MORPHOLOGY-DEPENDENT INTERGRANULAR BEHAVIOR IN HIGH-TC SUPERCONDUCTING Y-BA-CU-O

Isothermal zero-field-cooled magnetization behavior [M(H(a),T)], for increasing applied field H(a) in the range 0 less-than-or-equal-to H(a) less-than-or-equal-to H(c1g) (where H(c1g) is lower critical field for the Y-Ba-Cu-O grains) and temperatures in the range 69 less-than-or-equal-to T less-than-or-equal-to T(c), on two polycrystalline untextured Y-Ba-CuO specimens of different grain size is compared with a proposed critical-state model. The model assumes a weak-link network of Josephson coupled grains with a critical current that has two components; a field-independent component [J(c1j)(T)] and a field-dependent component [J(c0j) (H, T) ], which decays exponentially with internal field H over a characteristic range H0(T), which is in turn quantitatively modeled for the weak-link network. The fitting procedure yields the J(cj) (0, T) behavior, which is shown to be in agreement over most of the temperature range investigated with a model based on the Ambegaokar-Baratoff theory for a single superconductor-insulator-superconductor Josephson junction modified to include order-parameter suppression in the junction. Near T(c) the J(cj)(T) behavior of both specimens is shown to be consistent with the Ambegaokar-Halperin theory for thermally activated phase slippage in the Josephson junction. J(c1j) is found to scale linearly with J(c0j) with a morphology-dependent factor that is independent of temperature. The results of this investigation are discussed in relation to current understanding.