Anion height dependence of T$_c$ 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), Ba$_2$Fe$_2$As, (Sr,Ca)FFeAs, Sr$_4$Sc$_2$O$_6$Fe$_2$P$_2$, 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 $\Delta z_a\sim$1.37Å, attributed to changing Fe – As (P,Se,Te) hybridization. This leads to a similar dependence of superconducting transition temperature $T_c$ as observed in the experiments. The fit of this dependence to elementary BCS theory produces semiquantitative agreement with experimental data for $T_c$ for the whole class of iron based superconductors. The similar fit to Allen–Dynes formula underestimates $T_c$ 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 $T_c$ variation between different types of iron based superconductors is due to the corresponding variation of the density of states at the Fermi level.