Topological Phase Transitions Induced by Giant Strains Produced by Chemical Pressure*

Monolayer germanene has to be chemically functionalized to be stable. Though -CH₃ functionalized germanene has been predicted to undergo a topological phase transition when subjected to an external biaxial strain, achieving the requisite large values of strain by mechanical means is essentially impossible. We show that instead, chemical functionalization using -CX₃ groups (X = F, Cl, Br, I) induces giant chemical strains (of 9.5%, 37.4%, 48.9% and 62.8%, respectively) on the germanene lattice, relative to GeCH₃. For X = F and Cl, this causes the topological insulating phase to become stable at either a small or zero strain (with respect to the ground state geometry of GeCX₃), respectively. When X = Cl, the system undergoes a symmetry-lowering distortion that shifts the valence band maximum and conduction band minimum away from the zone center, while preserving the topological insulator phase. For X = Br, we obtain a trivial insulator, and for X = I, the system is unstable. Our finding that monolayer GeCCl₃ is a topological insulator under ambient conditions is of interest for possible applications in future devices.