  The Su-Schrieffer-Heeger (SSH) model in a two-dimensional rectangular lattice
features gapless or gapped Dirac cones with topological edge states along
specific peripheries. While such a simple model has been recently realized in
photonic/acoustic lattices and electric circuits, its material realization in
condensed matter systems is still lacking. Here, we study the atomic and
electronic structure of a rectangular Si lattice on Ag(001) by angle-resolved
photoemission spectroscopy and theoretical calculations. We demonstrate that
the Si lattice hosts gapped Dirac cones at the Brillouin zone corners. Our
tight-binding analysis reveals that the Dirac bands can be described by a 2D
SSH model with anisotropic polarizations. The gap of the Dirac cone is driven
by alternative hopping amplitudes in one direction and staggered potential
energies in the other one and hosts topological edge states. Our results
establish an ideal platform to explore the rich physical properties of the 2D
SSH model.
