Effective mass analysis of Bose-Einstein condensates in optical lattices   -- stabilization and levitation
  We investigate the time evolution of a Bose-Einstein condensate in a periodic optical potential. Using an effective mass formalism, we study the equation of motion for the envelope function modulating the Bloch states of the lattice potential. In particular, we show how the negative effective mass affects the dynamics of the condensate. 
H. Pu, L. O. Baksmaty, W. Zhang, N. P. Bigelow and P. Meystre