Quantum transport in HgTe topological insulator nanostructures

In this thesis, a number of transport effects in topological insulator nanostructures are investigated. A novel wet-chemical etching approach enabled the fabrication of high-quality nanostructures from strained HgTe thin films and CdTe/HgTe quantum wells and the investigations of topological boundary states in HgTe-based 2D and 3D topological insulators. Measurements of Hall bars, Aharonov-Bohm rings, antidots superlattices, and nanowires were carried out at dilution refrigerator temperatures. The properties of topological surface states were studied in high-mobility, wet-etched macroscopic Hall bars in large magnetic fields. Distinct Landau levels from bulk holes, topological surface states, and bulk electrons were observed. The high mobilities in wet-etched devices brought coexisting Landau levels from bulk holes and topological surface states to light. In the nanostructures, a number of mesoscopic effects were investigated. The Aharonov-Bohm effect was studied for low and high magnetic fields in a 3D topological insulator ring structure, where the surface states wrap around a torus-shaped insulating bulk. In 2D antidot superlattices in HgTe quantum wells, commensurable oscillations and the interference of quantum spin Hall edge channels was probed. Topological nanowires were probed in in- and out-of-plane magnetic fields, with regard to Aharonov-Bohm conductance oscillations and the quantum Hall effect. Furthermore, the subband-structure of quasi-ballistic topological nanowires was probed via subband-induced oscillations. The topological nature of the surface states could be conclusively proven with the help of a quantitative model. The model is justified by theoretical simulations, which give further insight into the properties of the coherent surface states.