Quantum Hall drag of exciton condensate in graphene

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An exciton condensate is a Bose–Einstein condensate of electron and hole pairs bound by the Coulomb interaction1,2. In an electronic double layer (EDL) subject to strong magnetic fields, filled Landau states in one layer bind with empty states of the other layer to form an exciton condensate3,4,5,6,7,8,9. Here we report exciton condensation in a bilayer graphene EDL separated by hexagonal boron nitride. Driving current in one graphene layer generates a near-quantized Hall voltage in the other layer, resulting in coherent exciton transport4,6. Owing to the strong Coulomb coupling across the atomically thin dielectric, quantum Hall drag in graphene appears at a temperature ten times higher than previously observed in a GaAs EDL. The wide-range tunability of densities and displacement fields enables exploration of a rich phase diagram of Bose–Einstein condensates across Landau levels with different filling factors and internal quantum degrees of freedom. The observed robust exciton condensation opens up opportunities to investigate various many-body exciton phases.

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Figure 1: Quantized Hall drag for νtot = 1 state in bilayer graphene double layers.
Figure 2: Exciton BEC in various LL fillings.
Figure 3: Phase transition of νtot = 1 exciton BEC induced by transverse electric field.


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We thank A. Yacoby, A. Macdonald, A. Young and L. Anderson for helpful discussions. The major experimental work is supported by DOE (DE-SC0012260). The theoretical analysis was supported by the Science and Technology Center for Integrated Quantum Materials, NSF Grant No. DMR-1231319. P.K. acknowledges partial support from the Gordon and Betty Moore Foundation’s EPiQS Initiative through Grant GBMF4543. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan. T.T. acknowledges support from a Grant-in-Aid for Scientific Research on Grant262480621 and on Innovative Areas ‘Nano Informatics’ (Grant 25106006) from JSPS. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-1157490 and the State of Florida. Nanofabrication was performed at the Center for Nanoscale Systems at Harvard, supported in part by an NSF NNIN award ECS-00335765.

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X.L. performed the experiments and analysed the data. X.L. and P.K. conceived the experiment. X.L., B.I.H. and P.K. wrote the paper. K.W. and T.T. provided hBN crystals.

Correspondence to Philip Kim.

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Liu, X., Watanabe, K., Taniguchi, T. et al. Quantum Hall drag of exciton condensate in graphene. Nature Phys 13, 746–750 (2017) doi:10.1038/nphys4116

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