Novel axisymmetric coherent vortex state in arrays of Josephson junctions far from equilibrium

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Abstract
We present a numerical study of a topologically disordered, overdamped array of Josephson junctions subjected to dc and ac currents. We find that vortices are nucleated by the ac current, apart from the ones produced at defects. After a complex transient, these vortices settle into a parity-broken, time-periodic, axisymmetric coherent vortex state characterized by rows of vortices lying along a tilted axis. This locked-in state leads to giant half-integer pseudo-steps in the I-V characteristics that are hysteretic in nature. Possible connections of these results to recent experiments are discussed.