Electronic structure and transport in exotic nanostructures

This thesis explores the physics of nanostructures involving nanowires, quantum dots, superconductors, and topological insulators. These systems serve as excellent platforms for fundamental physics studies and quantum technology applications.

The introduction contains information on the band structures of crystalline materials and transport phenomena in quantum dots. It is followed by discussions on nanostructures involving superconductors. One-dimensional topological superconductors and Majorana bound states, as well as two-dimensional topological insulators and relevant material systems, are also presented. The theoretical tools used for modeling the various nanostructures are discussed.

The thesis includes six research articles. The first two articles theoretically investigate the possibility of creating high-quality Majorana bound states in a system with two quantum dots coupled via a third quantum dot that is proximitized by a superconductor. The study not only confirms the possibility of creating these states, but also offers a roadmap for their detection, quality assessment, and the demonstration of their nonabelian properties. The third and fourth articles experimentally and theoretically study a parallel-coupled double quantum dot system epitaxially defined in an InAs nanowire. It was found that certain orbital crossings lead to the formation of ring-like states associated with giant g-factors. The same system was studied at higher magnetic fields. The main finding was that, for an increasing magnetic flux through the structure, crossings with ring-like states periodically turn to crossings without ring-like states and vice versa, with a period equal to one flux quantum. The fifth article focused on a similar double quantum dot system coupled to superconducting leads to form a Josephson junction. We found that, control over the hybridization between the quantum dot orbitals can induce a π-0 transition in the current-phase relation. In the sixth article, a core/shell/shell InAs/GaSb nanowire was theoretically studied. The study revealed that the structure exhibits a finite hybridization gap and hosts highly-localized end states,which are only partially protected against disorder.