What superconducts in sulfur hydrides under pressure, and why

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.

Supplementary material contains crystallographic information

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