NMR study on S-substituted FeSe: spin fluctuations from Bogoliubov Fermi surfaces

The study of the iron-based superconductor, FeSe, has resulted in various topics, such as the interplay among superconductivity, nematicity, and magnetism, BCS-BEC crossover, and FFLO superconductivity. Recently, topologically protected nodal Fermi surfaces, referred to as Bogoliubov Fermi surfaces (BFSs), have garnered much attention. A theoretical model for the S-substituted FeSe system demonstrated that BFSs can manifest under the conditions of spin-orbit coupling, multi-band systems, and superconductivity with time-reversal symmetry breaking.

To investigate electronic properties of the superconducting (SC) state on a microscopic level, we performed ⁷⁷Se-NMR measurements on FeSe_1-xS_x (x=0.05, 0.12, and 0.18) at very low temperatures to 100 mK. For x=0.18, we found anomalous spin fluctuations originating from BFSs deep in the SC state. The relaxation rate divided by temperature (1/T₁T), which provides a measure of low-energy spin fluctuations, exhibited an anomalous increase with decreasing temperature deep in the SC state despite that spin fluctuations should decrease due to the opening of the SC gap. Such unusual behavior cannot be explained by an impurity effect and implies the presence of significant spin fluctuations of Bogoliubov quasiparticles, which are associated with possible nesting properties between BFSs. We will discuss how the anomalous spin fluctuations evolve upon S substitution in the meeting.