Quantum simulation of low-temperature metallic liquid hydrogen

TL;DR

This study uses advanced quantum simulations to explore low-temperature metallic liquid hydrogen at high pressures, revealing a stable atomic liquid phase at temperatures as low as 50 K, with quantum proton motion being crucial.

Contribution

First ab initio path-integral molecular dynamics study demonstrating quantum effects enable low-temperature metallic liquid hydrogen at high pressures.

Findings

Atomic solid phase melts below 200 K at 500-800 GPa

Metallic atomic liquid stable at 50 K up to 1200 GPa

Classical simulations overestimate melting temperature by ~300 K

Abstract

Experiments and computer simulations have shown that the melt-ing temperature of solid hydrogen drops with pressure above about 65 GPa, suggesting that a liquid state might exist at low temperatures. It has also been suggested that this low temperature liquid state might be non-molecular and metallic, although evidence for such behaviour is lacking. Here, we report results for hydrogen at high pressures using ab initio path-integral molecular dynamics methods, which include a description of the quantum motion of the protons at finite temperatures. We have determined the melting temperature as a function of pressure by direct simulation of the coexistence of the solid and liquid phases and have found an atomic solid phase from 500 to 800 GPa which melts at <200 K. Beyond this and up to pressures of 1200 GPa a metallic atomic liquid is stable at temperatures as low as 50 K. The quantum motion of the protons is critical to the low melting temperature in this system as ab initio simulations with classical nuclei lead to a considerably higher melting temperature of \sim300 K across the entire pressure range considered.