Observation of 2D fermionic Mott insulators of $^{40}$K with single-site   resolution
  We report on the site-resolved observation of characteristic states of the two-dimensional repulsive Fermi-Hubbard model, using ultracold $^{40}$K atoms in an optical lattice. By varying the tunneling, interaction strength, and external confinement, we realize metallic, Mott-insulating, and band-insulating states. We directly measure the local moment, which quantifies the degree of on-site magnetization, as a function of temperature and chemical potential. Entropies per particle as low as $0.99(6)\,k_B$ indicate that nearest-neighbor antiferromagnetic correlations should be detectable using spin-sensitive imaging. 
Lawrence W. Cheuk, Matthew A. Nichols, Katherine R. Lawrence, Melih   Okan, Hao Zhang, Martin W. Zwierlein
cond-mat.quant-gas cond-mat.stat-mech cond-mat.str-el