Chiral superfluid States in hybrid graphene heterostructures.

We study the "hybrid" heterostructure formed by one sheet of single-layer graphene (SLG) and one sheet of bilayer graphene (BLG) separated by a thin film of dielectric material. In general, it is expected that interlayer interactions can drive the system to a spontaneously broken-symmetry state characterized by interlayer phase coherence. The peculiarity of the SLG-BLG heterostructure is that the electrons in the two layers have different chiralities. We find that this difference causes the spontaneously broken-symmetry state to be N-fold degenerate. Moreover, we find that some of the degenerate states are chiral superfluid states, topologically distinct from the usual layer ferromagnetism. The chiral nature of the ground state opens the possibility to realize protected midgap states. The N-fold degeneracy of the ground state makes the physics of SLG-BLG hybrid systems analogous to the physics of 3He, in particular given the recent discovery of chiral superfluid states in this system.