Abstract
High-spatial-resolution scanning Auger microscopy indicates the presence of non-superconducting interfaces between superconducting grains in partially fired YBaCuO oxides. From X-ray diffraction, the structure looks almost single phase with the well known orthorhombic structure, the resistivity measurements show a superconducting transition in a zero magnetic field which is comparable with the best specimen obtained following optimum heat treatment. However, the resistive behaviour of these specimens near the superconducting transition temperature is very different when a small (<or approximately=1 kG) magnetic field is applied. The intergranular resistance, the critical current density and the upper critical field gradient have been studied as a function of the sample preparation. The results are discussed in terms of the granular superconductor model consisting of superconducting grains embedded in a non-superconducting host and connected by a network of Josephson tunnel junctions.
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