Spin-polarised conducting edge currents, flowing in opposite directions in a spin quantum Hall (QSH) insulator, are protected by time-reversal symmetry from disorder-induced localisation. Using these spin-currents for device applications requires materials with a large bandgap and fast switchable QSH states.

Employing in-depth first-principles calculations, IFPAN researchers Carmine Autieri, Rajibul Islam, Ghulam Hussain in collaboration with other theory groups have demonstrated the presence of just such a large bandgap and fast switchable QSH states 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 requires 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.