NMR and NQR on LiFeAs
This Chapter will present 75As NQR, 75As NMR and 7Li NMR measurements on three different single crystals and a polycrystalline sample of LiFeAs. As will be shown, no common behavior could be found. Static and dynamic NMR properties vary among the samples, which are distinguishable by their slightly different quadrupole frequencies. Samples with a lower quadrupole frequency ν q exhibit a decrease of the Knight shift and the spin-lattice relaxation rate in the superconducting state, indicating usual spin-singlet superconductivity. For the sample with the highest V q , a constant Knight shift and a strange upturn of the 75As NQR spin-lattice relaxation rate in the superconducting state provide evidence for spin-triplet superconductivity. Surprisingly, all samples seem to be very homogeneous, according to their very narrow 75As NQR and 7Li NMR spectra and other physical properties (please refer to Section 5.3.2 for a detailed discussion of the sample quality). These results provide evidence for the proximity of LiFeAs to a ferromagnetic instability, where tiny changes in the stoichiometry, seen only by NQR, can lead to different Cooper pairing states. As will be discussed, these data are in agreement with recent band structure calculations. By including special characteristics of LiFeAs measured by ARPES, such as the absence of Fermi surface nesting and the shallow hole pockets around the Γ point (see Fig. 4.9), these calculations found dominant “almost ferromagnetic” incommensurate fluctuations in LiFeAs and thus a proximity to a ferromagnetic instability .