Theory of mixed-state effect on NMR relaxation measurements in iron pnictide superconductors

Based on a phenomenological model with s _± or s-wave pairing symmetry, the spatially resolved nuclear magnetic resonance (NMR) relaxation rate in the iron pnictides is investigated by solving Bogoliubov-de Gennes equations. Taking into account the presence of a magnetic field, our result for the s± pairing is in qualitative agreement with recent NMR experiments, while for the s-wave pairing, a coherence peak shows up right below T _c in apparent contradiction with experimental observations, thus excluding the s-wave pairing. We also propose that the spin-lattice relaxation rate (SLRR) should follow an exponential relation when the temperature is lowered below T/T _c-0.1 down to 0.01. It is noted that the SLRR cannot be entirely determined by the local density of states; the mixed-state effect and multiorbital physics must be considered.