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Nat Nanotechnol. 2016 Dec;11(12):1060-1065. doi: 10.1038/nnano.2016.158. Epub 2016 Aug 29.

Gate-controlled topological conducting channels in bilayer graphene.

Author information

1
Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
2
ICQD, Hefei National Laboratory for Physical Sciences at Microscale, and Synergetic Innovation Center of Quantum Information and Quantum Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
3
CAS Key Laboratory of Strongly-Coupled Quantum Matter Physics, and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, China.
4
Department of Electrical Engineering, Grove City College, Grove City, Pennsylvania 16127, USA.
5
National Institute for Material Science, 1-1 Namiki, Tsukuba 305-0044, Japan.
6
Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.

Abstract

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. In materials with broken inversion symmetry, such as an electrically gated bilayer graphene (BLG), the momentum-space Berry curvature Ω carries opposite sign in the K and K' valleys. A sign reversal of Ω 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 backscattering in the absence of valley-mixing scattering, and thus can carry current ballistically. In BLG, the reversal of Ω can occur at the domain wall of AB- and BA-stacked domains, or at the line junction of two oppositely gated regions. The latter approach can provide a scalable platform to implement valleytronic operations, such as valves and waveguides, 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 4e2/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.

PMID:
27570941
DOI:
10.1038/nnano.2016.158

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