Electronic Structure of New AFFeAs Prototype of Iron Arsenide Superconductors

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.

High-temperature superconductivity in iron-based layered iron compounds

“…First calculations of band structure of Sa(Ca)FFeAs compounds were done in Refs. [120,121]. Naturally enough, the band structure and Fermi surfaces in these compounds are also very similar to those obtained before for REOFeAs systems.…”

“…In the following, similar calculations were performed also for the other 1111 systems, as well as for 122 [112,114], 111 [113,114,115] and α-FeSe [116]. As results obtained in all these references were more or less similar, in the following we shall concentrate in more details on our group works [117,118,119,120], referring to other authors where necessary. We shall also limit ourselves mainly to the results obtained for nonmagnetic tetragonal phase of 1111 and 122 systems (as well as 111), as superconductivity is realized in this phase.…”

High-T c superconductivity induced by doping rare-earth elements into CaFeAsF

“…This phenomena could be attributed to the emergence of a spin-density-wave order in lower temperature region. Band structure calculations showed that the Fermi surfaces of CaFeAsF is similar to that in LaFeAsO with two electron pockets in M point and two hole pockets at Γ point 20 , the SDW order removes the density of states on some Fermi pockets and may leave one of the hole pockets partially of fully ungapped thus causes the low temperature behavior of R H 13 . Through partial substitution of Ca 2+ with Nd 3+ or Pr 3+ , electrons were introduced into this system.…”