All-Electrical Spin Pumping in Topological Insulators

Generating spin currents without magnetic or optical methods is important for an easy integration of spintronics in electronic devices. The use of spin-orbit coupling to create and control spins with electric fields has consequently become a major topic of research. However, most mechanisms to convert charge to spin (such as the spin Hall effect) rely on applying a bias voltage, which inevitably produces energy losses in gapless systems. Here, we predict that a time-dependent change of the chemical potential in the surface states of a topological insulator or other spin-orbit coupled systems generates a non-equilibrium AC spin current, under standard assumptions about the spin current in crystals. This driving can be achieved with e.g. an AC gate voltage without any charge current. The resulting spin polarization is locked with the current direction and the magnitude of the current increases linearly with the chemical potential, indicating that larger doping of the surface states is favorable. We discuss how this effect can be measured in an experiment, either by an AC inverse spin Hall effect or by measuring the time evolution of the spin density with optical probes. Finally, since the effect is not only limited to topological insulators but to any Rashba-like system, we also show similar results for a Rashba 2DEG. Our work opens the door to efficiently use topological surface states for spin current generation.