  Electrons in a simple correlated system behave either as itinerant charge
carriers or as localized moments. However, there is growing evidence for the
coexistence of itinerant electrons and local moments in transition metals with
nearly degenerate $d$-orbitals. It demands one or more selective electron
orbitals undergo the Mott transition while the others remain itinerant. Here we
report the first observation of such an orbital selective Mott transition
(OSMT) in Ca$_{1.8}$Sr$_{0.2}$RuO$_4$ by angle-resolved photoemission
spectroscopy (ARPES). While we observed two sets of dispersing bands and Fermi
surface associated with the doubly-degenerate $d_{yz}$ and $d_{zx}$ orbitals,
the Fermi surface associated with the wider $d_{xy}$ band is missing, a
consequence of selective Mott localization. Our theoretical calculations
demonstrate that this novel OSMT is mainly driven by the combined effects of
interorbital carrier transfer, superlattice potential, and orbital degeneracy,
whereas the bandwidth difference plays a less important role.
