Quantum phase transitions in two-dimensional tilted optical lattices

We discuss the quantum phase transition between the Mott-insulator state and the density-wave state of cold Bose atoms in a two-dimensional (2D) square lattice as the lattice is adiabatically tilted along one of its primary axes. It is shown that a small misalignment of the tilt drastically changes the result of the adiabatic passage and, instead of the density-wave state, one obtains a disordered state. An intrinsic relation of the problem to Bloch oscillations of hard-core bosons in a 2D lattice is illuminated.