Diffusion anomalies in ac driven Brownian ratchets

TL;DR

This paper investigates diffusion behavior in an ac-driven Brownian ratchet system, revealing controllable anomalous diffusion stages and a novel mechanism linked to ratchet potential asymmetry, with potential experimental verification.

Contribution

It uncovers a new mechanism for anomalous diffusion in ratchet systems and demonstrates how crossover times can be controlled by external parameters like temperature and magnetic flux.

Findings

Initial superdiffusion, intermediate subdiffusion, and long-term normal diffusion observed.

Crossover times are tunable via temperature and magnetic flux.

Normal diffusion coefficient shows non-monotonic temperature dependence.

Abstract

We study diffusion in ratchet systems. As a particular experimental realization we consider an asymmetric SQUID subjected to an external ac current and a constant magnetic flux. We analyze mean-square displacement of the Josephson phase and find that within selected parameter regimes it evolves in three distinct stages: initially as superdiffusion, next as subdiffusion and finally as normal diffusion in the asymptotic long-time limit. We show how crossover times that separates these stages can be controlled by temperature and an external magnetic flux. The first two stages can last many orders longer than characteristic time scales of the system thus being comfortably detectable experimentally. The origin of abnormal behavior is noticeable related to the ratchet form of the potential revealing an entirely new mechanism of emergence of anomalous diffusion. Moreover, a normal diffusion coefficient exhibits non-monotonic dependence on temperature leading to an intriguing phenomenon of thermal noise suppressed diffusion. The proposed setup for experimental verification of our findings provides a new and promising testing ground for investigating anomalies in diffusion phenomena.