Black metal hydrogen above 360 GPa driven by proton quantum fluctuations

Numerical calculations that include the quantum fluctuations of protons explain the optical properties of hydrogen at high pressure. Hydrogen metallization under stable conditions is a substantial step towards the realization of the first room-temperature superconductor. Recent low-temperature experiments(1-3)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 data(2). 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 scenarios(1-3).

Automated summary

Numerical calculations considering the quantum fluctuations of protons under high pressure explain the optical properties of hydrogen, with hydrogen metallization predicted to occur at 380 GPa in phase III. Replacing hydrogen with deuterium is expected to substantially increase the conductivity and optical gap, emphasizing the isotope dependence of metallization driven by quantum fluctuations.