Abstract: We report on the behavior of high T$-c$/ superconductor coupled multilayer structures in magnetic field perpendicular to the ab plane. In these structures the number N of superconducting layers in the sample has been systematically varied. We extract the activation energy for flux motion from resistive measurements of the superconducting transitions. For samples containing N 24 angstroms thick DyBa$-2$/Cu$-3$/O$-7$/ layers, each separated from the next by 96 angstroms of an (Y$-0.45$/Pr$-0.55$/)Ba$-2$/Cu$-3$/O$-7$/ alloy, we find that the activation energy U increases linearly with the number N of layers for N $LS 3 - 4, and then saturates. The linear increase of U is taken as an evidence that coupling through (Y$-0.45$/Pr$-0.55$/)Ba$-2$/Cu$-3$/O$-7$/ increases the correlated volume involved in the flux motion and thus U. For N $EQ 3 stacked pancake vortices move rigidly and the vortex lattice is 2D. For N $GRT 3 - 4 shear of the vortex lattice becomes important, the vortex lattice dimensionally crosses-over to 3D, and the activation energy saturates.!58
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