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

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We report the 75As-NQR and NMR studies on the iron arsenide superconductor LiFeAs with $T_{\text{c}} \sim 17$ K. The spin lattice relaxation rate, $1/T_{1}$, decreases below $T_{\text{c}}$ 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 $1/T_{1}T$ and the Knight shift decrease with decreasing temperature but become constant below $T \leq 50$ 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 $T_{\text{c}}$ in this material.
2010-08-15