The quantum Hall phase transition at ν_tot =1 in graphene double layers

We investigated the quantum Hall effect in graphene double layers which consist of two stacked graphene monolayers that are slightly rotated with respect to one another. We observe unexpected signatures of interlayer coherence resulting in odd-integer quantum Hall states (QHSs) at a balanced density between the layers (e.g ν_tot =1 = 1/2 + 1/2). We attribute the emergence of these QHSs to spontaneous layer coherence driven by interlayer Coulomb interactions, which are tunable using the magnetic field and the layer carrier densities. At fixed field a phase transition from the layer coherent state to the layer incoherent state is induced upon increasing density. This phase transition weakens and finally vanishes with increasing magnetic field.