Transport studies on proximity-coupled bismuth selenide
Topological insulators form a class of materials that features robust surface states with spin-momentum locking. Coupling topological insulators to superconductivity is expected to facilitate the study of these unique surface states and to potentially yield novel physics with exciting technological applications. The work in this thesis is focused on characterizing the transport properties of proximity-coupled 3D topological insulators. To do this, we fabricate Bi2Se3 Josephson junctions and subject devices to in-plane and out-of-plane magnetic fields. This allows us to probe and manipulate the superconducting states of Bi2Se3 and therefore test predictions for what can happen at the intersection of superconductivity and magnetism in topological insulators. First, we study how superconductivity can persist in strong magnetic fields in spite of the antithetical natures of spin-polarizing magnetic fields and Cooper pairs with anti-parallel spins. We do this by measuring the effect of parallel in-plane magnetic fields on the superconducting states of Bi2Se3 using Fraunhofer spectroscopy, and we find that the Fraunhofer pattern evolves in a way that is consistent with finite momentum Cooper pairing. Furthermore, we demonstrate how simulations of experimental data can be made more accurate by taking into account device imperfections that exist in real systems. Preliminary data on Fraunhofer evolution due to an orthogonal in-plane field is also presented. In the context of the hunt for novel physics in superconducting 3D topological insulators, we also study Fraunhofer pattern signals in the absence of in-plane fields. Preliminary data is presented that demonstrate the susceptibility of Fraunhofer patterns to fabrication imperfections, illustrating complications that can arise in the interpretation of Fraunhofer patterns in Josephson junction studies. Finally, we briefly discuss proximity-coupled Bi2Se3 nanowires, which is a configuration that can potentially enhance topological properties. Data taken on superconducting nanowires as well as nanowires subjected to in-plane magnetic fields demonstrate a promising initial step towards studying superconducting nanowires near the Dirac point.
superconductivity, topological insulators, fraunhofer patterns, nanostructures, magnetism, josephson junctions
University of Illinois - Urbana-Champaign
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University of Illinois at Urbana-Champaign