Electronic structure, topological phase transitions and superconductivity in (K, Cs) x Fe2Se2

Abstract

We present LDA band structure of novel hole doped high temperature superconductors (T _c ∼ 30 K) K _x Fe₂Se₂ and Cs _x Fe₂Se₂ and compare it with previously studied electronic structure of isostructural FeAs superconductor BaFe₂As₂ (Ba122). We show that stoichiometric KFe₂Se₂ and CsFe₂Se₂ have rather different Fermi surfaces as compared with Ba122. However at about 60% of hole doping Fermi surfaces of novel materials closely resemble those of Ba122. In between these dopings we observe a number of topological Fermi surface transitions near the Γ point in the Brillouin zone. Superconducting transition temperature T _c of new systems is apparently governed by the value of the total density of states (DOS) at the Fermi level.