isentropic compression of strongly nonideal plasmas: correlations, degeneracy and plasma phase transitions Vladimir E. Fortov, Radii I. Il'kaev, Mikhail I. Kulish, Pavel R. Levashov, Viktor B. Mintsev, Mikhail A. Mochalov, Dmitry N. Nikolaev, Vladimir V. Stegailov A series of experiments with intense quasi-isentropic compression of the plasma D2 and He were carried out to get strongly coupled nonideal plasma. Two-stage spherical explosively driven devices were used in these experiments. Recordary high parameters of strongly coupled deuterium plasma were obtained in the experiments: pressures up to 18 TPa and density up to 14 g/cc. Helium plasma was compressed ~ 200 times up to density of 8 g/cc and pressures of P ~ 5 TPa. Remarkable features of helium and deuterium plasma behaviour at these extremely high pressures, temperatures and densities are discussed with the use of nonideal plasma models as well as of the results of computer simulation calculations using first principle approaches. The different theoretical models of plasma phase transitions are discussed in comparison with shock wave experiments. Exclusively important role of quantum degeneracy effects as well as of strong correlation effects is analysed on the basis of the experimental data obtained. |
Wednesday, March 4, 2020 5:18PM - 5:30PM |
P03.00013: Measurement of the fs laser-induced structural modification process inside transparent materials Lin Zhang, Zhicheng Zhong, Hao Jiang, Shiyuan Liu Femtosecond laser pulses can cause structural modification when focused inside transparent material, which can be used in the formation of optical waveguides, 3D micromachining and other photonic machining applications. However, the structural modification process of the material is still unclear, which often occurs in the picosecond scale. |
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