Chiral symmetry breaking by a magnetic field in graphene

We report a novel many-body mechanism for the chiral symmetry breaking in the lowest Landau levels (LLs) of graphene, which is responsible for the new quantum Hall states near the charge neutral Dirac points corresponding to ν=±1 plateaus observed in magnetotransport measurements (Zhang et al., 2006 [31], Jiang et al., 2007 [32]). We show that, when a uniform magnetic field is switched on, valley symmetry breaking emerges in graphene due the interaction of electrons with zone-boundary phonons having A1g-symmetry. The induced valley gap is about 14.68KkBB(T) whose magnitude and field dependence are compatible with the observed quantum Hall plateaus in graphene.