Proximity induced topological superconductivity in bulk insulating Bi0.8Sb1.2Te3using Josephson junctions

Topological Superconductivity (TSC) is an exciting area of condensed matter physics research because of the possibility of systems demonstrating it being able to host Majorana Bound States (MBS), which can be used to realize topological qubits to build a fault-tolerant topological quantum computer. In this work we are interested in examining emergent TSC in an engineered system consisting of an s - wave superconductor and a 3D topological insulator(TI). To accurately study the topological superconductivity, we need to suppress the bulk carriers. We do this by selecting a TI that is a ternary alloy, BixSb2-xTe3 . For all compositions this material is a TI, with pure Bi2Te3 being naturally n-doped and pure Sb2Te3 being naturally p-type. Using the Molecular Bean Epitaxy thin film growth process, we can fine tune the stoichiometry of this material to end up with a material that is insulating in the bulk. Various microscopy techniques are used to confirm that a high quality and uniform thin film is grown. In terms of the electronic band structure, the Fermi energy level should lie between the bulk bands and only cross topological surface states. From various transport property and spectroscopy measurements, we can verify that this is indeed the case. The actual device used to study the TSC is the Josephson Junction(JJ), where the proximity induced TSC state will be interrupted by the weak link of the non-proximitized TI. We induce proximity coupled superconductivity in the TI layer using Niobium electrodes. Two generations of wafers totaling over 25 samples were processed and fabricated using JJs of different dimensions and geometry. DC current-voltage measurements were taken to measure contact transparency and the proximity induced superconducting energy gap in the TI. We introduce a new graphical method to accurately extract the critical currents at various temperatures by accounting for the thermal noise current. This is important since all our devices have small critical currents due to not having any bulk carriers. Differential conductance measurements gave us more information into the nature of the proximity induced TSC, with some interesting geometric resonances systematically observed in all devices. They cannot be accounted for using more commonly observed phenomena like Multiple Andreev Reflection or any other known scattering interaction. Magnetic diffraction studies showed anomalous Fraunhofer diffraction patterns which were understood as long junction effects. Node lifting indicating the presence of MBS couldn’t be confirmed or ruled out, because we predict the currents are too small to be accurately measured in our setup.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2024

Author dc:creator

• Suresh Babu, Soorya

Contributors dc:contributor

• Eckstein, James N
• Mahmood, Fahad
• Clark, Bryan K
• Wang, Pengjie

• Experimental Condensed Matter
• Molecular Beam Epitaxy
• Nanofabrication
• Quantum Computing
• Topological Materials
• Junctions

Statement dc:rights

• Copyright 2024 Soorya Suresh Babu

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/127144

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F127144