75As NMR-NQR study in superconducting LiFeAs

TL;DR

This study uses 75As NMR and NQR to investigate how slight variations in stoichiometry affect the superconducting properties of LiFeAs, revealing different magnetic responses in samples with different quadrupole frequencies.

Contribution

It demonstrates that small stoichiometric differences in LiFeAs significantly influence NMR and NQR behavior in the superconducting state, highlighting the sensitivity of magnetic properties to local electronic environments.

Findings

Samples with higher quadrupole frequency show constant Knight shift below Tc.

Samples with lower quadrupole frequency exhibit expected singlet superconductor behavior.

Tiny stoichiometric variations cause notable differences in NMR parameters in 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, $\nu_Q$, 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 Tc for a certain direction of the magnetic field and a peculiar upturn of the NQR spin lattice relaxation rate 1/T1 below Tc. In contrast, samples with a lower $\nu_Q$ exhibit the expected behavior for a singlet superconductor: a drop of K and 1/T1 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.