  A recent experiment in CuBi alloys obtained a large spin Hall angle (SHA) of
-0.24 (Niimi et al. Phys. Rev. Lett. 109, 156602 (2012)). We find that the SHA
can be dramatically enhanced by Bi impurities close to the Cu surface. The
mechanisms of this enhancement are two-fold. One is that the localized impurity
state on surface has a decreased hybridization and combined with Coulomb
correlation effect. The other comes from the low-dimensional state of
conduction electrons on surface, which results in a further enhancement of skew
scattering by impurities. Furthermore, we note that a discrepancy in sign of
SHA between the experiment and previous theories is simply caused by different
definitions of SHA. This re-establishes skew scattering as the essential
mechanism underlying the spin Hall effect in CuBi alloys.
