  Electrons in artificial lattices enable explorations of the impact of
repulsive Coulomb interactions in a tunable system. We have trapped
two-dimensional electrons belonging to a gallium arsenide quantum well in a
nanofabricated lattice with honeycomb geometry. We probe the excitation
spectrum in a magnetic field identifying novel collective modes that emerge
from the Coulomb interaction in the artificial lattice as predicted by the
Mott-Hubbard model. These observations allow us to determine the Hubbard gap
and suggest the existence of a novel Coulomb-driven ground state. This approach
offers new venues for the study of quantum phenomena in a controllable
solid-state system.
