A large-gap quantum spin Hall state in exfoliated Na3Bi-like two-dimensional materials

Abstract

Two-dimensional (2D) topological insulators (TIs) with gapped 2D electronic structures and metallic edge states protected by time-reversal symmetry possess the intriguing quantum spin Hall effect that has attracted intensive attention recently. A large-gap quantum spin Hall state has been sought for decades, which is essential for realizing high-temperature topological devices based on 2D TIs. Here, we designed a class of large-gap 2D TIs by exfoliating Na₃Bi-like alkali bismide three-dimensional Dirac semimetals, followed by modulation via external strain and an external electric field, predicting a quantum spin Hall state with a large gap of up to 0.473 eV. The designed 2D TIs (K₂NaBi and Rb₂NaBi monolayers) with benign dynamical, thermal, and mechanical stability have calculated exfoliation energies of 0.35–0.59 J m⁻² from density functional theory, which is much lower than the corresponding Na₃Bi monolayer TI (0.81 J m⁻²) that is found to be dynamically unstable. The near band edge spin splitting magnitude of the designed 2D TIs increases significantly under an external electric field, with the corresponding spin texture changing from the non-helical Dresselhaus spin texture to the partially helical Rashba–Dresselhaus-like spin texture. Our work proposes a new arena to search for 2D topological insulators with large-gap quantum spin Hall states, which can be used for future spintronic devices and quantum computing.