Evidence for a topological exciton Fermi sea in bilayer graphene

The quantum Hall physics of bilayer graphene is extremely rich due to the interplay between a layer degree of freedom and delicate fractional states. Recent experiments show that when an electric field perpendicular to the bilayer causes Landau levels of opposing layers to cross in energy, an even-denominator Hall plateau can coexist with a finite density of interlayer excitons. We present theoretical and numerical evidence that this observation is due to a new phase of matter: a Fermi sea of topological excitons.