Observation of harmonic generation and nonlinear coupling in the   collective dynamics of a Bose condensate
  We report the observation of harmonic generation and strong nonlinear coupling of two collective modes of a condensed gas of rubidium atoms. Using a modified TOP trap we changed the trap anisotropy to a value where the frequency of the m=0 high-lying mode corresponds to twice the frequency of the m=0 low-lying mode, thus leading to strong nonlinear coupling between these modes. By changing the anisotropy of the trap and exciting the low-lying mode we observed significant frequency shifts of this fundamental mode and also the generation of its second harmonic. 
G. Hechenblaikner, O.M. Marago', E. Hodby, J. Arlt, S. Hopkins, and   C.J. Foot