Abstract Submitted to the NANOTUBE'02 Workshop:

Extended single-walled carbon nanotubes can be grouped based on the graphene sheet model as either semiconducting or metallic. However, within the subset of “metallic” tubes only the armchair ones should be truly metallic due to symmetry with the remaining quasimetallic tubes having small band gaps that depend on the chiral angle and vary as the inverse square of the nanotube radius. Also, within this set of metallic and quasimetallic tubes theory predicts that the zigzag and armchair tubes yield the upper and lower limits for the band gap for a given tube radius. Recently, experimental results have confirmed that only the armchair tubes are truly metallic while the zigzag tubes do have gaps that vary as the inverse square of the nanotube radius. We present an approximate analytic expression for the band gaps of the quasimetallic zigzag tubes derived from an all valence tight-binding model. While comparing well to our earlier numerical results and experiment, this expression shows that the band gap in these tubes should depend sensitively on both on nearest neighbor carbon-carbon pi and sigma interactions. The possible consequences of these results for the use of quasimetallic tubes as electromechanical gauges will be discussed.