Vortex distributions and dynamics in superconductors near surface steps and sample edges studied by scanning SQUID microscopy and critical current measurements

We have studied the interactions of vortices in superconductors with surface features and sample edges. Surface steps can generate an asymmetric vortex pinning potential and can influence the distribution of vortices upon cooling into the superconducting state in an applied magnetic field. The edge of a sample acts as a barrier to vortex entry into the superconductor and can have a substantial effect on the transport properties of vortices. We have used a Scanning SQUID Microscope (SSM) to directly image the vortex distributions in various superconducting systems, including crystals with naturally cleaved surface steps as well as patterned superconducting thin films with etched surface trenches. The vortex distributions are significantly altered in the vicinity of the surface steps, and we have found evidence for a possibly paramagnetic compression of flux into surface trenches on thin film samples. We have studied vortex interactions with edge barriers primarily through transport measurements. By measuring the critical current at which the superconductor first develops a finite voltage, we are able to probe the entry of vortices at the edge of a superconducting strip in a perpendicular magnetic field. Our results display many of the characteristics expected for a thin superconducting strip with only weak bulk pinning based on recent theoretical treatments. We propose new experiments to investigate the possible paramagnetism around surface trenches and we discuss future work to study the edge barrier mechanism further.U of I Onlydissertatio