Theory of electronic structure and transport in heterostructure nanowires
Abstract: Today, semi-conductor nanowires can be grown with very high precision, using epitaxy. This allows for studies of new nanowire-based quantum devices, likely to be part of novel quantum technologies in the future. This thesis concerns nanostructures based on InAs nanowires. More specifically, it concerns the theoretical treatment of two types of nanowire-based structures: InAs/GaSb core-shell (and core-shell-shell) nanowires, and parallel double quantum dots, made from epitaxially defined single quantum dots in InAs, confined using wurtzite barriers.For the core-shell(-shell) nanowires, we focus on the band structure of the system, using k · p theory to calculate the k-dependent energies and space dependent wave functions. We find that, for the right core and shell thicknesses, a so-called hybridization gap opens up. A hybridization gap is a band gap where conduction- and valence band-like states are inverted, but a band gap appears because of coupling between these bands. In a quantum well such an inverted band gap gives rise to topological edge states. For hollow core-shell-shell InAs/GaSb nanowires, that can be thought up as “rolled-up” quantum wells, we study the wave functions for a finite system, and establish that there are localized end-states, with energies inside the bulk gap. However, in contrast to the topological edge states in two dimensions, the end states are not robust against perturbations.We have also studied parallel double quantum dots in InAs nanowires. The double quantum dots are created by our experimental collaborators, using a new method, from a single quantum dot in a nanowire, subject to positive side gates. By tuning the side gates the interaction between the two dots formed can be tuned. The double quantum dots are investigated in collaboration with experimentalists, where they are subject to a bias voltage and an external magnetic field. The measured transport data can be very well reproduced using a few-body Hamiltonian for the dot system together with a master equation. We find that in the two-electron regime, the ground state can be tuned between a singlet and a triplet state, either by tuning the side gates or the external magnetic field. At the ground state transition the singlet and triplet states anticross, resulting in an energy difference that can be affected by changing the dot interaction.It is likely that nanowire-based structures like these will be used in future quantum technologies, and it is then important to understand and model the structure-dependent electronic energies depending on e.g., spin-orbit coupling and large g-factors.
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