Using controlled disorder to distinguish s_± and s ₊₊ gap structure in Fe-based superconductors

We reconsider the effect of disorder on the properties of a superconductor characterized by a sign-changing order parameter appropriate for Fe-based materials. Within a simple two-band model, we calculate simultaneously T _c, the change in residual resistivity Δρ₀, and the zero-energy density of states, and show how these results change for various types of gap structure and assumptions regarding the impurity scattering. The rate of T_c suppression is shown to vary dramatically according to details of the impurity model considered. We search therefore for a practical, experimentally oriented signature of a gap of the s_± type, and propose that observation of a particular evolution of the penetration depth, nuclear magnetic resonance relaxation rate, or thermal conductivity temperature dependence with disorder would suffice.