Aharonov-Bohm effect and symmetry crossover in carbon nanotubes

Localization effects in a metallic carbon nanotube are studied as a function of Aharonov-Bohm magnetic flux. The conductance exhibits the behavior of strong localization except in the absence of a flux. When the Fermi level lies in higher bands, the localization effect becomes smaller at a half flux quantum due to the recovery of the time reversal symmetry. A clear symmetry crossover occurs due to perturbations such as trigonal band-warping and scatterers with potential range smaller than the lattice constant of the two-dimensional graphite.