BiCh2-based (Ch: S, Se) layered superconductors have attracted extensive attentions because of variation of materials and physical characteristics, which include relatively large spin-orbit coupling originating from bismuth 6porbitals, and the possibility of anisotropic superconducting gap. Some of theoretical studies suggested that anisotropic superconductivity is realized in the BiCh2-based superconductors. In experimental studies, angle-resolved photoemission spectroscopy measurement on the superconducting states of Nd(O,F)BiS2have revealed the anisotropic structure of the superconducting gap, and the absence of isotope effect have been reported, indicating unconventional superconductivity pairing. Furthermore, two-fold-symmetric in-plane anisotropy of magnetoresistance have been observed in the superconducting states of some of Bi(S,Se)2-based systems like La(O,F)Bi(S,Se)2while the crystal structure possesses a tetragonal square plane with four-fold symmetry. Those results indicate nematic superconductivity is emerging in BiCh2-based superconductors. On the basis of the observations suggesting unconventional superconductivity in BiCh2-based systems, clarification of pairing mechanisms of superconductivity in BiCh2-based superconductors have been highly desired. In this article, we review experimental results on the superconducting gap structure, the pairing mechanism, and related phenomena of BiCh2-based superconductors.
Keywords: BiCh2-based superconductor; pairing mechanism; superconducting gap structure; unconventional superconductivity.
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