Pairing states of composite fermions in double-layer graphene

Heterostructures of vertically stacked graphene double layers, separated by a thin tunnel barrier, provide a highly tunable system to explore strongly interacting electron states. This is because the interlayer Coulomb interactions can be sensitively tuned simply by varying the barrier thickness. Recent studies of double-layer graphene have shown that, in the quantum Hall effect regime, strong interlayer coupling can induce electron–hole pairing across the two layers, resulting in a superfluid phase of interlayer excitons1–3. Here, we report a series of emergent fractional quantum Hall effect (FQHE) states appearing under similar conditions. We find excellent agreement between the sequence of observable FQHE states and the theoretically proposed two-component composite-fermion (CF) model, where the CF quasiparticle construction results from both interlayer and intralayer interactions4,5. Most remarkably, we observe an additional series of incompressible states at fractional filling that do not fit within either the single- or two-component CF models. We interpret these states to result from residual pairing interactions between CFs, representing a new type of correlated ground state that is unique to graphene double-layer structures and not described by the conventional CF model.