Nonequilibrium Josephson-like effects in wide mesoscopic S-N-S junctions

TL;DR

This paper investigates nonequilibrium supercurrents in wide mesoscopic S-N-S junctions, revealing dominant second harmonic effects and how external biases and noise can excite supercurrents even when equilibrium effects are suppressed.

Contribution

It provides a simple Resistively-Shunted-Junction model to describe nonequilibrium supercurrents and predicts magnetic flux trapping in units of hc/4e due to phase fluctuations.

Findings

Second harmonic dominates supercurrents in wide junctions.

External biases and noise can excite supercurrents.

Thermal noise can lead to flux trapping in hc/4e units.

Abstract

Mesoscopic superconducting-normal-metal-superconducting (S-N-S) junctions with a large separation between the superconducting electrodes (i.e. wide junctions) exhibit nonequilibrium supercurrents, even at temperatures for which the equilibrium Josephson effect is exponentially small. The second harmonic of the Josephson frequency dominates these currents, as observed in recent experiments. A simple description of these effects, in the spirit of the Resistively-Shunted-Junction model, is suggested here. It is used to calculate dc I-V characteristics, and to examine the effects of various types of noise and of external microwave radiation (Shapiro steps). It is found that the nonequilibrium supercurrents are excited when the junction is driven by a dc bias or an ac bias, or even by external noise. In the case of junctions which are also long in the direction perpendicular to the current flow, thermodynamic phase fluctuations (thermal noise) alone can drive the quasiparticles out of local equilibrium. Magnetic flux is then predicted to be trapped in units of Phi_0 /2 = hc/4e.