Semimetallic molecular hydrogen at pressure above 350 GPa

TL;DR

This study provides experimental evidence that molecular hydrogen exhibits semimetallic behavior at pressures around 360 GPa, with conductivity increasing under higher pressures, and suggests a transition to metallic or atomic states at even higher pressures.

Contribution

First experimental observation of semimetallic behavior in molecular hydrogen at high pressures, bridging theoretical predictions and experimental data.

Findings

Hydrogen becomes conductive at ~360 GPa and exhibits semimetallic properties.

Conductivity increases significantly with pressure up to 440 GPa.

Raman spectra suggest hydrogen remains molecular up to 440 GPa, then transitions at higher pressures.

Abstract

According to the theoretical predictions, insulating molecular hydrogen dissociates and transforms to an atomic metal at pressures P~370-500 GPa. In another scenario, the metallization first occurs in the 250-500 GPa pressure range in molecular hydrogen through overlapping of electronic bands. The calculations are not accurate enough to predict which option is realized. Here we show that at a pressure of ~360 GPa and temperatures <200 K the hydrogen starts to conduct, and that temperature dependence of the electrical conductivity is typical of a semimetal. The conductivity, measured up to 440 GPa, increases strongly with pressure. Raman spectra, measured up to 480 GPa, indicate that hydrogen remains a molecular solid at pressures up to 440 GPa, while at higher pressures the Raman signal vanishes, likely indicating further transformation to a good molecular metal or to an atomic state.