75As NMR-NQR study in superconducting LiFeAs
Abstract
We report results of 75As nuclear magnetic resonance (NMR) and nuclear
quadrupole resonance (NQR) experiments as well as 7Li 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, νQ, 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
νQ, yet all the samples are clearly
superconducting. For sample S1 which has the largest
νQ, we find constant Knight shift
$$\mathcal{K}$$ across Tc for a certain
direction of the magnetic field and a peculiar upturn of the NQR spin lattice relaxation
rate (T1T)−1 below
Tc. In contrast, samples with a lower
νQ exhibit the expected behavior for a
singlet superconductor: a drop of $$\mathcal{K}$$ and
(T1T)−1 for both NMR and NQR
below Tc. 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.
