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.
Highlights
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
We confirmed that the structural characteristics of the examined samples are comparable on the basis of powder X-ray diffraction (XRD) analyses (Fig. 1a,b)
The reason for proposing the scenario is the recent observation of nematic superconductivity in La(O,F) BiSSe33,34; nematic superconductivity has been observed in unconventional superconductors like Fe-based and Bi2Se3-based superconductors[35,36]
Summary
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 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. BiCh2-based compounds are semiconductors with a band gap, electron doping of the BiCh2 layers makes the system metallic, and superconductivity is induced. An example of this is F substitution in REOBiCh2 (RE: rare earth)[9,10,11]. The emergence of bulk superconductivity due to chemical pressure effects can be explained by the suppression of local structural disorder, which is caused by the presence of Bi lone pair e lectrons[18,19,20]
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