Reaction-controlled diffusion: Monte Carlo simulations

We study the coupled two-species nonequilibrium reaction-controlled diffusion model introduced by Trimper et al. [Phys. Rev. E 62, 6071 (2000)] by means of detailed Monte Carlo simulations in one and two dimensions. Particles of type A may independently hop to an adjacent lattice site, provided it is occupied by at least one B particle. The B particle species undergoes diffusion-limited reactions. In an active state with nonzero, essentially homogeneous B particle saturation density, the A species displays normal diffusion. In an inactive, absorbing phase with exponentially decaying B density, the A particles become localized. In situations with algebraic decay rho(B)(t) approximately t(-alpha(B)), as occurring either at a nonequilibrium continuous phase transition separating active and absorbing states, or in a power-law inactive phase, the A particles propagate subdiffusively with mean-square displacement (t)(2)(A)> approximately t(1-alpha(A)). We find that within the accuracy of our simulation data, alpha(A) approximately alpha(B) as predicted by a simple mean-field approach. This remains true even in the presence of strong spatiotemporal fluctuations of the B density. However, in contrast with the mean-field results, our data yield a distinctly non-Gaussian A particle displacement distribution n(A)(x-->,t) that obeys dynamic scaling and looks remarkably similar for the different processes investigated here. Fluctuations of effective diffusion rates cause a marked enhancement of n(A)(x-->,t) at low displacements /x-->/, indicating a considerable fraction of practically localized A particles, as well as at large traversed distances.