Probing the order parameter symmetry in the cuprate high temperature superconductors by SQUID microscopy

The orbital component of the order parameter in the cuprate high- T c cuprate superconductors is now well established, in large part because of phase sensitive tests. Although it would be desirable to use such tests on other unconventional superconductors, there are a number of favorable factors associated with the properties of the cuprates, and a number of technical advances, that were required for these tests to be successful. In this review I will describe the development of phase sensitive pairing symmetry tests using SQUID microscopy, underlining the factors favoring these experiments in the cuprates and the technical advances that had to be made.