Subdiffusion of mixed origins: when ergodicity and nonergodicity coexist

Phys Rev E Stat Nonlin Soft Matter Phys. 2010 Jan;81(1 Pt 1):010101. doi: 10.1103/PhysRevE.81.010101. Epub 2010 Jan 4.

Abstract

Single particle trajectories are investigated assuming the coexistence of two subdiffusive processes: diffusion on a fractal structure modeling spatial constraints on motion and heavy-tailed continuous time random walks representing energetic or chemical traps. The particles' mean squared displacement is found to depend on the way the mean is taken: temporal averaging over single-particle trajectories differs from averaging over an ensemble of particles. This is shown to stem from subordinating an ergodic anomalous process to a nonergodic one. The result is easily generalized to the subordination of any other ergodic process (i.e., fractional Brownian motion) to a nonergodic one. For certain parameters the ergodic diffusion on the underlying fractal structure dominates the transport yet displaying ergodicity breaking and aging.