Impurity effect in multi-orbital, sign-reversing s-wave superconductors

We study the impurity effects on the transition temperature Tc with use of the T-matrix approximation. We propose a way to visualize the multi-orbital effect by introducing the hybridization function characterizing the multi-orbital effect for the impurity scattering. Characterizing function does not depend on the superconducting pairing symmetry, since this function is defined by the eigenvectors in normal states. The result indicates that an impurity-robust superconductivity does not necessarily imply a sign- preserving pairing. Visualizing the hybridization effect in the effective five-band model for LaFeAsO, we show that the impurity effect on Tc is relatively weaker than that in single-band models.