BiSbF2 Monolayer: A 2D Inversion-Asymmetric Topological Insulator With Linearly Tunable Giant Spin-Splitting and Bulk Gap

Using first-principles calculations, an intriguing 2D topological insulator (TI), fluorinated β-BiSb monolayer (BiSbF₂ ML) is identified, which not only harbors topologically protected gapless edge states, but also contains spin-split bulk states with opposite Berry curvature and spin moment in inequivalent valleys. Specifically, its topological edge states reside in a sizable bulk gap of up to 252 meV, sufficiently large for realizing room-temperature quantum spin Hall effect. For its bulk states, there exist giant spin-orbit induced spin-splittings in both the uppermost valence band (390 meV) and the lowermost conduction band (478 meV) due to the breaking of inversion symmetry. In particular, both of the spin-splitting and the bulk gap can be linearly tuned by external strains from −5% to 5% in a considerable energy window of about 100 meV. Moreover, the intrinsic electronic structure of BiSbF₂ ML near the Fermi level can be well preserved in the substrate-supported BiSbF₂ ML. The results establish a new 2D inversion asymmetric TI with distinguished bulk state, which provides an ideal platform for exploring the combined effects among spintronics, valleytronics, and topological physics.