Impurity-induced bound states in iron-based superconductors with s-wave cos kx•cos ky pairing symmetry

Using both the self-consistent Bogoliubov-de Gennes formulation and non-self-consistent T-matrix approach, we perform a comprehensive investigation of the in-gap bound states induced by a localized single impurity in iron-based superconductors. We focus on studying signatures associated with the unconventional sign-changed s-wave pairing symmetry. For a nonmagnetic impurity, we find that there are two in-gap bounds, symmetric with respect to zero energy, only in the sign-changed s-wave pairing state, not in the sign-unchanged s-wave state, due to the existence of nontrivial Andreev bound states caused by the sign change. For a magnetic impurity, we find that due to the breakdown of the local time-reversal symmetry, there exist only bound-state solutions (with orbital degeneracy) carrying one of the electron-spin polarizations around the impurity. As increasing the scattering strength, the system undergoes a quantum phase transition (level crossing) from a spin-unpolarized ground state to a spin-polarized one. While the results for the magnetic impurity are qualitatively similar in both the sign-changed and sign-unchanged s-wave superconducting (SC) states, the bound states in the first case are more robust and there is no pi phase shift of the SC gap near the impurity in the strong scattering regime.