Black metal hydrogen above 360 GPa driven by proton quantum fluctuations

Hydrogen metallization under stable conditions is a substantial step towards the realization of the first room-temperature superconductor. Recent low-temperature experiments1–3 report different metallization pressures, ranging from 360 GPa to 490 GPa. In this work, we simulate the structural properties and vibrational Raman, infrared 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 lineshape into the Raman spectra and reduce the optical gap by 3 eV. We show that hydrogen metallization occurs at 380 GPa in phase III due to band overlap, in good agreement with transport data2. Our simulations predict that this state is a black metal—transparent in the infrared—so the shiny metal observed at 490 GPa (ref. 1) is not phase III. We predict that the conductivity onset and optical gap will substantially increase if hydrogen is replaced by deuterium, underlining that metallization is driven by quantum fluctuations and is thus isotope-dependent. We show how hydrogen acquires conductivity and brightness at different pressures, explaining the apparent contradictions in existing experimental scenarios1–3. Numerical calculations that include the quantum fluctuations of protons explain the optical properties of hydrogen at high pressure.