High temperature superconductivity in sulfur hydride under ultrahigh pressure: A complex superconducting phase beyond conventional BCS

TL;DR

This paper challenges the conventional BCS explanation for high-temperature superconductivity in sulfur hydride under high pressure, proposing a multi-band approach that accounts for the observed Tc and isotope effects.

Contribution

It introduces a multi-band superconductivity model to explain high Tc in sulfur hydride, beyond traditional BCS/Eliashberg theories, considering anharmonic phonon effects.

Findings

Conventional BCS theories cannot explain the high Tc or isotope dependence.

A multi-band approach with small interband coupling accounts for observed phenomena.

Anharmonic hydrogen bonds vanish under pressure, but sulfur-related phonons remain active.

Abstract

The recent report of superconductivity under high pressure at the record transition temperature of Tc=203K in sulfur hydride has been identified as conventional in view of the observation of an isotope effect upon deuteration. Here it is demonstrated that conventional theories of superconductivity in the sense of BCS or Eliashberg formalisms can neither account for the observed values of Tc nor the pressure dependence of the isotope coefficient. The only way out of the dilemma is a multi-band approach of superconductivity where already small interband coupling suffices to achieve the high values of Tc together with the anomalous pressure dependent isotope effect. In addition, it is shown that anharmonicity of the hydrogen bonds vanishes under pressure whereas anharmonic phonon modes related to sulfur are still active