Vortex stabilization in a small rotating asymmetric Bose-Einstein condensate

We use a variational method to investigate the ground-state phase diagram of a small, asymmetric Bose-Einstein condensate with respect to the dimensionless interparticle interaction strength gamma and the applied external rotation speed Omega. For a given gamma, the transition lines between no-vortex and vortex states are shifted toward higher Omega relative to those for the symmetric case. We also find a re-entrant behavior, where the number of vortex cores can decrease for large Omega. In addition, stabilizing a vortex in a rotating asymmetric trap requires a minimum interaction strength. For a given asymmetry, the evolution of the variational parameters with increasing Omega shows two different types of transitions (sharp or continuous), depending on the strength of the interaction. We also investigate transitions to states with higher vorticity; the corresponding angular momentum increases continuously as a function of Omega.