SQUID microscopy for fundamental studies

The scanning SQUID microscope has proved to be a valuable tool for the study of fundamental properties of solid state systems. It has the advantages of high magnetic field sensitivity, a linear, easily calibrated response over a huge dynamic range, and uses sensors which (at least for the low-T, SQUIDS) incorporate a mature technology. It has the disadvantages of the requirement of a cooled tensor and modest spatial resolution. I will briefly describe the current status and potential advances in this instrument, and then discuss some example fundamental applications. These examples include pairing symmetry tests, tests of the interlayer tunneling model for the pairing mechanism, the search for ‘visons’, and evidence for a precursor to superconductivity well above T,, all in the high-T, cuprate superconductors; and measurements of the properties of very small Magnetic particles.