This thesis explores the initial steps towards integrating high temperature scanning SQUID-on-tip (SOT) with quartz tuning fork atomic force microscopy (QTF-AFM). By combining these imaging...Show moreThis thesis explores the initial steps towards integrating high temperature scanning SQUID-on-tip (SOT) with quartz tuning fork atomic force microscopy (QTF-AFM). By combining these imaging techniques into one sensor, the local magnetic field variations and surface topography of a sample can be mapped simultaneously. This allows the SQUID to scan with ultra-sensitive flux sensitivity and nanometer spatial resolution, while its position on the surface remains identifiable. Specifically, this thesis addresses the low operating temperature of conventional SOT by developing a fabrication method that uses BSCCO, a high temperature superconductor, to create SQUIDs through gallium focused ion beam (FIB) milling. The electrical contacting procedure of BSCCO involves mechanical exfoliation and electron-beam lithography. The results yield contact resistances in the order of 100 ohm, which are sufficiently low to perform current transport experiments. The flakes are then structured into 1 micrometer SQUIDs. The Josephson junctions are created by introducing ion beam induced damage to the crystal lattice of BSCCO to suppress superconductivity. The transport measurements reveal no conclusive evidence of SQUID features. However, it is shown that milling sub-200 nm wide structures does not alter the electronic properties of BSCCO, indicating that this nanostructuring method can potentially be applied in fundamental research into high temperature superconductors. This thesis also focuses on depositing multiple SQUID electrodes along a QTF, while keeping the self-sensing and -actuating capabilities of the force sensor intact. The QTF is insulated with a 100 nm thick SiOx layer. It is then covered with a laser micro-machined hardmask, through which 50 nm of titanium is evaporated in the shape of SQUID electrodes. Through fabrication alterations, certain issues involving alignment and electrode interruption can be solved. However, the evaporation method inexplicably compromises the integrity of the insulating barrier, thereby forming electrical shorts. Overall, the findings indicate that while substantial progress has been made in developing fabrication methods for the different components, significant technical hurdles remain. These need to be addressed to realize the potential of BSCCO scanning SQUID-on-tip for atomic force microscopy.Show less
When cuprate compounds are sufficiently doped with extra holes, the Mott insulating phase gives way to the puzzling phenomenon of high-temperature superconductivity. Here, we use spectroscopic...Show moreWhen cuprate compounds are sufficiently doped with extra holes, the Mott insulating phase gives way to the puzzling phenomenon of high-temperature superconductivity. Here, we use spectroscopic-imaging scanning tunnelling microscopy (SI-STM) to probe two overdoped cuprate samples belonging to the family of BSCCO. The two samples have slightly different doping levels and critical temperatures TC of 3 K and 12 K. At this doping level, the band structure contains a saddle point close to the Fermi surface. As such, one expects to see a van Hove singularity (vHS) peak in the local density of states at every spatial position, i.e. in every STM dI/dV spectrum. Surprisingly, we find that the vHS peak is absent in part of the measured dI/dV spectra. Hence, to enable further investigation into the partial absence of the vHS peak, we developed a phenomenological model that is capable of fitting all the single dI/dV spectra. Using this model, we are able to spatially map the presence of the van Hove singularity and to correlate its energy to the width of the measured gap.Show less
