MAGNETIC-FLUX DYNAMICS IN STACKED JOSEPHSON-JUNCTIONS

Vertical stacking of Josephson junctions leads to new interesting effects in their dynamics. The magnetic coupling between the junctions was provided by their common electrodes of the thickness comparable with the London penetration depth lambda(L). When Josephson vortices (fluxons) are present in neighboring junctions, they interact with each other via their screening currents flowing in a thin superconducting layer between the junctions. We investigated experimentally various stacks of Nb/Al-AlOx/Nb Josephson tunnel junctions. Vortex interaction effects are observed in the current-voltage (I - V) characteristics of these structures. Two phase-locked regimes characterized by different fluxon propagation velocities were found in two-fold stacks. In the lower velocity (antiphase) mode fluxons located in different Josephson layers repel each other. In contrast, the high velocity (in-phase) mode corresponds to an attraction between unipolar fluxons. Our experimental results may be considered in the line with modelling the physical properties of intrinsically stacked high-T-c superconducting materials.