Final Report Summary - NANOSQUID (Scanning Nano-SQUID on a Tip)
We designed and constructed two scanning SQUID-on-tip microscopes that allow sensitive imaging of local magnetic fields on the nanoscale at 300 mK and at 4 K over a wide range of magnetic fields. We developed a feedback mechanism in which the SQUID-on-tip is glued to a resonating quartz tuning fork. By monitoring the frequency and the amplitude of the resonance, a closed feedback loop is achieved that allows approach and scanning of the SOT within few nm above the surface of the sample. We have also developed sensitive readout electronics using a cryogenic amplifier based on series array of 100 SQUIDs. As a result we were able to obtain simultaneous sensitive magnetic and topographic imaging of superconducting samples. These microscopes were applied to study of topological insulators and vortices in superconductors. The outstanding sensitivity of our technique allows imaging of vortex displacements with subatomic precision down to 10 pm level. These studies of vortex dynamics with single vortex resolution reveal complex vortex trajectories within individual pinning centers, hysteretic hopping paths, flow patterns of moving lattices, dynamic instabilities, plastic flow, and ordering.