PhD Defense: Henri Suominen

Title: Two-dimensional Semiconductor-Superconductor Hybrids

Abstract

This thesis investigates hybrid two-dimensional semiconductor-superconductor (Sm-S) devices and presents a new material platform exhibiting intimate Sm-S coupling straight out of the box.

Starting with the conventional approach, we investigate coupling superconductors to buried quantum well heterostructures, observing clear evidence of supercurrent, and the first direct spectroscopy of an induced superconducting gap in a two-dimensional electron gas.

Nonetheless, these experiments reveal inhomogeneous contacts and a soft-induced superconducting gap, likely due to disorder at the Sm-S interface.

To overcome these issues we integrate the superconductor directly into the semiconducting material growth stack, depositing it in-situ in a molecular beam epitaxy system under high vacuum. We present a number of experiments on these hybrid heterostructures, demonstrating near unity interface transparency and a hard induced superconducting gap. Furthermore the thin superconducting (<10 nm) aluminum films allow for the application of large in-plane magnetic fields without destroying superconductivity. In such a scenario we investigate the magneto-transport properties in S-Sm-S junctions, revealing anomalous Fraunhofer diffraction, qualitatively in agreement with a complex interplay between Zeeman coupling, spin-orbit interaction and disorder.

Finally by patterning quasi-one-dimensional structures we observe coalescing Andreev bound states stabilizing at zero energy in large magnetic fields, in agreement with previous reports of Majorana modes in semiconductor nanowires. By offering a patternable two-dimensional platform, our approach opens up the door to experiments probing the predicted topological properties in this system.