Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session B13: Focus Session: Simulations of Matter at Extreme Conditions II: Beryllium, Carbon, and Metals |
Hide Abstracts |
Sponsoring Units: DCOMP GSCCM Chair: Eric Schwegler, Lawrence Livermore National Laboratory Room: Morial Convention Center 204 |
Monday, March 10, 2008 11:15AM - 11:27AM |
B13.00001: The Role of Anharmonicity in the Beryllium Equation of State Lorin X. Benedict, Andrea Trave, Christine Wu, Tadashi Ogitsu, Phil Sterne, Eric Schwegler We discuss the construction of a multiphase equation of state for Be from first principles, aimed at understanding the material's properties at extreme conditions. In addition to the usual computation of cold, quasiharmonic ion-thermal, and (negligible here) electron-thermal contributions, we consider the effects of strong anharmonicity in the bcc phase, and argue that the inclusion of such effects may greatly perturb the picture (in particular, the phase diagram) derived from assuming quasi-harmonic lattice dynamics. Our analysis involves studying the mean displacement from equilibrium of Be atoms in the lattice by DFT-molecular dynamics methods and comparing the results to those of the quasiharmonic theory. [Preview Abstract] |
Monday, March 10, 2008 11:27AM - 11:39AM |
B13.00002: Quantum molecular dynamics simulations of beryllium at high pressures Michael Desjarlais, Marcus Knudson The phase boundaries and high pressure melt properties of beryllium have been the subject of several recent experimental and theoretical studies. The interest is motivated in part by the use of beryllium as an ablator material in inertial confinement fusion capsule designs. In this work, the high pressure melt curve, Hugoniot crossings, sound speeds, and phase boundaries of beryllium are explored with DFT based quantum molecular dynamics calculations. The entropy differences between the various phases of beryllium are extracted in the vicinity of the melt curve and agree favorably with earlier theoretical work on normal melting. High velocity flyer plate experiments with beryllium targets on Sandia's Z machine have generated high quality data for the Hugoniot, bulk sound speeds, and longitudinal sound speeds. This data provides a tight constraint on the pressure for the onset of shock melting of beryllium and intriguing information on the solid phase prior to melt. The results of the QMD calculations and the experimental results will be compared, and implications for the HCP and BCC phase boundaries of beryllium will be presented. [Preview Abstract] |
Monday, March 10, 2008 11:39AM - 11:51AM |
B13.00003: Equation of state of beryllium from first-principles calculations Andrea Trave, Lorin Benedict, Tadashi Ogitsu, Christine J. Wu, Philip A. Sterne, Eric Schwegler The design of experiments of materials at extreme conditions of pressure and temperature is often based on hydrodynamic simulations, which make use of equation of state (EOS) models for the description of the systems under study. The validity of these models is extremely critical, and first-principles calculations can provide consistent and accurate parameters for the determination of the EOS in a wide range of thermodynamic conditions. Extensive density functional theory calculations at zero temperature have been performed for beryllium in various solid structures, in order to obtain accurate predictions for their compression curves, and phonon and electronic densities of states. Finite-temperature simulations have been used to further improve the models to include anharmonic effects. The melting line of beryllium has been obtained with first-principles two-phase simulations, which enables the construction of a multi-phase EOS for both liquid and solid beryllium. The results of these simulations provide useful indications on the relative stability of the various solid and liquid phases of beryllium in a region of the phase diagram lacking any experimental study so far. Prepared by LLNL in part under Contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 11:51AM - 12:03PM |
B13.00004: Construction of a multi-phase equation of state for carbon at extreme pressures Alfredo A. Correa, Lorin X. Benedict, Stanimir A. Bonev, David A. Young, Eric Schwegler We describe the construction of a multi-phase equation of state for carbon at extreme pressures that is based on the results of first principles electronic structure calculations. Two solid phases (diamond, BC8) and the liquid are considered. Solid-phase free energies are built from a knowledge of cold curve and phonon calculations, together with first principles molecular dynamics calculations of the equation of state itself to extract anharmonic terms. The liquid free energy is constructed from a combination of molecular dynamics calculations and constraints determined from previously calculated melt curves, assuming a simple solid-like free energy model. The resulting equation of state is extended to more extreme densities and temperatures with a plasma-based free energy model. Comparisons to available experimental results are discussed. [Preview Abstract] |
Monday, March 10, 2008 12:03PM - 12:15PM |
B13.00005: Clustering in dense molten lithium Isaac Tamblyn, Jean-Yves Raty, Stanimir A. Bonev Molten lithium is investigated from zero to over nine-fold compression using first principles theory. Over this pressure range, we observe several electronic and structural transitions. The changes that lithium undergoes with increasing pressure are initially analogous to those predicted for liquid sodium [1]. However, upon further compression, effects due to increased core overlap lead to a new liquid phase composed of weakly bound lithium clusters. The properties of the proposed new liquid phases, the melting curve of lithium, and the implications of our findings for the stability of low-symmetry lithium solids will be discussed. [1] J.-Y. Raty, E.R. Schwegler, S.A. Bonev, Nature, 449, 448-451 (2007) [Preview Abstract] |
Monday, March 10, 2008 12:15PM - 12:27PM |
B13.00006: Lithium at ultra-high pressures Andre Kietzmann, Ronald Redmer, Michael P. Desjarlais, Thomas R. Mattsson Lithium is a prototypical simple metal at standard conditions. However, by changing the density towards expanded or compressed states, the electrical conductivity shows strong variations. We have performed quantum molecular dynamics simulations for fluid lithium covering a wide range of densities and temperatures in order to derive the equation of state, the electrical conductivity, and information about structural and electronic changes along the expansion or compression. The electrical conductivity changes from the nonmetallic expanded fluid via the fluid metal region up to the degenerate electron liquid at high densities. We find a largely ordered ion structure at ultra-high densities reflecting a multi-center bonding situation in the liquid as predicted earlier for solid lithium. Supported by the DFG within SFB 652. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States DOE's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Monday, March 10, 2008 12:27PM - 12:39PM |
B13.00007: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 12:39PM - 12:51PM |
B13.00008: Optical Properties of LiH From Mixing Rules Daniel Horner, Joel Kress, Lee Collins We investigate the use of pressure and density matching mixing rules for predicting the optical properties and equation-of-state (EOS) of lithium hydride for densities from half to twice solid [0.78 g/cc] and temperatures from 0.5 to 3.0 eV. The mixing rules allow us to perform simulations of lithium and hydrogen separately and, from them, calculate properties of the mixture. Using the VASP code, we performed constant $(N, V, T)$ quantum molecular dynamics simulations for H, Li, and LiH with the results for the mixture (LiH) as a benchmark of the mixing procedures. A finite-temperature density functional theory formulation produces the electronic wave function at each time step within the generalized gradient approximation with projector augmented wave pseudopotentials. Optical properties were determined using the electronic wave function in a Kubo-Greenwood formula. We compare the frequency-dependent absorption coefficient, Rosseland Mean Opacity, and EOS computed via the mixing rules and those from a full LiH simulation. [Preview Abstract] |
Monday, March 10, 2008 12:51PM - 1:03PM |
B13.00009: First principles investigation of the dielectric function of gold under ultrafast laser excitation Tadashi Ogitsu, David Prendergast, Eric Schwegler, Yuan Ping, Andrew Ng Recently, a quasi-steady state in ultrathin, $\sim$30 nm gold foils exposed to an ultrafast laser pulse has been observed, which includes an enhanced interband transition peak at 2.6 eV in the imaginary part of the dielectric function [1]. Simulations of this system assuming a two-temperature model for the electronic and ionic degrees of freedom do not provide this enhancement in optical absorption, possibly indicating that both of these degrees of freedom are not in equilibrium. Our approach is to treat this as an inverse problem: to reproduce experiment by sampling various states of electrons and ions. We employ an efficient first principles technique to quickly estimate the dielectric function of this fcc metal for various finite temperature and non-equilibrium model distributions. Converged Brillouin zone sampling is achieved using a compact k-dependent Hamiltonian derived from first principles calculations [2]. [1] Y. Ping et al., Phys Rev Lett {\bf 96}, 25503 (2006). [2] E. L. Shirley, Phys Rev B {\bf 54}, 16464 (1996). [Preview Abstract] |
Monday, March 10, 2008 1:03PM - 1:15PM |
B13.00010: High pressure lattice dynamics and elasticity of transition metals Daniel Orlikowski, Lorin Benedict, John Klepeis For continuum-level description of transition metals using equation of state and strength models, a large concerted calculation effort is required. We present here a subset of that work to provide Debye temperatures and elastic moduli for the equation of state (EOS) and strength models. DFT calculations for the phonons are performed to obtain the Debye temperature over the pressure range required by the EOS model. For the strength model, we have combined several sets of quantum-based, atomistic calculations with density functional theory (DFT) to develop elastic moduli over a wide range of temperatures (12,000 K) and pressures (4 Mbar). Our focus is upon vanadium but other transition metals will be presented tantalum and molybdenum. Our results are comparable to available experimental data. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 1:15PM - 1:27PM |
B13.00011: A new wide-range equation of state for tungsten John H. Carpenter, Michael P. Desjarlais, Ann E. Mattsson, Kyle R. Cochrane A new wide-range equation of state for tungsten is described. Quantum molecular dynamics calculations in the warm dense matter region are combined with other experimental and theoretical calculations, providing a set of information on which to tune a model of the free energy landscape. The resulting model, describing the liquid, gas, and bcc solid phases, provides a good description of the liquid-vapor critical point, melt curve, static compression data, isobaric expansion data, and the Hugoniot. Finally, improvements in table generation greatly improve the resolution of phase boundaries. [Preview Abstract] |
Monday, March 10, 2008 1:27PM - 1:39PM |
B13.00012: ABSTRACT WITHDRAWN |
Monday, March 10, 2008 1:39PM - 1:51PM |
B13.00013: Pressure Induced Solidification of Ta and Cu: A Comparison David Richards, James Glosli, Frederick Streitz Using powerful computers such as Blue Gene/L it is now possible to use classical molecular dynamics to simulate pressure induced solidification at size scales that are free of finite size effects. We present a comparison of the nucleation, growth, and coalescence of clusters during pressure induced solidification in large scale MD simulations of liquid Ta and Cu. We extract growth and nucleation rates from our simulations, as well as cluster size distributions that can be compared against the predictions of simple models. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 1:51PM - 2:03PM |
B13.00014: New Phase Diagram of Ta: Bridging Laser Heated Diamond-Anvil Cell and Shock Melting Christine J. Wu, Per A. S\"oderlind, James N. Glosli, John E. Klepeis Determination of the melt line of materials under high pressures is essential for establishing its phase diagrams and has important implications for geophysics, material science, and high-pressure physics. So far, melting temperatures at high pressure are primarily measured by \textit{in situ} laser-heated diamond-anvil cell (DAC) or shock wave experiments. Often, these two methods yield significantly different results, particularly for non close-packed metals, such as bcc metals. For instance, anomalously flat melting slopes were reported for numerous bcc metals by laser-heated DAC. The flatness of the melting slope is in sharp contrast to the classical Lindemann behavior which shock-melting temperatures follow closely. In this presentation, we will report a novel phase diagram of Ta obtained from \textit{ab initio} methods, and molecular dynamics (MD) simulations, which resolves the long-standing controversy, and has significant impact on our understanding of phase diagrams of bcc metals. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700