Exponential vs. slower decay of time dependent structure factors

This article deals with the possible forms of decay of time dependent structure factors in rather generic systems. It concentrates on the distinction between exponential and slower forms of decay. The common form of decay is assumed traditionally to be exponential or approximately so. Slow decay on the other hand, is characteristic of systems such as glasses that may be viewed as being very special. The purpose of the present article is to show, that slow decay is not related necessarily to the system under consideration but rather to the structure factor we prefer to measure. We may think of the decay of structure factors in certain systems as being exponential or slow but this may just reflect experimental practical preference to measure certain correlations and not others.