Theoretical Prediction of Superconductivity in Boron Kagome Monolayer: MB_3 (M = Be, Ca, Sr) and the Hydrogenated CaB_3

Using first-principles calculations, we predict a new type of two-dimensional (2D) boride MB_3 (M = Be, Ca, Sr), constituted by boron kagome monolayer and the metal atoms adsorbed above the center of the boron hexagons. The band structures show that the three MB_3 compounds are metallic, thus the possible phonon-mediated superconductivity is explored. Based on the Eliashberg equation, for BeB_3, CaB_3, and SrB_3, the calculated electron–phonon coupling constants \lambda are 0.46, 1.09, and 1.33, and the corresponding superconducting transition temperatures T_\rm c are 3.2, 22.4, and 20.9 K, respectively. To explore superconductivity with higher transition temperature, hydrogenation and charge doping are further considered. The hydrogenated CaB_3, i.e., HCaB_3, is stable, with the enhanced \lambda of 1.39 and a higher T_\rm c of 39.3 K. Moreover, with further hole doping at the concentration of 5.8\times 10^11 hole/cm^2, the T_\rm c of HCaB_3 can be further increased to 44.2 K, exceeding the McMillan limit. The predicted MB_3 and HCaB_3 provide new platforms for investigating 2D superconductivity in boron kagome lattice since superconductivity based on monolayer boron kagome lattice has not been studied before.