New quantum Hall ferromagnetic states in graphene bilayers

Quantum Hall ferromagnetic states are expected in graphene bilayers because of the degeneracy of the eight Landau levels which appear near the neutral system Fermi level. Working within the Hartree-Fock approximation, we derive the phase diagram of the two-dimensional electron gas (2DEG) at integer filllings $\nu=-4,-3,...,3,4$ as a function of the magnetic field and an external potential difference between the layers. We show that coulomb interaction leads to broken symmetry ground states that lift the degeneracies associated with spin, valley pseudospin and orbital pseudospin (the $n=0$ and $n=1$ orbital Landau level states are degenerate in graphene bilayers). The phase diagram of the 2DEG in this system is very rich and contains states with interlayer and/or orbital coherence that can be abruptly modified by an interlayer potential difference. We show that some of the broken symmetry states have collective excitations with unusual dispersion relations due to the coupling between valley and orbital pseudospins whose fluctuations give rise to electric dipoles.