Effect of inelastic scattering on the nuclear magnetic relaxation rate 1/T1T in iron-based superconductors

We present a microscopic study of the nuclear magnetic relaxation rate 1/T-1 based on the five-orbital model for iron-based superconductors. We mainly discuss the effect of the 'inelastic' quasi-particle damping rate gamma due to many-body interaction on the size of the coherence peak, for both s(++) and s(+/-)-wave superconducting states. We focus on Ba(Fe1-xCox)(2)As-2, and systematically evaluate gamma in the normal state from the experimental resistivity, from optimally to overdoped compounds. Next, gamma in the superconducting state is calculated microscopically based on second-order perturbation theory. In optimally doped compounds (T-c similar to 30 K), it is revealed that the coherence peak on 1/T1T is completely suppressed due to large gamma for both s(++) and s(+/-)-wave states. On the other hand, in heavily overdoped compounds with T-c < 10 K, the coherence peak could appear for both pairing states, since gamma at T-c is quickly suppressed in proportion to T-c(2). By making careful comparison between theoretical and experimental results, we conclude that it is difficult to discriminate between s(++) and s(+/-)-wave states from the present experimental results.