Quasiparticle interaction originating from Bogoliubov Fermi Surfaces under pressure in 18%-S substituted FeSe studied via NMR

TL;DR

This study uses NMR to explore quasiparticle interactions from Bogoliubov Fermi Surfaces in S-substituted FeSe under pressure, revealing persistent low-energy excitations and insights into its unconventional superconducting pairing mechanism.

Contribution

It provides experimental evidence of Bogoliubov quasiparticle interactions in FeSe$_{1-x}$S$_x$ under pressure, supporting the BFS theoretical model and revealing pressure-dependent suppression of low-energy excitations.

Findings

Anomalous low-energy spin fluctuations persist under pressure.

Quasiparticle interactions weaken with increasing pressure.

Normal state spin fluctuations differ from those in the superconducting state.

Abstract

S-substituted FeSe superconductors in the tetragonal phase display several unique features among iron-based superconductors, particularly the presence of zero-energy excitations in the superconducting (SC) state. The recent concept of Bogoliubov Fermi Surfaces (BFSs), a theoretical model describing ultranodal states, has attracted considerable interest. Nuclear magnetic resonance (NMR) studies on FeSe$_{1-x}$S$_x$ (x=0.18) have revealed an anomalous low-energy spin fluctuations deep in the SC state. The low-energy spin fluctuations are enhanced with decreasing temperature, supporting strong Bogoliubov quasiparticle interactions associated with BFSs. Here, we further investigate these correlation effects through $^{77}$Se-NMR measurements of FeSe$_{1-x}$S$_x$ (x=0.18) under pressures up to 2.0 GPa and temperatures down to ~100 mK. The results demonstrate that the anomalous enhancement is suppressed but persists under pressure, implying that quasiparticle interactions become weak by applying pressure. Furthermore, spin fluctuations in the normal state exhibit different temperature dependence from those deep in the SC state, suggesting that the nesting properties of normal electrons differ from those of Bogoliubov quasiparticles. These findings are consistent with the theoretical model of BFSs with C$_2$ symmetry and strengthen evidence for Bogoliubov quasiparticle interactions, providing insights into the unconventional pairing state of this system.