Topological origin of subgap conductance in insulating bilayer graphene

TL;DR

This paper demonstrates that topologically derived edge states in gapped bilayer graphene contribute significantly to conductance, remaining robust despite edge imperfections, explaining recent experimental observations.

Contribution

It provides a theoretical analysis showing the robustness of topological edge states in bilayer graphene against disorder, clarifying their role in conductance.

Findings

Edge modes contribute significantly to conductance in realistic devices.

Edge conductance dominates over bulk when the gap is large.

Topological origin explains experimental observations despite disorder.

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.