SciPost Thesis Link
| Title: | Towards a realistic description of topological hybrid semiconductor, superconductor and ferromagnetic-insulator systems | |
| Author: | Samuel D. Escribano | |
| As Contributor: | Samuel D. Escribano | |
| Type: | Ph.D. | |
| Field: | Physics | |
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| Approaches: | Theoretical, Computational | |
| URL: | http://hdl.handle.net/10486/706437 | |
| Degree granting institution: | Universidad Autonoma de Madrid | |
| Supervisor(s): | Elsa Prada, Alfredo Levy Yeyati | |
| Defense date: | 2022-12-16 |
Abstract:
Semiconductor-based heterostructures are suitable platforms to engineer and manipulate exotic quasiparticles that emerge in low dimensions. One example, with promising applications to quantum technologies, is a semiconductor nanowire partially covered by a superconductor layer. These wires support quasi one-dimensional states that acquire superconducting correlations by proximity effect. When applying an external magnetic field, the system may enter a topological superconducting phase giving rise to the so-called Majorana bound states at the ends of the wire. Alternatively, magnetic-free platforms have been proposed in which the hybrid nanowire is additionally covered by a ferromagnetic-insulator layer. Ideally, the ferromagnet induces a proximity-induced spin polarization on the wire that leads to the same kind of topological quasiparticles. During the last decade, several simplified models have been introduced to predict and explain the features emerging in experimental devices based on these nanostructures. However, the phenomenology arising in these systems seems to be richer and more complex than what these models can capture. In this thesis, we seek to describe this kind of heterostructures in an accurate and realistic way. To this end, we use a microscopic numerical approach in which we consider the three-dimensionality of the heterostructure, the different materials involved, their interaction when they are placed together, as well as the interaction of the hybrid system with the surrounding electrostatic environment. We apply this formalism to hexagonal nanowires, superlattice nanowires, as well as effective wires in planar stacking geometries. Particularly, we study how the different layers induce their properties into the semiconductor as well as the behavior of the electrostatic potential and the spin-orbit coupling inside the wire. These quantities establish the appearance, extension and robustness of the topological phase and, thus, their understanding is crucial to design topological qubits based on these nanostructures. We find that, in order to acquire topological properties, the wavefunction inside the semiconductor needs to be close to both the superconductor and ferromagnetic insulator layers so that the hybridization among the different materials is enhanced. We show that this can be controlled using external potential gates or by means of a strong confinement if the semiconductor is thin. We furthermore explore the intricate dependence of the spin-orbit coupling on the electrostatic potential profile and strain, showing that it can be strong enough to support a topological phase under certain conditions. We use this knowledge to propose new hybrid wire designs with improved topological performance.