Impurity Effects in Sign-Reversing Fully Gapped Superconductors: Analysis of FeAs Superconductors

To understand the impurity effect on T-c in FeAs superconductors, we analyze a simple two-band BCS model with a repulsive interband interaction. The realized fully gapped superconducting state with sign reversal, which is predicted by spin fluctuation theories in this compound, is suppressed by impurities due to the interband hopping of Cooper pairs, if the interband impurity scattering I' is equal to the intraband one I. When vertical bar I'/I vertical bar<1, in high contrast, T-c is almost unchanged by strong impurity scattering since interband scattering is almost prohibited by the multiple scattering effect. Since vertical bar I'/I vertical bar similar to 0.5 is expected, the robustness of superconductivity against impurities in FeAs superconductors is naturally understood in terms of the sign-reversing fully gapped state. (C)2008 The Physical Society of Japan