Impurity-induced subgap bound states in alkali-doped iron chalcogenide superconductors

Measurements of the local density of states near impurities can be useful for identifying the superconducting gap structure in alkali-doped iron chalcogenide superconductors K_xFe_2-ySe₂. Here, we study the effects of nonmagnetic and magnetic impurities within a nearest-neighbor d-wave and next-nearest-neighbor s-wave superconducting state. For both repulsive and attractive nonmagnetic impurities, it is shown that subgap bound states exist only for d-wave superconductors with the positions of these bound states depending rather sensitively on the electron doping level. Further, for such disorder, Coulomb interactions can lead to local impurity-induced magnetism in the case of d-wave superconductivity. For magnetic impurities, both s-wave and d-wave superconducting states support subgap bound states. The above results can be explained by a simple analytic model that provides a semiquantitative understanding of the variation of the impurity bound states energies as a function of impurity potential and chemical doping level.