First-principles study of the pressure and crystal-structure dependences of the superconducting transition temperature in compressed sulfur hydrides

We calculate the superconducting transition temperatures (T-c) in sulfur hydrides H2S and H3S from first principles using the density functional theory for superconductors. At pressures of less than or similar to 150 GPa, the high values of T-c(>= 130 K) observed in a recent experiment (A. P. Drozdov, M. I. Eremets, and I. A. Troyan, arXiv: 1412.0460) are accurately reproduced by assuming that H2S decomposes into R3m H3S and S. For higher pressures, the calculated T-c's for Im3m H3S are systematically higher than those for R3m H3S and the experimentally observed maximum value (190 K), which suggests the possibility of another higher-T-c phase. We also quantify the isotope effect from first principles and demonstrate that the isotope effect coefficient can be larger than the conventional value (0.5) when multiple structural phases energetically compete.