Effect of structure on the superconductivity of CaAlSi and SrAlSi: Density functional calculations

Using the full-potential, density-functional-based method, we study the superstructure effect on superconductivity in CaAlSi compounds and disordered effect on superconductivity in SrAlSi compounds. We propose a 2H superstructure model for CaAlSi, in which the Al and Si atoms form a flat graphite-type sheet with Al and Si atoms alternated within the layer as well as along the c axis. By making a comparison in phonon spectra between the present 2H superstructure model and the previous 1H model, it is found that the superstructure is more stable for CaAlSi. A higher phonon logarithmically averaged frequency and a weaker electron-phonon coupling strength obtained in the 2H superstructure are in good agreement with related experimental data. For a disordered phase of SrAlSi, the calculated result is well consistent with the experimental results of a weakly coupled BCS superconductor, indicating that the distribution of Al/Si is probably disordered