Doping dependence of correlation effects in K1 − x Fe2 − y Se2 superconductors: LDA’ + DMFT investigation

Abstract

We present detailed LDA'+DMFT investigation of doping dependence of correlation effects in novel K{1-x}Fe{2-y}Se2 superconductor. Calculations were performed at four different hole doping levels, starting from hypothetical stoichiometric composition with total number of electrons equal to 29 per unit cell through 28 and 27.2 electrons towards the case of 26.52, which corresponds to chemical composition K{0.76}Fe{1.72}Se2 studied in recent ARPES experiments. In general case the increase of hole doping leads to quasiparticle bands in wide energy window +/-2 eV around the Fermi level becoming more broadened by lifetime effects, while correlation induced compression of Fe-3d LDA' bandwidths stays almost the same and of the order of ~1.3 for all hole concentrations. However close to the Fermi level situation is more complicated. Here in the energy interval from -1.0 eV to 0.4 eV the bare Fe-3d LDA' bands are compressed by significantly larger renormalization factors up to 5 with hole doping increase, while the value of Coulomb interaction remains the same. This fact manifests the increase of correlation effects with hole doping in K{1-x}Fe{2-y}Se2 system. Moreover in contrast to typical pnictides K{1-x}Fe{2-y}Se2 does not have well defined quasiparticle bands on the Fermi leves but pseudogap like "dark" region instead. We also find that with the growth of hole doping Fe-3d orbitals of various symmetries are affected by correlations in a different way in different parts of Brillouin zone. To illustrate this we determine quasiparticle mass renormalization factors and energy shifts, which transform the bare Fe-3d LDA' bands of various symmetries into LDA'+DMFT quasiparticle bands. These renormalization factors effectively mimic more complicated energy dependent self-energy effects and can be used to analyze the available ARPES data.