NMR evidence of strongly correlated superconductivity in LiFeAs: Tuning toward a spin-density-wave ordering

In this Rapid Communication, we reported the results of NMR study on LiFeAs single crystals. We find a strong evidence of the low-temperature spin fluctuations; by changing sample preparation conditions, the system can be tuned toward a spin-density-wave (SDW) quantum-critical point. The detection of an interstitial Li(2) ion, possibly locating in the tetrahedral hole, suggests that the off-stoichiometry and/or lattice defect can probably account for the absence of the SDW ordering in LiFeAs. These facts show that LiFeAs is a strongly correlated system and the superconductivity is likely originated from the SDW fluctuations.