Parallel and perpendicular field investigations of coupled high-T_c multilayer structures

We report on the behavior of high Tc superconductors coupled multilayer structures in magnetic field parallel and perpendicular to the ab plane. These structures consist of N 24 angstroms thick DyBa2Cu3O7 layers, each separated from the next by 96 angstroms of an (formula available in paper) alloy. This alloy allows a coupling of the superconducting layers as demonstrated by measurements of the activation energies for flux motion U. For a N equals 15 sample one finds, in perpendicular fields, that a correlated motion of pancake vortices occurs with a characteristic length of about 480 angstroms. Additionally, U is found to be proportional to logB at low fields while at higher fields a power law behavior is observed. In parallel field we find U to be field independent up to 1 T, suggesting that the vortices are trapped at low fields in the material where weak superconductivity is induced by proximity effect