Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors

This work is provoked by recent discovery of new class prototype systems AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-$T_c$ superconductors ($T_c=36$ K). Here we report ab initio LDA results for electronic structure of the AFFeAs systems. We provide detailed comparison between electronic properties of both new systems and reference LaOFeAs (La111) compound. In the vicinity of the Fermi level all three systems have essentially the same band dispersions. However for iron fluoride systems F(2$p$) states were found to be separated in energy from As(4$p$) ones in contrast to La111, where O(2$p$) states strongly overlaps with As(4$p$). Thus it should be more plausible to include only Fe(3$d$) and As(4$p$) orbitals into a realistic noninteracting model than for La111. Moreover Sr substitution with smaller ionic radius Ca in AFFeAs materials leads to a lattice contruction and stronger Fe(3$d$)-As(4$p$) hybridization resulting in smaller value of the density of states at the Fermi level in the case of Ca compound. So to some extend Ca system reminds RE111 with later Rare Earths. However Fermi surface of new fluorides is found to be nearly perfect two-dimensional. Also we do not expect strong dependence of superconducting properties with respect to different types of A substitutes.