Abstract
Josephson- (intergrain-) flux depinning was studied in a Josephson-junction network of granular YBCO and YBCO/Ag superconductors. We measured temperatures and magnetic fields at which the Josephson-flux depinning occurs and analyzed the corresponding relaxation processes. A direct relationship between depinning of the Josephson vortices and dissipation of the transport (Josephson) critical current was found. Special experimental techniques were applied to achieve those objectives, allowing (a) detection of the Josephson-flux motion without the influence of the intragrain (Abrikosov) flux; and (b) direct measurement of changes in the effective activation energy for the Josephson-flux motion and corresponding changes in the Josephson critical current. The results were interpreted in the framework of the models of granular superconductivity developed by Tinkham and Lobb [Solid State Phys. 43, 91 (1989)], and by Clem [Physica C 153-155, 50 (1988)]. The results support the idea given in those models, that the pinning barrier for the Josephson-flux motion originates from the variation of the Josephson coupling energy within the junction network.
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