Spin fluctuations from Bogoliubov Fermi surfaces in the superconducting state of S-substituted FeSe

• Published in: Communications physics Vol. 6; no. 1; pp. 175 - 6
• Main Authors: Yu, Zhongyu, Nakamura, Koya, Inomata, Kazuya, Shen, Xiaoling, Mikuri, Taketora, Matsuura, Kohei, Mizukami, Yuta, Kasahara, Shigeru, Matsuda, Yuji, Shibauchi, Takasada, Uwatoko, Yoshiya, Fujiwara, Naoki
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 13.07.2023; Nature Publishing Group; Nature Portfolio
• Subjects: 639/301/119/997; 639/766/119/1003; Broken symmetry; Electrons; Elementary excitations; Fermi surfaces; Impurity effects; Iron; Magnetism; NMR; Nuclear magnetic resonance; Physics; Physics and Astronomy; Spin-orbit interactions; Substitutes; Superconductivity; Superconductors
• ISSN: 2399-3650; 2399-3650
• DOI: 10.1038/s42005-023-01286-x

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. Over the years the iron-based superconductors have become a platform to investigate many different phenomena such as the interplay between magnetism and superconductivity, nematicity and topological superconductivity. Here, the authors apply nuclear magnetic resonance to S-substituted FeSe in order to probe spin fluctuations, which indicate the presence of Bogoliubov Fermi surfaces.