FINE SPIN-DEPENDENT SPLITTING OF ELECTRONIC EXCITATIONS AND THEIR DISPERSION IN SINGLE-LAYER GRAPHENE AND GRAPHITE

The dispersion dependences of electronic excitations in single-layer graphene and crystalline graphite have been studied taking the electron spin into consideration. Compatibility conditions for two-valued irreducible projective representations characterizing the symmetry of spinor excitations in the above structures and the distributions of spinor quantum states over projective classes and irreducible projective representations at all high-symmetry points in the corresponding Brillouin zones are determined for the first time. The principal existence of the spin-dependent splitting (or merging) of the electronic energy states, in particular, the electronic it-bands at the Dirac points, is established. The magnitude of spin-dependent splitting can be significant, e.g., for the transition-metal chalcogenides belonging to the same spatial symmetry group as crystalline graphite. However, because of the weak spin-orbit interaction for carbon atoms, it turns out small for all carbon structures including single-layer graphene and crystalline graphite.