Abstract
Spatially resolved images of the dynamic states of curent-biased two-dimensional arrays of Nb/${\mathrm{AlO}}_{\mathit{x}}$/Nb Josephson junctions were obtained using low-temperature scanning electron microscopy. The arrays were square or rectangular and the maximum size was 20 junctions \ifmmode\times\else\texttimes\fi{}20 junctions. In overdamped arrays our images at zero or small applied perpendicular magnetic field together with model calculations confirm the nucleation of a vortex at one sample edge (or an antivortex at the opposite edge) and its subsequent motion into the array interior. Vortex annihilation due to vortex-antivortex collision was observed to take place in the middle of the array or at the edge opposite to the nucleation edge. These dynamics and the underlying model considerations are similar to that for Abrikosov vortices in the current-induced resistive state of thin film type-II superconductors. The phenomenon of ``row switching'' is directly confirmed in images of underdamped arrays. The specific rows experiencing this process change randomly when the same bias point on the current-voltage characteristic is established many times, starting each time from zero current.
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