MULTISCALE ELECTRON MICROSCOPY OF DEFECTS IN NB-BASED SUPERCONDUCTING MATERIALS
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The performance of superconducting radio-frequency (SRF) cavities and superconducting quantum devices is fundamentally limited by defects that cause dissipation, vortex nucleation, and dielectric losses. These effects often originate from subtle variations in local structure and composition that are not accessible through conventional characterization. The thesis develops and applies advanced electron microscopy methods to establish a unified, multiscale understanding of how defects modify the structural and electronic landscape of superconducting materials.At the mesoscale, focused ion beam–scanning electron microscopy of Nb₃Sn coatings reveals defect populations associated with different growth methods. Electrochemically synthesized (ES) films exhibit large thickness variations while maintaining near-stoichiometric composition, whereas vapor-diffused (VD) coatings are more uniform but contain Sn-depleted regions in both the surface and bulk. These compositional inhomogeneities define preferential sites for vortex nucleation and likely sources of residual RF losses. To quantify compositional variations in tin-depleted grains with high precision, an iterative zeta-factor framework based on energy-dispersive X-ray spectroscopy is applied to correct absorption effects. For structural characterization at nanoscale, a 4D-STEM–based method is developed to reconstruct the full three-dimensional strain tensor. This approach reveals metastable tetragonal regions in Nb₃Sn stabilized by internal strain, demonstrating that local strain is a key parameter shaping the effective phase diagram. The multimodal approaches developed in this work provide a general framework for linking microstructure to performance in complex superconducting materials, offering guidance for the optimization of SRF cavities and superconducting quantum devices.