Quantum Phase Transition in Germanene and Stanene Bilayer: From Normal Metal to Topological Insulator.

Two-dimensional (2D) topological insulators (TIs) that exhibit quantum spin Hall effects are a new class of materials with conducting edge and insulating bulk. The conducting edge bands are spin-polarized, free of back scattering, and protected by time-reversal symmetry with potential for high-efficiency applications in spintronics. On the basis of first-principles calculations, we show that under external pressure recently synthesized stanene and germanene buckled bilayers can automatically convert into a new dynamically stable phase with flat honeycomb meshes. In contrast with the active surfaces of buckled bilayer of stanene or germanene, the above new phase is chemically inert. Furthermore, we demonstrate that these flat bilayers are 2D TIs with sizable topologically nontrivial band gaps of ∼0.1 eV, which makes them viable for room-temperature applications. Our results suggest some new design principles for searching stable large-gap 2D TIs.