Fast Electrically Switchable Large Gap Quantum Spin Hall States in MGe₂Z₄

Spin-polarized conducting edge currents counterpropagate in quantum spin Hall (QSH) insulators and are protected against disorder-driven localizations by the time-reversal symmetry. Using these spin-currents for device applications requires materials with a large bandgap and fast switchable QSH states. By means of in-depth first-principles calculations, this study demonstrates the large bandgap and fast switchable QSH state in a newly introduced 2D material family with 1T′-MGe₂Z₄ (M = Mo or W and Z = P or As). These Ge-based compounds show superior properties with respect to other members of the same family. For the WGe₂As₄ monolayer it can stabilize the 1T′-phase, while for the other members of the family, this study needs an appropriate strain. The dynamically stable 1T′-MGe₂Z₄ monolayers have a large energy gap up to 237 meV for WGe₂As₄. These materials undergo a phase transition from a QSH insulator to a trivial insulator with a Rashba-like spin splitting under the influence of an out-of-plane electric field, demonstrating a fast tunability of the bandgap and its band topology for the Ge-based compounds. Fast topological phase switching in a large gap 1T′-MGe₂Z₄ QSH insulators have potential applications in low-power devices, quantum computation, and quantum communication.