  Two-dimensional (2D) atom lattices provide model setups for Coulomb
correlations inducing competing ground states, partly with topological
character. Hexagonal SiC(0001) is an intriguing wide-gap substrate,
spectroscopically separated from the overlayer and hence reduced screening. We
report the first study of an artificial high-Z atom lattice on SiC(0001) by Sn
adatoms, based on combined experimental realization and theoretical modeling.
Density-functional theory of our $\sqrt{3}$-structure model closely reproduces
the scanning tunneling microscopy. Instead of metallic behavior, photoemission
data show a deeply gapped state (~2 eV gap). Based on our calculations
including dynamic mean-field theory, we argue that this reflects a pronounced
Mott insulating scenario. We also find indications that the system is
susceptible to antiferromagnetic superstructures. Such spin-orbit-coupled
correlated heavy atom lattices on SiC(0001) thus form a novel testbed for
peculiar quantum states of matter, with potential bearing for spin liquids and
topological Mott insulators.
