Abstract
Valley pseudospin, the quantum degree of freedom characterizing the degenerate valleys in energy bands1, is a distinct feature of two-dimensional Dirac materials1,2,3,4,5. Similar to spin, the valley pseudospin is spanned by a time-reversal pair of states, although the two valley pseudospin states transform to each other under spatial inversion. The breaking of inversion symmetry induces various valley-contrasted physical properties; for instance, valley-dependent topological transport is of both scientific and technological interest2,3,4,5. Bilayer graphene is a unique system whose intrinsic inversion symmetry can be controllably broken by a perpendicular electric field, offering a rare possibility for continuously tunable topological valley transport. We used a perpendicular gate electric field to break the inversion symmetry in bilayer graphene, and a giant nonlocal response was observed as a result of the topological transport of the valley pseudospin. We further showed that the valley transport is fully tunable by external gates, and that the nonlocal signal persists up to room temperature and over long distances. These observations challenge the current understanding of topological valley transport in a gapped system, and the robust topological transport may lead to future valleytronic applications.
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Acknowledgements
We thank Q. Niu, D.-H. Lee, F. Wang, J. Shi, J. Xiao, P. Kim and J. Zhu for helpful discussions. Part of the sample fabrication was performed at the Fudan Nano-fabrication Lab. M.S., G.C., L.M. and Y.Z. acknowledge the financial support of the National Basic Research Program of China (973 Program) under grant nos 2013CB921902 and 2011CB921802, and NSF of China under grant nos 11034001 and 11425415. W.Y. acknowledges support from the University of Hong Kong (OYRA), and the RGC of Hong Kong SAR (HKU706412P). W.-Y.S. and D.X. acknowledge support from the US Department of Energy, Office of Science, Office of Basic Energy Science, under award no. DE-SC0012509. D.T. and X.J. are supported by MOST (grants no. 2015CB921400 and no. 2011CB921802) and NSF of China (grants no. 11374057, no. 11434003 and no. 11421404). K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan. T.T. acknowledges support from Grant-in-Aid for Scientific Research (Grant 262480621) and Innovative Areas ‘Nano Informatics’ (Grant 25106006) from JSPS.
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M.S. fabricated the samples, carried out the measurements, and analysed the data. G.C. helped with sample fabrication. L.M. helped with electrical measurements. W.-Y.S. and D.T. helped with data analysis. K.W. and T.T. grew hBN crystals. Y.Z., D.X., W.Y. and X.J. co-supervised the project. All authors contributed to the writing of the manuscript.
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Sui, M., Chen, G., Ma, L. et al. Gate-tunable topological valley transport in bilayer graphene. Nature Phys 11, 1027–1031 (2015). https://doi.org/10.1038/nphys3485
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DOI: https://doi.org/10.1038/nphys3485
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