Effects Of Defects And External Electric Field On The Conductance And Density Of States Of Carbon Nanotubes

In this thesis, we have made a deep investigation on the conductance and the density of states of carbon nanotubes, exploring the influences of defects, nonmagnetic impurities and external electrical field. On the base of understanding the physical essence of these influences, we explain important experimental results and predict some new effects.We first discuss the symmetry effects of defects in the single-walled armchair carbon nanotubes. We use a new formula to calculate the conductance of transmissive eigenchannels, and arrive at a detailed understanding of the conducting behavior which is caused by the symmetry and relative distance of defects and the quasibound states. The effects of defect strength and nanotube size on the conductance are also discussed.Then, we investigated quantum interference induced by non-magnetic impurities in a single-walled carbon nanotube. The scanning tunnelling spectroscopy of a doped single-walled armchair carbon nanotube is calculated. The spatial oscillations arising from quantum interference between the forward and backward electron waves are found in the local density of states (LDOS). These oscillations are strongly dependent on the geometric arrangement of the relationship between the doped non-magnetic impurities. Three typical oscillations are predicted and their characteristic features have been studied in detail. It is found that the symmetry and parity of the quasibound states induced by impurities are important to distinguish the oscillation types. The second typical spatial oscillations in the LDOS have been observed in recent STM experiment. The other typical oscillations are expected to be observed in the future experiments.