Theory-based interpretation of recent experiments on metallic hydrogen*

A recent experiment tracked the direct band gap and vibron frequency of hydrogen via infrared measurements up to ~425 GPa [1]. Above this pressure, the direct gap has a discontinuous drop to below ~0.1 eV. The authors interpreted this as a structural phase transition from the C2/c-24 molecular phase to another molecular phase. In our ab initio study of pressure dependent vibron frequency and direct band gap, we find that the experimental data is consistent with the C2/c-24 phase up to 425 GPa. We also find that the pressure induced changes in the band structure of the C2/c-24 phase lead to a discontinuous drop of the direct band gap, which can explain the observed drop without a structural transition [2]. We also investigate the superconducting properties of the C2/c-24 phase and compare it with the other candidate phases.

[1] P. Loubeyre, F. Occelli, P. Dumas, Synchrotron Infrared Spectroscopic Evidence of the Probable Transition to Metal Hydrogen, Nature 577, 7792 (2020).
[2] M. Dogan, S. Oh, M. L. Cohen, Observed Metallization of Hydrogen Interpreted as a Band Structure Effect, J. Phys.: Condens. Matter 33, 03LT01 (2020).