  The physics of doped Mott insulators is at the heart of some of the most
exotic physical phenomena in materials research including insulator-metal
transitions, colossal magneto-resistance, and high-temperature
superconductivity in layered perovskite compounds. Advances in this field would
greatly benefit from the availability of new material systems with similar
richness of physical phenomena, but with fewer chemical and structural
complications in comparison to oxides. Using scanning tunneling microscopy and
spectroscopy, we show that such a system can be realized on a silicon platform.
Adsorption of one-third monolayer of Sn atoms on a Si(111) surface produces a
triangular surface lattice with half-filled dangling bond orbitals. Modulation
hole-doping of these dangling bonds unveils clear hallmarks of Mott physics,
such as spectral weight transfer and the formation of quasi-particle states at
the Fermi level, well-defined Fermi contour segments, and a sharp singularity
in the density of states. These observations are remarkably similar to those
made in complex oxide materials, including high-temperature superconductors,
but highly extraordinary within the realm of conventional sp-bonded
semiconductor materials. It suggests that exotic quantum matter phases can be
realized and engineered on silicon-based materials platforms.
