  The study of the iron-based superconductor, FeSe, has resulted in various
topics, such as the interplay among superconductivity, nematicity, and
magnetism, Bardeen-Cooper-Schrieffer Bose-Einstein-condensation (BCS-BEC)
crossover, and Fulde-Ferrell-Larkin-Ovchinnikov (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. Here we report the
observation of spin fluctuations originating from BFSs in the superconducting
(SC) state via $^{77}$Se-nuclear magnetic resonance measurements to 100 mK. In
a heavily S-substituted FeSe, we found an anomalous enhancement of low-energy
spin fluctuations deep in the SC state, which cannot be explained by an
impurity effect. Such unusual behavior implies the presence of significant spin
fluctuations of Bogoliubov quasiparticles, which are associated with possible
nesting properties between BFSs.
