Electronic structure and superconductivity of CaAlSi and SrAlSi

We report full-potential linear augmented plane-wave calculations for CaAlSi and SrAlSi in ordered structures and in the virtual-crystal approximation, at normal and elevated pressures. We also estimate the electron-phonon coupling using frozen-phonon calculations at the zone center, and the rigid muffin-tin approximation. We conclude that there is no simple way to explain the recently reported qualitative disparity in the superconducting properties of the two compounds. An assumption of an ultrasoft phonon mode, on the other hand, allows to reconcile the experimental findings with the theory in a reasonable way.