Bose-Einstein Condensates with Tunable Atom-atom Interactions: The First Experiments with 85Rb BECs

<p>In this thesis, the creation of magnetically trapped atomic gas Bose-Einstein Condensates (BECs) with tunable atom-atom interactions is described. The atom-atom interactions are tuned by adjusting an external magnetic field through the phenomenon of a Feshbach resonance. The strength of the interactions can be tuned to any desired value, and the interactions can be made to be repulsive or attractive. Since many properties of BECs are determined by the strength of the atom-atom interactions, such as the spatial size, the collective excitation frequencies, and the stability of the BECs, the ability to tune the interactions allows a wide range of experiments to be performed.</p> <p>The work in this thesis details the characterization of a Feshbach resonance in cold collisions between 85Rb atoms. Both the elastic and inelastic collision rates are measured as a function of the external magnetic field, showing variations of four or more orders of magnitude as the external magnetic field is changed by a few gauss. Using the knowledge gained about the variation of the elastic and inelastic collisions near the Feshbach resonance, a stable BEC of up to 18,000 85Rb atoms was created through evaporative cooling. The spatial size of these BECs responded as predicted by mean-field theory as the strength of repulsive interactions was changed by tuning the external magnetic field. The mean-field description is predicted to have a limited range of validity as the strength of the repulsive interactions is increased further, however, and in a different experiment we observed deviations from the mean-field predicted values of collective excitation frequencies of the BECs as the strength of the interactions was increased. Work was also performed on BECs with attractive interactions. For strong enough attractive interactions, the BECs are predicted to become unstable, and the onset of this instability as a function of the number of atoms in the BEC and the strength of the attractive interactions is reported here. The dynamics of the unstable BECs as they collapsed are also described in this work.</p>
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University of Colorado Boulder
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