Splitting in the Excitation Spectrum of A Bose-Einstein Condensate   Undergoing Strong Rabi Oscillations
  We report on a measurement of splitting in the excitation spectrum of a condensate driven by an optical travelling wave. Experimental results are compared to a numerical solution of the Gross Pitaevskii equation, and analyzed by a simple two level model and by the more complete band theory, treating the driving beams as an optical lattice. In this picture, the splitting is a manifestation of the energy gap between neighboring bands that opens on the boundary of the Brillouin zone. 
E. Rowen, R. Ozeri, N. Katz, E. Gershnabel, N. Davidson