Conductivity and Dissociation in Metallic Hydrogen: Implications for Planetary Interiors

Liquid metallic hydrogen (LMH) was recently produced under static compression and high temperatures in bench-top experiments. Here, we report a study of the optical reflectance of LMH in the pressure region of 1.4-1.7 Mbar and use the Drude free-electron model to determine its optical conductivity. We find static electrical conductivity of metallic hydrogen to be 11,000-15,000 S/cm. A substantial dissociation fraction is required to best fit the energy dependence of the observed reflectance. LMH at our experimental conditions is largely atomic and degenerate, not primarily molecular. We determine a plasma frequency and the optical conductivity. Properties are used to analyze planetary structure of hydrogen rich planets such as Jupiter

Diamond: Molten under Pressure

Physic

Finite element simulation of the liquid-liquid transition to metallic hydrogen

Hydrogen at high temperature and pressure undergoes a phase transition from a liquid molecular phase to a conductive atomic state, or liquid metallic hydrogen, sometimes referred to as the plasma phase transition (PPT). The PPT phase line was observed in a recent experiment studying laser-pulse heated hydrogen in a diamond anvil cell in the pressure range $\sim 100 - 170 \text{GPa}$ for temperatures up to $\sim 2000 \text{K}$. The experimental signatures of the transition are (i) a negative pressure-temperature slope, (ii) a plateau in the heating curve, assumed to be related to the latent heat of transformation, and (iii) an abrupt increase in the reflectance of the sample. We present a finite element simulation that accurately takes into account the position and time dependence of the heat deposited by the laser pulse. We calculate the heating curves and the sample reflectance and transmittance. This simulation confirms that the observed plateaus are related to the phase transition, however we find that large values of latent heat are needed and may indicate that dynamics at the transition are more complex than considered in current models. Finally, experiments are proposed that can distinguish between a change in optical properties due to a transition to a metallic state or due to closure of the bandgap in molecular hydrogen.Comment: 23 pages, 4 figure

Pathways to Metallic Hydrogen

The traditional pathway that researchers have used in the goal of producing atomic metallic hydrogen is to compress samples with megabar pressures at low temperature. A number of phases have been observed in solid hydrogen and its isotopes, but all are in the insulating phase. The results of experiment and theory for this pathway are reviewed. In recent years a new pathway has become the focus of this challenge of producing metallic hydrogen, namely a path along the melting line. It has been predicted that the hydrogen melt line will have a peak and with increasing pressure the melt line may descend to zero Kelvin so that high pressure metallic hydrogen may be a quantum liquid. Even at lower pressures hydrogen may melt from a molecular solid to an atomic liquid. Earlier attempts to observe the peak in the melting line were thwarted by diffusion of hydrogen into the pressure cell components and other problems. In the second part of this paper we present a detailed description of our recent successful demonstration of a peak in the melting line of hydrogenPhysic

The Melting Line of Hydrogen at High Pressures

The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature vs. pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and final peak in the melting line at $P=64.7 \pm 4$ GPa and $T=1055 \pm 20$ K.Physic

Electron Emission in Superfluid and Low-temperature Vapor Phase Helium

Tungsten filaments used as sources of electrons in a low temperature liquid or gaseous helium environment have remarkable properties of operating at thousands of degrees Kelvin in surroundings at temperatures of order 1 K. We provide an explanation of this performance in terms of important changes in the thermal transport mechanisms. The behavior can be cast as a first-order phase transition.Comment: 12 pages, 3 figure

The Melting Line of Hydrogen at High Pressures

The insulator to metal transition in solid hydrogen was predicted over 70 years ago but the demonstration of this transition remains a scientific challenge. In this regard, a peak in the temperature vs. pressure melting line of hydrogen may be a possible precursor for metallization. However, previous measurements of the fusion curve of hydrogen have been limited in pressure by diffusion of hydrogen into the gasket or diamonds. To overcome this limitation we have used an innovative technique of pulsed laser heating of the sample and final peak in the melting line at P=64.7+-4GPa and T=1055+-20 K.Comment: 11 page

Metallic Hydrogen: The Most Powerful Rocket Fuel Yet To Exist

Wigner and Huntington first predicted that pressures of order 25 GPa were required for the transition of solid molecular hydrogen to the atomic metallic phase. Later it was predicted that metallic hydrogen might be a metastable material so that it remains metallic when pressure is released. Experimental pressures achieved on hydrogen have been more than an order of magnitude higher than the predicted transition pressure and yet it remains an insulator. We discuss the applications of metastable metallic hydrogen to rocketry. Metastable metallic hydrogen would be a very light-weight, low volume, powerful rocket propellant. One of the characteristics of a propellant is its specific impulse, $I_{sp}$. Liquid (molecular) hydrogen-oxygen used in modern rockets has an Isp of $\sim460s$; metallic hydrogen has a theoretical $I_{sp}$ of 1700s! Detailed analysis shows that such a fuel would allow single-stage rockets to enter into orbit or carry economical payloads to the moon. If pure metallic hydrogen is used as a propellant, the reaction chamber temperature is calculated to be greater than 6000 K, too high for currently known rocket engine materials. By diluting metallic hydrogen with liquid hydrogen or water, the reaction temperature can be reduced, yet there is still a significant performance improvement for the diluted mixture.Physic