Persistent Current In Finite-width Mesoscopic Ring And Carbon Nanotorus

The emergence of mesoscopic physics has led to the discovery of many striking new phenomena in solid state physics in the last two decades. One very different and uniquely mesoscopic phenomenon is the persistent current in normal mesoscopic rings. The fulx-induced persistent current is a thermodynamic equilibrium, nondissipative current flowing around a nonsuperconducting ring, which has attracted much attention. In this thesis, we have investigated the energy spectra and persistent currents of the finite-width mesoscopic ring and carbon nanotorus. The present work is organized as follows:In chapter 1, first some characteristic scales and basic concepts of mesoscopic systems are introduced, and then a brief review of previously theoretical and experimental investigations are presented.In chapter 2, we study the effects of edge disorder on the persistent current in a finite-width mesoscopic ring. It is shown that, in the stronger disorder regime the persistent current increases with the disorder strength, while it decreases in the weaker disorder regime. It is also found that the disorder-induced changes in the persistent current strongly depend on both the ring width and radius.In chapter 3, a novel approach is presented to calculate the energy spectra in carbon nanotorus. Compared to previous calculations, the approach has the advantage that it does not require to diagonalize large Hamiltonian matrix. It is applied to study the influence of curvature and disorder on the magnetic moments of single-walled carbon nanotorus. It is found that the intrinsic curvature induce an energy gap, which result in a novel magnetic response. An armchair carbon nanotorus exhibits a diamagnetic response, while a metallic zigzag nanotorus presents a much weaker paramagnetic response. Both curvature effect and high temperature decrease the amplitude of the magnetic moments. In the presence of disorder, the amplitude of the magnetic moments decreases with the disorder strength increasing, and there may appear a transition from diamagnetic to paramagnetic.The chapter 4 is devoted to the persistent current in double-walled carbon nanotorus. The calculations show that the inter-wall interaction induces the dramatic changes in the energy spectra of both armchair and zigzag mutli-walled carbon nantorus such as level splitting, level crossings and/or anticrossings, and even an energy gap opening at Fermi energy, which lead to a decrease of persistent current and changes in the shape of flux-dependent current. In the presence of outer-wall disorder, it is found that in the regime of weak disorder, the persistent current decreases with disorder strength, while it increases in the regime of strong disorder. The results show that mutli-walled carbon nantorus may be a promising candidate for realizing a new type of surface-doping-like molecule devices.Finally, a conclusion for this thesis is given and a further investigation is discussed.