Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session A23: Focus Session: High Pressure I - Earth and Planetary Materials |
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Sponsoring Units: DMP DCOMP Chair: Choong-shik Yoo, Lawrence Livermore National Laboratory Room: Colorado Convention Center 110 |
Monday, March 5, 2007 8:00AM - 8:12AM |
A23.00001: Characterization of Jupiter's Interior with First Principles Computer Simulations Burkhard Militzer, Jan Vorberger, William Hubbard We report results from recent investigations of the interior structures of Jupiter using density-functional molecular dynamics (DFT) simulations of dense fluid hydrogen-helium mixtures [1]. The equation of state (EOS) is derived on a grid of temperature and density points spanning Jupiter's interiors. The properties of both fluids in dynamic shock compression experiments are compared [2]. Based on the DFT-EOS, we derive models for the interior of giant planets. Our models update the suite of models that were based on the widely used Saumon-Chabrier-Van Horn (SCVH) EOS. Unlike SCVH, the computed DFT-EOS does not predict any first-order thermodynamic discontinuities associated with pressure-dissociation and metallization of hydrogen. Deviations of the DFT-EOS from SCVH are up to about +/- 5{\%} depending on the pressure. As a result our models predict a significantly larger rocky core for Jupiter than SCVH. We will discuss inferred core mass and make predictions for properties of core. [1] J. Vorberger, I. Tamblyn, B. Militzer, S.A. Bonev, ``Hydrogen-Helium Mixtures in the Interiors of Giant Planets,'' cond-mat/0609476. [2] B. Militzer, PRL 97 (2006) 175501. Supported by NASA PGG04-0000-0116 and NSF Grant 0507321. [Preview Abstract] |
Monday, March 5, 2007 8:12AM - 8:24AM |
A23.00002: High pressure bonding properties of hydrogen Isaac Tamblyn, Eric Schwegler, Stanimir Bonev There has been considerable experimental and theoretical effort to describe the transition in hydrogen from a molecular to non-molecular fluid. Resolution of discrepancies that continue to exist between different investigations of hydrogen is expected to have significant implications in fields such as planetary science. We have preformed three sets of first-principles simulations, constant density, pressure, and temperature, in order to study the molecular, non-molecular, and transition regimes of the hydrogen (deuterium) phase diagram. Constrained and unconstrained bond length simulations were used to examine changes that occur in the inter-atomic potential upon disassociation. By forcing the destruction of molecules in the molecular regime, and by considering the catalyzing effect of single hydrogen atoms, we have probed the mechanisms that drive this transition. Finally, spatial distributions of species surrounding molecules at the time of dissociation have provided insight into the structure of the liquid. [Preview Abstract] |
Monday, March 5, 2007 8:24AM - 8:36AM |
A23.00003: Computational study of the Hydrogen equation of state using the Coupled Electron-Ion Monte Carlo method Miguel Morales, Kris Delaney, David Ceperley, Carlo Pierleoni We study the equation of state of liquid Hydrogen at Mbar pressures, in the regime of pressure dissociation/ionization, using the Coupled Electron-Ion Monte Carlo (CEIMC) method. Our aim is to accurately describe the crossover from the molecular to the atomic regime. The CEIMC method is based on the Born-Oppenheimer approximation and consists of a Monte Carlo simulation of the ionic degrees of freedom (either with path integals or classical Metropolis) using a potential energy surface obtained from a zero temperature QMC method. The electronic calculation is done using either Variational Monte Carlo or the more accurate Reptation Quantum Monte Carlo. A Slater-Jastrow wavefunction is used, with an analytical RPA Jastrow term and one-body orbitals obtained from a fast band structure calculation. Recently, we incorporated backflow corrections to the orbitals obtained from DFT. This results in a much improved wavefunction over the entire crossover regime. We report preliminary results using this new wavefunction. We also compare our results with recent calculations obtained using Born-Oppenheimer Molecular Dynamics. [Preview Abstract] |
Monday, March 5, 2007 8:36AM - 8:48AM |
A23.00004: First-principles study of the effect of helium on the onset of dissociation in liquid hydrogen Kyle Caspersen, Francois Gygi, Eric Schwegler The molecular to non-molecular liquid-liquid phase transition that occurs in high-temperature/high-pressure hydrogen has been speculated to be first-order-like, where the onset of dissociation occurs abruptly. However, a small concentration of non-interacting particles, specifically helium, has been postulated to retard and smooth the transition. To study this transition in hydrogen and hydrogen-helium mixtures we performed a series of large-scale Born-Oppenheimer molecular-dynamic simulations. Additionally, we have studied the electronic properties of hydrogen-helium mixtures by using hybrid density functional theory to analyze snapshots from our molecular dynamics simulations. The simulations show that the transition is smooth and continuous without any indication of any first-order-like behavior. The simulations also predict that small concentrations of helium have a significant effect on the phase transition; most notably, the pressure profile is much smoother, and the band gap closes at a higher temperature, for the hydrogen-helium mixtures relative to pure liquid-hydrogen. This work was performed under the auspices of the U.S. Department of Energy by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48. [Preview Abstract] |
Monday, March 5, 2007 8:48AM - 9:00AM |
A23.00005: Equation of state and electronic structure of liquid Helium at high pressure Lars Stixrude, Raymond Jeanloz As the second most abundant element, the properties of fluid Helium form an important part of our understanding of stellar and giant planetary structure. Yet the physics of Helium at pressure-temperature conditions characteristic of these bodies is uncertain. We perform first principles molecular dynamics simulations of fluid Helium over a wide range of pressure ($< 1$ Gbar) and temperature ($< 5$ eV). The simulations are based on finite-temperature density functional theory in the generalized gradient approximation, and are performed in the canonical ensemble with a Nose thermostat. We find that both temperature and compression have a strong influence on the electronic structure as revealed by the band gap. At a density of 1 g cm$^{-3}$ the band gap varies from 20 eV for the static crystal to 0 for the fluid at 4 eV. The gap is closed at all temperatures for density greater than 20 g cm$^{-3}$. We find that the equation of state varies smoothly through the band gap closure transition with no indication of a high-order phase transformation. The decrease in band gap with increasing temperature at constant density results from enhanced mixing of 1s- and 2s-like states with increasing disorder (i.e., enhanced vibrational amplitudes and melting) that has profound implications for understanding the deep interiors of planets. [Preview Abstract] |
Monday, March 5, 2007 9:00AM - 9:12AM |
A23.00006: Liquid-liquid transitions in low-Z materials: Parallel with high-pressure solid phase transitions Stanimir Bonev, Isaac Tamblyn, Adam Chaffey, Jean-Yves Raty First-principles molecular dynamics simulations reported in [1] predict structural and electronic transitions in dense liquid sodium that are responsible for its exotic melting curve. In this talk, the possibility for observing similar behavior in other low-Z materials will be discussed. Results from ab initio calculations of several materials will be presented and compared with sodium. [1] Jean-Yves Raty, Eric Schwegler and Stanimir A. Bonev, submitted. [Preview Abstract] |
Monday, March 5, 2007 9:12AM - 9:24AM |
A23.00007: Probing Dense States of Hydrogen and Oxides in Giant Planets using Multiple and Single- Shock Compression and Laser-Pulse-Heated Diamond-Anvil Cells. W.J. Nellis, I.F. Silvera Pressures and temperatures of hydrogen on adiabats deep in gas giants are achieved using a shock wave reverberating between incompressible oxide anvils and by pulsed heating in a diamond-anvil cell. At 100 GPa in gas giants, temperature varies from $\sim $20,000 K in hot Jupiters down to $\sim $1,000 K in cold Jupiters. The Hugoniot curve of hydrogen crosses these adiabats at $\sim $15 GPa and $\sim $4,000 K for Jupiter and $\sim $100 GPa for hot Jupiters, both at compressions of $\sim $4 fold. Reverberating shocks and diamond cells produce compressions up to $\sim $12 fold. Since dense hydrogen has a huge diffusion coefficient, experiments must be done sufficiently slowly that hydrogen is in thermal equilibrium and sufficiently fast that hydrogen remains in the cell. Dynamic experiments occur within this constraint. DAC experiments require heating by multiple laser pulses each of $\sim $100 ns duration. Pressures and temperatures achieved by multiple shock compression are tuned by variation of the density of oxide anvils. An oxide (Gd$_{3}$Ga$_{5}$O$_{12})$ has been found that is stiffer than diamond above 100 GPa. This oxide will enable higher pressures and lower temperatures in metallic fluid hydrogen by multiple shock and might be representative of new oxide phases in deep interiors of giant extrasolar rocky planets. Experiments and systematics will be described. [Preview Abstract] |
Monday, March 5, 2007 9:24AM - 9:36AM |
A23.00008: Coherent anti-Stokes Raman Spectroscopy Study of Highly Compressed Deuterium Bruce Baer, William Evans, Choong-Shik Yoo High density ($>$ 0.3 mol/cm$^{3})$ hydrogen and its isotopes have been studied intensely over the past three decades. Although many spectroscopic methods have been applied, none utilizes a multiphoton technique. Coherent anti-Stokes Raman Spectroscopy (CARS) has now been applied to samples over one megabar for the first time to accurately determine the density at which the bandgap of deuterium is 4.66 eV. This method yields very precise Raman shifts, linewidths and third order polarizability ratios since it avoids the problems associated with strain induced diamond fluorescence above a megabar. The pressure dependent third order polarizability ratios can indicate the location of the bandgap. We will present evidence for extrapolating the metallization pressure using these results and the implications on the phase diagram. This work has been supported by the LDRD and PDRP programs at Lawrence Livermore National Laboratory, University of California under the auspices of the U.S. Department of Energy under Contract No. W-7405-ENG-48. [Preview Abstract] |
Monday, March 5, 2007 9:36AM - 9:48AM |
A23.00009: Simple Molecular Systems at High Pressures and Temperatures. Alexander Goncharov, Jonathan Crowhurst Knowledge of the elastic, optical and vibrational properties of materials under extreme conditions of high pressure and temperature is crucial for interpreting the results of seismological and planetary observations, for materials science, and for improving our understanding of fundamental physics and chemistry under such conditions. We will present the results of Raman, infrared, and x-ray diffraction measurements of hydrogen, water, nitrogen, and oxygen under conditions of high static pressure and temperature in the diamond anvil cell. High temperatures were generated mainly by laser heating, but also using internal resistive heating. These studies revealed novel phase transitions, complex phase diagrams, unexpected chemical transformations and also helped to established the behavior of interatomic interactions in molecular materials. We thank the following individuals for contributing to this work: N. Goldman, L. Fried, C. Mundy, J. Zaug, R. J. Hemley, E. Gregoryanz, C. Sanloup, M. Somayazulu, Y. Meng, N. Guignot, M. Mezour. [Preview Abstract] |
Monday, March 5, 2007 9:48AM - 10:24AM |
A23.00010: Bonding Changes in Compressed Carbon Dioxide: A New Stishovite-like Phase of CO$_{2}$ Invited Speaker: At ambient conditions, carbon dioxide (CO$_{2})$ is a prototypical molecular system, with strong covalent O=C=O molecular bonds and relatively weak quadrupolar interactions between molecules. At high pressures and temperatures, CO$_{2}$ transforms to a series of solid polymorphs with differing crystal structures, intermolecular interactions and chemical bonding. In particular, two fully covalent (extended) solid phases have been reported above 40GPa, with characteristics analogous to SiO$_{2}$ polymorphs. First, CO$_{2}$-V (above 40GPa and 1500K), consists of a network of corner sharing CO$_{4}$ tetrahedra and is structurally similar to SiO$_{2}$ tridymite$^{1}$. And, recently, an extended-solid amorphous phase (\textit{a-carbonia}), similar to amorphous silica, has been reported at room temperature above 40GPa$^{2}$. Here, we present a new stishovite-like CO$_{2}$ phase VI, formed by compressing CO$_{2}$-II above 50GPa and 550K. We define the PT stability domain for the new solid, and present Raman and X-Ray diffraction results consistent with a 6-fold average coordination within a P42/mnm structure. Finally, we propose a phase/bonding diagram for carbon dioxide describing the systematic relationship between its molecular and extended phases at high pressures and temperatures. 1] V. Iota, \textit{et al.}, Science \textbf{283}, 1510 (1999). 2] M. Santoro,\textit{ et al}. Nature \textbf{441}, 857 (2006). [Preview Abstract] |
Monday, March 5, 2007 10:24AM - 10:36AM |
A23.00011: Phase diagram of Nitrogen at high pressures and temperatures Zsolt Jenei, Jung-Fu Lin, Choong-Shik Yoo Nitrogen is a typical molecular solid with relatively weak van der Waals intermolecular interactions but strong intramolecular interaction arising from the second highest binding energy of all diatomic molecules. The phase diagram of solid nitrogen is, however, complicated at high pressures, as inter-molecular interaction becomes comparable to the intra-molecular interaction. In this paper, we present an updated phase diagram of the nitrogen in the pressure-temperature region of 100 GPa and 1000 K, based on in-situ Raman and synchrotron x-ray diffraction studies using externally heated membrane diamond anvil cells. While providing an extension of the phase diagram, our results indicate a ``steeper'' slope of the $\delta $/$\varepsilon $ phase boundary than previously determined$^{1}$. We also studied the stability of the $\varepsilon $ phase at high pressures and temperatures. Our new experimental results improve the understanding of the Nitrogen phase diagram. 1. Gregoryanz et al, Phys. Rev. B 66, 224108 (2002) [Preview Abstract] |
Monday, March 5, 2007 10:36AM - 10:48AM |
A23.00012: Theoretical precursors to polymeric nitrogen Razvan Caracas, Russell J. Hemley We predict the existence of new structures of nitrogen based on new observations in analog systems from first-principles density-functional calculations. A series of structures was examined. A structure with orthorhombic symmetry is stable relative to the $\epsilon$ and {\em cubic gauche} phases in LDA, whereas GGA shows the $\epsilon$ and the new orthorhombic structure are energetically competitive. This structure is dynamically stable at least from ambient pressure to 90 GPa and thus may be observed as a stable or metastable polynitrogen phase prior to the transition to the atomic phases of nitrogen. [Preview Abstract] |
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