Van Hove singularity, flat bands, Dirac states, and superconductivity in van der Waals–bonded and covalently bonded bilayer borophene with a coloring triangular lattice

The coloring triangular (CT) lattice, which is equivalent to the kagome lattice, incorporates van Hove singularities, flat bands, and Dirac states, significantly advancing the field of materials discovery in kagome physics. Using first-principles calculations, this study identifies six stable bilayer borophene structures composed of two CT-latticed ${\mathrm{B}}_{9}$ monolayers arranged in three distinct stacking modes. These six bilayer structures can be categorized into two classes based on their interlayer interactions: three van der Waals-bonded and three covalently bonded bilayers. Interestingly, bilayer structures of the same stacking mode can interchange between vdW-bonded and covalently bonded forms under suitable conditions, such as biaxial strain or external pressure. As anticipated, the vdW-bonded bilayer structures display the characteristic kagome band topology, attributed to their weak interlayer interactions. In contrast, the covalently bonded bilayers exhibit versatile properties, including ideal Dirac semimetal behavior and phonon-mediated superconductivity, with a maximum ${T}_{\text{c}}$ of 17.96 K. This superconducting phase features Dirac states and van Hove singularities near the Fermi level, indicating the potential for realizing a topological superconductor. These findings offer valuable insights and opportunities for the design of CT-latticed materials and to create a platform to investigate the interplay between band topology, electron correlation, and superconductivity.