Thermal depinning of fluxons in discrete Josephson rings

TL;DR

This paper investigates how thermal fluctuations cause fluxons in Josephson ring arrays to depin and transition to high-voltage states, revealing conditions where fluxon behavior deviates from single-particle models.

Contribution

It provides a detailed analysis of thermal depinning mechanisms in Josephson rings, highlighting deviations from classical models and identifying diffusion phenomena without frequency-dependent damping.

Findings

Depinning follows a Kramers-type escape law under certain conditions.

Fluxon diffusion can occur before switching to the high-voltage mode.

Depinning rates can exceed single-particle predictions at low discreteness and damping.

Abstract

We study the thermal depinning of single fluxons in rings made of Josephson junctions. Due to thermal fluctuations a fluxon can be excited from its energy minima and move through the array, causing a voltage across each junction. We find that for the initial depinning, the fluxon behaves as a single particle and follows a Kramers-type escape law. However, under some conditions this single particle description breaks down. At low values of the discreteness parameter and low values of the damping, the depinning rate is larger than the single particle result would suggest. In addition, for some values of the parameters the fluxon can undergo low-voltage diffusion before switching to the high-voltage whirling mode. This type of diffusion is similar to phase diffusion in a single junction, but occurs without frequency-dependent damping. We study the switching to the whirling state as well.