John Kirtley,
IBM T. J. Watson Research Center

The scanning SQUID microscope images magnetic fields above samples with excellent sensitivity but modest spatial resolution. It is a powerful tool for doing fundamental studies of high temperature superconductors. I will briefly describe this type of microscope, and then talk about: 1) Tests of the orbital symmetry of the Cooper pairs: By observing magnetic vortices with exactly half of the conventional quantum of flux in specialized sample geometries, we have shown that all of the successfully tested optimally doped cuprates, both hole- and electron- doped, have predominantly d-wave pairing symmetry. 2) Possible broken time-reversal symmetry in the cuprate superconductors: Most of our experiments indicate that time-reversal symmetry is observed within experimental uncertainties; other types of experiments indicate at least some broken time-reversal symmetry. I will present some new results that may help to resolve this discrepancy.