Dynamics of multiple-junction stacked flux-flow oscillators: Comparison between theory and experiment

Abstract

Flux-flow dynamics in long $N$-layered ${\mathrm{N}\mathrm{b}/\mathrm{A}\mathrm{l}\ensuremath{-}\mathrm{A}\mathrm{l}\mathrm{O}}_{x}/\mathrm{Nb}$ Josephson tunnel junctions is investigated experimentally and by numerical simulations. Magnetic-field-dependent current-voltage characteristics show the collective flow of Josephson vortices in the experiments with $N=7$ and $N=9.$ In order to interpret the observed characteristics we performed numerical analysis using a finite difference method. The structure of cavitylike resonances displayed in the $I\ensuremath{-}V$ characteristics is accounted for by the characteristic frequencies calculated using the coupled sine-Gordon equations model. A static $I\ensuremath{-}V$ curve obtained numerically for a seven-junction stack shows voltage-locked flux-flow motion among the inner five junctions. The numbers of vortices in the top and bottom junction is found to be larger than those of the inner junctions because of the thicker top and bottom Nb electrodes. Numerical data show very good overall agreement with the experiment.