  We propose a mechanism of angular momentum conversion from optical transverse
spin in surface plasmon polaritons (SPPs) to conduction electron spin. Free
electrons in the metal follow the transversally spinning electric field of SPP,
and the resulting orbital motions create inhomogeneous static magnetisation in
the metal. By solving the spin diffusion equation in the SPP, we find that the
magnetisation field generates an electron spin current. We show that there
exists a resonant condition where the spin current is resonantly enhanced, and
the polarisation of the spin current is flipped. Our theory reveals a novel
functionality of SPP as a spin current source.
