Tunneling through Nanostructures - Interactions, Interference and Broadening

In this thesis, quantum transport through nanostructures is addressed theoretically by considering simplified model systems representing the most important features of quantum dots or molecules. The generic model consists of a central region coupled to noninteracting leads. The key ingredients are a discrete level spectrum of the central region and complicated many-body interactions present therein, the coupling between the leads and the dot, and the finite temperatures of the leads. After a general introduction to quantum transport through nanostructures, different theoretical methods are briefly reviewed with a particular focus on density-matrix based approaches. Then a new method denoted the second order von Neumann (2vN) approach is presented, which forms the core of this thesis. By working in a basis of many-particle states for the central region, Coulomb interactions are taken fully into account and correlated transitions by up to two different contact states are included. The latter extends standard rate equation approaches by including level-broadening effects and interference due to different transport paths through the nanostructure. The method is applied to various model systems in three of the four papers, Paper II-IV, contained in the thesis, and supplementary material is presented in the main part of the thesis. The models discussed are the spinless single and double quantum dot models, the Anderson model, and, finally, a spintronics model with a single spin-degenerate level coupled to ferromagnetic contacts and subjected to a magnetic field. Furthermore, cotunneling through single quantum dots is treated using the 2vN method. In Paper I, experimental data obtained from measurements on an InAs-InP nanowire containing a double quantum is analyzed on a microscopic basis using a capacitance model