JOSEPHSON-FLUX DEPINNING IN GRANULAR YBA2CU3O7-DELTA

Josephson- (intergrain-) flux depinning was studied in a Josephson-jun ction network of granular YBCO and YBCO/Ag superconductors. We measure d temperatures and magnetic fields at which the Josephson-flux depinni ng occurs and analyzed the corresponding relaxation processes. A direc t relationship between depinning of the Josephson vortices and dissipa tion of the transport (Josephson) critical current was found. Special experimental techniques were applied to achieve those objectives, allo wing (a) detection of the Josephson-flux motion without the influence of the intragrain (Abrikosov) flux; and (b) direct measurement of chan ges in the effective activation energy for the Josephson-flux motion a nd corresponding changes in the Josephson critical current. The result s were interpreted in the framework of the models of granular supercon ductivity developed by Tinkham and Lobb [Solid State Phys. 43, 91 (198 9)], and by Clem [Physica C 153-155, 50 (1988)]. The results support t he idea given in those models, that the pinning barrier for the Joseph son-flux motion originates from the variation of the Josephson couplin g energy within the junction network.