Possible pairing mechanism switching driven by structural symmetry breaking in BiS2-based layered superconductors

TL;DR

This study shows that the pairing mechanism in BiS2-based superconductors can switch from unconventional to conventional due to structural symmetry changes induced by pressure, as evidenced by isotope effect measurements.

Contribution

It provides evidence that structural symmetry changes can alter the pairing mechanism in layered superconductors, highlighting the role of pressure-induced structural transitions.

Findings

Conventional isotope effects observed in high-pressure monoclinic phase.

Unconventional isotope effects in tetragonal structures.

Structural symmetry influences pairing mechanisms.

Abstract

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.