  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.
