Hyperballistic transport in dense systems of charged particles under ac electric fields

Abstract

The Langevin equation is ubiquitously employed to numerically simulate plasmas, colloids and electrolytes. However, the usual assumption of white noise becomes untenable when the system is subject to an external AC electric field. This is because the charged particles in the system, which provide the thermal bath for the particle transport, become themselves responsive to the AC field and the thermal noise is field-dependent and non-Markovian. We theoretically study the particle diffusivity in a Langevin transport model for a tagged charged particle immersed in a dense system of charged particles (plus also, possibly, other neutral particles) that act as the thermal bath, under an external AC electric field. This is done by properly accounting for the effects of the AC field on the thermal bath statistics. We analytically derive the time-dependent generalized diffusivity $D(t)$ for different initial conditions. The generalized diffusivity exhibits damped oscillatory-like behaviour with initial very large peaks, where the generalized diffusion coefficient is enhanced by orders of magnitude with respect to the infinite-time steady-state value. The latter coincides with the Stokes-Einstein diffusivity in the absence of external field. For initial conditions where the external field is already on at $t=0$ and the system is thermalized under DC conditions for $t \leq 0$, the short-time behaviour is hyperballistic, $MSD \sim t^4$ (where MSD is the mean-squared displacement), leading to giant enhancement of the particle transport. Finally, the theory elucidates the role of medium polarization on the local Lorentz field, and allows for estimates of the effective electric charge due to polarization by the surrounding charges.