Rapidly rotating Bose-Einstein condensates in anharmonic confinement

We study a rapidly rotating Bose-Einstein condensate in anharmonic confinement and find that many properties, such as the critical rotating frequency and phase diagram, are different from those in a harmonic trap. We investigate the phase transitions between various vortex lattices and find that a hole emerges in the center of the cloud when the rotating frequency Θ reaches Θ_h but it becomes invisible when Θ > 1.0842ω _⊥.