WEAK LINK BEHAVIOR AND CRITICAL CURRENTS IN CERAMIC HIGH T(C) SUPERCONDUCTORS

Low-field d.c. magnetization measurements, at various temperatures above 65 K, have been made on two untextured YBCO specimens of different grain size and on an untextured BISCCO(Pb) specimen. These results are compared with the predictions of a critical-state model which assumes a critical current density which has a field-independent component, J(cj1)(T), and a component J(cj0)(H, T) which decreases exponentially with internal field H over a characteristic range H0. The J(cj0)(T) behaviour for the YBCO specimen is found, for T less than or similar T(c), to be in agreement with the Ambegaokar-Baratoff theory for SIS Josephson junctions, modified for order-parameter suppression in the junction boundaries. Departure from this behaviour near T(c) is found to be consistent with the Ambegaokar - Halperin theory f or thermally activated phase slippage (TAPS) in the weak links. The BISCCO(Pb) specimen shows a different J(cj0)(T) behaviour which is discussed in relation to microstructural properties of this system. The ratio J(cj1)/J(cj0) is found to be a constant independent of T in both the YBCO and BISCCO(Pb) specimens and the physical basis for J(cj1) is discussed.