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Nonlinear dependence observed in quadrupolar collective excitation of a trapped BEC

We report the experimental observation of the collective excitations induced in a magnetically trapped 87Rb Bose-Einstein condensate. Low-lying mode excitations were studied by tracking the condensate's center-of-mass displacement, and its aspect ratio as a function of the hold time in the trap. We were able to partially control the modes onset by modulating the amplitude of the additional field gradient used to excite the BEC. The measured excitation frequencies were found to be in good agreement with the literature. We have also found that the modulation amplitude was able to change the phase of the center-of-mass oscillation. Finally, an interesting, non-linear dependence was observed on the condensate aspect ratio as a function of the perturbing amplitude which induces the quadrupolar mode.