Figure 3.

Equi-energy lines of the valence band in wave-vector space. The hexagon denotes the first Brillouin zone, L is the chiral vector and η is the chiral angle. Dashed lines denote allowed wave vectors when graphene is rolled into a cylinder to form a carbon nanotube, and show the orientation perpendicular to L. The K and K' points lie on the dashed lines in metallic carbon nanotubes (a), and lie away from the dashed lines in semiconducting nanotubes (b).