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Title:Proximity effect in Josephson junction arrays fabricated on topological insulators
Author(s):Bai, Yang
Director of Research:Eckstein, James N
Doctoral Committee Chair(s):Bezryadin, Alexey
Doctoral Committee Member(s):Chiang, Tai-Chang; Stone, Michael
Department / Program:Physics
Discipline:Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Degree:Ph.D.
Genre:Dissertation
Subject(s):topological insulator
superconductivity
Josephson junction array
proximity effect
Abstract:Topological Insulators (TIs) have attracted quite an amount of attention recently because of their nontrivial surface states and the promising possibility of hosting Majorana Zero Modes (MZMs) when proximity-coupled to normal s-wave superconductors (SC) predicted by Fu and Kane in 2008. Although extensive efforts have been dedicated to the realization of Topological Superconductivity (TSC) via the proximity effect and evidence of TSC have been successfully reported, the superconducting proximity effect between TIs and SC is poorly understood. To tackle this problem, we fabricated and tested a series of Josephson Junction Arrays (JJAs) on TIs of different carrier densities with two kinds of geometrical designs and various spacings. JJA is a great platform for studying 2D superconductivity with tunable critical temperature and current by adjusting island sizes and spacings. In this work, two kinds of islands were fabricated, square shape and dot shape islands, their spacings also vary. From the transport study of Josephson junction made of big square-island arrays, it is found that the contact conductance is related to surface geometry and band structure of TIs, bumper surface and presence of bulk carriers can increase contact conductance, hence enhance the proximity effect. In the second kind of arrays, TI films with and without bulk carriers are both used. To eliminate bulk superconductivity contamination to TSC, we carefully tuned the composition ratio of the TI material grown by molecular beam epitaxy and successfully obtained TI films with intrinsic bulk insulting state, and the TSC induced in JJAs on this kind of TI film is 2D superconductivity and confined only in the top surface state. Temperature dependent resistive transition results and current-voltage characteristics are presented. The contact conductance of 2D carriers is found smaller than the 3D carriers. Results suggest that TSC confined in the top surface state is significantly different from TSC assisted by the bulk state. This work may provide important guidance for future study of TSC and search for MZMs in TI based systems.
Issue Date:2020-11-06
Type:Thesis
URI:http://hdl.handle.net/2142/109353
Rights Information:Copyright 2020 Yang Bai
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12


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