Visualizing Electronic Transport in Quantum Matter

170 pagesThis thesis describes a set of local magnetic imaging experiments on quantum materials using a scanning superconducting quantum intereference device (SQUID) microscope. First the construction and performance of a the scanning SQUID microscope operated in a cryogen-free dilution refrigerator is described. The effect of the pulse tube cryo-cooler on vibrations in the system is investigated and different strategies for minimizing the influence of these vibrations on measurements while maintaining good thermal conductivity to the sample are discussed. Different modes of operatuion are described, including magnetometry, susceptometry and current imaging. Next, local magnetic susceptibility measurements are used to visualize the superconducting transition in microstructured crystals of {\CeIrIn} carved with a focused ion beam. The spatial structure of the local transition temperature is modeled as a strain effect arising from the microstructuring process. The geometry of the superconducting regions are in good agreement with global electrical transport measurements and local magnetic imaging. Finally, samples of magnetically doped topological insulators are studied, with an emphasis on visualizing the transport current distribution in the quantum anomalous Hall regime. The samples exhibit different magnetic phenomena that are explored via the excitation of different combinations of Ohmic contacts and electrostatic gates. Depending on the back gate voltage, many different qualitatively different microscipic current distributions can give rise to a quantized Hall effect. Included amongst these current distribtuions are those where electrical transport is dominated by bulk conduction. These observations are interpreted in the context of a microscopic picture for electronic transport originally developed for the integer quantum Hall effect.2023-03-0