1. Lawrence Livermore National Laboratory, University of California, Livermore, California 94550, USA

Correspondence to: Stanimir A. Bonev¹ Email: bonev1@llnl.gov

Abstract

It is generally assumed^{1, 2, 3} that solid hydrogen will transform into a metallic alkali-like crystal at sufficiently high pressure. However, some theoretical models^{4, 5} have also suggested that compressed hydrogen may form an unusual two-component (protons and electrons) metallic fluid at low temperature, or possibly even a zero-temperature liquid ground state. The existence of these new states of matter is conditional on the presence of a maximum in the melting temperature versus pressure curve (the 'melt line'). Previous measurements^{6, 7, 8} of the hydrogen melt line up to pressures of 44 GPa have led to controversial conclusions regarding the existence of this maximum. Here we report ab initio calculations that establish the melt line up to 200 GPa. We predict that subtle changes in the intermolecular interactions lead to a decline of the melt line above 90 GPa. The implication is that as solid molecular hydrogen is compressed, it transforms into a low-temperature quantum fluid before becoming a monatomic crystal. The emerging low-temperature phase diagram of hydrogen and its isotopes bears analogies with the familiar phases of ³He and ⁴He (the only known zero-temperature liquids), but the long-range Coulomb interactions and the large component mass ratio present in hydrogen would result in dramatically different properties⁹.

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