Microscopic Thoery of the Conductivity in Carbon Nanotubes

The electrical conductivity of carbon nanotubes varies, depending on the temperature and the radius and pitch of the sample. In majority cases, the resistance decreases with increasing temperature, suggesting a thermally activated process. The standard band theory based on the Wigner-Seitz(WS) cell model predicts a gapless semiconductor, which does not account for the thermal activation. A new band model in which an ``electron'' (``hole'') has a carbon hexagon size for graphene is proposed. The normal charge carriers in graphene transport are ``electrons'' and ``holes.'' The ``electrons'' (``holes'') wavepackets extend over the carbon hexagon and carry the charges --e(+e). ``Electrons'' or ``holes'' thermally activated are shown to generate the observed temperature behavior of the conductivity in the nanotubes.