Anomalous non-gaussian diffusion in small disordered rings

We analyze an equilibrium classical diffusion of a Brownian particle confined to a ring coated on a two-dimensional disordered film. The random potential modeling the interaction with the inhomogeneous medium is assumed to be Gaussian with a finite correlation length. With a microscopic method, we derive the second and fourth cumulant function of the particle's displacement at large times. It is shown that the disorder gives rise to a quadratic time dependence of the fourth cumulant (anomalous non-Gaussian diffusion), whereas the usual diffusion covered by the second cumulant remains normal. This points to the fact that the motion of a Brownian particle along the disordered ring is attended by nonergodic fluctuations in its diffusion coefficient.