Role of Doping Effect and Chemical Pressure Effect Introduced by Alkali Metal Substitution on 1144 Iron-Based Superconductors

CaAFe (Formula presented.) As (Formula presented.) with A = K, Rb, and Cs are close to the doped 122 system, and the parent material can reach a superconducting transition temperature of 31–36 K without doping. To study the role of alkali metals, we investigated the induced hole doping and chemical pressure effects as a result of the introduction of alkali metals using density-functional-based methods. These two effects can affect the superconducting transition temperature by changing the number of electrons and the structure of the FeAs conductive layer, respectively. Our study shows that the (Formula presented.) and (Formula presented.) orbitals, which are degenerate in CaFe (Formula presented.) As (Formula presented.), become nondegenerate in CaAFe (Formula presented.) As (Formula presented.) due to two nonequivalent arsenic atoms (As1 and As2). The unusual oblate ellipsoid hole pocket at (Formula presented.) point in CaAFe (Formula presented.) As (Formula presented.) results from a divalent cation Ca+ replaced by a monovalent cation A. It shows one of the main differences in fermiology compared to a particular form of CaFe (Formula presented.) As (Formula presented.) with reduced 1144 symmetry, due to the enhancement of As2-Fe hybridization. The unusual band appears in CaFe (Formula presented.) As (Formula presented.) (1144) and gradually disappears in the change of K to Cs. Further analysis shows that this band is contributed by As1 and has strong dispersion perpendicular to the FeAs layer, suggesting that it is related to the peculiar van Hove singularity below the Fermi level. In addition, various aspects of CaFe (Formula presented.) As (Formula presented.) (1144) and CaAFe (Formula presented.) As (Formula presented.) in the ground state are discussed in terms of the influence of hole doping and chemical pressure.