What superconducts in sulfur hydrides under pressure, and why

TL;DR

This paper explains why sulfur hydrides under pressure exhibit record-high superconductivity, identifying H3S as the key compound with high vibrational frequencies of hydrogen driving its strong-coupling BCS superconductivity.

Contribution

It demonstrates that H3S, not H2S, is responsible for high Tc superconductivity under pressure, clarifying the material's composition and the origin of its high critical temperature.

Findings

H2S decomposes into H3S and S under pressure.

H3S has a record high Tc due to covalent bonds and high vibrational frequencies.

High hydrogen vibrational frequencies are crucial for high Tc in H3S.

Abstract

The recent discovery of superconductivity at 190~K in highly compressed H$_{2}$S is spectacular not only because it sets a record high critical temperature, but because it does so in a material that appears to be, and we argue here that it is, a conventional strong-coupling BCS superconductor. Intriguingly, superconductivity in the observed pressure and temperature range was predicted theoretically in a similar compound H$_{3}$S. Several important questions about this remarkable result, however, are left unanswered: (1) Does the stoichiometry of the superconducting compound differ from the nominal composition, and could it be the predicted H$_{3}$S compound? (2) Is the physical origin of the anomalously high critical temperature related only to the high H phonon frequencies, or does strong electron-ion coupling play a role? We show that at experimentally relevant pressures H$_2$S is unstable, decomposing into H$_3$S and S, and that H$_3$S has a record high $T_c$ due to its covalent bonds driven metallic. The main reason for this extraordinarily high $T_c$ in H$_3$S as compared with MgB$_2$, another compound with a similar superconductivity mechanism, is the high vibrational frequency of the much lighter H atoms.