Excitonic superfluid phase in double bilayer graphene

By J. Li, Takashi Taniguchi, Kenji Watanabe, James C. Hone, Cory R. Dean. Nature Physics, 2016

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Strongly interacting bosons have been predicted to display a transition into a superfluid ground state, similar to Bose–Einstein condensation. This effect is now observed in a double bilayer graphene structure, with excitons as the bosonic particles. 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.

Subfields: Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/72536fa028c287f732ef3904e697b1d91e470574