Bimodal momentum distribution of laser-cooled atoms in optical lattices
  We study, numerically and experimentally, the momentum distribution of atoms cooled in optical lattices. Using semi-classical simulations, we show that this distribution is bimodal, made up of a central feature corresponding to "cold", trapped atoms, with tails of "hot", untrapped atoms, and that this holds true also for very shallow potentials. Careful analysis of the distribution of high-momentum untrapped atoms, both from simulations and experiments, shows that the tails of the distribution does not follow a normal law, hinting at a power-law distribution and non-ergodic behavior. We also revisit the phenomenon of d\'ecrochage, the potential depth below which the temperature of the atoms starts increasing. 
Claude M. Dion, Svante Jonsell, Anders Kastberg, and Peder Sj\"olund
physics.atom-ph