We suggest that physics underlying the recently observed removal of sublattice and spin degeneracies in graphene in a strong magnetic field describes a phase transition connected with the generation of excitonic and spin gaps. The strong-coupling regime is described using a phenomenological model with enhanced Zeeman splitting (spin gap) and excitonic gaps. The experimental form of the Hall conductivity $\sigma_{ xy}$ with the additional $\nu= 0, \pm 1$ plateaus is reproduced. The form of $\sigma_{ xy}$ in the case of a strong-coupling regime with no enhanced Zeeman splitting is also discussed.
Categories: cond-mat.str-el cond-mat.mes-hall