POSSIBLE ENTANGLEMENT OF THE VORTICES IN DYBA2CU3O7 (Y1-XPRX)BA2CU3O7COUPLED HETEROSTRUCTURE

Thermally activated flux motion is studied in proximity coupled multil ayers, containing a number N of 24 Angstrom thick DyBa2Cu3O7 layers se parated by 96 Angstrom (or more) of(Y1-xPrx)Ba2Cu3O7 alloy (x=0.4, x=0 .55). In contrast to ultra-thin decoupled layers, for which we find th at the activation energy for flux motion U approximate to=logB (H//c), coupled samples show a cross-over to a power law behaviour U approxim ate to B--0.5 at magnetic field H. This characteristic field depends on the anisotropy and decreases as N increases, a behaviour which can be understood in terms of the entanglement of the vortices, resulting in a relative increase of U due to the flux line crossing energy.