The influence of the interference of electron waves, which were scattered by single impurities, on nonlinear quantum conductance of metallic microconstrictions (as was recently investigated experimentally 1,2 ) is studied theoretically. The dependence of the interference pattern in the conductance G (V ) on the contact diameter and the spatial distribution of impurities is analyzed. It is shown that the amplitude of conductance oscillation is strongly depended on the position of impurities inside the constriction.

Using a π-orbital tight-binding (TB) model within a perturbative formalism, the effects of substitutional impurities on the conductance of infinite metallic single-wall carbon nanotubes (MSWCNTs) are studied. The perturbative scheme is based on the energy dissipation of electrons travelling through the nanotube. A general expression for the differential conductance (DC) is presented, and scattering processes are investigated. It is demonstrated how the DC depends sensitively on the nature of the electronic band structure and velocity of carriers moving in the nanotube. We have shown that the quantum interference (QI) of electronic waves scattered by impurities plays a meaningful role. In particular, for the case of a couple of impurities the DC exhibits periodic oscillations comprising both positive and negative values. The negative differential conductance (NDC) stemming from the QI and rotational symmetry selection rule is very sensitive to the relative distance and symmetry of two impurities. This signature is absent for the case of a single impurity. In fact, the NDC can be attributed to the zero-temperature/elastic weak-localization correction to the conductance. As a result, the faster/higher and slower/shorter oscillations can then effectively be achieved by metallic zigzag and armchair nanotubes, respectively. A corrigendum for this article has been published in 2007 J. Phys.: Condens. Matter 19 469001

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It has been shown that the two different orientations of Stone-Wales (SW) defects, i.e. longitudinal and circumferential SW defects, on carbon nanotubes (CNTs) result in two different electronic structures. Based on density functional theory we have shown that the longitudinal SW defects do not open a bandgap near the Fermi energy, while a relatively small bandgap emerges in tubes with circumferential defects. We argue that the bandgap opening in the presence of circumferential SW defects is a consequence of long-range symmetry breaking which can spread all the way along the tube. Specifically, the distribution of contracted and stretched bond lengths due to the presence of defects, and hopping energies for low-energy electrons, i.e. the 2p(z) electrons, show two different patterns for the two types of defects. Interplay between the geometric features and the electronic properties of the tubes have also been studied for different defect concentrations. Considering π-orbital charge density, it has also been shown that the deviations of bond lengths from their relaxed length result in different doping for two defect orientations around the defects-electron-rich for a circumferential defect and hole-rich for a longitudinal one. We have also shown that, in the tubes having both types of defects, circumferential defects would dominate and impose their electronic properties.