Ab initio investigation on the experimental observation of metallic hydrogen

TL;DR

This study uses ab initio methods to interpret experimental observations of metallic hydrogen at high pressure, highlighting the importance of electron-phonon interactions and anharmonic effects, and supporting recent experimental realization.

Contribution

It identifies the I41/amd structure as consistent with experiments and emphasizes the role of electron-phonon interactions and anharmonic effects in metallic hydrogen.

Findings

I41/amd structure explains experimental spectra

Electron-phonon interactions are crucial for interpretation

Anharmonic effects are significant but not sufficient

Abstract

The optical spectra of hydrogen at $\sim$500 GPa were studied theoretically using a combination of \emph{ab initio} methods. Among the four most competitive structures, i.e. C2/c-24, Cmca-12, Cmca-4, and I41/amd, only the atomic phase I41/amd can provide satisfactory interpretations of the recent experimental observation, and the electron-phonon interactions (EPIs) play a crucial role. Anharmonic effects (AHEs) due to lattice vibration are non-negligible but not sufficient to account for the experimentally observed temperature dependence of the reflectance. The drop of the reflectance at 2 eV is not caused by diamond's band gap reducing or interband plasmon, but very likely by defects absorptions in diamond. These results provide theoretical support for the recent experimental realization of metallic hydrogen. The strong EPIs and the non-negligible AHEs also emphasize the necessity for quantum treatments of both the electrons and the nuclei in future studies.