Non-uniform vortex lattices in inhomogeneous rotating Bose-Einstein   condensates
  We derive a general framework, in terms of elastic theory, for describing the distortion of the vortex lattice in a rotating Bose-Einstein condensate at arbitrary rotation speed and determining the dependence of the distortion on the density inhomogeneity of the system. In the rapidly rotating limit, we derive the energetics in terms of Landau levels, including excitation to higher levels; the distortion depends on the excitation of higher levels as well as on the density gradient. As we show, the dominant effect of higher Landau levels in a distorted lattice in equilibrium is simply to renormalize the frequency entering the lowest Landau level condensate wave function -- from the transverse trap frequency, $\omega$, to the rotational frequency, $\Omega$, of the system. Finally, we show how the equilibrium lattice distortion emerges from elastohydrodynamic theory for inhomogeneous systems. 
Gordon Baym, C. J. Pethick, S. Andrew Gifford, Gentaro Watanabe