Superconducting Proximity Effect in Nanowire Josephson Junctions

Abstract

Semiconducting nanowires contacted with superconductors are an interesting class of hybrid mesoscopic devices, in which charge transport is quantum mechanical due to the confinement potential of the nanowire, as well as the quantum mechanical nature of superconductivity. Especially interesting is the case where transport is phase coherent, resulting in the semiconductor inheriting certain properties of the superconductor (e.g. sustaining a dissipation-less current), a phenomenon called proximity superconductivity. Proximity effects allow for rich and interesting physics to occur at the intersection of superconductivity and mesoscopic transport, which are the subject of study in this thesis. Furthermore, proximitized nanowire devices with a strong spin-orbit coupling are promising candidates for the realization of Majorana bound states — quasiparticle states that are topological in nature and have enjoyed much recent attention due to their applications to topological quantum computing. As well as fundamental curiosity about proximity phenomena, it is imperative to fully understand them in order to utilize hybrid nanowire devices as the building blocks of a topological quantum computer.

In this thesis we present experimental studies of three generations of Nb/InAs nanowire/Nb Josephson junctions in which proximity superconductivity is observed. Cryogenic transport measurements allow us to identify Andreev reflection as the mechanism behind the proximity effects — a mechanism wherein an electron incident on the superconductor/semiconductor interface is retro-reflected as a (conduction band) hole, carrying a charge equal to twice the electronic charge from the semiconductor into the superconductor, where it is carried as a Cooper pair. This mechanism is critically dependent on the transparency of the superconductor/semiconductor interface, whose qualities are successively improved over the three generations of devices. Further interesting and rich phenomena are also observed in the nanowire junctions, including Multiple Andreev reflections, Andreev bound states, and the likelihood of a novel form of Josephson interference called Orbital Josephson interference. We present theoretical and numerical studies that model these observed phenomena. Finally, we explore the relevance of this work to the topological quantum computing community by describing future challenges and experiments which can reveal the physics of Majorana bound states in similar systems. We give an in-depth proposal involving a proximitized nanowire and a quantum dot which can be used to verify the topological nature of the system, as well as read out the parity state of the Majorana bound states within the system.