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Quantum Hall drag of exciton condensate in graphene

2017; Nature Portfolio; Volume: 13; Issue: 8 Linguagem: Inglês

10.1038/nphys4116

1745-2481

Xiaomeng Liu, Kenji Watanabe, Takashi Taniguchi, Bertrand I. Halperin, Philip Kim,

Semiconductor Quantum Structures and Devices

An exciton condensate is a Bose-Einstein condensate of electron and hole pairs bound by the Coulomb interaction 1,2 .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 condensate 3-9 .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 transport 4,6 .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 di erent filling factors and internal quantum degrees of freedom. The observed robust exciton condensation opens up opportunities to investigate various many-body exciton phases.An exciton Bose-Einstein condensate (BEC) is formed when a large fraction of excitons occupy the ground state, establishing macroscopic coherence with weak dipolar repulsion 1,3 .However, optically generated excitons have short lifetimes.They can quickly recombine and release a photon, which leads to the annihilation of excitons.By trapping the released photon in an optical cavity, recent studies have shown the BEC of exciton-polaritons, consisting of a superposition of an exciton and a photon [10][11][12][13] .Another way to achieve a large density of long-lived excitons is to place electrons and holes in spatially separated parallel conducting layers, where excitons can form across the layers.In semiconducting EDLs, such indirect excitons can be formed by optical excitation 14 or electrical doping 15 .One salient feature of the exciton BEC is dissipationless exciton transport, consisting of counter-flowing electrical currents carried by co-travelling electrons and holes 4 .The first experimental observation of this superfluid exciton flow was demonstrated in GaAs EDLs under a strong magnetic field, in which a strong correlation is formed between electron-like and hole-like quasiparticles in quantizing orbits [3][4][5][6][7][8][9] .The magnetic-field-induced layer coherence of the EDL can be established in the following way.When a two-dimensional (2D) electron gas of density n is subject to a perpendicular magnetic field B, the kinetic energy of electrons is quantized to discrete Landau levels (LLs).Each LL contains n 0 = (eB/h) degenerate Landau orbits per unit area, where e is electron charge and h is Planck's constant.If all the orbits in a LL are occupied (that is, the filling factor ν = n/n 0 is an integer), the 2D electron system forms a quantum Hall state.In the EDL, the filling factor of the individual layer can be specified by ν top = n top /n 0 and ν bot = n bot /n 0 , where n top and n bot are the density of top and bottom layer, respectively.If LLs in