Dilatation symmetry of the Fokker-Planck equation and anomalous diffusion.

Based on the canonical formalism, the dilatation symmetry is implemented to the Fokker-Planck equation for the Wigner distribution that describes atomic motion in an optical lattice. This reveals the symmetry principle underlying the recent result obtained by Lutz [Phys. Rev. A 67, 051402(R) (2003)] on the connection between anomalous transport in the optical lattice and Tsallis statistics in the high-energy regime. A condition is derived for the general linear Fokker-Planck equation to admit a nonstationary solution distribution that decays as a power law.