Glimpsing order within the disarray

We describe some of the recent advances in the fluctuation microscopy technique for probing medium-range structural correlations in disordered materials. In particular we show that fluctuation microscopy is a surprisingly sensitive method for detecting trace quantities of C₆₀ in a disordered graphite matrix. This surprising sensitivity arises because C ₆₀ does not have the same forbidden reflections as graphite. Modeling shows that the method should readily distinguish between C₆₀, C ₇₀ and C₇₂. This result indicates that the technique can be used to discern dilute distributions of macromolecules in an otherwise disordered matrix. We also describe preliminary interferometric fluctuation microscopy studies using cross-correlations in diffraction between coherent double probes. This is a form of holography where the diffraction patterns from two neighboring regions are allowed to overlap and interfere. Young's fringes appear wherever both regions scatter strongly. The cross-correlation can be examined as a function of probe separation to estimate a structure correlation length. At present, the method is being applied to x-ray and optical microscopies, but could also be applied to TEM. Since it isolates the essential four-body terms underpinning the fluctuation microscopy technique, this method holds much promise for studying medium-range order.