Impurity suppression of the critical temperature in the iron-based   superconductors
  We study the impurity suppression of the critical temperature $T_c$ of the FeAs superconductors theoretically based on the the $\pm$s-wave pairing state of a two band model. The effects of non-magnetic and magnetic impurities are studied with the $\mathcal{T}$-matrix approximation, which can continuously treat impurity scattering from weak to strong coupling limit. We found that both magnetic and non-magnetic 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 negative s-wave order parameter (OP) magnitudes. This is because the density of states enters together with the OP magnitude for the scattering process. 
Yunkyu Bang, Han-Yong Choi, Hyekyung Won