Electron-phonon coupling and superconductivity in LiB1+xC1−x

Abstract

By means of the first-principles density-functional theory calculation and Wannier interpolation, electron-phonon coupling and superconductivity are systematically explored for boron-doped LiBC (i.e., ), with x between 0.1 and 0.9. Hole doping introduced by boron atoms is treated through virtual-crystal approximation. For the investigated doping concentrations, our calculations show that the optimal doping concentration corresponds to 0.8. By solving the anisotropic Eliashberg equations, we find that LiB1.8C0.2 is a two-gap superconductor, whose superconducting transition temperature, Tc, may exceed the experimentally observed value of MgB2. Similar to MgB2, the two-dimensional bond-stretching E2g phonon modes along the line have the largest contribution to electron-phonon coupling. More importantly, we find that the first two acoustic phonon modes B1 and A1 around the midpoint of the line play a vital role for the rise of Tc in LiB1.8C0.2. The origin of strong couplings in B1 and A1 modes can be attributed to enhanced electron-phonon coupling matrix elements and softened phonons. It is revealed that all these phonon modes couple strongly with σ-bonding electronic states.