Spin-Current-Driven Spin Pumping in Rashba Spin-Orbit Coupled Systems: A Spin Torque Effect

There exists a direct connection between spin torque and spin current in two-dimensional electron systems with linear in momentum Rashba spin-orbit (SO) coupling. In terms of the spin-current continuity equation, we show that the spin torque of this type generates a divergent spin current due to spin injection, which we call the spin-current-driven spin pumping. We quantitatively investigate the spin pumping from SO coupled systems in contact with spin-polarized reservoirs using the nonequilibrium Green's function formalism, demonstrating that the spin torque effect efficiently produces a pure spin current which is orders of magnitude larger than the spin Hall current.