What superconducts in sulfur hydrides under pressure and why

Abstract

The recent discovery of superconductivity at 190 K in highly compressed ${\mathrm{H}}_{2}\mathrm{S}$ 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, ${\mathrm{H}}_{3}\mathrm{S}.$ 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 ${\mathrm{H}}_{3}\mathrm{S}$ 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 ${\mathrm{H}}_{2}\mathrm{S}$ is unstable, decomposing into ${\mathrm{H}}_{3}\mathrm{S}$ and S, and that ${\mathrm{H}}_{3}\mathrm{S}$ has a record high ${T}_{c}$ due to its covalent bonds driven metallic, which make this compound rather similar to ${\mathrm{MgB}}_{2}$, but unlike most other good conventional superconductors.