Defects, vortices, and critical current in Josephson-junction arrays

The breakdown phenomena of resistively shunted two-dimensional Josephson-junction array with a single defect driven by an external current at zero temperature are studied numerically. The nonlinear Josephson relation causes the formation of vortices at the tips of the defect at $i_v$ and thus lowers the current enhancement there. Above a higher critical current $i_c$ the vortices depin from the defect and march across the sample producing a voltage. The critical current $i_c$ is studied versus defect size. Various dynamic properties and the I-V characteristics of the array are explained in the context of the vortex motion. From the observed features the critical behavior of a randomly disordered array is predicted.