Direct observation of effective ferromagnetic domains of cold atoms in a shaken optical lattice

One of the intriguing properties of quantum many-body systems is the emergence of long-range order from particles with short-range interactions. For example, magnetism involves the long-range ordering of electron spins. Systems of ultracold atoms are rapidly emerging as precise and controllable simulators of magnetism and other phenomena. Spinor condensates(1,2) are a powerful tool in this regard; however, the spin interaction is typically weak and accessible only when multiple atomic internal states are collisionally stable. Here we demonstrate a lattice-shaking technique for hybridizing Bloch bands in optical lattices to introduce a strong effective spin interaction and the formation of large ferromagnetic domains. Our band hybridization method is independent of the atomic internal state, and can be widely applied to quantum simulators to explore new magnetic phases in optical lattices with tunable band structure.