Resonant impurity scattering in the $\pm$s-gap state of the Fe-based superconductors

We study the impurity scattering on the $\pm$s-wave superconductor, with realistic parameters for the Fe-pnictide superconductors. Using $\mathcal{T}$-matrix method, generalized for the two bands, we found that impurity scattering of the unitary limit forms off-centered bound states inside of the superconducting gap, which modifies, surprisingly, the density of states (DOS) of a fully opened gap to a V-shaped one as in the case of a d-wave superconductor. This behavior provides coherent explanations to the several conflicting experimental issues of the Fe-pnictide superconductors: the V-shaped DOS but with an isotropic gap observed in the photoemission and tunneling experiments; the power law behavior of the nuclear spin-lattice relaxation rate ($1/T_1 \approx T^α$ ; $α\approx 3$), down to very low temperatures.

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