  Investigation of isotope effects on superconducting transition temperature
(Tc) is one of the useful methods to examine whether electron-phonon
interaction is essential for pairing mechanisms. The layered BiCh2-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 Bi4O4S3. In this study, we investigated the isotope effects of
32S and 34S in the high-pressure phase of (Sr,La)FBiS2, which has a monoclinic
crystal structure and a higher Tc of 10 K under high pressures, and observed
conventional-type isotope shifts in Tc. The conventional-type isotope effects
in the monoclinic phase of (Sr,La)FBiS2 are different from the unconventional
isotope effects observed in La(O,F)BiSSe and Bi4O4S3, which have a tetragonal
structure. The obtained results suggest that the pairing mechanisms of
BiCh2-based superconductors could be switched by a structural-symmetry change
in the superconducting layers induced by pressure effects.
