Surface inducing high-temperature superconductivity in layered metal carborides Li ₂ BC ₃ and LiBC by metallizing σ electrons

Metallizing σ electrons provides a promising route to design high-temperature superconducting materials, such as MgB₂ and high-pressure hydrides. Here, we focus on two MgB₂-like layered carborides Li₂BC₃ and LiBC; their bulk does not have superconductivity because the B-C σ states are far away from the Fermi level (E_F), however, based on first-principles calculations, we found that when their bulk systems are cleaved into surfaces with B-C termination, high T_c of ∼80 K could be observed in the exposed B-C layer on the surfaces. Detailed analysis reveals that surface symmetry reduction, due to lattice periodic breaking, not only introduces hole self-doping into surface B-C layers and shifts the σ-bonding states towards the E_F - associated with emergent large electronic occupation, but also makes in-plane stretching modes on the surface layer experience significant softness. The enhanced σ states and softened phonon modes work to produce strong coupling, thus yielding high-T_c surface superconductivity, which distinctly differs from the superconducting features of the MgB₂ film, which generates phonon stiffness accompanied by suppressed superconductivity. Our findings undoubtedly provide a novel platform to realize high-T_c surface superconductivity, and also clearly elucidate the microscopic mechanism of surface-enhanced superconductivity in favor of creating more high-T_c surface superconductors among MgB₂-like layered materials.