Theoretical Prediction of Superconductivity in Boron Kagome Monolayer: MB3 (M = Be, Ca, Sr) and the Hydrogenated CaB3

Using first-principles calculations, we predict a new type of two-dimensional (2D) boride M B 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 M B 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 λ are 0.46, 1.09, and 1.33, and the corresponding superconducting transition temperatures T 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 λ of 1.39 and a higher T c of 39.3 K. Moreover, with further hole doping at the concentration of 5.8 × 10 11 hole/cm 2 , the T c of HCaB 3 can be further increased to 44.2 K, exceeding the McMillan limit. The predicted M B 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.