Competition between electron-phonon coupling and spin fluctuations in superconducting hole-doped CuBiSO

CuBiSO is a band insulator that becomes metallic upon hole doping. Superconductivity was recently reported in doped Cu_1-xBiSO and attributed to spin fluctuations as the pairing mechanism. Based on first-principles calculations of the electron-phonon coupling, we argue that the latter is very strong in this material, and probably drives superconductivity. The critical temperature is, however, strongly depressed by the proximity to magnetism. Thus Cu_1-xBiSO is a quite unique compound where both a conventional phonon-driven and an unconventional triplet superconductivity are possible, and compete with each other. We argue that, in this material, it may be possible to switch from conventional to unconventional superconductivity by varying such parameters as doping or pressure.