Magnetic imaging of fluxoid behavior in superconductors and spin-polarization effects in chiral materials

Abstract

This dissertation details the utilization of a custom-built scanning Hall-probe microscopy system to image a range of superconducting and magnetic phenomena. Our system offers approximately 1 μm2 spatial and 10 mG field resolution (depending greatly on the conditions of the scan), serving as a powerful tool for measuring local magnetic fields. This section provides an extensive overview of scanning Hall-probe microscopy; Hall-probe fabrication; and the scanning, positioning, and cryogenic systems. Much of the work presented will focus on imaging magnetic signatures of exotic superconductivity. An introduction to superconductivity will be presented as necessary background to understand the systems described in later chapters. We will start with the most well-known characteristics such as the transition to zero resistance and the Meissner effect, introduce various theoretical descriptions, and eventually delve into the subtle properties that underlie unconventional superconductivity. A point of emphasis will be the behavior of superconducting vortices—material-penetrating magnetic fibers circulated by supercurrent. The importance of London's fluxoid and its quantization will be discussed at length. An imaging study of a thin-film type II superconducting ring will be presented, focusing on the evolution of fluxoids in applied magnetic field along the axis of the ring. The interplay between fluxoids hosted in the ring's geometrical hole, and those associated with nucleated superconducting vortices in the bulk of the ring will be examined using complementary techniques. A theoretical and numerical project aimed at understanding and predicting the equilibrium behavior of this system will be presented. This experiment serves as a basis for later imaging studies of exotic fluxoid quantization conditions. Superconducting vortices capable of supporting half-quantized fluxoids (HQFs) hold promise in topological quantum computing in addition to fundamental scientific interest. HQFs have been experimentally verified in geometric holes, however those associated with superconducting vortices (HQVs) have not been observed. Certain type II superconductors with spin-triplet electron pairing have been theorized to host vortices with HQFs. Our work on detecting HQVs in one such material will be detailed. The chirality-induced spin-selectivity effect is a property of certain chiral materials that exhibit a spin-dependent resistance along their chiral axis, leading to spin-filtering. These materials have garnered interest in the spintronics community as a platform for manipulating spin without the need for an external magnetic field or cryogenic temperatures. A novel CISS detection method and preliminary experimental results will be presented. A proposal for a device geometry that allows for more sensitive detection will be discussed.