Precision Measurements on a Tunable Mott Insulator of Ultracold Atoms
  We perform precision measurements on a Mott-insulator quantum state of ultracold atoms with tunable interactions. We probe the dependence of the superfluid-to-Mott-insulator transition on the interaction strength and explore the limits of the standard Bose-Hubbard model description. By tuning the on-site interaction energies to values comparable to the interband separation, we are able to quantitatively measure number-dependent shifts in the excitation spectrum caused by effective multi-body interactions. 
M. J. Mark, E. Haller, K. Lauber, J. G. Danzl, A. J. Daley, and H.-C.   N\"agerl
quant-ph cond-mat.quant-gas