Superfluid to Mott-insulator transition of cold atoms in optical lattices

We review the superfluid (SF) to Mott-insulator (MI) transition of cold atoms in optical lattices. The experimental signatures of the transition are discussed and the random-phase-approximation (RPA) theory of the Bose–Hubbard model briefly described. We point out that the critical behavior at the transition, as well as the prediction by the RPA theory of a gapped mode (besides the Bogoliubov sound mode) in the SF phase, is difficult to understand from the Bogoliubov theory. On the other hand, these findings appear to be intimately connected to the non-trivial infrared behavior of the SF phase as recently studied within the non-perturbative renormalization group.