A free-energy landscape model for primary relaxation in glass-forming liquids: Rotations and dynamic heterogeneities

Higher order correlation functions as observed in four-dimensional NMR experiments on glass-forming liquids are interpreted in terms of a simple model for molecular reorientations. Its key assumption is an intrinsic link between the rotational and structural relaxation of the liquid. It is shown that the introduction of an additional time scale as proposed previously is not necessary. Furthermore, the model naturally accounts for a number of features associated with rotational motions in supercooled liquids. Applications of the model to other aspects of the α relaxation in supercooled liquids are discussed.