Evidence for a three-layer-coherent quantum Hall state in graphene triple layers

Heterostructures consisting of two graphene monolayers separated by a thin BN spacer provides a rich platform in which to study correlated states driven by strong interlayer interactions. In the quantum Hall effect regime, tuning this system to total filling fraction \nu_tot = 1 (i.e. half filling of the lowest Landau level in each layer) results in a spontaneously formed interlayer coherent state believed to be a Bose-Einstein condensate of indirect excitons or a pseudo spin ferromagnet. Here we report the first realization of a triple layer system where three monolayer graphene layers are strongly coupled via coulomb interaction while maintaining negligible interlayer tunneling. We demonstrate independent control of the density in each layer, giving access to the entire phase space of this system. Tuning the relative filling fraction between all three layers allows us to effectively shift the exciton condensate between different layer pairs and then probe this phase with the third layer. When the total filling factor of all three layers equals 1, we observe simultaneous quantization of Hall resistance in the drive and drag layer and vanishing longitudinal resistance, suggesting a generalized exciton condensate phase with macroscopic phase coherence across all three layers.