Phase-Sensitive Determination of the Superconducting-Gap Structure in Tetragonal FeSe<sub>1-x</sub>S<sub>x</sub>
ORAL
Abstract
FeSe exhibits superconductivity within an electronic nematic state, which is suppressed by substituting sulfur for selenium. When the sulfur concentration exceeds 17%, nematicity vanishes, yet superconductivity persists. In this tetragonal phase, the superconducting state is marked by a substantial residual density of states at the Fermi level [1,2]. Although several theoretical models have been proposed to explain this "ultranodal" superconducting state [3,4], experimental evidence remains insufficient to comprehensively understand this exotic phase. In particular, the momentum-space structure of the superconducting gap remains elusive.
In this study, we employ spectroscopic-imaging scanning tunneling microscopy on FeSe0.8S0.2 to visualize Bogoliubov quasiparticle interference (QPI) patterns, which encode information about the superconducting gap. Phase-referenced QPI analysis enables the extraction of both the phase information in momentum space and the superconducting gap dispersion [5]. Our results identify a nodal d+s-wave gap structure, suggesting that nematicity continues to influence the system even in the nominally tetragonal phase. The spatially averaged superconducting gap spectrum is well reproduced by this d+s-wave model, provided energy-dependent quasiparticle damping is assumed. The large residual density of states originates from a low-lying van Hove singularity in the Bogoliubov quasiparticle dispersion, broadened by this damping.
[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, arXiv:1710.09089.
In this study, we employ spectroscopic-imaging scanning tunneling microscopy on FeSe0.8S0.2 to visualize Bogoliubov quasiparticle interference (QPI) patterns, which encode information about the superconducting gap. Phase-referenced QPI analysis enables the extraction of both the phase information in momentum space and the superconducting gap dispersion [5]. Our results identify a nodal d+s-wave gap structure, suggesting that nematicity continues to influence the system even in the nominally tetragonal phase. The spatially averaged superconducting gap spectrum is well reproduced by this d+s-wave model, provided energy-dependent quasiparticle damping is assumed. The large residual density of states originates from a low-lying van Hove singularity in the Bogoliubov quasiparticle dispersion, broadened by this damping.
[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, arXiv:1710.09089.
*This work was supported by KAKENHI Grants No. 24H00198 and No. 25H01249 from JSPS and MEXT of Japan. This work was also supported by the RIKEN TRIP initiative (Many-body Electron Systems).
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Presenters
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Tetsuo Hanaguri
- RIKEN