Although superconductivity is arguably one of the best understood phenomena in solid state physics, there is still much to learn. Despite nearly 25 years of effort, there is still no consensus on the mechanism producing the high critical temperatures in the cuprate perovskite superconductors. The heavy fermion superconductors have large carrier masses, strong interaction between spin and charge degrees of freedom, and potentially a wide variety of Cooper pairing symmetries. The non-cuprate perovskite superconductor Sr2RuO4 is believed to have a p-wave pairing state that breaks time reversal symmetry. Interest in unconventional superconductors has been reinvigorated with the discovery of iron-based compounds with high critical temperatures.
In this talk I will review some of the past and potential contributions of scanning magnetic imaging to our fundamental understanding of superconductivity. For example, scanning SQUID microscopy (SSM) played a central role in the demonstration of dx2-y2 pairing symmetry in the cuprate high-Tc superconductors. Magnetic force microscopy provides a means to measure the absolute value, temperature dependence, and spatial homogeneity of the superfluid density in superconductors, providing clues to their pairing symmetry. Scanning SQUID susceptometry images inhomogeneous superfluid density, including striking “stripes” in the pnictide superconductor Ba(Fe1- xCox)2As2 (Ba-122), and reveals paramagnetic shielding just above the critical temperature in this material. Finally, SQUID microscopy reveals a novel “pairing” of superconducting vortices in Ba-122.