Transport in a Levy ratchet: Group velocity and distribution spread

TL;DR

This paper investigates the transport properties of an overdamped particle in a symmetric potential driven by symmetric Lévy noise, introducing robust measures like median-based group velocity and distribution spread to characterize directed motion.

Contribution

It introduces new measures for analyzing Lévy ratchet transport, addressing divergence issues of traditional moments, and explores escape behaviors and splitting probabilities in this context.

Findings

Group velocity characterized by median displacement shows directional bias.

Distribution width measured by interquantile distance reveals spread behavior.

Escape times are independent of potential structure.

Abstract

We consider the motion of an overdamped particle in a periodic potential lacking spatial symmetry under the influence of symmetric L\'evy noise, being a minimal setup for a ``L\'evy ratchet.'' Due to the non-thermal character of the L\'evy noise, the particle exhibits a motion with a preferred direction even in the absence of whatever additional time-dependent forces. The examination of the L\'evy ratchet has to be based on the characteristics of directionality which are different from typically used measures like mean current and the dispersion of particles' positions, since these get inappropriate when the moments of the noise diverge. To overcome this problem, we discuss robust measures of directionality of transport like the position of the median of the particles displacements' distribution characterizing the group velocity, and the interquantile distance giving the measure of the distributions' width. Moreover, we analyze the behavior of splitting probabilities for leaving an interval of a given length unveiling qualitative differences between the noises with L\'evy indices below and above unity. Finally, we inspect the problem of the first escape from an interval of given length revealing independence of exit times on the structure of the potential.