Observation of intrinsic time-reversal invariant Z₂ topological phase in bilayer graphene

Topological insulators, along with Chern insulators and Quantum Hall insulator phases, are considered as paradigms for symmetry protected topological phases of matter. In this letter, we report the experimental realization of the time-reversal invariant Z₂ topological phase [Phys. Rev. Lett. 95, 146802 (2005)] in bilayer graphene-based heterostructures -- a phase generally considered as a precursor to the field of generic topological insulators. Our observation of this elusive phase depended crucially on our ability to create mesoscopic devices comprising of both a moir\'e superlattice potential and strong spin-orbit coupling; this resulted in materials whose electronic band structure could be tuned from trivial to topological by an external displacement field. We find that the topological phase is characterized by a bulk bandgap and helical edge modes with electrical conductance quantized exactly to 2e²/h in zero external magnetic field. We also present results of a theoretical study of a realistic model of the Z₂ topological insulator which, in addition to replicating our experimental results, explains the origin of the topological insulating bulk and helical edge modes.