SEARCH FOR TOPOLOGICAL SUPERCONDUCTIVITY IN SUPERCONDUCTOR-SEMICONDUCTOR HETEROSTRUCTURES
Creators: name: Sundaresh, Ananthesh
Publisher: name: Purdue University Graduate School
Year: 2023
Subjects: Electronic and magnetic properties of condensed matter; superconductivity

description: Scientific progress often relies on unexpected discoveries and unique observations. In fact, many of the most groundbreaking scientific advances throughout history have been the result of serendipitous events. For instance, the discovery of penicillin by Alexander Fleming was a result of him noticing a mold growing on a petri dish that was contaminating his bacterial culture. Similarly, the discovery of the cosmic microwave background radiation, which is considered one of the strongest pieces of evidence for the Big Bang theory, was the result of two scientists accidentally stumbling upon it while conducting a completely different experiment. These types of unexpected discoveries can lead to new avenues of research and open up entirely new fields of study. During my PhD, I experienced a similar phenomenon when I stumbled upon an anomaly in my experimental data that led me down a completely new path of investigation. This unexpected discovery not only provided me with new insights into the underlying mechanisms of my research, but also opened new avenues for future research directions. It was a reminder that sometimes the greatest scientific progress can come from the most unexpected places. My primary focus was initially directed towards topological superconductivity. However, this research direction was modified by unexpected findings while characterizing a SQUID. Specifically, a unique response by a Josephson junction was observed when exposed to an inplane magnetic field. Chapter 1 details our experimental results on the SQUID. We observed intriguing effects resulting from the in-plane magnetic field in the asymmetric evolution of the Fraunhofer pattern suggesting the existence of additional underlying physics in the heterostructure, which may have been previously overlooked. This serendipitous finding served as the impetus to explore simpler superconducting devices such as nanowires and rings. Remarkably, subsequent investigations into the critical current of a superconducting ring revealed a bi-modal histogram arising from the application of an in-plane magnetic field, which was an unforeseen outcome. This adds to our observations made in chapter 1. Chapter 2 details the unique properties of Al-InAs superconducting rings. Further experiments involving a superconducting nanowire resulted in the observation of non-reciprocal critical current under an in-plane magnetic field perpendicular to the current direction, subsequently referred to as the superconducting diode effect. Chapter 3 delves into the non-reciprocal properties of an Al-InAs superconducting nanowire. Our findings revealed the diamagnetic source of non-reciprocity generic to multi-layer superconductors. Finally, chapter 4 provides a detailed account of the fabrication processes for the superconducting devices, along with a discussion of the measurement techniques employed to unveil the underlying physics.