Excitonic superfluid phase in double bilayer graphene

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Abstract

A spatially indirect exciton is created when an electron and a hole, confined to separate layers of a double quantum well system, bind to form a composite boson1,2. Such excitons are long-lived, and in the limit of strong interactions are predicted to undergo a Bose–Einstein condensate-like phase transition into a superfluid ground state1,2,3. Here, we report evidence of an exciton condensate in the quantum Hall effect regime of double-layer structures of bilayer graphene. Interlayer correlation is identified by quantized Hall drag at matched layer densities, and the dissipationless nature of the phase is confirmed in the counterflow geometry4,5. A selection rule for the condensate phase is observed involving both the orbital and valley indices of bilayer graphene. Our results establish double bilayer graphene as an ideal system for studying the rich phase diagram of strongly interacting bosonic particles in the solid state.

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Figure 1: Double bilayer graphene.
Figure 2: Superfluid exciton condensate.
Figure 3: Tunability of the condensate phase.
Figure 4: Interlayer bias.

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Acknowledgements

The authors thank A. Levchenko for helpful discussions. This work was supported by the National Science Foundation (DMR-1507788). C.R.D. acknowledges partial support by the David and Lucille Packard Foundation. 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.

Author information

J.I.A.L., J.H. and C.R.D. designed the experiment. Experimental work and analysis was carried out by J.I.A.L., advised by J.H. and C.R.D. All authors contributed to writing the manuscript.

Correspondence to J. I. A. Li or C. R. Dean.

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The authors declare no competing financial interests.

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Li, J., Taniguchi, T., Watanabe, K. et al. Excitonic superfluid phase in double bilayer graphene. Nature Phys 13, 751–755 (2017) doi:10.1038/nphys4140

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