The nature of grain-boundary links in high-qualitytapes is clarified. The investigations focus on the role that various types of links play in the restriction of the critical current. The multiple exponential function is found to be a good approximation for the magnetic-field dependence of critical current. The behaviour ofat different temperatures gives clear evidence of the two-dimensional character of critical current. It is shown that at all temperatures higher than the Kosterlitz - Thouless transition temperature of isolated superconducting layers the critical current is totally defined by the Josephson interaction between the layers.
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