Quantum Transport and Interaction in Carbon Nanotubes

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Graphite needles called carbon nanotubes (CN's) were discovered recently and have been a subject of an extensive study. A CN is a few concentric tubes of two-dimensional (2D) graphite consisting of carbon-atom hexagons arranged in a helical fashion about the axis. The diameter of CN's is usually between 20 and 300 Å and their length can exceed 1μm. The distance of adjacent sheets or walls is larger than the distance between nearest neighbor atoms in a graphite sheet and therefore electronic properties of CN's are dominatecl by those of a single layer CN.A single-wall nanotubes are produced in a form of ropes. The purpose of this project is to study electronic and transport properties of carbon nanotubes theoretically.Carbon nanotubes can be either a metal or semiconductor, depending on their diameters and helical arrangement. The condition whether a CN is metallic or semi-conducting can be obtained based on the band structure of a 2D graphite sheet and periodic boundary conditions along the circumference direction. This result was first predicted by means of a tight-binding model ignoring the effect of the tube curvatureThese properties can be well reproduced in a k p method or an effective-mass approximation. In fact, the effective-mass scheme has been used successfully in the study of wide varieties of electronic properties of CN.In this project we have discussed transport properties of nanotubes based on the k p method combined with a tight-binding model. Effects of impurity scattering have been studied and the total absence of backward scattering has been pointed out except for scatterers with a potential range smaller than the lattice constant. The conductance quantization in the presence of lattice vacancies, i.e., strong and short-range scatterers, has been predicted. The transport across a junction of nanotubes with different diameters through a pair of topological defects such as five-and seven-membered rings has also been clarified.