Electronic Structure and Superconducting Gap Structure in BiS₂-based Layered Superconductors

Abstract

BiS2-based layered superconductors have attracted considerable interest owing to the relatively large spin–orbit coupling of bismuth 6p orbitals, the strong coupling nature by anharmonic phonons, and some evidence of an anisotropic superconducting gap structure. In the early stage of study, both conventional electron–phonon and unconventional mechanisms were examined as the superconducting pairing mechanism. Most experimental results can be well explained by the fully gapped strong-coupling s-wave state, which is driven by the electron–phonon interactions. Nevertheless, the recent angle-resolved photoemission measurement has indicated the possibility of a sign change of the superconducting gap. Furthermore, the absence of the isotope effect has been reported. These recent findings are at odds with the conventional s-wave pairing state, and rather imply unconventional pairing states. This means that the pairing glue also remains unclear. In this review article, we provide an overview of the crystal structures, electronic band structures, phonon properties, and candidates for the superconducting gap structures/pairing mechanisms in these superconductors. We also discuss the possibility of a nodal gap structure driven by purely attractive interactions.