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Transport spectroscopy of symmetry-broken insulating states in bilayer graphene

Nature Nanotechnology volume 7, pages 156–160 (2012)Cite this article

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Abstract

Bilayer graphene is an attractive platform for studying new two-dimensional electron physics1,2,3,4,5, because its flat energy bands are sensitive to out-of-plane electric fields and these bands magnify electron–electron interaction effects. Theory6,7,8,9,10,11,12,13,14,15,16 predicts a variety of interesting broken symmetry states when the electron density is at the carrier neutrality point, and some of these states are characterized by spontaneous mass gaps, which lead to insulating behaviour. These proposed gaps6,7,10 are analogous17,18 to the masses generated by broken symmetries in particle physics, and they give rise to large Berry phase effects8,19 accompanied by spontaneous quantum Hall effects7,8,9,20. Although recent experiments21,22,23,24,25 have provided evidence for strong electronic correlations near the charge neutrality point, the presence of gaps remains controversial. Here, we report transport measurements in ultraclean double-gated bilayer graphene and use source–drain bias as a spectroscopic tool to resolve a gap of ∼2 meV at the charge neutrality point. The gap can be closed by a perpendicular electric field of strength ∼15 mV nm−1, but it increases monotonically with magnetic field, with an apparent particle–hole asymmetry above the gap. These data represent the first spectroscopic mapping of the ground states in bilayer graphene in the presence of both electric and magnetic fields.

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Figure 1: Device image, proposed gapped states in bilayer graphene and quantum Hall data at 300 mK.
Figure 2: Transport data at B = 0 and T = 300 mK.
Figure 3: Transport data in magnetic field at n = 0 and E⊥ = 0.
Figure 4: Transport data at constant B.

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Acknowledgements

The authors thank R. Nandkishore, B. Feldman, A. Yacoby, L. Levitov, P. Jarillo-Herrero and K. Novoselov for stimulating discussions, and D. Humphrey, G. Liu, A. Zhao and H. Zhang for assistance with fabrication. This work was supported in part by the UC LabFees programme, NSF CAREER DMR/0748910, NSF/1106358, ONR N00014-09-1-0724, ONR/DMEA H94003-10-2-1003 and the FENA Focus Center. D.S. acknowledges support from NHMFL UCGP #5068. Part of this work was performed at NHMFL, which is supported by NSF/DMR-0654118, the State of Florida, and DOE. A.M., J. J. and F.Z. acknowledge support from the Welch Foundation (grant TBF1473), NRI-SWAN and DOE (grant DE-FG03-02ER45958). C.V. acknowledges support from NSF DMR-0906530. V.A. acknowledges support from UCR I.C.

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Authors and Affiliations

  1. Department of Physics and Astronomy, University of California, Riverside, 92521, California, USA

    J. Velasco Jr, L. Jing, W. Bao, Y. Lee, P. Kratz, V. Aji, M. Bockrath, C. N. Lau & C. Varma

  2. National High Magnetic Field Laboratory, Tallahassee, 32310, Florida, USA

    R. Stillwell & D. Smirnov

  3. Department of Physics, University of Texas at Austin, Austin, 78712, Texas, USA

    Fan Zhang, J. Jung & A. H. MacDonald

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  1. J. Velasco Jr
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Contributions

C.N.L and J.V. conceived the experiments. Y.J. and P.K. isolated and identified the graphene sheets. R.S. assisted with sample preparation. J.V., L.J., W.B., Y.J. and D.S. performed transport measurements. C.N.L, M.B., W.B. and J.V. interpreted and analysed the data. V.A., C.V., F.Z., J.J. and A.H.M. interpreted data and performed theoretical calculations. C.N.L., J.V., F.Z., J.J. and A.H.M. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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Correspondence to C. N. Lau.

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Velasco, J., Jing, L., Bao, W. et al. Transport spectroscopy of symmetry-broken insulating states in bilayer graphene. Nature Nanotech 7, 156–160 (2012). https://doi.org/10.1038/nnano.2011.251

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