Gate-controlled topological conducting channels in bilayer graphene

The existence of inequivalent valleys K and K' in the momentum space of 2D hexagonal lattices provides a new electronic degree of freedom, the manipulation of which can potentially lead to new types of electronics, analogous to the role played by electron spin(1-3). In materials with broken inversion symmetry, such as an electrically gated bilayer graphene (BLG)(45), the momentum-space Berry curvature Omega carries opposite sign in the K and K' valleys. A sign reversal of Omega along an internal boundary of the sheet gives rise to counterpropagating 1D conducting modes encoded with opposite-valley indices. These metallic states are topologically protected against back scattering in the absence of valley-mixing scattering, and thus can carry current ballistically(1.6-11). In BLG, the reversal of Omega can occur at the domain wall of AB- and BA-stacked domains(12-14), or at the line junction of two oppositely gated regions(6). The latter approach can provide a scalable platform to implement valleytronic operations, such as valves and waveguides(9,15), but it is technically challenging to realize. Here, we fabricate a dual split -gate structure in BLG and present evidence of the predicted metallic states in electrical transport. The metallic states possess a mean free path (MFP) of up to a few hundred nanometres in the absence of a magnetic field. The application of a perpendicular magnetic field suppresses the backscattering significantly and enables a junction 400 nm in length to exhibit conductance close to the ballistic limit of 4e(2)/h at 8 T. Our experiment paves the way to the realization of gate-controlled ballistic valley transport and the development of valleytronic applications in atomically thin materials.