On the origin of the shallow and “replica” bands in FeSe monolayer superconductors

Resumo

We compare the electronic structures of single FeSe layer films on SrTiO₃ substrate (FeSe/STO) and K_xFe_2-ySe₂ superconductors obtained from extensive LDA and LDA + DMFT calculations with the results of ARPES experiments. It is demonstrated that correlation effects on Fe-3d states are sufficient in principle to explain the formation of the shallow electron-like bands at the M(X)-point. However, in FeSe/STO these effects alone are apparently insufficient for the simultaneous elimination of the hole-like Fermi surface around the Γ-point which is not observed in ARPES experiments. Detailed comparison of ARPES detected and calculated quasiparticle bands shows reasonable agreement between theory and experiment. Analysis of the bands with respect to their origin and orbital composition shows, that for FeSe/STO system the experimentally observed “replica” quasiparticle band at the M-point (usually attributed to forward scattering interactions with optical phonons in SrTiO₃ substrate) can be reasonably understood just as the LDA calculated Fe-3d_xy band, renormalized by electronic correlations. The only manifestation of the substrate reduces to lifting the degeneracy between Fe-3d_xz and Fe-3d_yz bands near M-point. For the case of K_xFe_2-ySe₂ most bands observed in ARPES can also be understood as correlation renormalized Fe-3d LDA calculated bands, with overall semi-quantitative agreement with LDA + DMFT calculations.