Impurity states in multiband s-wave superconductors: Analysis of iron pnictides

We examine the effect of a single, nonmagnetic impurity in a multiband, extended s-wave superconductor allowing for anisotropy of the gaps on the Fermi surfaces. We derive analytic expressions for the Green's functions in the continuum and analyze the conditions for the existence of sharp impurity-induced resonant states. Underlying band structure is more relevant for the multiband than for the single-band case, and mismatch between the bands generically makes the formation of the impurity states less likely in the physical regime of parameters. We confirm these conclusions by numerically solving the impurity problem in a tight-binding parametrization of the bands relevant to pnictide superconductors.