High-temperature superconductivity in SrB_3C_3 and BaB_3C_3 predicted from first-principles anisotropic Migdal-Eliashberg theory

摘要

Very recently, carbon-boron clathrate SrB_3C_3 has been successfully synthesized, in which carbon and boron atoms form sp~3-bonded truncated octahedral cages. Interestingly, the sp~3-hybridized σ-bonding bands are partially occupied. This may drive SrB_3C_3 into a superconducting state, like boron-doped diamond. By means of density functional first-principles calculations and Wannier interpolation technique, we have investigated the electron-phonon coupling and phonon-mediated superconductivity in SrB_3C_3. Our calculations reveal that there exists strong coupling between sp~3-hybridized σ-bonding bands and boron-associated E_g phonon modes. Based on the Migdal-Eliashberg theory, we self-consistently solve the anisotropic Eliashberg equations. It is found that SrB_3C_3 is a single-gap superconductor, with superconducting transition temperature being 40 K. The anisotropic ratio of superconducting energy gap is computed to be 32.8%. Further replacing Sr with Ba, the transition temperature can be boosted to 43 K in BaB_3C_3 due to phonon softening. These findings suggest that SrB_3C_3 and BaB_3C_3 are phonon-mediated high-temperature anisotropic s-wave superconductors.