So far we have discussed the basic bulk properties of
graphene. Nevertheless, graphene has very interesting
surfaceedgestates that do not occur in other systems.
A semi-infinite graphene sheet with a zigzag edge has a
band of zero-energy states localized at the surface
Fujitaet al., 1996; Nakadaet al., 1996; Wakabayashiet
al., 1999. In Sec. II.H, we discuss the existence of edge
states using the Dirac equation. Here we discuss the
same problem using the tight-binding Hamiltonian. To
see why these edge states exist, we consider the ribbon
geometry with zigzag edges shown in Fig.14. The ribbon
width is such that it hasNunit cells in the transverse
cross sectionydirection. We assume that the ribbon
has infinite length in the longitudinal directionxdirec-tion