Phase-sensitive probes of extended topological Josephson junctions

The combination of superconductors and topological materials has yielded a rich area of study in condensed matter physics. In particular, the possibility of generating Majorana bound states (MBSs) in these hybrid systems has provided motivation for many experiments targeting signatures of MBSs. In recent years, theoretical and experimental progress has been made in studying the Josephson effect in topological-insulator-based systems. The Josephson junction formed by a topological insulator in proximity to two superconductors (S-TI-S) is predicted to exhibit 4π-periodic signatures in the Josephson phase due to MBSs. We study multiple approaches to measuring phase-sensitive phenomena in S-TI-S Josephson junctions through indirect and direct measurements of the current-phase relation (CPR) using Josephson interferometry. Through indirect observations involving measurements of the maximum supercurrent through the junction versus magnetic flux, we observe entering features and a lifting of the odd-numbered nodes in an interference pattern, which is indicative of 4π-periodicity. A powerful tool that we employ in direct measurements of CPR is the superconducting quantum interference device, or SQUID. The most unique component of this work, the SQUID interferometer technique with feedback, is used to measure the CPR of S-TI-S Josephson junctions directly without the need for extracting the CPR from a measurement offset. Using this technique, we measure a CPR that is primarily 2π-periodic and exhibits skewness. We follow up the SQUID interferometer feedback measurement with a more standard technique for measuring CPR, the asymmetric dc-SQUID, and obtain similar results. We compare our results to different models for CPR in the presence of MBSs and conclude that signatures in the CPR may be dominated by low-energy, high-transparency Andreev bound states (ABSs). We discuss methods for tuning the device and materials parameters to improve the sensitivity of CPR signatures of MBSs, and ultimately, more definitive measurements that may be required for the detection of MBSs in S-TI-S Josephson junctions.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Montone, Jessica H

Contributors dc:contributor

• Van Harlingen, Dale J
• Kou, Angela
• Vishveshwara, Smitha
• Filippini, Jeffrey
• Wang, Pengjie

• Josephson junction, topological insulator

Statement dc:rights

• Copyright 2025 Jessica Montone

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132471

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132471