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 $γ$ and the applied external rotation speed $Ω$. For a given $γ$, the transition lines between no-vortex and vortex states are shifted toward higher $Ω$ relative to those for the symmetric case. We also find a re-entrant behavior, where the number of vortex cores can decrease for large $Ω$. 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 $Ω$ 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 $Ω$.