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.