Electronic Structure, Topological Phase Transitions and Superconductivity in (K, Cs)xFe2Se2

We present LDA band structure of novel hole doped high temperature superconductors (T-c similar to 30 K) KxFe2Se2 and CsxFe2Se2 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 Gamma 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.