Ab initio investigation on the experimental observation of metallic hydrogen

The optical spectra of hydrogen at similar to 500 GPa were studied theoretically using a combination of 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 nonnegligible 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 defect absorptions in diamond. These results provide theoretical support for the recent experimental realization of metallic hydrogen. The strong EPIs and the nonnegligible AHEs also emphasize the necessity for quantum treatment of both the electrons and the nuclei in future studies.