Ratchets in homogeneous extended systems: internal modes and the role of noise

TL;DR

This paper investigates how symmetry-breaking ac forces induce directed motion of solitons in extended systems, revealing the role of internal modes, initial phase, and noise in controlling the ratchet effect.

Contribution

The study introduces a collective coordinate approach to explain how width oscillations drive soliton motion and explores noise effects, providing new insights into ratchet phenomena in extended systems.

Findings

Directed energy current depends on symmetry-breaking ac forces.

Initial phase influences the direction of soliton motion.

Noise enhances velocity, indicating possible resonance effects.

Abstract

We revisit the issue of directed motion induced by zero average forces in extended systems driven by ac forces. It has been shown recently that a directed energy current appears if the ac external force, $f(t)$, breaks the symmetry $f(t) = - f(t+T/2)$, $T$ being the period, if topological solitons (kinks) existed in the system. In this work, a collective coordinate approach allows us to identify the mechanism through which the width oscillation drives the kink and its relation with the mathematical symmetry conditions. Furthermore, our theory predicts, and numerical simulations confirm, that the direction of motion depends on the initial phase of the driving, while the system behaves in a ratchet-like fashion if averaging over initial conditions. Finally, the presence of noise overimposed to the ac driving does not destroy the directed motion; on the contrary, it gives rise to an activation process that increases the velocity of the motion. We conjecture that this could be a signature of resonant phenomena at larger noises.