# 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 in a superconducting material under pressure, supporting the existence of Bogoliubov Fermi surfaces.

## Contribution

The paper provides experimental evidence for Bogoliubov quasiparticle interactions under pressure in FeSe1−xSx.

## Key findings

- Anomalous low-energy spin fluctuations are suppressed but persist under pressure.
- Spin fluctuations in the normal and superconducting states show different temperature dependencies.
- Findings align with the theoretical model of Bogoliubov Fermi surfaces with C2 symmetry.

## 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 FeSe1 − xSx (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 77Se-NMR measurements of FeSe1 − xSx (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 C2 symmetry and strengthen evidence for Bogoliubov quasiparticle interactions, providing insights into the unconventional pairing state of this system.

The online version contains supplementary material available at 10.1038/s41598-025-13717-6.