Ultracold bosons in disordered superlattices: Mott insulators induced by tunneling

Abstract

We analyze the phase diagram of ultracold bosons in a one-dimensional superlattice potential with disorder, using the time-evolving block decimation algorithm for infinite-sized systems. For degenerate potential energies within the unit cell of the superlattice, loophole-shaped insulating phases with noninteger filling emerge with a particle-hole gap proportional to the boson hopping. Addition of a small amount of disorder destroys this gap. For not too large disorder, the loophole Mott regions detach from the axis of vanishing hopping, giving rise to insulating islands. Thus the system shows a transition from a compressible Bose glass to a Mott-insulating phase with increasing hopping amplitude. We present a straightforward effective model for the dynamics within a unit cell which provides a simple explanation for the emergence of Mott-insulating islands. In particular, it gives rather accurate predictions for the inner critical point of the Bose glass to Mott insulator transition.