Two-dimensional surface state in the quantum limit of a topological insulator

Abstract

The edges of graphene-based systems possess unusual electronic properties, originating from the non-trivial topological structure associated to the pseudo-spinorial character of the electron wave-functions. These properties, which have no analogue for electrons described by the Schrodinger equation in conventional systems, have led to the prediction of many striking phenomena, such as gate-tunable ferromagnetism and valley-selective transport. In most cases, however, the predicted phenomena are not expected to survive the influence of the strong structural and chemical disorder that unavoidably affects the edges of real graphene devices. Here, we present a theoretical investigation of the intrinsic low-energy states at the edges of electrostatically gapped bilayer graphene (BLG), and find that the contribution of edge modes to the conductance of realistic devices remains sizable even for highly imperfect edges. This edge conductance dominates over the bulk contribution if the electrostatically induced gap is sufficiently large, and accounts for seemingly conflicting observations made in recent transport and optical spectroscopy experiments. Our results illustrate the robustness of phenomena whose origin is rooted in the topology of the electronic band-structure, even in the absence of specific protection mechanisms.