Electronic, vibrational, and superconducting properties of CaBeSi: First-principles calculations

We report first-principles calculations on the normal and superconducting state of CaBe_x Si_2-x (x=1), in the framework of density-functional theory for superconductors. CaBeSi is isostructural and isoelectronic to MgB₂ and this makes possible a direct comparison of the electronic and vibrational properties and the electron-phonon interaction of the two materials. Despite many similarities with MgB₂ (e.g., σ and π bands at the Fermi level and an even larger density of states), according to our calculations CaBeSi has a very low critical temperature T _c ≈ 0.4 K consistent with the experiment. CaBeSi exhibits a complex gap structure, with three gaps at the Fermi level: besides the σ and π gaps (present also in MgB₂), the appearance of a third gap is related to the anisotropy of the Coulomb repulsion, acting in different ways on the bonding and antibonding electronic π states.