Inhomogeneous Knight shift in vortex cores of superconducting FeSe

TL;DR

This study uses $^{77}$Se NMR to investigate the vortex core electronic structure in FeSe, revealing inhomogeneous Knight shifts and spin-singlet pairing with anisotropic behavior in the superconducting state.

Contribution

It provides new insights into the inhomogeneous electronic states in FeSe vortex cores and their relation to nematicity and spin-orbit coupling.

Findings

Knight shift suppressed in-plane below T_c but not zero at zero temperature

Little or no Knight shift reduction for out-of-plane fields

Enhanced electronic inhomogeneity in the superconducting state

Abstract

We report $^{77}$Se NMR data in the normal and superconducting states of a single crystal of FeSe for several different field orientations. The Knight shift is suppressed in the superconducting state for in-plane fields, but does not vanish at zero temperature. For fields oriented out of the plane, little or no reduction is observed below $T_c$. These results reflect spin-singlet pairing emerging from a nematic state with large orbital susceptibility and spin-orbit coupling. The spectra and spin-relaxation rate data reveal electronic inhomogeneity that is enhanced in the superconducting state, possibly arising from enhanced density of states in the vortex cores. Despite the spin polarization of these states, there is no evidence for antiferromagnetic fluctuations.