-based) superconductor*

We have investigated the selenium isotope effect on T_c in the layered bismuth chalcogenide (BiCh₂-based) superconductor LaO_0.6F_0.4Bi(S,Se)₂ using ⁷⁶Se and ⁸⁰Se. For all examined samples, the Se concentration, which is linked to the superconducting properties, is successfully controlled within x =1.09–1.14 in LaO_0.6F_0.4BiS_2−xSe_x. From the transition temperature estimated form the magnetization and electrical resistivity measurements, we found that the changes in T_c between ⁷⁶Se and ⁸⁰Se were apparently smaller than that expected from phonon-mediated pairing mechanism. The estimated isotope effect exponent α_Se is close to zero (-0.04 ≦ α_Se ≦ +0.04) [1]. Our results may indicate that the pairing in LaO_0.6F_0.4Bi(S,Se)₂ is not mediated by phonons, which is consistent with recent theoretical and experimental studies suggesting that unconventional pairing mechanisms are essential for the BiCh₂-based superconductor. We will show the recent results on sulfur isotope effect on the BiS₂-based superconductors.

[1] K. Hoshi, Y. Goto, Y. Mizuguchi, Phys. Rev. B 97, 094509 (2018).