A quantum fluid of metallic hydrogen suggested by first-principles calculations

TL;DR

This study uses first-principles calculations to explore the melting behavior of hydrogen under high pressure, predicting a transition to a quantum fluid phase before becoming a metallic crystal, with implications for understanding hydrogen's exotic states.

Contribution

It provides the first ab initio predictions of hydrogen's melt line up to 200 GPa, revealing a decline above 90 GPa and suggesting a low-temperature quantum fluid phase.

Findings

Melt line predicted up to 200 GPa with a maximum around 90 GPa.

Hydrogen transforms into a quantum fluid before crystallizing.

Implications for exotic low-temperature phases of hydrogen and isotopes.

Abstract

It is generally assumed that solid hydrogen will transform into a metallic alkali-like crystal at sufficiently high pressure. However, some theoretical models 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 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 Coulombic interactions and the large component mass ratio present in hydrogen would ensure dramatically different properties