Inhomogeneous Knight shift in vortex cores of superconducting FeSe

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 Tc. 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.

Non-symmorphic symmetry and field-driven odd-parity pairing in CeRh$_2$As$_2$

“…Remarkably, there exist experimental results on superconducting FeSe, which also crystallizes in a P4/nmm structure, that suggest similar considerations apply. In particular, Knight shift measurements indicate that there is no change in the spin susceptibility upon entering the superconducting state for the field applied along the c-axis [25,26]. Within the framework we have discussed here, this could be explained by a nearly-vanishing g-factor for a c-axis Zeeman field due to strong spin-orbit coupling.…”