Abstract

Exciton bound states in solids between electrons and holes are predicted to form a superfluid at high temperatures. We show that by employing atomically thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium superfluidity of electron- hole pairs should be achievable for the first time. The transition temperatures are well above liquid helium temperatures. We find that Coulomb screening is progressively suppressed in the strongly coupled regime of pairing approaching low densities, because of the large Fermi surface smearing that occurs when the superfluid gap is of order of the noninteracting Fermi energy and the pairs become more localized. This is a self-consistent effect, which is included in our calculation. Because the sample parameters needed for the device have already been attained in similar graphene devices, our work suggests a new route towards realizing high-temperature superfluidity in existing quality graphene samples [1]. Our proposed system consists of a pair of parallel bilayer graphene sheets. The lower bilayer sheet is an electron bilayer and the upper bilayer sheet is a hole bilayer. The two bilayer sheets are separated by a hexagonal boron nitride insulating barrier to prevent tunneling between the sheets. There are separate electrical contacts to the two layers and by tuning the biases on top and bottom metal gates a wide range of carrier densities can be achieved. We discuss bilayer graphene here since it has been well characterized, but a number of other such crystals are possible. Finally, we will report a quantitative comparison between our theoretical results for the condensate fraction based on the zero temperature BCS -RPA approach and the recent difussion Quantum Monte Carlo simulations for the same quantity [2]. We found a remarkable agreement for the onset of the electron-hole superfluidity and a satisfactory description of the BCS -BEC crossover induced in this system by approaching the low density regime [3].

[1] A. Perali, D. Neilson, and A. R. Hamilton, Phys. Rev. Lett. 110, 146803 (2013). [2] R. Maezono, P. L. Rios, T. Ogawa, R. J. Needs, Phys. Rev. Lett. 110, 216407 (2013). [3] D. Neilson, A. Perali, in preparation.

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