Superconductivity of highly spin-polarized electrons in FeSe probed by $^{77}$Se NMR

TL;DR

This study uses $^{77}$Se NMR to investigate the superconducting state of FeSe under high magnetic fields, revealing a highly spin-polarized, homogeneous superconducting phase in the BCS-BEC crossover regime.

Contribution

It provides direct evidence of a bulk, highly spin-polarized superconducting state in FeSe at high magnetic fields, advancing understanding of superconductivity in the BCS-BEC crossover.

Findings

Superconductivity persists with high spin polarization at strong magnetic fields.

The static spin susceptibility decreases significantly near the superconducting transition.

The superconducting state is spatially homogeneous and bulk in nature.

Abstract

A number of recent experiments indicate that the iron-chalcogenide FeSe provides the long-sought possibility to study bulk superconductivity in the cross-over regime between the weakly coupled Bardeen--Cooper--Schrieffer (BCS) pairing and the strongly coupled Bose--Einstein condensation (BEC). We report on $^{77}$Se nuclear magnetic resonance experiments of FeSe, focused on the superconducting phase for strong magnetic fields applied along the $c$ axis, where a distinct state with large spin polarization was reported. We determine this high-field state as bulk superconducting with high spatial homogeneity of the low-energy spin fluctuations. Further, we find that the static spin susceptibility becomes unusually small at temperatures approaching the superconducting state, despite the presence of pronounced spin fluctuations. Taken together, our results clearly indicate that FeSe indeed features an unusual field-induced superconducting state of a highly spin-polarized Fermi liquid in the BCS-BEC crossover regime.