Probing interaction-induced ferromagnetism in optical superlattices

We propose a method for controllable preparation and detection of interaction-induced ferromagnetism in ultracold fermionic atoms loaded in optical superlattices. First, we discuss how to probe and control Nagaoka ferromagnetism in an array of isolated plaquettes (four lattice sites arranged in a square). Then, we allow for weak interplaquette tunneling. Since ferromagnetism is unstable in the presence of weak interplaquette couplings, we propose to mediate long-range ferromagnetic correlations via double-exchange processes by exciting atoms to the first vibrational band. We calculate the phase diagram of the two-band plaquette array and discuss conditions for the stability and robustness of the ferromagnetic phases in this system. Experimental implementation of the proposed schemes is discussed.