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We study the existence and topological stability of Fermi points in a graphene layer and stacks with many layers. We show that the discrete symmetries (space-time inversion) stabilize the Fermi points in monolayer, bilayer, and multilayer graphenes with orthorhombic stacking. The bands near k=0 and ϵ=0 in multilayers with the Bernal stacking depend on the parity of the number of layers, and Fermi points are unstable when the number of layers is odd. The low-energy changes in the electronic structure induced by commensurate perturbations which mix the two Dirac points are also investigated.
Mañes et al. (2007) studied this question.