Stationary states of a rotating Bose-Einstein condensate: Routes to vortexnucleation

Using a focused laser beam we stir a Rb-87 Bose-Einstein condensate confine d in a magnetic trap. We observe that the steady states of the condensate c orrespond to an elliptic cloud, stationary in the rotating frame. These ste ady states depend nonlinearly on the stirring parameters (amplitude and fre quency), and various solutions can be reached experimentally depending on t he path followed in this parameter space. These states can be dynamically u nstable and we observe that such instabilities lead to vortex nucleation in the condensate.