Black metal hydrogen above 360 GPa driven by proton quantum fluctuations

TL;DR

This paper uses simulations including proton quantum effects to show hydrogen becomes metallic at around 380 GPa, predicting a black metallic state transparent in IR, with properties influenced by isotope substitution, clarifying experimental discrepancies.

Contribution

It demonstrates the role of proton quantum fluctuations in hydrogen metallization and predicts a black metallic phase with specific optical properties at high pressures.

Findings

Hydrogen metallizes at 380 GPa due to band overlap.

Proton quantum effects cause a 25% downshift in vibron frequencies.

Metallization and optical properties are isotope-dependent.

Abstract

Production of metallic hydrogen is one of the top three open quests of physics. Recent low-temperature experiments report different metallization pressures, varying from 360GPa to 490GPa. In this work, we simulate structural properties, vibrational Raman, IR and optical spectra of hydrogen phase III accounting for proton quantum effects. We demonstrate that nuclear quantum fluctuations downshift the vibron frequencies by 25%, introduce a broad line-shape in the Raman spectra, and reduce the optical gap by 3eV. We show that hydrogen metallization occurs at 380GPa in phase III due to band overlap, in good agreement with transport data. By simulating the optical properties, we predict this state to be a peculiar black metal, transparent in the IR. The transparent window closes at 450GPa, but the reflectivity remains low, discarding phase III as the shiny metal observed at 490GPa. We predict the conductivity onset to increase by 70GPa and the transparent window to increase by 1.3eV when replacing hydrogen by deuterium at 0K, underlining that metallization is driven by quantum fluctuations and is thus isotope dependent. We show how hydrogen acquires metallic features (conductivity and brightness) at different pressures, explaining the apparent contradictions in existing experimental scenarios.