Direct observation of vortex dynamics in two-dimensional Josephson-junction arrays

Spatially resolved images of the dynamic states of current-biased overdamped two-dimensional arrays of Nb/AlO/sub x//Nb Josephson junctions were obtained using low-temperature scanning electron microscopy. We present two-dimensional imaging results describing various vortex dynamic regimes in zero applied magnetic field. The nucleation of current-induced vortices at the array boundaries and their subsequent motion into the array interior are observed for bias currents slightly above the array critical current. With increasing bias current, vortex-vortex interaction becomes important. Discussions on the coherent microwave radiation emission are presented.>