  In transition metals and their compounds, the orbital degrees of freedom
gives rise to an orbital current, in addition to the ordinary spin and charge
currents. We reveal that considerably large spin and anomalous Hall effects
(SHE and AHE) observed in transition metals originate from an orbital Hall
effect (OHE). To elucidate the origin of these novel Hall effects, a simple
periodic s-d hybridization model is proposed as a generic model. The giant
positive OHE originates from the orbital Aharonov-Bohm phase factor, and
induces spin Hall conductivity that is proportional to the spin-orbit
polarization at the Fermi level, which is positive (negative) in metals with
more than (less than) half-filling.
