Ab initio study of electronic structure properties of CaAFe4As4(A = K, Rb and Cs) superconductors

The electronic and magnetic structures of CaAFe(4)As(4) where A = K, Rb, Cs (denoted as Ca-1144) were investigated through the full-potential augmented plane wave method and the generalized gradient approximation. The Self-consistent Field calculations for several magnetic configurations revealed a nonmagnetic to be the ground state of the studied superconductors. A comparison of Density of States, DOS, of the Ca-1144 to those of their parent compounds (Ca, K, Cs and Rb) Fe2As2 (denoted as 122) (in both orthorhombic and tetragonal phase) suggested that the high DOS M Fermi level and the presence of Van Hove singularities in Ca-1144 are not different enough from the 122 compounds to explain the T-c enhancement in the Ca-1144 superconductors. However, there is pronounced dissimilarity in the Electronic Band Structures between the Ca-1144 and 122 materials, namely, the chemical A-doping increases the number of bands crossing E-F and forms hole-type pockets around the Gamma point as well as electron-type ones at the corners of Brillouin zone. This observed distinction implies a close correlation with the enhanced T-c values. It is worth pointing out that the Fermi surface of the CaCsFe4As4 is found to reminiscence the structure of nodal gap. The Electron Localization Function discloses an anisotropic behaviour and the presence of both valence and metallic bonds in all studied compounds but the weaker covalent bonding is the necessary adjunct to the T-c enlargement in the Ca-1144 superconductors.