Person:

Liu, Xiaomeng

Exciton condensate is a Bose-Einstein condensate (BEC) of electron and hole pairs bound by the Coulomb interaction1,2. In an electronic double layer (EDL) under strong magnetic fields, filled Landau states in one layer bind with empty states of the other layer to form exciton condensate3–9. Here we report exciton condensation in bilayer graphene EDL separated by hexagonal boron nitride (hBN). Driving current in one graphene layer generates a near-quantized Hall voltage in the other layer, signifying 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 GaAs EDL. The wide-range tunability of densities and displacement fields enables exploration of a rich phase diagram of BEC 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.

When a strong magnetic field is applied to a two-dimensional (2D) electron system, interactions between the electrons can cause fractional quantum Hall (FQH) effects. Bringing two 2D conductors close to each other, a new set of correlated states can emerge due to interactions between electrons in the same and opposite layers. Here we report interlayer correlated FQH states in a device consisting of two parallel graphene layers separated by a thin insulator. Current flow in one layer generates different quantized Hall signals in the two layers. This result is interpreted by composite fermion (CF) theory with different intralayer and interlayer Chern-Simons gauge-field coupling. We observe FQH states corresponding to integer values of CF Landau level (LL) filling in both layers, as well as "semi-quantized" states, where a full CF LL couples to a continuously varying partially filled CF LL. We also find a quantized state between two coupled half-filled CF LLs and attribute it to an interlayer CF exciton condensate.