Model for collisions in ultracold-atom mixtures

We present a model for the collisional properties of a mixture of Cs-133 and Rb-87 atoms in a magnetic trap at mu K temperatures. The experimental sequence we model corresponds to a selective evaporation of the Rb atoms using a radio-frequency field, leading to sympathetic cooling of the Cs atoms or, if selective evaporation is carried out fast, to a difference in temperature between the two atomic species. In the latter case, the two atomic clouds reached an equilibrium temperature starting from an Rb temperature lower than that of Cs. By supposing that each atomic cloud was in thermal equilibrium we modeled this rethermalization process through differential equations for the two atomic temperatures. An alternative approach was based on the Monte Carlo simulations of the individual collisional processes. The sympathetic cooling and the rethermalization were analyzed in terms of the inter-species collisional cross-section.