Localized states and conductance gaps in metallic carbon nanotubes

We have found that a purely metallic carbon nanotube consisting of a nanometer-size metallic island connected to another two metallic leads gives rise to discrete energy levels, thus behaving as metallic quantum dot. Remarkably, we find discrete levels not only when the constituent tubes have a symmetry gap, but also in structures made of tubes without a common symmetry. In the latter case, the states are actually sharp resonances; we attribute these quantum size effects to the wave-vector mismatch between the available states of the constituent tubes. When there is a symmetry gap, the conductance vanishes for a wide energy window. Otherwise, nonzero conductance values occur for a series of resonances coupled to the leads continua, yielding interesting transport characteristics that may be exploited for carbon nanotube single-electron transistors.