Extended s+ scenario for the neclear spin-lattice relaxation rate in superconducting pnictides

Recently, several measurements of the nuclear spin-lattice relaxation rate T₁^-1 in the superconducting Fe pnictides have been reported. These measurements generally show no coherence peak below Tc and indicate a low-temperature power-law behavior, the characteristics commonly taken as evidence of unconventional superconductivity with lines of nodes crossing the Fermi surface. In this work we show that (i) the lack of a coherence peak is fully consistent with the previously proposed nodeless extended s±-wave symmetry of the order parameter (whether in the clean or dirty limit) and (ii) the low-temperature power-law behavior can be also explained in the framework of the same model but requires going beyond the Born limit.