Transport spectroscopy of semiconductor superconductor nanowire hybrid devices

Abstract

In this thesis, we investigate integrated quantum dots as tunnel spectrometers in semiconductor – superconductor nanowire hybrid devices, by means of low temperature transport experiments. The goal is, to probe the density of states and sub-gap states in an adjacent nanowire lead segment. By investigating a large variety of effects in nanowire lead segments, when they are coupled to a superconducting electrode, we pave the way towards systematic spectroscopy of topological states in nanowires. We can use our platform essentially in two different ways: 1) as a controllable single tunnel barrier, when the quantum dot is in Coulomb blockade and 2) as an energy filter, when the quantum dot is on resonance. By using both regimes, we demonstrate the evolution of the proximity induced superconducting gap in the nanowire lead segment, when the nanowire is coupled to one superconducting electrode. We can explain the observed characteristic features as a transition between the long and the short junction limit of the device (see chapter 5). Furthermore, we present spectroscopy measurements on a nanowire segment, in which discrete sub-gap states form. These sub-gap states exhibit rich physics, due to a competition between Coulomb interactions and the Kondo effect (see chapter 6). Moreover, we discuss electronic spectroscopy measurements performed in nanowire devices with integrated quantum dots with two superconducting electrodes. Here, we can access a large variety of transport regimes. Most remarkably, we find a hybridization of the quantum dot resonances with Andreev bound states in the intermediate coupling regime (see chapter 7). In addition, we present a first step towards devices, based on two nanowires, a potential candidate for the detection of parafermions. For the first time, we present a splitting of Cooper pairs into two separated individual nanowires (see chapter 8).