Session K42: Focus Session: Planetary Materials II

Shock Hugoniot Calculations of Dense Liquid Helium

By combining two first-principles computer simulation techniques, path integral Monte Carlo and density functional molecular dynamics, the properties of dense liquid helium are studied. From the equation of state we derive the shock Hugoniot curves. Results at low pressures agree well with gas gun experiments. For higher pressures, we predict that helium is compressible to more than five times its initial density. This behavior is in contrast to hydrogen, for which we predicted a maximum compression ratio of only 4.25. In the case of helium, the conditions for 5-fold compression are attainable with existing shock facilities. Studying this material will enable us to verify new experimental and theoretical techniques, and may help us to understand the existing controversy in the experimental shock results for deuterium. The characterization of dense liquid hydrogen and helium allows us to build models describing the interior of Giant Planets and answer fundamental questions of their evolution.   

Simulations of dense hydrogen and hydrogen-helium mixtures at conditions relevant to gas planet interiors

The principle components of all gas giants are hydrogen and helium. In order to improve models describing the formation and evolution of planets such as Jupiter and Saturn, we investigate the properties of these materials under extreme conditions. \textit{Ab initio} molecular dynamics simulations are performed on both pure hydrogen and hydrogen-helium mixtures. The equation of state and structural properties of these liquids are determined at characteristic temperatures, pressures, and mixing ratios relevant for the interior of Jupiter-like planets. Results are compared with previous investigations, both experimental and theoretical, with improvements highlighted. In particular, effects originating from the dissociation of molecular hydrogen are discussed.   

Hydrogen-Helium Mixtures under Giant Gas Planet Conditions

We use density functional molecular dynamic simulations to investigate equilibrium properties of mixtures of hydrogen and helium. We consider a range of temperature, density and mixing ratio which enables us to study the equation of state for these mixtures under extreme conditions relevant, e.g., for giant gas planets. We focus on the atomic and molecular phase of hydrogen helium mixtures. We consider the structure of the liquid and how the presence of helium influences the bond length and suppresses the dissociation of the hydrogen molecules. We present binary distribution functions illustrating that helium leads to lower dissociation in the system. We will demonstrate constraints concerning the validity of the linear mixing rule. We will show comparisons to different methods and present results for Jupiters isentrope.   

Equation of state and electronic structure of liquid Helium at high pressure

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.   

Raman spectroscopy of hot compressed hydrogen and nitrogen - implications for the intramolecular potential

Raman measurements of molecular hydrogen, deuterium, and nitrogen have been made under simultaneous conditions of high temperature and high static pressure. Measurements have been made on hydrogen and deuterium to 50 GPa and 1600 K, and on nitrogen to 50 GPa and 2000 K. In all three materials the familiar molecular stretching mode (vibron) is accompanied in the high temperature Raman spectra by one or more lower frequency peaks due to transitions from excited vibrational states. We find the frequency differences between these bands decreases with pressure, implying that the anharmonicity of the corresponding part of the intramolecular potential also decreases. This is accompanied by an increase in the measured line widths of the bands that is consistent with a decrease of the depth of the potential and an approaching molecular dissociation.   

First-principles study of the effect of helium on the onset of dissociation in liquid hydrogen

The onset of molecular dissociation in liquid hydrogen under high-pressures is known to occur abruptly, possibly involving a first-order liquid-liquid phase transition [1,2]. We have examined this transition in detail by performing a series of large-scale first-principles molecular dynamics simulations of liquid hydrogen and mixtures of hydrogen with small concentrations of helium. In addition, we have examined the electronic properties of hydrogen-helium mixtures by using hybrid density functional theory to analyze snapshots from our molecular dynamics simulations. 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. [1] S. Scandolo, PNAS 100, 3051 (2003). [2] S.A. Bonev, et al. Phys. Rev. B 69, 014101 (2004).   

Reentrant Phase Diagram in Ortho-Para Mixtures of Solid H$_{2}$ at High Pressure

Quantum effects dominate the low temperature phase diagram of solid molecular hydrogen in a wide range of pressures from ambient up to about 100 GPa. Important differences exist in the behavior of pure even-$J $(para-H$_{2}$ and ortho-D$_{2})$, and odd-$J$ (ortho-H$_{2}$ and para-D$_{2})$ species, but little is known about the phase diagram of ortho-para mixtures. We develop a multiorder parameter mean-field formalism for systems of coupled quantum rotors and apply it to solid H$_{2}$ and D$_{2}$. For a thermal distribution of ortho-para molecules we find an anomalous reentrant orientational phase transition in the pressure - temperature phase diagram of both systems [Hetenyi et al., PRL 94, 125503 (2005)]. The correlation functions of the order parameter indicate short-range order at low temperatures. As the temperature is increased the correlation increases along the phase boundary. We also find that even extremely small \textit{odd}-$J $concentrations (1{\%}) can trigger short-range orientational ordering.   

Observability of a projected new state of matter: a metallic superfluid hydrogen

Dissipationless quantum states, such as superconductivity and superfluidity, have attracted interest for almost a century. A variety of systems exhibit these macroscopic quantum phenomena, ranging from superconducting electrons in metals to superfluid liquids, atomic vapours, and even large nuclei. It was recently suggested that liquid metallic hydrogen could form two new unusual dissipationless quantum states, namely the metallic superfluid and the superconducting superfluid. Liquid metallic hydrogen is projected to occur only at an extremely high pressure of about 400 GPa, while pressures on hydrogen of 320 GPa having already been reported. The issue to be adressed is if this state could be experimentally observable in principle. We propose experimental probes for detecting it and discuss recent developments in superconducting/superfluid properties of the projected metallic state of hydrogen or its isotopes.   

Observation of a metallic superfluid in a numerical experiment

We report the observation, in Monte Carlo simulations, of a novel type of quantum ordered state: {\it the metallic superfluid}. The metallic superfluid features ohmic resistance to counter-flows of protons and electrons, while featuring dissipationless co-flows of electrons and protons. One of the candidates for a physical realization of this remarkable state of matter is hydrogen or its isotopes under high compression. This adds another potential candidate to the presently known quantum dissipationless states, namely superconductors, superfluid liquids and vapours, and supersolids.   

The melting of water under pressure

We have investigated the melting of water under high-pressure conditions with a series of first-principles molecular dynamics simulations. In particular, the two-phase approach [1] has been used to determine the melting temperature of water under pressures ranging from 10 to 50 GPa. The effect of molecular dissociation on the structural, dynamical and melting properties of water will be discussed in detail. [1] T. Ogitsu, E. Schwegler, F. Gygi and G. Galli, \textit{Phys. Rev. Lett}. 91, 175502 (2003)   

Scaling fields and equation of state near the liquid-liquid critical point in supercooled water

We have developed a scaled parametric equation of state to describe and predict thermodynamic properties of water in supercooled conditions. The equation of state is built on the assumption that in the supercooled water an additional critical point, the critical point of liquid-liquid separation, does exist. Although this second critical point of water is not accessible experimentally, the pre-critical anomalies affect thermodynamic and transport properties of water in the metastable and even in stable regions and can be observed experimentally. Our approach is based on the principle of critical-point universality. The equation of state is universal in terms of theoretical scaling fields and belongs to the three-dimensional Ising-model class of universality. The theoretical scaling fields are postulated to be analytical combinations of physical fields (pressure and temperature). The proposed equation of state enables us to accurately locate the ``Widom line'' (the locus of stability minima) and the position of the critical point, as well as to predict the thermodynamic properties in the regions that are not accessible to experiments.   

The metastable limit of isentropically compressed water

Although freezing is normally a slow process, it can be observed on very short time scales using isentropic compression techniques. For isentropic compression beyond 2 GPa, liquid water becomes metastable with respect to the ice VII phase and can freeze on nanosecond time scales if heterogeneous nucleation sites are present [D.H. Dolan et al., J. Chem. Phys. \textbf{123}, 64702 (2005)]. Such nucleation sites are typically found on the surfaces of crystalline and amorphous silica windows used to compress a water sample; in the absence of such windows, water remains in a metastable liquid state for some time. Recent gas gun and Z machine experiments at Sandia National Laboratories suggest a sharp metastable limit for isentropically compressed water, beyond which the liquid phase rapidly transforms to a solid without the aid of a nucleating window. This rapid transition is expected because the liquid phase is increasingly unfavorable at high pressure, but has not been previously observed. Comparison of the new freezing observations with prior results reveals stark qualitative differences, suggesting that this newly observed freezing is very different from a heterogeneously nucleated transition.   

Infrared study of high-pressure HD: observation of the A-phase

Infrared absorption was used to investigate the phase diagram of solid HD up to pressures of 156 GPa at temperatures ranging from 4 to 200K. A re-entrant phase line between the low-pressure and broken-symmetry phases (BSP) was observed, in agreement with Raman scattering results, with a 0 K transition pressure of $\sim $ 65-70 GPa. A phase transition was observed with an onset at $\sim $154 GPa (at 5K), consistent with the transition to the A-phase, previously observed in H$_{2}$ and D$_{2}$. The infrared spectra in the compressed low-pressure and BSP phases complement existing Raman spectra in these phases.   

Water in MgSiO$_{3}$ melt at high pressure

The presence of water is thought to play important role in modifying the equilibrium and transport properties of earth materials including silicate liquids. In our study we want to address the following questions: How does the presence of water modifies density of melts? What is the partial molar volume of water in melts? What structural species of H$_{2}$O are present at high pressure? In order to address these issues we explore the high-pressure behaviour of hydrous MgSiO$_{3}$ melt with 11 wt{\%} H$_{2}$O, using first principles molecular dynamics simulation, based on local density approximation (LDA) and the plane-wave-pseudopotential method. The simulations are performed in the canonical ensembles with periodic boundary conditions and a Nose' thermostat. Melting was confirmed by the radial distribution function displaying no long-range order. By comparing our results with that of the anhydrous counterpart (Stixrude and Karki, 2005), we find that the partial molar volume of water decreases along the 3000 K isotherm from 20 cc/mol at 2 GPa to 6.6 cc/mol at 80 GPa. The water component is substantially more compressible than the silicate component. The partial molar volume of water is much less than the volume of pure water at the same conditions (Pitzer and Sterner, 1994), indicating a difference in structure. Analysis of our simulations shows a range of H$_{2}$O species including hydroxyls, water molecules and H-O-H-O groups.   

Optical studies of compressed silane up to 40 GPa

Under sufficiently strong compression, hydrogen is believed to be a metal and eventual superconductor with high transition temperatures. Despite an unrelenting experimental assault at ultra-high pressures, dense solid hydrogen has so far defined all attempts at metallization. Recently, Ashcroft suggested that the dense group IVa hydrides would undergo a transition to eventual metallic and superconducting states at pressures considerably lower than may be necessary for hydrogen. We have performed the vibrational study of silane at high pressures up to 31.6 GPa by Raman spectroscopy. By using the fully symmetrical stretching mode as a probe for exploring the phase transition, we find one fluid-solid transition at around 4.0 GPa and two solid-sold transitions near 6.5 GPa and 26.5 GPa at 300 K. After 26.5 GPa, the solid silane becomes opaque. Our x-ray diffraction data also confirmed the high-pressure phase transitions. Moreover, the observed high-pressure structure is irreversible. Although there is no evidence for possible metallization in this pressure regime from our IR measurements, the observation of black hydrogen in solid silane is significant since the pressure used is ten times smaller than that in solid hydrogen.