Phase coherent electron transport in carbon nanotube devices

The electrical properties of multiwalled carbon nanotubes have been studied by transport measurements. The devices were fabricated by e-beam lithography after carefully locating the nanotubes on silicon/silicon oxide substrates. Low contact resistances have been achieved by plasma etch prior to metal deposition. Measurements were carried out at low temperature using a 3He refrigerator or a dilution refrigerator to study the phase coherent electron transport in the nanotube devices.; The four-terminal differential resistance dV/ dI has been measured as a function of various parameters such as dc bias current Idc, gate voltage V g, temperature T and magnetic field B. The devices we measured show large sample to sample variations. Even in the same sample, the characteristics change with thermal cycling and/or time. Asymmetric features with respect to dc bias current (voltage) have been found in these nanotube devices that can be well explained by Fano resonances, a natural consequence of interference effects between direct and resonant transmission. The sample specific behavior and metastability suggests defects or impurities play an important role in nanotube electron transport. Simultaneous measurements of the contact resistances show correlation between the nanotube conductance and the metal-nanotube contact conductance. These results give us some insights into the influence of the defects or impurities on the conductance of the whole nanotube device. However, the exact locations of the defects or impurities cannot be determined due to nonlocal effects. The sample phase coherence length has been deduced from magnetic field dependent measurements.; To explain the behavior we observed in the multiwalled carbon nanotube devices, we proposed a model which explains the origin of the direct transmission and the transmission through a resonant state. This model also offers an explanation of the controversies arising from previous measurements of the multiwalled carbon nanotubes.