FeSe exhibits a number of unusual superconducting properties. A key feature is that superconductivity emerges within the electronic nematic phase associated with orbital ordering. Substituting more than 17% of Se with S suppresses the nematic order, while superconductivity persists. In this tetragonal phase, a large residual density of states remains at the Fermi level [1,2], suggesting an unconventional gap structure [3,4]. To identify the superconducting gap structure in this "ultranodal" state, we performed spectroscopic-imaging scanning tunneling microscopy on FeSe0.8S0.2, enabling the observation of Bogoliubov quasiparticle interference (QPI) patterns. Phase-referenced QPI analysis [5,6] allows us to extract momentum-resolved phase information of the superconducting gap. Our results reveal a nodal d+s-wave gap structure, indicating that nematicity persists even in the nominally tetragonal phase. The measured tunneling spectra are well reproduced by the d+s-wave gap model, supporting its validity. The dominance of the d-wave channel provides important insight into the nature of pairing in Fe(Se,S).
This work was carried out in collaboration with A. Matsuno and S. Kasahara.

[1] T. Hanaguri et al., Sci. Adv. 4, eaar6419 (2018).
[2] Y. Sato et al., PNAS 115, 1227 (2018).
[3] C. Setty et al., Nature Commun. 11, 523 (2020).
[4] K. R. Islam and A. Chubukov, npj Quantum Mater. 9, 28 (2024).
[5] S. Chi et al., arXiv:1710.09088.
[6] S. Chi et al., arXiv:1710.09089.