Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D26: Materials at High Pressure: H plus |
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Sponsoring Units: DCOMP DMP GSCCM Chair: Kevin Driver, University of California, Berkeley Room: 502 |
Monday, March 3, 2014 2:30PM - 2:42PM |
D26.00001: Quantum Monte Carlo simulations of high pressure solid hydrogen Jonathan Lloyd-Williams, Bartomeu Monserrat, Pablo Lopez Rios, Neil Drummond, Richard Needs Several solid phases of hydrogen have been observed but the stable structures of hydrogen at high pressure remain experimentally undetermined because of the weak scattering of protons in x-ray diffraction studies. Theoretical identification of the structures is also difficult because of the small energy differences between competing phases and the large zero-point (ZP) contributions to the energies. We have performed static-nucleus diffusion Monte Carlo calculations for the candidate high pressure phases across a range of relevant densities and added ZP energies from both harmonic and anharmonic density functional theory calculations. We have used our calculated total energies to construct an enthalpy-pressure phase diagram from which we have evaluated the relative stability of the candidate structures. \\[4pt] This work was facilitated by a 2013 INCITE award of computing resources on Titan at Oak Ridge National Laboratory. It also made use of facilities provided by the High Performance Computing Service at the University of Cambridge and the N8 High Performance Computing Service which is coordinated by the Universities of Leeds and Manchester. Financial support was provided by the Engineering and Physical Sciences Research Council (UK). [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D26.00002: Graphene physics and insulator-metal transition in compressed hydrogen Ivan I. Naumov, R.E. Cohen, Russell J. Hemley As established recently both theoretically and experimentally, compressed hydrogen passes through a series of layered structures in which the layers can be viewed as distorted graphene sheets. These structures and their electronic properties can be understood by studying simple model systems-(i) a H$_6$ ring, (ii) an ideal single hydrogen graphene sheet and (iii) three-dimensional model lattices consisting of such sheets [1]. The energetically stable structures result from structural distortions of model graphene-based systems due to electronic instabilities towards Peierls or other distortions associated with the opening of a bandgap. Two factors play crucial roles in the metallization of compressed hydrogen: (i) crossing of conduction and valence bands in hexagonal or grapheme-like layers due to topology and (ii) formation of bonding states with 2p${_z}$ $\pi$ character.\\[4pt] [1] I. I. Naumov, R. E. Cohen and R. J. Hemley Phys. Rev. B, {\bf 88}, 045125 (2013). [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D26.00003: Optical Signature of Metallization of Hydrogen R.E. Cohen, Ivan Naumov, Russell J. Hemley All proposed high-pressure structures of hydrogen are based on distorted graphene-structured, honeycomb layers. These give unique signatures for metallization and optical response [1,2]. Theoretical calculations and an assessment of recent experimental results for dense solid hydrogen lead to a unique scenario for the metallization of hydrogen under pressure. The metallization of hydrogen is very different from that originally proposed via a phase transition to a close-packed monoatomic structure, and different from simple metallization recently used to interpret recent experimental data. These different mechanisms for metallization have very different experimental signatures. We show that the shift of the main visible absorption edge does not constrain the point of band gap closure, in contrast with recent claims. This conclusion is confirmed by measured optical spectra, including spectra obtained to low photon energies in the infrared region for phases III and IV of hydrogen. This work was supported as part of EFree, an Energy Frontier Research Center funded by the US Department of Energy.\\[4pt] [1] Naumov, I.I., R. E. Cohen, and R. J. Hemley, PRB {\bf 88}, 045125 (2013).\\[0pt] [2] Cohen, R. E., I. I. Naumov, and R. J. Hemley, PNAS {\bf 110} 13757 (2013). [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:42PM |
D26.00004: High-Pressure Hydrogen from First-Principles Invited Speaker: Miguel A. Morales The main approximations typically employed in first-principles simulations of high-pressure hydrogen are the neglect of nuclear quantum effects (NQE) and the approximate treatment of electronic exchange and correlation, typically through a density functional theory (DFT) formulation. In this talk I'll present a detailed analysis of the influence of these approximations on the phase diagram of high-pressure hydrogen, with the goal of identifying the predictive capabilities of current methods and, at the same time, making accurate predictions in this important regime. We use a path integral formulation combined with density functional theory, which allows us to incorporate NQEs in a direct and controllable way. In addition, we use state-of-the-art quantum Monte Carlo calculations to benchmark the accuracy of more approximate mean-field electronic structure calculations based on DFT, and we use GW and hybrid DFT to calculate the optical properties of the solid and liquid phases near metallization. We present accurate predictions of the metal-insulator transition on the solid, including structural and optical properties of the molecular phase. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D26.00005: Quasiparticle energies and excitonic effects of solid hydrogen under ultrahigh pressures Zhigang Wu, Marc Dvorak, Xaojia Chen We investigate the insulator-to-metal transition in the crucial $Cmca$-12 phase of solid hydrogen employing the many-body perturbation theory with Green's functions. In particular quasiparticle energies are calculated within the $GW$ approximation to accurately determine the insulator-to-metal transition pressure. We consider the effects of self-consistency, plasmon-pole models to the dielectric function, off-diagonal elements of the self-energy, and vertex corrections on $GW$ calculations, and our results show that the band gap of the $Cmca$-12 phase of solid hydrogen is sensitive to the choice of $GW$ procedures and approximations involved, leading to a change of $\sim 20$ GPa in transition pressure. We also compute the optical absorption and electron-hole binding energy by solving the Bethe-Salpeter equation, and the resulting optical absorption shows a redshift and enhancement of absorption peaks compared to the GW-RPA absorption with excitonic effects omitted. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D26.00006: Rotation of water molecules in plastic phase at extreme conditions from first principles molecular dynamics method Tomofumi Tasaka, Kazuo Tsumuraya Water has a variety of polymorphs in wide ranges of temperature and pressure. Ice VII phase transforms to ice X with increased pressure. However the ice VII transforms to a superionic phase at higher temperatures around 2000K and pressure 30GPa in which the protons migrate in the body centered cubic lattice of oxygens. The ice VII transforms into rotator phase (so called plastic phase at lower temperatures around 600K and 5 to 50GPa. The formation of the phase has been confirmed only with the empirical potentials, whereas the experimental confirmation has been postponed until now. The present study elucidates the mechanism of the rotation of the water molecules and the correlation between the molecules during the rotation with the first principles molecular dynamics method. The water molecules rotate around each oxygen atom to conserve the ice VII positions of the protons. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D26.00007: The Phase Diagram of Superionic Ice Jiming Sun, Bryan Clark, Roberto Car Using the variable cell Car-Parrinello molecular dynamics method, we study the phase diagram of superionic ice from 200GPa to 2.5TPa. We present evidence that at very high pressure the FCC structure of the oxygen sublattice [1] may become unstable allowing for a new superionic ice phase, in which the oxygen sublattice takes the P21 structure found in zero-temperature total energy calculations [2]. We also report on how the melting temperature of the hydrogen sublattice is affected by this new crystalline structure of the oxygen sublattice.\\[4pt] [1] Hugh F. Wilson, Michael L. Wong, and Burkhard Militzer. Superionic to superionic phase change in water: Consequences for the interiors of uranus and neptune. Phys. Rev. Lett.,110:151102, Apr 2013. \\[0pt] [2] Andreas Hermann, N. W. Ashcroft, and Roald Hoffmann. High pressure ices. Proceedings of the National Academy of Sciences, 109(3):745-750, 2012. [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D26.00008: Dielectric Properties of Water Under Extreme Conditions Invited Speaker: Ding Pan Water is a major component of fluids in the Earth's mantle, where its properties are substantially different from those at ambient conditions. At the pressures and temperatures of the mantle, experiments on aqueous fluids are challenging, and several fundamental properties of water are poorly known; e.g., its dielectric constant has not been measured. This lack of knowledge of water dielectric properties has greatly limited our ability to model water-rock interactions and, in general, our understanding of aqueous fluids below the Earth's crust. Using ab initio molecular dynamics, we computed the dielectric constant of water under the conditions of the Earth's upper mantle, and we predicted the solubility products of carbonate minerals [1]. We found that MgCO$_3$ (magnesite)---insoluble in water under ambient conditions---becomes at least slightly soluble at the bottom of the upper mantle, suggesting that water may transport significant quantities of oxidized carbon. We also computed the electronic dielectric constant of water as a function of pressure [2] and we found that, contrary to expectations based on widely used simple models, both the refractive index and the electronic band gap of water increase under pressure. \\[4pt] [1] D. Pan, L. Spanu, B. Harrison, D. A. Sverjensky and G. Galli, Proc. Natl. Acad. Sci. U. S. A. 110, 6646 (2013)\\[0pt] [2] D. Pan. Q. Wan, G. Galli (submitted for publication) [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D26.00009: Ab initio Simulations of Fluid and Superionic Water in the Interiors of Uranus and Neptune Burkhard Militzer, Shuai Zhang Water is one of the most prevalent substances in our solar system. Large quantities are assumed to be stored in the interiors of ice giant planets. Water has an unusually rich phase diagram with 15 solid phases that were determined experimentally and 6 additional ones that were predicted theoretically at high pressure. Water is predicted to assume a superionic state where the oxygen ions remain confined to specific lattice sites while the hydrogen ions move through the crystal structure like a fluid. In our recent article [Physical Review Letters 110 (2013) 151102], we predicted the oxygen sub-lattice to assume a face-centered cubic structure at pressures above 1 Mbar. For this presentation, we extended our density functional molecular dynamics simulations in order to determine the equation of state of fluid and superionic water. We employed a thermodynamics integration technique to derive the entropy and the Gibbs free energy of both phases. We discuss how a novel superionic state could be identified in high pressure experiments and talk about the implications for the interiors of Uranus and Neptune. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D26.00010: Anharmonic vibrational properties of solids and the metallization of solid helium Bartomeu Monserrat, Neil D. Drummond, Chris J. Pickard, Richard J. Needs We describe a first-principles method for the calculation of anharmonic vibrational properties in solids. The method is based on a principal axes mapping of the Born-Oppenheimer energy surface and the vibrational self-consistent field scheme, and it allows us to calculate, amongst other quantities, the anharmonic free energy, the band gap renormalizations due to electron-phonon coupling, and the vibrational stress tensor. We exemplify the method by determining the effects of electron-phonon coupling and thermal expansion on the metallization of solid helium. Our results have implications for the cooling of white dwarf stars and suggest a revision of current lower bounds to the age of the Universe as determined within cosmochronology. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D26.00011: Structure and Metallization of Hydrogen Iodide Stanimir Bonev, Vahid Askarpour The structure and the metallization mechanism of hydrogen iodide under pressure are investigated using GW and hybrid density functional theory methods with exact exchange. The band gap closure is explained in terms of overlapping iodine $p$ orbitals and a close correlation is shown to exist between evolving structural and electronic changes. The metallization transition in phase III of hydrogen iodide is determined to take place between 20 and 25 GPa at zero temperature. This result differs significantly from existing experimental data. [Preview Abstract] |
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