Atom pair persistence in disordered materials from fluctuation microscopy

We show that the kinematical theory of fluctuation microscopy for a disordered specimen can be reformulated in terms of an atom pair-pair correlation function. Resolution and coherence are two experimental variables which yield a two-dimensional fluctuation map. This map is a transformation of the pair-pair correlation function, but can be directly interpreted in terms of a pair persistence function (PPF). We show that the PPF reveals the correlation length and structure for a simple paracrystalline model.