DFT study of electronic structure properties of SrAFe(4)As(4) (A = Rb and Cs) superconductors

摘要

Using the full-potential linearised augmented plane wave method and the PBE generalized gradient approximation, the electronic structure properties of SrAFe(4)As(4) (A = Rb and Cs) superconductors were determined and compared with those of their parents SrFe2As2 (Fmmm), SrFe2As2 (I4/mmm), RbFe2As2 and CsFe2As2. It is found that associated with the Van Hove singularities, densities of states of SrAFe(4)As(4) around Fermi energy level, mostly caused by Fe-3d orbitals, are relatively high but not enough to account for enhancement of T-c in these materials. We suggest rather a relationship between T-c values and interband scattering strength, which depends on the number of electronic bands crossing E-F and forming hole-like pockets around the Gamma point and electron-like pockets at the corners of Brillouin zone. It is established that Fermi surfaces of SrAFe(4)As(4) are described by 2D and manifest the behaviour of +/- s-wave gap in Cs- and nodal gap in Rb-based. Electron Localization Function demonstrates the presence of both valence and metallic bondings. However, there is a stronger covalent bonding in AFe(2)As(2) than in SrAFe(4)As(4), suggesting that the weakness of covalency accompanies higher T-c values.