Transition to the Giant Vortex State in an Harmonic Plus Quartic Trap

We consider a rapidly rotating Bose-condensed gas in an harmonic plus quartic trap. At sufficiently high rotation rates the condensate acquires an annular geometry with the superposition of a vortex lattice. With increasing rotation rate the lattice evolves into a single ring of vortices. Of interest is the transition from this state to the giant vortex state in which the circulation is carried by only a central vortex. By analyzing the Gross-Pitaevskii energy functional variationally, we have been able to map out the phase boundary between these two states as a function of the rotation rate and the various trapped gas parameters. The variational results are in good qualitative agreement with those obtained by means of a direct numerical solution of the Gross-Pitaevskii equation.