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

Abstract

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 |I′⁄I|<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 |I′⁄I|∼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.