75As NMR-NQR study in superconducting LiFeAs

We report results of As-75 nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) experiments as well as Li-7 NMR on different samples of self flux grown LiFeAs and 5% Co doped LiFeAs single crystals, and a polycrystalline LiFeAs sample. We were able to distinguish the samples by their slightly different quadrupole frequencies, nu(Q), which is a direct measure of the electric field gradient (EFG) at the As site. Interestingly, samples with a large quadrupole frequency appear to show a different Knight shift and spin lattice relaxation in the superconducting state from those with a lower nu(Q), yet all the samples are clearly superconducting. For sample S1 which has the largest nu(Q), we find constant Knight shift K across T-c for a certain direction of the magnetic field and a peculiar upturn of the NQR spin lattice relaxation rate (T1T)(-1) below T-c. In contrast, samples with a lower nu(Q) exhibit the expected behavior for a singlet superconductor: a drop of K' and (T1T)(-1) for both NMR and NQR below T-c. Our results show that already tiny changes in stoichiometry uncovered by slightly different NQR frequencies lead to very different behavior of the NMR parameters in the superconducting state of LiFeAs. Different possibilities will be discussed which may explain the contrasting behavior.