A survey of some outstanding questions in dense hydrogen

Abstract

Dense hydrogen, sometimes thought of as the simplest condensed matter system, continues to be a cautionary tale against overconfidence in the predictive powers of condensed matter theory. Recent experimental advances using diamond anvil cells (DACs), and single and multi-shock compression all present new challenges to theory, and old questions such as the ground state insulator to metal transition pressure remain almost as unsettled as when originally proposed by Wigner and Huntington 60 years ago. This article gives an overview of efforts to understand two areas of the hydrogen phase diagram: the low to room temperature region probed by DAC experiments and the 0.5–2.0 Mbar pressure, 0.5–3.0 e.v. temperature range now being explored in deuterium using laser-generated shocks. The aim is to provide nonspecialists with a guide to this interesting and active field.