Vortex Rings in Fast Rotating Bose-Einstein Condensates

When Bose-Einstein condensates are rotated sufficiently fast, a giant vortex phase appears, that is, the condensate becomes annular with no vortices in the bulk but a macroscopic phase circulation around the central hole. In a former paper (Correggi et al. in Commun Math Phys 303:451-308, 2011) we have studied this phenomenon by minimizing the two-dimensional Gross-Pitaevskii (GP) energy on the unit disc. In particular, we computed an upper bound to the critical speed for the transition to the giant vortex phase. In this paper we confirm that this upper bound is optimal by proving that if the rotation speed is taken slightly below the threshold, there are vortices in the condensate. We prove that they gather along a particular circle on which they are uniformly distributed. This is done by providing new upper and lower bounds to the GP energy.