Two-gap topological superconductor LaB₂ with high T_c = 30 K

Since two gap superconductivity was discovered in MgB₂, research on multigap superconductors has attracted increasing attention because of its intriguing fundamental physics. In MgB₂, the Mg atom donates two electrons to the borophene layer, resulting in a stronger gap from the σ band and a weaker gap from the π bond. First-principles calculations demonstrate that the two gap anisotropic superconductivity strongly enhances the transition temperature of MgB₂ in comparison with that given by the isotropic model. In this work, we report a three-band (B-σ, B-π, and La-d) two-gap superconductor LaB₂ with very high T_c = 30 K by solving the fully anisotropic Migdal-Eliashberg equation. Because of the σ and π-d hybridization on the Fermi surface, the electron-phonon coupling constant λ = 1.5 is significantly larger than the λ = 0.7 of MgB₂. Our work paves a new route to enhance the electron-phonon coupling strength of multigap superconductors with d orbitals. On the other hand, our analysis reveals that LaB₂ belongs to the weak topological semimetal category, leading to a possible topological superconductor with the highest T_c to date. Moreover, upon applying pressure and/or doping, the topology is tunable between weak and strong with T_c varying from 15 to 30 K, opening up a flexible platform for manipulating topological superconductors.