Interaction broadening of Wannier functions and Mott transitions in atomic BEC

Superfluid to Mott-insulator transitions in atomic Bose - Einstein condensates (BEC) in optical lattices are investigated for the case of number of atoms per site larger than one. To account for mean field repulsion between the atoms in each well, we construct an orthogonal set of Wannier functions. The resulting hopping amplitude and on-site interaction may be substantially different from those calculated with single-atom Wannier functions. As illustrations of the approach, we consider lattices of various dimensionality and different mean occupations. We find that in three-dimensional optical lattices the correction to the critical lattice depth is significant enough to be measured experimentally even for a small number of atoms. Finally, we discuss the validity of the single-band model.