What superconducts in sulfur hydrides under pressure and why

The recent discovery of superconductivity at 190 K in highly compressed H2S 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, H3S. 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 H3S 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 H2S is unstable, decomposing into H3S and S, and that H3S has a record high T-c due to its covalent bonds driven metallic, which make this compound rather similar to MgB2, but unlike most other good conventional superconductors.