  We realize and study the ionic Hubbard model using an interacting
two-component gas of fermionic atoms loaded into an optical lattice. The
bipartite lattice has honeycomb geometry with a staggered energy-offset that
explicitly breaks the inversion symmetry. Distinct density-ordered phases are
identified using noise correlation measurements of the atomic momentum
distribution. For weak interactions the geometry induces a charge density wave.
For strong repulsive interactions we detect a strong suppression of doubly
occupied sites, as expected for a Mott insulating state, and the externally
broken inversion symmetry is not visible anymore in the density distribution.
The local density distributions in different configurations are characterized
by measuring the number of doubly occupied lattice sites as a function of
interaction and energy-offset. We further probe the excitations of the system
using direction dependent modulation spectroscopy and discover a complex
spectrum, which we compare with a theoretical model.
