Bipotential motion and anomalous transport in optical lattices

We present a semiclassical treatment of atomic motion in an optical lattice . A two-component kinetic equation with an adequate expression for the diff usion matrix is derived. We distinguish between three velocity domains with substantially different atomic kinetics. The motion of slow atoms in a suf ficiently strong field is governed by the Fokker-Planck equation with spati ally modulated kinetic coefficients. The kinetics of atoms with intermediat e velocities has a clearly pronounced bipotential character. The stationary solution in this range is represented as a sum of two Boltzmann distributi ons shifted in space. It is shown that the spatially homogeneous equation i s valid only for asymptotically high velocities. In this range, the station ary distribution decreases as a power function of the energy, giving rise t o an anomalous spatial diffusion and enhancing energy fluctuations. The cri tical depth of the optical potential calculated within the framework of our approach agrees well with the relevant experimental data.