Josephson junction ratchet: effects of finite capacitances

TL;DR

This paper investigates the directed transport phenomena in an asymmetric Josephson junction SQUID with finite capacitances, revealing complex behaviors like voltage reversals and noise-induced currents, and extends previous models to broader damping regimes.

Contribution

It extends the Stewart-McCumber model to include finite capacitances in a Josephson junction ratchet, analyzing transport across various damping regimes and controlling current direction via magnetic flux.

Findings

Detection of negative differential conductance

Observation of voltage reversals and noise-induced currents

Control of transport direction through magnetic flux

Abstract

We study transport in an asymmetric SQUID which is composed of a loop with three capacitively and resistively shunted Josephson junctions: two in series in one arm and the remaining one in the other arm. The loop is threaded by an external magnetic flux and the system is subjected to both a time-periodic and a constant current. We formulate the deterministic and, as well, the stochastic dynamics of the SQUID in terms of the Stewart-McCumber model and derive an equation for the phase difference across one arm, in which an effective periodic potential is of the ratchet type, i.e. its reflection symmetry is broken. In doing so, we extend and generalize earlier study by Zapata et al. [Phys. Rev. Lett. 77, 2292 (1996)] and analyze directed transport in wide parameter regimes: covering the over-damped to moderate damping regime up to its fully under-damped regime. As a result we detect the intriguing features of a negative (differential) conductance, repeated voltage reversals, noise induced voltage reversals and solely thermal noise-induced ratchet currents. We identify a set of parameters for which the ratchet effect is most pronounced and show how the direction of transport can be controlled by tailoring the external magnetic flux.