Prediction of topological phase transition in X2-SiGe monolayers.

Quantum spin Hall (QSH) insulators exhibit a bulk insulting gap and metallic edge states characterized by nontrivial topology. Here, we used first-principles calculations to investigate the electronic and topological properties of halogenated silicon germanide (X2-SiGe, with X = F, Cl, and Br) monolayers, which we found to be trivial semiconductors with energy band gaps ranging from 500 meV to 900 meV. Interestingly, we found that under 8% strain, X2-SiGe monolayers behave as QSH insulators with global band gaps between 53 meV and 123 meV. The underlying mechanism of the topological phase transition is the strain-induced s-p band inversion. The nontrivial topological features for the strained X2-SiGe monolayers were further confirmed by the presence of topologically protected edge states that form a single Dirac cone in the middle of the bulk band gaps. Therefore, our results reveal that this new family of QSH insulators is promising for room temperature applications in spintronics and quantum computation devices.