Robust dual topological character with spin-valley polarization in a monolayer of the Dirac semimetal Na3Bi

摘要

Topological materials with both insulating and semimetal phases can be protected by crystalline (e. g., mirror) symmetry. The insulating phase, called a topological crystalline insulator (TCI), has been investigated intensively and observed in three-dimensional materials. However, the predicted two-dimensional (2D) materials with TCI phase are explored much less than 3D TCIs and 2D topological insulators, while the 2D TCIs considered thus far possess almost exclusively a square-lattice structure with the mirror Chern number C-M = -2. Here, we predict theoretically that a hexagonal monolayer of Dirac semimetal Na3Bi is a 2D TCI with a mirror Chern number C-M = -1. The large nontrivial gap of 0.31 eV is tunable and can be made much larger via strain engineering, while the topological phases are robust against strain, indicating a high possibility for room-temperature observation of quantized conductance. In addition, a nonzero spin Chern number C-S = -1 is obtained, indicating the coexistence of a 2D topological insulator and a 2D TCI, i.e., the dual topological character. Remarkably, a spin-valley polarization is revealed in the Na3Bi monolayer due to the breaking of crystal inversion symmetry. The dual topological character is further explicitly confirmed via the unusual behavior of the edge states under the corresponding symmetry breaking.