Possible pairing mechanism switching driven by structural symmetry breaking in BiS(2)-based layered superconductors

Investigation of isotope effects on superconducting transition temperature (T(c)) is one of the useful methods to examine whether electron–phonon interaction is essential for pairing mechanisms. The layered BiCh(2)-based (Ch: S, Se) superconductor family is a candidate for unconventional superconductors, because unconventional isotope effects have previously been observed in La(O,F)BiSSe and Bi(4)O(4)S(3). In this study, we investigated the isotope effects of (32)S and (34)S in the high-pressure phase of (Sr,La)FBiS(2), which has a monoclinic crystal structure and a higher T(c) of ~ 10 K under high pressures, and observed conventional-type isotope shifts in T(c). The conventional-type isotope effects in the monoclinic phase of (Sr,La)FBiS(2) are different from the unconventional isotope effects observed in La(O,F)BiSSe and Bi(4)O(4)S(3), which have a tetragonal structure. The obtained results suggest that the pairing mechanisms of BiCh(2)-based superconductors could be switched by a structural-symmetry change in the superconducting layers induced by pressure effects.