Superconducting quantum interference in semiconducting Josephson junctions

Abstract

A topological superconductor is a new state of matter that attract a lot of interest for its potential application in quantum computers. However, there is no single material known to host this state of matter. In this thesis, combinations of superconductors and semiconductors are investigated experimentally with the goal to engineer such a topological superconductor. The materials chosen combine spin-orbit interaction, superconductivity and onedimensionality. Then, under influence of a magnetic field, the hybrid superconductor semiconductor system is predicted to become topological.
First, the theoretical background of the experiments is presented, with special attention to the superconducting quantum interference in semiconducting Josephson junctions. In addition, a description of the different materials used and the fabrication of the devices, is provided.
In the first experiment we explore hole transport through GeSi core-shell nanowires. Electronic measurements reveal two transport channels only, which underlines the onedimensionality of the nanowire. On top of that, high-quality induced superconductivity is observed in both the tunneling and open regime, and evidence for strong spin-orbit interaction is presented.
Then, we switch materials to a two-dimensional electron and hole gas in an InAs/GaSb double quantum well. The spin-orbit interaction is studied by measuring the difference between the densities of electrons with opposite spin orientation. Two types of spin-orbit interaction are identified by tuning the magnitude of one of them, with an applied electric field.
InAs quantum wells are known to exhibit enhanced conduction at their edges. We find supercurrent through these edges in Josephson junction devices using superconducting quantum interference measurements. The interference pattern reveals a flux periodicity of h/e. Interestingly, while this periodicity is observed in the trivial regime, it was considered a signature of topological superconductivity before. We argue and show that nonlocal processes lead to the h/e effect in our devices. The correlated occurence of enhanced edge conduction and the h/e periodicity is confirmed in Josephson junctions made of InSb flakes.
The final experimental chapter considers a superconducting quantum interference device, fabricated in an InAs quantum well. This geometry allows for control of the superconducting phase difference of the Josephson junction, potentially reducing the magnetic field needed for the device to become topological. Unfortunately, in the measurements we do not observe signatures of topological superconductivity.
At last, we describe what device geometry and material combination could be used to do reach the topological regime. In addition, we discuss ideas for future research of the othermaterial systems used in this thesis.