A cold-atom Fermi-Hubbard antiferromagnet

Many exotic phenomena in strongly correlated electron systems emerge from the interplay between spin ordering and motional degrees of freedom. For example, doping an antiferromagnet is expected to give rise to interesting phases including pseudogap states, stripe-ordering and incommensurate spin order. Ultracold fermions in optical lattices offer the potential to answer open questions on the low-temperature regime of the doped Hubbard model, which is thought to capture essential aspects of the cuprate superconductor phase diagram but is numerically intractable in that parameter regime.

We have observed antiferromagnetic long-range order in a repulsively interacting Fermi gas of Li-6 atoms on a 2D square lattice containing about 80 sites. At our lowest temperature of T/t=0.25, the ordered state is directly detected from a peak in the spin structure factor and a diverging correlation length of the spin correlation function. When doping away from half-filling into a numerically intractable regime, we find that long-range order extends to doping concentrations of about 15%. Our results open the path for a controlled study of the low-temperature phase diagram of the Hubbard model.