Vortex nucleation and quadrupole deformation of a rotating Bose-Einstein condensate

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Using a semi-analytic model based on the Thomas-Fermi approximation, we investigate the relevance of the quadrupole deformation of a trapped Bose-Einstein condensate for the nucleation of quantized vortices. For sufficiently high angular velocities Ω of the trap, the tendency of the system to exhibit spontaneous deformation is shown to lower the barrier which inhibits the nucleation of vortices at smaller Ω. The corresponding value of the critical angular velocity Ωc is calculated as a function of the deformation of the trap and of the chemical potential. The theoretical predictions for Ωc refer to the case of a sudden switch-on of the deformed rotating trap and they are compared with recent experimental data.