Converting Non-Equilibrium Charge Density into Spin Current

The interconversion between charge and spin degrees of freedom is of both fundamental and technological relevance in spintronics. While a non-equilibrium spin density and a charge current are related by the well known Rashba-Edelstein effect, here we theoretically model the generation of a time-dependent spin current due to a periodic modulation of the charge density, for example by a gate. By using the Boltzmann transport equation, we show that when the chemical potential is varied, a spin current is generated in the time scale it takes for the system to re-equilibrate in the new chemical potential. The effect is ubiquitous in many systems with spin-momentum locking; we compute the strength of the effect in four examples and propose a simple device scheme to measure the spin accumulation resulting from such time-dependent spin currents. Our findings test fundamental theoretical questions about charge-to-spin conversion mechanisms and provide an all-electrical way to generate spin currents without the need for charge currents, magnetic materials or optical methods.