Topology and Transport in Carbon Nanotubes

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A carbon nanotube consists of coaxially rolled graphite sheets. Because the distance between different sheets is much larger than the nearest neighbor carbon atoms, electronic properties are dominated by those of a single-shell nanotube. A single-shell CN can be either a metal or a semiconductor depending on the circumference length and the helical fashion. The purpose of this project is to study effects of the topological structure of nanotubes on their transport properties.Various calculations have been performed to predict energy bands of nanotubes. It has been found that the characteristic properties are all reproduced quite well in a k・p method, which is effective in the study of effects of external fields such as a magnetic and electric field. In fact, it has been successful in the study of magnetic properties including the Aharonov-Bohm effect on the band gap, optical absorption spectra, and lattice instabilities in the presence and absence of a magnetic field.Transport properties are interesting because CN's have a structure topologically different from that of conventional quantum wires fabricated using semiconductor heterostructures. There have been some reports on experimental study of transport in CN bundles. Quite recently, measurements of magnetotransport of a single nanotube became possible.During the period of two-year project, the conductivity of CN's was calculated using the Boltzmann transport equation and in Landauer' approach for a model of short-range scatterers. The study was made on transport across a junction of nanotubes having difference circumferences and on exciton effects in optical absorption spectra of semiconducting nanotubes and metallic nanotubes in the presence of an Aharonov-Bohm magnetic flux.