First-principles study of moderate phonon-mediated pairing in high-pressure monoclinic phase of BiS₂-based superconductors

Isotope effect on superconductive transition temperature (T_c) is an essential indicator to examine whether the mechanism of superconductors is conventional. Unconventional isotope effect of BiS₂-based superconductors has been previously reported in ambient-pressure tetragonal phase. However, to comprehensively ascertain the nature of superconductivity, the investigation of BiS₂-based system in high-pressure structure is highly desirable. In this work, we carried out the first-principles calculations of phonon spectra and superconductivity in high-pressure monoclinic phase of LaO_0.5F_0.5BiS₂ with ³²S and ³⁴S, and observed that the corresponding isotope coefficient is 0.13 ≤ α ≤ 0.20. This value is much greater than that of BiS₂-based superconductors in ambient-pressure phase, but slightly smaller than that of conventional MgB₂. Taking into account the calculated T_c lower than experimental results, we finally conclude that the moderate phonon-mediated pairing plays a significant role in forming superconductivity of BiS₂-based system in high-pressure phase, moreover, the cooperative multiple paring interactions should also be considered.