Anharmonic effects in atomic hydrogen: superconductivity and lattice dynamical stability

TL;DR

This study uses first-principles calculations to explore the stability and superconductivity of atomic hydrogen at high pressures, revealing that anharmonic effects slightly reduce the critical temperature but do not significantly alter the superconducting properties.

Contribution

It provides the first detailed analysis of anharmonic effects on superconductivity and lattice stability in atomic hydrogen under extreme pressures.

Findings

Atomic hydrogen remains dynamically stable with a tetragonal structure at 400-600 GPa.

Anharmonic effects slightly reduce the superconducting critical temperature from 318 K to 300 K.

Superconductivity is robust against anharmonicity due to uniform electron-phonon coupling distribution.

Abstract

We present first-principles calculations of metallic atomic hydrogen in the 400-600 GPa pressure range in a tetragonal structure with space group $I4_1/amd$, which is predicted to be its first atomic phase. Our calculations show a band structure close to the free-electron-like limit due to the high electronic kinetic energy induced by pressure. Bands are properly described even in the independent electron approximation fully neglecting the electron-electron interaction. Linear-response harmonic calculations show a dynamically stable phonon spectrum with marked Kohn anomalies. Even if the electron-electron interaction has a minor role in the electronic bands, the inclusion of electronic ex- change and correlation in the density response is essential to obtain a dynamically stable structure. Anharmonic effects, which are calculated within the stochastic self-consistent harmonic approxima- tion, harden high-energy optical modes and soften transverse acoustic modes up to a 20% in energy. Despite a large impact of anharmonicity has been predicted in several high-pressure hydrides, here the superconducting critical temperature is barely affected by anharmonicity, as it is lowered from its harmonic 318 K value only to 300 K at 500 GPa. We atribute the small impact of anharmoncity on superconductivity to the absence of softened optical modes and the fairly uniform distribution of the electron-phonon coupling among the vibrational modes.