Pathways to metallic hydrogen

Abstract

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 hydrogen.

Optical properties of high-pressure fluid hydrogen across molecular dissociation

“…The emergence of the metallic state was detected by a direct measure of sample resistivity, but they were unable to make a clear characterization of the insulator-metal (IM) transition [3]. Recent static compression experiments, using a Diamond Anvil Cell (DAC) with controlled laser heating (short-pulses DAC, sp-DAC) [7,8,[12][13][14], studied liquid hydrogen and deuterium measuring both temperature and pressure. An absorber was heated with short laser pulses; the heat was transferred to the sample by thermal conduction.…”

Melting of cryocrystals at high pressures. Computer simulation

“…Just after first maximum of the melting temperature predicted at P ~ ~ 170 GPa, there appears an apparent flattening (or even a notch) located just at pressure around 200 GPa. According to estimation of Silvera and Deemyad [29], the phase Iphase III polymorph transition line crosses the predicted melting line just in this region. Taking into account the fact that AAP model reproduces the orientational ordering of hydrogen molecules in solid phase [13], one may speculate that the right part of the melting line at P > 200 GPa corresponds to the melting of a more orientationally ordered phase of the solid hydrogen.…”

Pressure-induced superconductivity of Ac–B–H hydrides

“…To find most stable structure for each system, 100 GA generations were carried out. Then, the accurate enthalpies of the GA predicted structures were calculated via re-optimization using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional, 35,36 and ultrasoft pseudopotential implemented in the Cambridge Serial Total Energy Package (CASTEP) module; [37][38][39][40][41] the atomic configuration is 1s 1 (H), 2s 2 2p 1 (B), and 6s 2 p 6 d 1 7s 2 (Ac). A k-point density in the Brillouin zone of 2p  0.03 Å À1 and a cut off energy of 1000 eV for the plane wave basis were set.…”