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

CaAFe [Formula: see text] As [Formula: see text] 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: see text] and [Formula: see text] orbitals, which are degenerate in CaFe [Formula: see text] As [Formula: see text] , become nondegenerate in CaAFe [Formula: see text] As [Formula: see text] due to two nonequivalent arsenic atoms (As1 and As2). The unusual oblate ellipsoid hole pocket at [Formula: see text] point in CaAFe [Formula: see text] As [Formula: see text] results from a divalent cation Ca(2)+ replaced by a monovalent cation A(+). It shows one of the main differences in fermiology compared to a particular form of CaFe [Formula: see text] As [Formula: see text] with reduced 1144 symmetry, due to the enhancement of As2-Fe hybridization. The unusual band appears in CaFe [Formula: see text] As [Formula: see text] (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: see text] As [Formula: see text] (1144) and CaAFe [Formula: see text] As [Formula: see text] in the ground state are discussed in terms of the influence of hole doping and chemical pressure.