Strain-induced topological charge density wave in monolayer NbSe₂

Despite the progress made in the successful prediction of many classes of weakly-correlated topological materials, it is not clear whether a charge ordered topological state can exist in a two-dimensional (2D) material. Here, through first-principles modeling and analysis, we identify a 2×2 charge density wave (CDW) phase in monolayer 2H-NbSe₂ that harbors coexisting quantum spin Hall (QSH) insulator, topological crystalline insulator (TCI), and topological nodal line (TNL) semimetal states. The topology in monolayer NbSe₂ is driven by the formation of the CDW and the associated symmetry-breaking periodic lattice distortions and not via a pre-existing topology. Our finding of an emergent triple-topological state in monolayer 2H-NbSe₂ will offer novel possibilities for exploring connections between different topologies and a unique materials platform for controllable CDW-induced topological states for potential applications in quantum electronics and spintronics.