Possible entanglement of the vortices in DyBa2Cu3O7-(Y1−xPrx)Ba2Cu3O7 coupled hetero-structures

Thermally activated flux motion is studied in proximity coupled multilayers, containing a number N of 24Å thick DyBa2Cu3O7 layers separated by 96Å (or more) of (Y1−xPrx)Ba2Cu3O7 alloy (x=0.4, x=0.55). In contrast to ultra-thin decoupled layers, for which we find that the activation energy for flux motion U≈logB (Hc), coupled samples show a cross-over to a power law behaviour U≈B−0.5 at a 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