EFFECTS OF IMPURITY VERTEX CORRECTION ON NMR COHERENCE PEAK IN S-WAVESUPERCONDUCTORS

We study the effects of nonmagnetic impurity vertex correction on nucl ear spin-lattice relaxation rate 1/T-1 of conventional s-wave supercon ductors within the Eliashberg formalism. We obtain, with a self-consis tent t-matrix treatment of impurity scatterings, the expressions for i mpurity vertex function and nuclear spin-lattice relaxation rate. The coherence peak increases by about a factor of 2, as one goes from the clean to dirty limit, which is in good agreement with the NMR measurem ents on Al- and In-based superconductors. This observation raises an i nteresting possibility that the experimental observation of the NMR co herence peak increase may be understood in terms of impurity vertex co rrection rather than the gap anisotropy smearing by impurities.