Impurity-induced in-gap state and T_c in sign-reversing s-wave superconductors: analysis of iron oxypnictide superconductors

The sign-reversing fully gapped superconducting state, which is expected to be realized in oxypnictide superconductors, can be prominently affected by nonmagnetic impurities due to the interband scattering of Cooper pairs. We study this problem based on the isotropic two-band BCS model: in oxypnictide superconductors, the interband impurity scattering I ' is not equal to the intraband one I. In the Born scattering regime, the reduction in T_c is sizeable and the impurity-induced density of states (DOS) is prominent if I∼I ', due to the interband scattering. Although impurity-induced DOS can yield a power-law temperature dependence in 1/T₁, it is inevitably accompanied by a sizeable suppression of T_c. In the unitary scattering regime, in contrast, the effect of impurities is very small for both T_c and DOS except at I=I '. On comparing theory and experiments, we surmise that the degree of anisotropy in the s_±-wave gap function strongly depends on the compound.