Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors

TL;DR

This study uses ab initio calculations to analyze the electronic structure of new AFFeAs iron arsenide superconductors, revealing similarities and differences with known compounds and implications for their superconducting properties.

Contribution

First ab initio analysis of AFFeAs systems, comparing their electronic structures with LaOFeAs, and highlighting differences in orbital interactions and Fermi surface characteristics.

Findings

AFFeAs systems have similar band dispersions near the Fermi level.

F(2p) states are separated from As(4p) states in fluorides, unlike in LaOFeAs.

Fermi surface is nearly two-dimensional and less dependent on A-site substitution.

Abstract

This work is provoked by recent discovery of new class prototype systems AFFeAs (A=Sr,Ca) of novel layered ironpnictide High-Tc superconductors (Tc=36K). 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(2p) states were found to be separated in energy from As(4p) ones in contrast to La111, where O(2p) states strongly overlaps with As(4p). Thus it should be more plausible to include only Fe(3d) and As(4p) 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(3d)-As(4p) 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.