Anion height dependence of Tc and density of states in iron based superconductors

Systematic ab initio LDA calculations were performed for all the typical representatives of recently discovered class of iron based high-temperature superconductors: REOFe(As,P) (RE=La,Ce,Nd,Sm,Tb), Ba2Fe2As, (Sr,Ca)FFeAs, Sr4Sc2O6Fe2P2, LiFeAs and Fe(Se,Te). Non-monotonic behavior of total density of states at the Fermi level is observed as a function of anion height relative to Fe layer with maximum at about Dz_a~1.37A, attributed to changing Fe - As (P,Se,Te) hybridization. This leads to a similar dependence of superconducting transition temperature Tc as observed in the experiments. The fit of this dependence to elementary BCS theory produces semiquantitative agreement with experimental data for Tc for the whole class of iron based superconductors. The similar fit to Allen - Dynes formula underestimates Tc in the vicinity of the maximum, signifying the possible importance of non - phonon pairing in this region. These results unambiguously demonstrate that the main effect of Tc variation between different types of iron based superconductors is due to the corresponding variation of the density of states at the Fermi level.