Electron-phonon coupling and superconductivity in LiB_1+xC_1−x

¹ Department of Microelectronics Science and Engineering, Faculty of Science, Ningbo University, Zhejiang 315211, China
² College of Physics and Engineering, Qufu Normal University - Shandong 273165, China

^(a) gaomiao@nbu.edu.cn
^(b) wangjun2@nbu.edu.cn

Received: 8 February 2018
Accepted: 6 June 2018

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 LiB_1.8C_0.2 is a two-gap superconductor, whose superconducting transition temperature, T_c, may exceed the experimentally observed value of MgB₂. Similar to MgB₂, the two-dimensional bond-stretching E_2g 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 T_c in LiB_1.8C_0.2. 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.