  We show that there is a fundamental difference between spin Hall current and
orbital angular momentum Hall current in Rashba- Dresselhaus spin orbit
coupling systems. The orbital angular momentum Hall current has a pure
topological contribution which is originated from the existence of magnetic
flux in momentum space while there is no such topological nature for the spin
Hall current. Moreover, we show that the orbital Hall conductance is always
larger than the spin Hall conductance in the presence of both couplings. The
topological part is expected to be free from the effect of disorder due to the
topological nature. Therefore, the orbital angular momentum Hall current should
be the major effect in real experiments.
