Eur. Phys. J. B (2012) 85: 159
https://doi.org/10.1140/epjb/e2012-30164-5

Regular Article

⁷⁵As NMR-NQR study in superconducting LiFeAs

Leibniz Institute for Solid State and Materials Research IFW-Dresden, PF 270116, 01171 Dresden, Germany

^a e-mail: sbaek.fu@gmail.com

Received: 24 February 2012
Received in final form: 15 March 2012
Published online: 22 May 2012

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, ν_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 ν_Q, yet all the samples are clearly superconducting. For sample S1 which has the largest ν_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 ν_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.