UC Berkeley

We present Coulomb-drag measurements on a heterostructure comprising two Bernal-stacked bilayer graphene (BLG) sheets separated by a 2.5 nm hexagonal boron nitride (hBN) spacer in the quantum Hall (QH) regime. Using top and bottom gate control, together with an interlayer bias, we independently tune the two BLG layers into either the lowest (N=0) or second (N=1) Landau level (LL) orbital and probe their interlayer QH states. When both layers occupy the N=0 orbital, we observe both interlayer exciton condensates (ECs) at integer total filling and interlayer fractional QH states, echoing the results in double monolayer graphene. In contrast to previous studies, however, when both BLG layers occupy the N=1 orbital, we also observe quantized drag signals, signifying an interlayer exciton condensate formed between the second LLs. By tuning the layer degree of freedom, we find that this N=1 EC state arises only when the N=1 wave function in each BLG is polarized toward the hBN interface to maximize the interlayer Coulomb interaction.