The spectrum of order parameter fluctuations of superconductors can be determined through the measurement of the wave-vector and frequency-dependent generalized susceptibility, or pair-field susceptibility. The determination of the pair-field susceptibility is conceptually similar to other susceptibility measurements. In the case of paramagnets at temperatures above a ferromagnetic transition, the susceptibility is determined by the linear response to a magnetic field. Because the superconducting order parameter is off-diagonal in number space, for superconductors there is no classical field analogous to a laboratory magnetic field. However, an effective field can be applied to a fluctuating superconductor across a tunneling barrier through the Josephson coupling of the rigid order parameter of a second superconductor well below its transition temperature. This leads to an observable dc contribution to the tunneling current that is a higher order, "incoherent" Josephson current. The magnitude of this current determines the susceptibility. Its frequency and wave-vector dependence are determined by the dc voltage across the junction and the dc magnetic field applied in the plane of the junction, respectively. In conventional superconductors near, but above their transition temperatures, measurements of the pair-field susceptibility have revealed a diffusive dynamics that can be described by a simple time-dependent Ginzburg-Landau equation. Measurements of the pair-field susceptibility below the transition temperature have revealed the existence of a gapless, propagating order parameter collective mode that becomes quickly overdamped as the temperature is reduced below T c. The physics of these phenomena and the existing experiments will be reviewed. Opportunities for the application of these techniques to contemporary problems of high-temperature superconductors will be presented. Of particular interest are the possibilities for characterizing the nature of the pseudogap regime.
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The author would like to thank Doug Scalapino for suggesting this problem and John Anderson, Richard Carlson, and Frank Aspen for their heroic contributions to the experimental work. I would like to thank Alan Kadin for sharing his deep physical insight into the nature of the pair-field susceptibility and its connection with the proximity effect and other aspects of nonequilibrium superconductivity. This work was supported by the National Science Foundation under grant NSF/DMR-0455121.