  Investigation of the spin Hall effect in gold has triggered increasing
interest over the past decade, since gold combines the properties of a large
bulk spin diffusion length and strong interfacial spin-orbit coupling. However,
discrepancies between the values of the spin Hall angle of gold reported in the
literature have brought into question the microscopic origin of the spin Hall
effect in Au. Here, we investigate the thickness dependence of the spin-charge
conversion efficiency in single Au films and ultrathin Au/Si multilayers by
non-local transport and spin-torque ferromagnetic resonance measurements. We
show that the spin-charge conversion efficiency is strongly enhanced in
ultrathin Au/Si multilayers, reaching exceedingly large values of 0.99 +/- 0.34
when the thickness of the individual Au layers is scaled down to 2 nm. These
findings reveal the coexistence of a strong interfacial spin-orbit coupling
effect which becomes dominant in ultrathin Au, and bulk spin Hall effect with a
relatively low bulk spin Hall angle of 0.012 +/- 0.005. Our experimental
results suggest the key role of the Rashba-Edelstein effect in the
spin-to-charge conversion in ultrathin Au.
