First-principles calculations of pressure effects on the superconducting CaAlSi and SrAlSi

The pressure effects on superconducting transition temperature T-c in CaAlSi and SrAlSi are studied by calculating the electronic structure, phonon spectra and the electron-phonon (EP) interaction at different volumes using the full-potential, density-functional-based method. Our calculations show that the opposite signs of pressure coefficients of T, in CaAlSi and SrAlSi are not caused by their electronic structures, but by different pressure behaviors of the ultrasoft phonon mode. The phonon frequencies and EP coupling strengths for all phonon modes, except the ultrasoft mode, have similar pressure dependence in the two compounds.