TRAPPING ASPECTS IN ENHANCED DIFFUSION

We study the superlinear diffusion <x2(t)> approximately t(alpha), alpha > 1, in layered media containing random velocity fields. The superlinear behavior holds in the case of random velocities along the x direction accompanied by diffusional motion in the space transverse to it. The transverse space can be either Euclidean, fractal, or ultrametric. For a one-dimensional transverse space we derive exact expressions for the higher moments of the displacement. Furthermore, we investigate the propagator P(x, t) along the x direction and establish its scaling behavior. Our analysis highlights the resemblance between the stretched-Gaussian behavior of the propagator and the stretched-exponential form of the survival probability in the trapping problem; both show late crossover behavior.