Impurity effects on the ±s-wave state of the iron-based superconductors

We studied the impurity scattering on the +/- s-wave superconductor (SC), with realistic parameters for the Fe pnictide SCs. Using the T-matrix method, generalized for the two bands, we found that the strong scattering limit of impurities forms an off-centered resonance state inside the superconducting (SC) gap, which modifies, surprisingly, the density of states (DOS) of a fully opened gap to a V-shaped DOS as if in the case of a d-wave SC. This behavior provides coherent explanations to the several conflicting experiments of the Fe-based SC: (1) the V-shaped DOS observed in photoemission and tunneling spectroscopy but with an isotropic gap; (2) the power-law behavior of the nuclear-spin-lattice relaxation rate (1/T(1)similar to T(alpha) with alpha similar to 3) down to very low temperatures. We also extended the same T-matrix method to study the impurity suppression of the critical temperature T(c) of the +/- s-wave pairing state. We found that both magnetic and nonmagnetic impurities suppress T(c) with a rate that is practically indistinguishable from the standard d-wave case despite a possibly large difference of the positive and the negative s-wave order parameter magnitudes.