Current-carrying capacity of carbon nanotubes

The current carrying capacity of ballistic electrons in carbon nanotubes that are coupled to ideal contacts is analyzed. At small applied voltages, electrons are injected only into crossing subbands, the differential conductance is 4e(2)/h. At applied voltages larger than Delta E-NC/2e (Delta E-NC is the energy level spacing of first noncrossing subbands), electrons are injected into noncrossing subbands. The contribution of these electrons to current is determined by the competing processes of Bragg reflection and Zener-type intersubband tunneling. In small diameter nanotubes, Bragg reflection dominates, and the maximum differential conductance is comparable to 4e(2)/h. Intersubband Zener tunneling can be non-negligible as the nanotube diameter increases, because Delta E-NC is inversely proportional to the diameter. As a result, with increasing nanotube diameter, the differential conductance becomes larger than 4e(2)/h, though not comparable to the large number of subbands into which electrons are injected from the contacts. These results may be relevant to recent experiments in large diameter multiwall nanotubes that observed conductances larger than 4e(2)/h.