Two-dimensional topological insulators exfoliated from NaBi-like Dirac semimetals.

Topological insulation is widely predicted in two-dimensional (2D) materials realized by epitaxial growth or van der Waals (vdW) exfoliation. Such 2D topological insulators (TI's) host many interesting physical properties such as the quantum spin Hall effect and superconductivity. Here, we extend the search of 2D TI's into the exfoliatable non-vdW 2D crystals. We find that three-dimensional Dirac semimetals A3Bi (A = Na, K, Rb) (P3[combining macron]c1) can be exfoliated into 2D materials with exfoliation energies of 0.479-0.990 J m-2. Our careful examination of the topological invariants of exfoliated A3Bi monolayers/multilayers by using two well-established approaches reveals that bilayer and tetralayer Na3Bi are 2D TI's. It is found that the band gap of 2D TI's can be significantly increased by external strain. We further find that the predicted 2D TI's possess interesting hidden Rashba-like spin textures. Our results suggest a new arena to search for two-dimensional topological insulators and spintronic materials.