  Recently a positive spin Hall angle (SHA) of 0.021, was observed
experimentally in nonmagnetic CuIr alloys [Niimi et al., Phys. Rev. Lett. 106,
126601 (2011)] and attributed predominantly to an extrinsic skew scattering
mechanism, while a negative SHA was obtained from ab initio calculations
[Fedorov et al., Phys. Rev. B 88, 085116 (2013)], using consistent definitions
of the SHA. We reconsider the SHA in CuIr alloys, with the effects of the local
electron correlation U in 5d orbitals of Ir impurities, included by the quantum
Monte Carlo method. We found that the SHA is negative if we ignore such local
electron correlation, but becomes positive once U approaches a realistic value.
This may open up a way to control the sign of the SHA by manipulating the
occupation number of impurities.
