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Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene

Nature Materials volume 12, pages 887–892 (2013)Cite this article

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Abstract

Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions1,2,3 that have a bandgap tunable by a transverse electric field3,4. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene5,6,7, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist8,9,10, strain11,12, or electronic interactions7,13,14 that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. Here we show both by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ~0.1°, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum not only complements the framework of massive Dirac fermions, widely relevant to charge transport in bilayer graphene, but also supports the possibility of valley Hall transport15.

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Figure 1: Dirac spectrum of bilayer graphene.
Figure 2: n-doped Dirac spectrum and constant-energy maps.
Figure 3: Theoretical band calculations based on the twisted-AA model.
Figure 4: Low-energy band topology and gap structure plotted for U = 0.4 eV.

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Acknowledgements

This work and ALS were supported by the US Department of Energy, Office of Sciences under Contract No. DE-AC02-05CH11231. K.S.K. acknowledges support by an NRF Grant funded by the Korean Government (NRF-2011-357-C00022). A.L.W. acknowledges support by the Max Planck Society. L.M. acknowledges support by Grant PA00P2-136420 from the Swiss National Science Foundation (SNSF). T.S. was supported by the DFG in the framework of the Priority Program 1459 ‘Graphene’. We thank F. Speck, M. Ostler and F. Fromm for assistance during sample preparation, and J. Denlinger for assistance during measurement.

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Authors and Affiliations

  1. Advanced Light Source, E. O. Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    Keun Su Kim, Luca Moreschini, Eli Rotenberg & Aaron Bostwick

  2. Department of Molecular Physics, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany

    Keun Su Kim & Karsten Horn

  3. Donostia International Physics Centre, Manuel Lardizábal 4, E-20018 San Sebastián, Spain

    Andrew L. Walter

  4. Institut für Physik, Technische Universität Chemnitz, Reichenhainer Str. 70, 09126 Chemnitz, Germany

    Thomas Seyller

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  1. Keun Su Kim
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Contributions

E.R., K.H. and A.B. conceived the experiments. K.S.K. performed the experiments, data analysis and interpretations with help from L.M. and supervision by A.B. and E.R. A.L.W. and T.S. fabricated the samples, and performed the preliminary experiments. K.S.K., E.R., A.B. and K.H. wrote the manuscript with input from all other co-authors. E.R. rendered the three-dimensional bands and movie.

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Correspondence to Aaron Bostwick.

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The authors declare no competing financial interests.

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Kim, K., Walter, A., Moreschini, L. et al. Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene. Nature Mater 12, 887–892 (2013). https://doi.org/10.1038/nmat3717

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