Abstract
In order to explore the reason why the hexagonal phase of FeSe is insulating, in contrast with tetragonal FeSe which is superconducting, we study a model that incorporates all orbitals of the Iron d-shell. It is found, within the local-density approximation plus dynamical mean-field theory (LDA+DMFT) that hexagonal FeSe resembles an orbital-selective insulating state. Basing ourselves on the good semiquantitative accord with a range of experimental data for tetragonal-FeSe superconductor, we show the intrinsic nature of the pseudogap state in these chalcogenide systems leading to an orbital-selective violation of the Landau–Fermi-liquid description at low temperatures. We discuss the pseudogap regime microscopically, along with the nature of the insulating state of hexagonal FeSe.