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 LaO0.5F0.5BiS2 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. [ABSTRACT FROM AUTHOR]