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

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.

6 pages, 4 figures

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• Quasiparticle Excitations in the Superconducting State of FeSe Probed by Thermal Hall Conductivity in the Vicinity of the BCS-BEC Crossover
• Singular magnetic anisotropy in the nematic phase of FeSe

Cited by in corpus (5)

• Non-symmorphic symmetry and field-driven odd-parity pairing in CeRh${}_2$As${}_2$
• Quasiparticle Nodal Plane in the Fulde-Ferrell-Larkin-Ovchinnikov State of FeSe
• Spin fluctuations from Bogoliubov Fermi surfaces in the superconducting state of S-substituted FeSe
• Suppression of nematicity by tensile strain in multilayer FeSe/SrTiO${}_3$ films
• Quasiparticle interaction originating from Bogoliubov Fermi Surfaces under pressure in 18%-S substituted FeSe studied via NMR