Quantum mechanical effects on charge transport in two mesoscopic systems

Abstract: Carbon nanotubes are one of the most promising building blocks for the developing field of nanoelectromechanics. Due to their unique electronic and mechanical properties, these all-carbon systems have been suggested for a range of applications where one wishes to utilse the coupling between mechanical and electronic degrees of freedom on the nanometer length scale. In this thesis two carbon nanotube based systems are studied. the first system under consideration is a carbon pepaod, a caron nanotube encapsulating a single scattering C60 fullerene. By considering the scattering molecule to be weakly pinned along the axis of the tube we study the influence of its vibrational motion on the ballistic electron transport through the tube. This analysis shows that in the weak scattering limit the back-flow current is reduced as compared to the case when the fullerene is static. This effect is attributed to a renormalisation in the elastic backscattering channel due to smearing out of the fullerenes position, as well as a suppression of the inelastic backscattering channels due to the Pauli principle which restricts the number of final energy quantum states of the oscillator. The main result of this analysis is that we predict a temperature and bias voltage independent excess current which should be readily observable for realistic experimental parameters. Also, the differential and linear conductance is studied and is shown to exhibit clear signatures of measurable quantum effects in this system. The second system studied comprises a suspended carbon nanotube coupled to two superconducting leads in a transverse magnetic field. By voltage biasing the two leads an ac current is induced along the wire which, due to the coupling to the magnetic field, drives the wire into a n oscillatory state. This in turn induces an electromotive force along the wire which modulates the phase-difference over the leads, resulting in a system of coupled equations for the vibrational amplitude of the wire, which, in the low temperature limit, is shown to give rise to novel resonance phenomena. An analytic analysis of the subsequent oscillatory motion is presented which predicts the system to exhibit a region of bistability in the vibrational amplitude and measurable dc current, which is also confirmed by numerical simulations.