Radio-frequency-induced evaporative cooling of
87Rb atoms in the
F=2, m=2 ground state in a magnetic trap with a high bias field is found to be hampered, and eventually interrupted, by the presence of nonlinear Zeeman shifts. An experiment studying evaporation at a fixed adjustable rf frequency was used to illustrate this effect. These nonlinear shifts prevent us from reaching quantum degeneracy in the
F=2, m=2 ground state; however, evaporation in the
F=1, $m=\ensuremath{-}1$ ground state is not adversely affected, and we can produce a Bose-Einstein condensate (BEC). Our analysis shows that this effect can play a role in many BEC experiments.
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 87 Rb 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.
We report the experimental achievement of 87Rb Bose-Einstein condensation in a magnetic trap with microwave and radio frequency (RF) induced evaporation. Evaporative cooling is realized by using 6.8 GHz microwave radiation driving the 87Rb atoms to transit from the ground-state hyperfine state |F = 2,mF =2> to |F = 1,mF = 1>. Compared with RF-induced evaporation, 87Rb atoms are hardly to achieve pure condensate by microwave evaporation cooling due to the effect of atoms in the |F = 1,mF = 1> state being pumped back into the trapping |F = 2,mF = 1> state.