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, ${\nu }_Q$, 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.

10 pages, 7 figures, published in the European Physical Journal B

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Cited by in corpus (20)

• Functional Renormalization Group for multi-orbital Fermi Surface Instabilities
• Using Gap Symmetry and Structure to Reveal the Pairing Mechanism in Fe-based Superconductors
• Emergent Kondo scaling in iron-based superconductors AFe2As2 (A = K, Rb, Cs)
• Preformed Cooper pairs in layered FeSe-based superconductors
• Superconducting gap in LiFeAs from three-dimensional spin-fluctuation pairing calculations
• Incommensurate antiferromagnetic fluctuations in single-crystalline LiFeAs studied by inelastic neutron scattering
• Unusual temperature evolution of superconductivity in LiFeAs
• Anomalous superconducting state in LiFeAs implied by the ${}^{75}$As Knight shift measurement
• Magnetic fluctuations and spin freezing in nonsuperconducting LiFeAs derivatives
• Antiferromagnetic spin correlations and pseudogap-like behavior in Ca(Fe1-xCox)2As2 studied by 75As nuclear magnetic resonance and anisotropic resistivity
• Anomalous hysteresis as an evidence for a magnetic field-induced chiral superconducting state in LiFeAs
• Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance
• Spatial competition of the ground states in 1111 iron pnictides
• A unified phase diagram of F-doped LaFeAsO by means of NMR and NQR parameters
• Defect states in LiFeAs as seen by low temperature scanning tunneling microscopy and spectroscopy
• Superconductivity of highly spin-polarized electrons in FeSe probed by ${}^{77}$Se NMR
• Direct observation of spin-orbit coupling in iron-based superconductors
• Local electromagnetic properties of magnetic pnictides: A comparative study probed by NMR measurement
• Absence of hexagonal to square structural transition in LiFeAs vortex matter
• 31P NMR Investigation of the Superconductor LiFeP (Tc = 5 K)