  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-y}Se_2. 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 sub-gap 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 sub-gap bound states. The above results can be
explained by a simple analytic model that provides a semi-quantitative
understanding of the variation of the impurity bound states energies as a
function of impurity potential and chemical doping level.
