Gate-controlled topological conducting channels in bilayer graphene

Abstract

The existence of inequivalent valleys K and K′ in the momentum space of two-dimensional hexagonal lattices provides a new electronic degree of freedom, the manipulation of which can potentially lead to new types of electronics, in analogy to the role played by electron spin 1-3 . In materials with broken inversion symmetry, such as an electrically gated bilayer graphene 4,5 , the momentum-space Berry curvature  carries opposite sign in the K and K′ valleys. A sign reversal of  along an internal boundary of the sheet gives rise to counterpropagating one-dimensional conducting modes encoded with opposite valley indices. These metallic states are topologically protected against backscattering in the absence of valley-mixing scattering, and thus can carry current ballistically 1,6-11 . In bilayer graphene, the reversal of  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 is technically challenging to realize. Here we fabricate a dual-split-gate structure in bilayer graphene and demonstrate transport evidence of the predicted metallic states. They possess a mean free path of up to a few hundred nanometers in the absence of a magnet field. The application of perpendicular magnetic field suppresses backscattering significantly and enables a 400-nanometer-long junction to exhibit conductance close to the ballistic limit of 4 e 2 /h at 8 Tesla. Our experiment paves the path to the realization of gate-controlled ballistic valley transport and the development of valleytronic applications in atomically thin materials.Exploiting the valley degree of freedom in hexagonal lattices may offer an alternative pathway to achieving low-power-consumption electronics. Experiments have shown that a net valley polarization in the material can be induced by the use of circularly polarized light 2,16,17 or a net bulk current [18][19][20] . However the use of light is not always desirable in electronics and device proposals using bulk valley polarization often put stringent requirements on the size and edge orientation of the active area 3 . Alternatively, electrically created, valley-polarized topological conducting channels in high-mobility bilayer graphene may offer a robust, scalable platform to realize valleytronic operations 1,6,[8][9][10][11][12][13][14][15] . Figure 1a illustrates the dual-split-gating scheme proposed by Martin et al 6 , where an AB-stacked bilayer graphene (BLG) sheet is controlled by two pairs of top and bottom gates separated by a line junction. The device operates in the regime where both the left and the right regions of the BLG sheet are insulating due to a bulk band gap induced by the independently applied displacement fields DL and DR. In the "odd" field configuration, where DLDR < 0, theory predicts the existence of eight conducting modes (referred to as the "kink" states) propagating along the line ...

Bilayer Graphene as a Platform for Bosonic Symmetry-Protected Topological States

“…The edge state in our system is also very different from the cases studied in Ref. 27,28, since in those systems the states localized at the domain wall is unstable to disorder.…”

Topologically protected refraction of robust kink states in valley photonic crystals

“…Of the utmost interest to optical communications is their ability to support topologically protected chiral edge (kink) states [3][4][5][6][7][8][9] at the internal domain wall between two VPCs with opposite valley-Chern indices. Here we experimentally demonstrate valley-polarized kink states with polarization multiplexing in VPCs, designed from a spin-compatible four-band model.…”

Gate-tunable current partition in graphene-based topological zero lines

“…13,19 At the domain wall interfaces between different bulk gap signs, in particular for the one-dimensional zero-line modes appearing at the interface between regions with opposite valley Hall conductivities, [20][21][22][23][24][25][26][27] numerical studies have shown transport properties that are robust against backscattering, leading to practically ballistic transport through zero-line modes with mean free paths of the order of hundreds of microns in relatively clean samples. 28,29 This remarkable robustness against backscattering would be preserved in the presence of turns and bends of the zero lines suggesting that essentially arbitrary trajectories would preserve excellent transport qualities. Recent years have witnessed experimental progresses towards the realization of these zero-line channels through careful gate alignment 29,30 or in samples of bilayer graphene with stacking faults.…”

“…28,29 This remarkable robustness against backscattering would be preserved in the presence of turns and bends of the zero lines suggesting that essentially arbitrary trajectories would preserve excellent transport qualities. Recent years have witnessed experimental progresses towards the realization of these zero-line channels through careful gate alignment 29,30 or in samples of bilayer graphene with stacking faults. 31 While the field is yet in its infancy, it is expected that research of transport along topological zero lines will thrive in a near future.…”