Van Hove singularity induced phonon-mediated superconductivity above 77 K in hole-doped SrB3C3

To design a phonon-mediated superconductor with transition temperature above the liquid-nitrogen temperature under ambient pressure, we align the Fermi level of compound ${\mathrm{SrB}}_{3}{\mathrm{C}}_{3}$ to a Van Hove singularity by introducing 0.4 hole/f.u., namely ${\mathrm{Rb}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$. Based on density functional first-principles calculations and the Wannier interpolation technique, the electronic structure, lattice dynamics, and electron-phonon coupling (EPC) of ${\mathrm{Rb}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ are investigated. Both the enlarged density of states at the Fermi level and the softened phonons play important roles to enhance the EPC. By solving the anisotropic Eliashberg equations, we find that the transition temperature of ${\mathrm{Rb}}_{0.4}{\mathrm{Sr}}_{0.6}{\mathrm{B}}_{3}{\mathrm{C}}_{3}$ is 83 K, successfully exceeding the liquid-nitrogen temperature at ambient pressure condition. Moreover, our calculations reveal that there is a single-gap to two-gap superconductivity transition in comparison with ${\mathrm{SrB}}_{3}{\mathrm{C}}_{3}$.