BCS-BEC crossover in FeSe-based superconductors
ORAL · Invited
Abstract
The physics of the crossover between weak-coupling BCS and strong-coupling BEC limits gives a unified framework of quantum-bound (superfluid) states of interacting fermions. This crossover has been studied in the ultracold atomic systems but has been challenging to find a solid-state realization. Here we show that superconducting Fe(Se1-xSx) offers the possibility to enter the previously unexplored realm where the two energies, Fermi energy εF, and superconducting gap Δ, become comparable, indicating that this system is deep inside the BCS–BEC crossover regime [1]. Through scanning tunneling microscopy and laser-excited angle-resolved photoemission spectroscopy, we demonstrate that εF of Fe(Se1-xSx) is extremely small, with the ratio Δ/εF∼0.3-1 for all bands [2][3]. We discuss several unusual superconducting properties associated with the crossover, including non-Gaussian superconducting fluctuations [4], pseudogap, quantum vortex core [5], FFLO phase [6], and unusual Bogoliubov quasiparticle band dispersions [3]. Some of these properties are not expected for single-band superconductors, which calls for a new mechanism of BCS-BEC crossover in the multiband system.
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Publication: [1] T. Shibauchi, T. Hanaguri, and Y. Matsuda, J. Phys. Soc. Jpn. 89, 102002 (2020).<br>[2] S. Kasahara et al., PNAS 111, 16309 (2014).<br>[3] T. Hashimoto et al. Sci. Adv. 6, eabb9052 (2020).<br>[4] S. Kasahara et al. Nature Commun. 7, 12843 (2016).<br>[5] T. Hanaguri et al. Phys. Rev. Lett. 122, 077001 (2019).<br>[6] S. Kasahara et al. Phys. Rev. Lett. 124, 107001 (2020).
Presenters
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Yuji Matsuda
Kyoto University, Kyoto Univ
Authors
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Yuji Matsuda
Kyoto University, Kyoto Univ