Unconventional superconductivity in topological insulator and magnetic Josephson junctions

Beach, Alexander R

Permalink

https://hdl.handle.net/2142/129879 Copy

Description

Title

Unconventional superconductivity in topological insulator and magnetic Josephson junctions

Author(s)

Beach, Alexander R

Issue Date

2025-07-16

Director of Research (if dissertation) or Advisor (if thesis)

Mason, Nadya

Doctoral Committee Chair(s)

Kou, Angela

Committee Member(s)

• Bradlyn, Barry
• Cooper, Lance

Department of Study

Physics

Discipline

Physics

Degree Granting Institution

University of Illinois Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Superconductivity
• Condensed Matter Physics
• Josephson Junctions
• Quantum Transport
• Topological Insulators
• Magnetic Materials
• Nanofabrication
• Superconducting Diode Effect
• Unconventional Superconductivity
• Bi2Se3
• Chiral Helimagnets
• Cr1/3NbS2
• Josephson Diode Effect
• Fraunhofer Pattern
• Magnetic Diffraction
• Finite Momentum Cooper Pairing
• Abrikosov Vortices
• Magnetic Hysteresis
• Geometric Disorder
• Non-reciprocal Transport
• SnTe
• Mechanical Exfoliation
• Andreev Reflection
• Spin-Orbit Coupling

Abstract

In its century-long history, superconductivity has been applied in a host of technologies, including in medical devices, power grids, mag-lev trains, particle accelerators, incredibly sensitive quantum detectors, and many more areas. More recently, research has increasingly focused on unconventional forms of superconductivity that are not explained by the standard BCS theory. The potential applications of these unconventional forms of superconductivity are many, and so my research focuses on exploring and understanding superconductivity in different materials. I present experiments I have performed on unconventional superconductivity in Josephson junctions made with Bi2Se3 and Josephson junctions made with Cr1/3NbS2. Bi2Se3 is a 3D topological insulator, which means it has an insulating bulk, but conducting surface states. Cr1/3NbS2 is a chiral helimagnet, which means it has a magnetization that rotates in a helix. In both experiments the junction interlayers are made from mechanically exfoliated flakes of the materials. The primary measurements of the Bi2Se3 junctions are Fraunhofer patterns that measure the critical current of the junction under a changing magnetic field. These patterns have considerable deviations from the standard Fraunhofer form, showing asymmetry in magnetic field, offsets from zero field, aperiodicity, and lifted nodes. Simulations of junctions with geometric disorder in the form of flake thickness jumps also show these phenomena. Atomic force microscopy scans of the Bi2Se3 flakes in the junctions show that the surface of the flakes is very rough, with jumps in height that are linked to the observed features of the measured Fraunhofer patterns. The measurements of Cr1/3NbS2 junctions are also mostly Fraunhofer patterns, but with an emphasis on asymmetry in current. The positive critical current and negative critical current of the junctions are not the same magnitude, + ≠ − , and this is called the superconducting/Josephson diode effect. This effect is explained by the chiral nature of Cr1/3NbS2 with the additional presence of a small out-of-plane magnetic field. There is also an anomalous magnetic hysteresis in the junction that could be explained by the presence of one or more Abrikosov vortices in the junction, which themselves may also be caused by a small out-of-plane field. Whether an out-of-plane field comes from a conical spin configuration in the Cr1/3NbS2, flux focusing effects, or another source is unknown. I also briefly describe three secondary experiments that have not yet produced published results. The first of these experiments is the creation of Josephson junctions with the topological crystalline insulator SnTe. In SnTe the topological surface states are protected by crystalline symmetries, rather than time-reversal symmetry, meaning the surface states will appear and disappear under different conditions than in a 3D topological insulator. The SnTe junctions I fabricated did not show any Josephson behavior, but did show weak superconductivity at very low temperatures. The second experiment consists of capacitance and leakage current measurements of La2O3 thin films. The measurements are used to determine the dielectric quality of several thicknesses of the La2O3 and the efficacy of a forming gas annealing process. The final experiment attempts to observe a superconducting transition in Co thin films. Co is ferromagnetic and does not superconduct under standard conditions, but is predicted to do so when under strain. Resistance measurements show some transition in response to temperature changes and current changes, while measurements with applied magnetic fields are more ambiguous. The temperature transition also shows an anomalous hysteresis.

Graduation Semester

2025-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/129879

Copyright and License Information

Copyright © 2025 Alexander Richard Beach

Owning Collections