Two-dimensional Weyl and type-III Dirac semimetals in BaCu monolayer and twisted α/β-BaCu/BN systems

Two-dimensional (2D) topological insulators with symmetry-protected helical edge states have drawn significant interest. The recently synthesized layered 2D electride BaCu features a monolayer structure with intriguing band crossings near Fermi level and a low exfoliation energy. In this study, first-principles calculations combined with symmetry analysis reveal that the BaCu monolayer behaves as a 2D topological insulator (TI) nature. When integrated with a 30° twisted \(\sqrt{3}\times \sqrt{3}\) hexagonal boron nitride (h-BN) supercell, the resulting twisted α/β-BaCu/BN heterobilayers exhibit 2D Weyl points and type-III Dirac points, respectively, demonstrating that twist angle can effectively modulate topological properties. Interestingly, ab initio molecular dynamics (AIMD) simulations reveal a spontaneous transition from the metastable β-BaCu/BN to α-BaCu/BN configuration, indicating a low energy barrier and highlighting the potential for property modulation, emphasizing the versatility of twisted structures for tuning topological states. This work establishes a robust platform for exploring twist-angle-induced topological electride states, broadening the scope for future investigations.