Electronic states of metallic and semiconducting carbon nanotubes with bond and site disorder

Disorder effects on the density of states in carbon nanotubes are analyzed by a tight-binding model with Gaussian bond or site disorder. Metallic armchair and semiconducting zigzag nanotubes are investigated. In the strong disorder limit, the conduction- and valence-band states merge, and a finite density of states appears at the Fermi energy in both metallic and semiconducting carbon nanotubes. The bond disorder gives rise to a huge density of states at the Fermi energy differently from that of the site-disorder case. Consequences for experiments are discussed.