Structural and electronic properties of pentagon-heptagon pair defects in carbon nanotubes

General features of the energetics and electronic states of carbon nanotubes, containing pentagon-heptagon pair (5/7) topological defects in the hexagonal network of the zigzag configuration, are investigated within simple tight-binding models. This 5/7 pair defect on the nanotube structure is not only responsible for a change in nanotube diameter, but also governs the electronic behavior around the Fermi level. Furthermore, nanotubes having different helicities and different electronic properties can be connected using such a pair. Various configurations including one, two, and three 5/7 pairs are explored in order to understand the influence of several defects on the electronic properties of the system. Topological aspects associated with the presence of 5/7 pair defects in armchair and chiral nanotubes are also discussed.