  Localized defects, unavoidable in real solids, may be simulated in
(generically defect-free) cold-atom systems, e.g., via modifications of the
optical lattice. We study the Hubbard model on a square lattice with single
impurities, pairs of nearby impurities, or lines of impurities using
numerically exact determinantal quantum Monte Carlo simulations. In all cases,
correlations on the "impurity" sites are enhanced either by larger on-site
interactions or by a reduced coupling to the environment.
  We find highly nontrivial magnetic correlations, which persist at elevated
temperatures and should be accessible in cold-atom systems with current
experimental techniques. With improved cooling techniques, these features could
be followed towards generic quantum antiferromagnetism in the homogeneous
limit. More generally, tunable crossing points between different correlation
functions could be used, in a quantum steelyard balance setup, as robust
thermometers.
