Nonlocal electrodynamics of long ultra-narrow Josephson junctions: Experiment and theory

TL;DR

This study combines experiments and theory to explore how electromagnetic cavity modes and Fiske steps in ultra-narrow Josephson junctions are affected by stray magnetic fields, revealing size and vortex properties.

Contribution

It introduces a nonlocal wave propagation model to explain experimental observations and determines vortex characteristics through a variational approach.

Findings

Voltage spacing of Fiske steps increases with decreasing junction width.

Stray magnetic fields significantly influence electromagnetic modes in narrow junctions.

The nonlocal model accurately predicts experimental Fiske step voltages.

Abstract

We experimentally and theoretically investigate electromagnetic cavity modes in ultra-narrow Al-AlO$_x$-Al and Nb-AlO$_x$-Nb long Josephson junctions. Experiments show that the voltage spacing between the Fiske steps on the current-voltage characteristics of sub-$\mu$m wide and several hundred $\mu$m long Al-AlO$_x$-Al and Nb-AlO$_x$-Nb Josephson junctions increases when decreasing the width of a junction. This effect is explained by stray magnetic fields, which become important for narrow junctions. Theoretical estimates of the Fiske step voltage based on a nonlocal wave propagation equation are in good agreement with our experimental data. Using the nonlocal model, we determine the size and mass of a Josephson vortex by means of a variational approach, and relate vortex size to the experimentally measured critical magnetic field of the junction.