  Recent experimental observation of unexpectedly large current-induced
spin-orbit torque in surface oxidized Cu on top of a ferromagnet suggested a
possible role of the orbital Rashba effect (ORE). With this motivation, we
investigate the ORE from first principles by considering an oxygen monolayer on
top of a Cu(111) film. We show that surface oxidization of Cu film leads to
gigantic enhancement of the ORE for states near the Fermi surface. The
resulting chiral orbital texture in the momentum space is exceptionally strong,
reaching $\sim 0.5\hbar$ in magnitude. We find that resonant hybridization
between O $p$-states and Cu $d$-states is responsible for the emergence of the
ORE. We demonstrate that application of an external electric field generates
huge orbital Hall current, which is an order of magnitude larger than the spin
Hall current found in heavy metals. This implies that "orbital torque"
mechanism may be significant in surface oxidized Cu/ferromagnet structures. It
also encourages experimental verification of the orbital texture in surface
oxidized Cu through optical measurements such as angle-resolved photoemission
spectroscopy.
