Imaging topological superconductivity in UTe_2
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Date
2024
Authors
Carroll, Joseph P.
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Publisher
University College Cork
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Abstract
Superconductivity has been a topic of significant interest to the physics community for over a century. However, despite persistent effort, there remain many unsolved problems and unmeasured phenomena. Although the physics of p-wave superconductivity was developed in the 1960s there have been little to no conclusive measurements of a p-wave superconductor. The subject of this thesis, UTe_2, has ignited a new wave of interest in p-wave superconducting states as it offers the exciting prospect of measuring unusual surface electronic states, such as zero-energy Andreev bound states, and helical or chiral topological surface states. The latter state being a prerequisite for non-Abelian fermion statistics and Majorana zero modes.
Low-temperature spectroscopic-imaging scanning tunnelling microscopy is an ideal experimental platform with which to study such a superconductor as it allows direct, high energy resolution measurement of the surface electronic states. Using this tool, we present research conducted over the last four years which, we hope, contributes to the understanding of this new material and outlines how one could search for the effects of p-wave superconductivity in other compounds.
In Chapter 1 we discuss the theoretical framework frequently used in superconductivity research. Where relevant, we highlight the differences between conventional s-wave superconductors and their p-wave counterparts. In Chapter 2 we explain the essential operating principles of a spectroscopic-imaging scanning tunnelling microscope (SI-STM) and the experimental techniques used to acquire the results presented herein. Chapter 3 provides an overview of the wealth of theoretical and experimental work which has taken place since the discovery of UTe_2’s superconducting properties. Taking these results with the experimental results presented in this thesis, there is strong support for the presence of a p-wave superconducting state in UTe_2.
Chapter 4 features the results of our first SI-STM research project, which makes use of a superconducting scan tip to enhance the spectroscopic resolution of our STM system. By measurement of the superconducting energy gap across a representative field of view we discover the presence of a spatially modulating superconducting state, a pair density wave (PDW) at the (0 -1 1) crystal surface. This is likely to be the first observation of a spatially modulating p-wave superconducting state, highlighting the ubiquity of such states in varying material systems. While performing superconducting scan tip measurements of this PDW, we discovered a zero-energy Andreev bound state at the (0 -1 1) surface. We discuss the theory of how such states come about and how they can be measured in Chapter 5. We also discuss the theory of topological superconductivity and the diverse classification scheme associated with it. Chapter 6 then presents direct measurement of the surface Andreev bound state supported by a newly developed s-wave - insulator - p-wave tunnelling model. Using this model and tip-sample dependent tunnelling measurements, we place restrictions on the form of superconducting order present in UTe_2. Finally, in Chapter 7 we present a detailed band structure model which reveals the electronic states responsible for low-energy scattering at the surface of UTe_2.These measurements reveal unique scattering signatures of the quasiparticle surface band expected from a crystal whose superconducting order parameter transforms as the B_{3u} irreducible representation of the crystal point symmetry group D_{2h}.
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Keywords
Superconductivity , Topological superconductivity , Condensed Matter Physics , Scanning tunnelling microscopy , Heavy fermion , Strongly-correlated systems
Citation
Carroll, J. P. 2024. Imaging topological superconductivity in UTe_2. PhD Thesis, University College Cork.