Nucleation of vortices in a rotating Bose-Einstein condensate

For the precise experimental procedure used at ENS to stir a Bose-Einstein condensate, we have shown that the vortex nucleation is driven by a dynamic, hydrodynamic type instability rather than by a thermodynamic mechanism. The universal value of the rotation frequency, Ω = 0.7ω_⊥ leading to vortex formation in a very slightly anisotropic rotating trap corresponds simply to a resonance between the stirring harmonic potential and the condensate, at the origin of the dynamic instability. The fact that a resonance is involved also explains simply why vortex nucleation takes place in a narrow interval of rotation frequencies.