  Doped Mott insulators are the starting point for interesting physics such as
high temperature superconductivity and quantum spin liquids. For multi-band
Mott insulators, orbital selective ground states have been envisioned. However,
orbital selective metals and Mott insulators have been difficult to realize
experimentally. Here we demonstrate by photoemission spectroscopy how
Ca$_2$RuO$_4$, upon alkali-metal surface doping, develops a single-band metal
skin. Our dynamical mean field theory calculations reveal that homogeneous
electron doping of Ca$_2$RuO$_4$ results in a multi-band metal. All together,
our results provide compelling evidence for an orbital-selective Mott insulator
breakdown, which is unachievable via simple electron doping. Supported by a
cluster model and cluster perturbation theory calculations, we demonstrate a
novel type of skin metal-insulator transition induced by surface dopants that
orbital-selectively hybridize with the bulk Mott state and in turn produce
coherent in-gap states.
