Nonlocal transport signatures of Andreev bound states

TL;DR

This thesis investigates nonlocal transport signatures of Andreev bound states in semiconductor-superconductor hybrid nanowires, revealing their extended wavefunction properties and conditions for their detection via tunneling spectroscopy.

Contribution

It introduces novel device geometries and experimental methods for nonlocal tunneling spectroscopy, advancing the understanding of Andreev bound states in hybrid nanowires.

Findings

Extended Andreev bound states show signatures in nonlocal conductance.

No signatures observed when measurement points are more than 0.8 μm apart.

Devices enable spatially resolved tunneling spectroscopy of bound states.

Abstract

In this PhD thesis, quantum devices based on molecular-beam epitaxy grown InAs semiconductor with an in-situ grown epitaxial Al film were investigated. Novel device geometries were realized that allow for the study of bound states that emerge at low temperatures in semiconducting nanowires due to the presence of spin-orbit coupling, Zeeman effect, and superconducting proximity effect. Experiments utilizing either gate-defined nanowires in two-dimensional heterostructures or vapor-liquid-solid grown nanowires with a full-shell of Al are presented. Devices and experimental methods were developed that allow for tunneling spectroscopy at multiple locations of a nanowire. Results of nonlocal conductance transport measurements of Andreev bound states are discussed. Due to their extended wavefunction, bound states that are locally hybridized with a quantum dot show characteristic signatures in tunneling spectroscopy at neighboring measurement locations. If the distance between neighboring tunneling measurement locations is larger than 0.8 $\mathrm{\mu m}$ no signatures of extended bound states was observed.