Mode-locking transitions and vortex flows in current-driven Josephson-junction arrays

The dynamical behavior of overdamped dc-driven Josephson-junction arrays is studied numerically in two dimensions. Currents varying linearly along an edge are injected into the array and drawn out at the opposite edge either uniformly or through a busbar. The system is found to undergo a series of dynamical transitions as the gradient of the current drive is increased. We show that, for ladder arrays, these transitions mark the loss of mode locking across specific bonds. The transitions can, alternatively, be associated with the onset of well-defined vortex flows. Spatial localization of vortices in individual plaquettes of a ladder, driven in the direction of its length, is seen to stablize quasiperiodicity of order N>3 in a certain region of the underlying parameter space. We also discuss the extension of each of these features to full-fledged rectangular arrays.