DEPENDENCE OF THERMAL-ACTIVATION ENERGY ON THICKNESS AND MAGNETIC-FIELD IN (PR0.6Y0.4)BA(2)CU(3)O(7) YBA2CU3O7/(PR0.6Y0.4)BA2CU3O7 TRILAYERS/

We have studied the activation energy of thermally activated vortex mo tion using (Pr0.6Y0.4)Ba2Cu3O7/YBa2Cu3O7/(Pr0.6Y0.4)Ba2Cu3O7 trilayer structures. The dependence of the activation energy on the thickness o f the YBa2Cu3O7 layer and magnetic field was studied for YBa2Cu3O7 lay er thicknesses of 1.2 similar to 120 nm and fields up to 7 T. We obtai n a longitudinal vortex correlation length of 15 similar to 20 nm In Y Ba2Cu3O7. When the YBa2Cu3O7 layer is a few unit-cells thick, the acti vation energy shows a logarithmic field dependence. In the thickness r ange between 10 to 30 nm, the dependence of the activation energy on m agnetic field shows a crossover behavior from logarithmic dependence a t law fields to one of power law, U-O similar to H--alpha (with alpha similar to 0.5), at high fields. This crossover field does not depend on the thickness of the YBa2Cu3O7 layer.