Energy gap closure of crystalline molecular hydrogen with pressure

TL;DR

This study investigates how the electronic energy gap in crystalline molecular hydrogen closes under high pressure, revealing critical pressures where metallic behavior emerges, and supports recent experimental findings.

Contribution

It introduces a combined quantum Monte Carlo approach to accurately determine gap closure pressures in crystalline hydrogen, accounting for nuclear zero point effects.

Findings

The indirect gap closes between 380-530 GPa for ideal crystals.

Quantum effects lower the closure pressure to 330-380 GPa.

The system transitions to a bad metal phase with increased density of states at the Fermi level.

Abstract

We study the gap closure with pressure of crystalline molecular hydrogen. The gaps are obtained from grand-canonical Quantum Monte Carlo methods properly extended to quantum and thermal crystals, simulated by Coupled Electron Ion Monte Carlo. Nuclear zero point effects cause a large reduction in the gap ($\sim 2eV$). \CP{Depending on the structure,} the fundamental indirect gap closes \CP{between 380GPa and} 530GPa for ideal crystals and 330-380GPa for quantum crystals. Beyond this pressure the system enters into a bad metal phase where the density of states at the Fermi level increases with pressure up to $\sim$450\CP{-500} GPa when the direct gap closes. Our work partially supports the interpretation of recent experiments in high pressure hydrogen.