Collective Excitation of Bose-Einstein Condensates Induced by Evaporative Cooling

We demonstrate novel methods for exciting the collective oscillations of Bose-Einstein condensates without directly deforming the confinement potential. The collective excitations were induced through the radio-frequency (rf) forced evaporative cooling of Rb-87 atoms confined in a magnetic trap. We studied two methods, the first involving a sinusoidal amplitude modulation of the rf field, and the other a fast sweep of the rf frequency, both performed during evaporation. An almost-pure low-energy quadrupole oscillation mode was observed with both methods. In the case of the rf-amplitude modulation (fast rf-frequency sweep), the collective excitations are likely to be induced by the periodic (bulk) emission of atoms from the trap. We also observed substantial loss of atoms from the trap when the rf amplitude was modulated with a frequency near the axial-trap frequency. This allows a precise determination of the magnetic-trap frequency.