Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session V23: Focus Session: Extreme Conditions and High Pressure III: Electronic Transitions and Mixtures |
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Sponsoring Units: DCOMP GSCCM Chair: Stephane Mazevet, French Atomic Energy Commission Room: 325 |
Thursday, March 19, 2009 8:00AM - 8:12AM |
V23.00001: Phase separation in hydrogen-helium mixtures at high pressure Miguel Morales, Eric Schwegler, David Ceperley, Carlo Pierleoni, Sebastian Hamel, Kyle Caspersen We study the properties of hydrogen-helium mixtures at Mbar pressures and intermediate temperatures (4000 to 10000 K) using first-principles molecular dynamics simulations. Our main goal is to calculate the temperature, as a function of pressure, at which helium becomes insoluble in dense metallic hydrogen. We perform an extensive study of the equation of state of the mixture as a function of density, temperature, and composition and, together with a variety of thermodynamic integration techniques, we calculate the Gibbs free energy of mixing. We will show how to calculate the entropic contribution of the free energy using coupling constant integration methods, which allows us to directly calculate immiscibility temperatures without the need to resort to approximations of the entropy of mixing. These results are relevant to models of the interior structure and evolution of Jovian planets. We find demixing temperatures that are sufficiently high to cross the planetary adiabat of Saturn at pressures around 5 Mbar, implying the existence of partially miscible regions over a significant portion of the interior of the planet. [Preview Abstract] |
Thursday, March 19, 2009 8:12AM - 8:24AM |
V23.00002: Molecular dynamics simulations of electron-ion temperature equilibration in an SF6 plasma Lorin X. Benedict, James N. Glosli, David F. Richards, Frederick H. Streitz, Stefan P. Hau-Riege, Frank R. Graziani, Michael S. Murillo, John F. Benage We use classical MD to investigate electron-ion temperature equilibration in a two- temperature SF$_{6}$ plasma. We choose a density of 1.0$\times 10^{19}$ (dissociated) SF$_{6}$ molecules per cm$^{3}$ and initial temperatures of $T_{e}\sim 100$ eV and $T_{S}\sim T_{F}\sim 15$ eV in accordance with experiments currently underway at Los Alamos National Laboratory. Our computed relaxation time is significantly longer than that predicted by the classic theory of Landau and Spitzer. Similar discrepancies are found when comparing to predictions made be more recent theories of electron-ion equilibration. These differences should be large enough to be measured in the upcoming experiments. [Preview Abstract] |
Thursday, March 19, 2009 8:24AM - 8:36AM |
V23.00003: Optical signature of the OCP crystallization in a boron plasma Flavien Lambert, St\'ephane Mazevet, Jean Cl\'erouin We have calculated the DC conductivity of a boron plasma along the 1 eV isotherm up to 25 times the normal density. We use both the quantum and orbital free molecular dynamics coupled with, respectively, the Kubo-Greenwood formulation and the semi-classical Ziman theory. We find that the DC conductivity obtained using a full quantum mechanical treatment exhibits a significant jump at the one component plasma phase transition -- specifically the OCP crystallization -- jump that is not reproduced using the semi-classical Ziman description. This difference -- reaching up to a factor of four -- remains well beyond the phase transition and up to the highest density explored. This shows that a full quantum mechanical treatment of the optical and electrical quantities is required in this regime even if semi-classical theories are reliable to obtain both the thermodynamical, and, ionic dynamical and structural properties. [Preview Abstract] |
Thursday, March 19, 2009 8:36AM - 9:12AM |
V23.00004: Mixtures in the Warm, Dense Matter Regine Invited Speaker: The bulk of normal matter from planets to the intergalactic medium exists as a composite of various elemental constituents. The interactions among these different species determine the basic properties of such diverse environments. For dilute systems, simple gas laws serve well to describe the mixing. However, once the density and temperature increase, more sophisticated treatments of the electronic component and dynamics become necessary. For the warm, dense matter (WDM) region [10$^{22}$-10$^{25}$ atoms/cm$^3$ and 300K - 10$^6$ K], quantum Monte Carlo and molecular dynamics, utilizing finite-temperature density functional theory (DFT), have served as the basic exploratory tools and benchmarks for other methods. The computational intensity of both methods, especially for mixtures, which require large sample sizes to attain statistical accuracy, has focused considerable attention on mixing prescriptions based on the properties of the pure atomic constituents. Though extensively utilized in many disciplines, these rules have received very little verification [1,2]. We examine the validity of two such rules, density and pressure mixing, for several systems and concentrations by comparing against quantum calculations for the fully-interacting composite. We find considerable differences in some regimes, especially for optical properties. We also probe dynamical properties such as diffusion and viscosity as well as the role of impurities. Finally, as a means of extending DFT results to higher temperature regimes, we also study orbital-free molecular dynamics (OFMD) approaches [3] based on various approximations to the basic density functional. These OFMD schemes permit a smooth transition from the WDM region to simpler one-component plasma and ideal gas models. Research in collaboration with J.D. Kress (LANL), D.A. Horner (LANL), and Flavien Lambert (CEA). \\[4pt] [1] D.A. Horner, J.D. Kress, and L.A. Collins, Phys. Rev. B {\bf{77}}, 064102 (2008).\\[0pt] [2] F. Lambert {\em{et. al.}} Phys. Rev. E {\bf{77}}, 026402 (2008); J. Clerouin {\em{et. al.}} Phys. Rev. B {\bf{76}}, 064204 (2007).\\[0pt] [3] F. Lambert, J. Clerouin, and G. Zerah, Phys. Rev. E {\bf{73}}, 016403 (2006). [Preview Abstract] |
Thursday, March 19, 2009 9:12AM - 9:24AM |
V23.00005: Electrical Conductivity of Synthetic Uranus Sebastien Hamel, Eric Schwegler Mixtures of accreted water, ammonia, and methane at high pressures and temperatures are thought to be major components of the giant planets such as Uranus and Neptune. The pressures and temperatures in their deep interiors can reach several Mbar and several thousands Kelvin, conditions corresponding to the fluid phase. At such extreme interior conditions it is expected that these molecules react chemically to produce a complex mixture. Observables properties such as the magnetic field of these planets are thought to be determined by the physical and chemical properties of matter within this water mixture layer. Using quantum molecular dynamics, we explore the properties of water mixtures at planetary conditions. In particular we discuss the electrical conductivity at high pressure and high temperature of those mixtures in comparison to pure water. 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] |
Thursday, March 19, 2009 9:24AM - 9:36AM |
V23.00006: Magnetism and vibrations in the phase $\epsilon$ of Oxygen Tuan Anh Pham, Ralph Gebauer, Sandro Scandolo Sandwiched between a set of magnetic phases at lower pressure, and a non magnetic phase at higher pressure, the magnetic state of phase $\epsilon$ of oxygen has so far been elusive, together with its crystal structure. Neutron diffraction data indicate absence of antiferromagnetism, but do not exclude a ferromagnetic order. The recent refinement of the internal atomic positions from single-crystal diffraction finally provides us with a correct structural model to study the possible occurrence of a magnetic ground state. By employing non-collinear spin-polarized density-functional theory we show that the ground state of $\epsilon$-O$_2$ is non magnetic. We also calculate vibrational spectra and show that $\epsilon$-O$_2$ possesses an additional vibron mode with large Raman cross section, not seen in experiments yet. [Preview Abstract] |
Thursday, March 19, 2009 9:36AM - 9:48AM |
V23.00007: Ab-initio calculations of the X-ray absorption spectra of shocked compressed aluminum Stephane Mazevet, Vanina Recoules, Gilles Zerah Molecular dynamics (MD) simulations, using density functional (DF) electronic structure techniques, provide a powerful, predictive tool for examining materials from solids to plasmas over a wide range of densities and temperatures. Using the Kubo-Greenwood formulation, we can access to the frequency-dependent electrical conductivity as well as additional optical properties consistent with the Equation of State. Due to the use of pseudopotentials, the calculations of these properties have, so far, been limited to low photon frequencies, i.e. below 100eV, where only the valence electrons contribute. We recently extended those calculations into the X-ray domain within the PAW formalism. This allows us to describe for the first time, X-ray absorption spectra of shocked compressed aluminum from an ab-initio standpoint. [Preview Abstract] |
Thursday, March 19, 2009 9:48AM - 10:00AM |
V23.00008: High-pressure spectroscopic studies on solid germane Xiaojia Chen, Viktor V. Struzhkin, Zhen-Xian Liu, Muhetaer Aihaiti, Yue Meng, Ho-kwang Mao, Russell J. Hemley, Chao Zhang, Rui-Qin Zhang, Yanling Li, Hai-Qing Lin We performed extensive spectroscopic studies on dense germane up to 110 GPa. Pressure -- temperature phase diagram has been established from Raman and infrared spectra. There is no any trace of decomposition of Ge and H$_{2}$ over the pressure range studied. Infrared measurements provide spectroscopic evidence of the metallization of this material at pressure around 16 GPa which is much lower than that observed in sister system - silane. Angle-dispersive powder x-ray diffraction studies reveal that only a structural transition is accompanying when germane enters its metallic state. These experimental observations are examined by \textit{ab initio} calculations. The theoretical results of the electronic, lattice dynamical, and superconducting properties of metallic phase of this material are also presented. [Preview Abstract] |
Thursday, March 19, 2009 10:00AM - 10:12AM |
V23.00009: High pressure optical properties of sodium Amy Lazicki, Alexander Goncharov, Viktor Struzhkin, Zhenxian Liu, Eugene Gregoryanz, Christophe Guillaume, Ho-Kwang Mao, Russel Hemley Sodium displays significant complexity at high pressure. The melting temperature drops above a critical pressure, nearly reaching ambient temperature by 120 GPa. In the same pressure regime, phase transitions to low-symmetry and incommensurate structures are observed. Computational studies predict a decrease in the metallic character, and experimental observations have indicated this as well. We present infrared reflectivity for Na through the high pressure incommensurate phase, revealing a significant drop near 120 GPa. First principles calculations of the optical properties are compared to values derived from a Kramer's Kronig analysis of the experimental IR reflectivity, revealing the detailed nature of the pressure-induced deviations from simple metallic behavior. [Preview Abstract] |
Thursday, March 19, 2009 10:12AM - 10:24AM |
V23.00010: Influence of Fermi Surface Topology on Superconductivity in High-Pressure Phases of Silane Tian Cui, Xilian Jin Both a semimetallic molecular phase with P2$_{1}$/m symmetry and a metallic atomic phase with P2$_{1}$/c symmetry are found using \textit{ab initio} geometry optimization method from initial configurations of random molecular SiH$_{4}$ and atoms Si and H, respectively. The molecular phase shows a pressure-induced metallization, which can be described quantitatively by Fermi surface (FS) filling constant defined in our work. The lower superconducting critical temperature (\textit{Tc}) about 16.2 K at 175 GPa and its peculiar superconductive behavior that its \textit{Tc} decreases initially and increases later with pressure agree with a recent experimental results. Different electron-phonon coupling mechanisms are uncovered during the increase and decrease of \textit{Tc} with pressure. The atomic phase shows a higher Tc of about 47 K at 190GPa and its \textit{Tc} increases with pressure in its dynamically stable range. The FS filling constant and FS topology transitions under pressure mostly account for the different superconductivity between the molecular and atomic phases. [Preview Abstract] |
Thursday, March 19, 2009 10:24AM - 10:36AM |
V23.00011: Structural Properties of Superconducting CaLi$_{2}$ At High Pressures Hahnbidt Rhee, Warren Pickett, Richard Scalettar, William Evans, David Young The hexagonal Laves phase of CaLi$_{2}$, a superconductor at high pressures, has been studied in the diamond anvil cell at varying pressures and temperatures. CaLi$_{2}$ is known to have a maximum superconducting transition temperature of 13 K at 40 GPa. X-ray diffraction measurements were done up to 40 GPa, from room temperature down to 10 K, and phase stability in relation to pressure and temperature has been examined. We present our study to provide more insight into phonon-mediated superconductors and simple-metal systems such as Li.\\[4pt] This work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344 and under Contract DE-FG01-06NA26204. HPCAT use is supported by DOE-BES, DOE-NNSA, NSF, and the W.M. Keck Foundation. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 19, 2009 10:36AM - 10:48AM |
V23.00012: Fe K pre-edge of Fe$_{2}$O$_{3}$ at High Pressure Shibing Wang, Wendy Mao, Yong Cai, Nozomu Hiraoka, Hirofumi Ishii, Yang Ding, Yuming Xiao, Paul Chow, Ho-kwang Mao, Jinfu Shu, Chichang Kao Hematite ($\alpha$-Fe$_{2}$O$_{3}$), as an archetypal 3d transition metal oxide and important earth mineral, undergoes a series of electronic transitions and structural changes at high pressure. At ambient conditions, Fe$_{2}$O$_{3}$ adopts the $\alpha$-Al$_{2}$O$_{3}$ structure and is an antiferromagnetic Mott insulator, with five 3d electrons in the high-spin state. Upon increasing pressure, it transforms from a high-spin state to a low-spin state in the 40-70 GPa range. Here we report experimental results for the Fe K-edge spectra of Fe$_{2}$O$_{3}$ collected in-situ at high pressure using synchrotron x-ray absorption spectroscopy in partial fluorescence yield geometry. The pre-edge features give explicit information about the crystal field splitting energy (CFSE) of octahedrally coordinated Fe$^{3+}$ in Fe$_{2}$O$_{3}$ as a function of pressure, mapping the electronic structure (high-spin to low-spin) transition. The K-$\alpha$ emission spectra at high pressure are also presented. [Preview Abstract] |
Thursday, March 19, 2009 10:48AM - 11:00AM |
V23.00013: High Pressure Phase Transitions in FeO from Density Functional Theory, Quantum Monte Carlo and Dynamical Mean Field Theory Luke Shulenburger, Ken Esler, Sergej Savrasov, Jeongnim Kim, R. E. Cohen FeO has a rich behavior under pressure, exhibiting a structural phase transition as well as an insulator-metal transition and a spin collapse. The electronic transitions have been particularly difficult to explain because of the failure of Density Functional Theory (DFT) to capture the electronic state of FeO. We present results from three different methods to better understand the nature of this material. First, from DFT calculations we explore competing explanations for the spin collapse, finding that the increase in bandwidth is at least as important as the crystal field splitting. Additionally, we find that the ligand field effects are responsible for the majority of the change in the local energy levels on the Fe rather than the electrostatic crystal field effect. Secondly, we have performed Dynamical Mean Field Theory (DMFT) calculations. From these we find that the metal insulator transition involves the reorganization of the existing bands and not the appearance of new states at the Fermi level. Finally we test the validity of the approximate results obtained by DFT and DMFT by performing highly accurate Diffusion Monte Carlo calculations. [Preview Abstract] |
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