Phys. Rev. Lett. (in the press); preprint at

The Bose–Hubbard model provides a basic description of the behaviour of interacting spinless bosons on a lattice. At low temperatures, it predicts an interaction-driven phase transition between a superfluid and a Mott insulating state. Under a mean-field approximation, the phase diagram of the Bose–Hubbard model is determined by the lattice coordination number, not the lattice geometry. Can this effect be tested in experiment?

Claire K. Thomas and colleagues address this question using ultracold atomic gases loaded into optical lattices with two different coordination numbers — four for a kagome lattice or six for a triangular lattice. The coherent fraction in two different lattices was monitored and plotted as a function of the rescaled interaction energy. The data points superimposed on each other confirmed the scaling prediction. A phase transition from the Mott insulating to the superfluid state barely driven by the change of the coordination number was also observed.