75As NQR and NMR Studies of Superconductivity and Electron Correlations in Iron Arsenide LiFeAs

We report the As-75-NQR and NMR studies on the iron arsenide superconductor LiFeAs with T-c similar to 17 K. The spin lattice relaxation rate, 1/T-1, decreases below T-c without a coherence peak, and can be fitted by gaps with s(+/-)-wave symmetry in the presence of impurity scattering. In the normal state, both 1/T1T and the Knight shift decrease with decreasing temperature but become constant below T <= 50 K. Estimate of the Korringa ratio shows that the spin correlations are weaker than those in other families of iron arsenides, which may account for the lower T-c in this material.