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Gate-controlled topological conducting channels in bilayer graphene

Nature Nanotechnology volume 11, pages 1060–1065 (2016)Cite this article

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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 spin1,2,3. In materials with broken inversion symmetry, such as an electrically gated bilayer graphene (BLG)4,5, 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 ballistically1,6,7,8,9,10,11. In BLG, the reversal of Ω can occur at the domain wall of AB- and BA-stacked domains12,13,14, or at the line junction of two oppositely gated regions6. The latter approach can provide a scalable platform to implement valleytronic operations, such as valves and waveguides9,15, 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 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.

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Figure 1: Device structure and characterization.
Figure 2: Evidences of kink states.
Figure 3: Calculated band structure and conductance of the kink states.
Figure 4: Kink-state resistance in a magnetic field.

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Acknowledgements

J.L., Z.Z. and J.Z. are supported by the Office of Naval Research under grant no. N00014-11-1-0730 and by the National Science Foundation (NSF) grant no. DMR-1506212. K.J.M. was supported by the NSF National Nanofabrication Infrastructure Network (NNIN) Research Experience for Undergraduates Program in 2013. K. Wang, Y.R. and Z.Q. are supported by the China Government Youth 1000-Plan Talent Program, the Fundamental Research Funds for the Central Universities (grant nos WK3510000001 and WK2030020027), the National Natural Science Foundation of China (grant no. 11474265) and the National Key R&D Program (grant no. 2016YFA0301700). K. Watanabe and T.T. are supported by the Elemental Strategy Initiative conducted by MEXT, Japan, and a Grant-in-Aid for Scientific Research on Innovative Areas ‘Science of Atomic Layers’ from Japan Society for the Promotion of Science (JSPS). T.T. is also supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘Nano Informatics’ (grant no. 25106006) from JSPS. Part of this work was performed at the National High Magnetic Field Laboratory (NHMFL), which was supported by NSF through NSF-DMR-0084173 and the State of Florida. The Supercomputing Center of the University of Science and Technology of China is acknowledged for the high-performance computing assistance. The authors acknowledge the use of facilities at the Pennsylvania State University site of NSF NNIN. We are grateful for helpful discussions with R. Du, H. Fertig, D. Goldhaber-Gordon, W. Halperin, S. Ilani, J. Jain, E. Kim, C. Liu, X. Li, A. H. MacDonald, Q. Niu, A. Paramekanti and A. Young. We thank J. Jaroszynski of the NHMFL for experimental assistance.

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Authors and Affiliations

  1. Department of Physics, The Pennsylvania State University, University Park, Pennsylvania 16802, USA

    Jing Li, Zachary Zern & Jun Zhu

  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, 230026, Anhui, China

    Ke Wang, Yafei Ren & Zhenhua Qiao

  3. and Department of Physics, CAS Key Laboratory of Strongly-Coupled Quantum Matter Physics, University of Science and Technology of China, Hefei, 230026, Anhui, China

    Ke Wang, Yafei Ren & Zhenhua Qiao

  4. Department of Electrical Engineering, Grove City College, Grove City, 16127, Pennsylvania, USA

    Kenton J. McFaul

  5. National Institute for Material Science, 1-1 Namiki, Tsukuba, 305-0044, Japan

    Kenji Watanabe & Takashi Taniguchi

  6. Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, 16802, Pennsylvania, USA

    Jun Zhu

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  1. Jing Li
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Contributions

J.Z. and J.L. conceived the experiment. J.L. designed and fabricated the devices and made the measurements. K.J.M. assisted in optimizing the procedure used to fabricate the bottom split gates. J.L. and Z.Z. performed the COMSOL simulations. J.L. and J.Z. analysed the data. K. Wang, Y.R. and Z.Q. did the theoretical calculations. K. Watanabe and T.T. synthesized the h-BN crystals. J.L., J.Z., K. Wang, Y.R. and Z.Q. wrote the manuscript with input from all authors.

Corresponding authors

Correspondence to Zhenhua Qiao or Jun Zhu.

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The authors declare no competing financial interests.

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Li, J., Wang, K., McFaul, K. et al. Gate-controlled topological conducting channels in bilayer graphene. Nature Nanotech 11, 1060–1065 (2016). https://doi.org/10.1038/nnano.2016.158

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