Excitonic Condensate Phase in Hybrid double-layer structures

In this talk I will present Coulomb transport measurement of exciton condensate phase in a hybrid double-layer structure, where twisted bilayer graphene (tBLG) and monolayer graphene (MLG) are separated by a thin insulating barrier. With a large twist angle, the lowest Landau level in twisted bilayer graphene features eight-fold degeneracy, where each broken symmetry state in the quantum Hall effect regime is described by Landau wavefunction with orbital index N=0 [1]. As such, excitonic pairing across the insulating barrier is expected to be robust over a large phase space, characterized by filling fractions n_MLG and n_tBLG, along with perpendicular electric field D. Using Coulomb drag measurement, we discovered multiple copies of fully developed exciton condensate phase in both the electron-electron and electron-hole quadrants of the phase space. Over a large filling fraction range, -5 < n_total < 5, the value of quantized Hall resistance plateaus varies with total filling fraction n_total. Most interestingly, the strength of exciton pairing is shown to be tunable with layer polarization in tBLG. This tunability adds an extra dimension to the phase space, which allows us to investigate the behavior of exciton condensate in a three-component system with three layers of graphene.