Flux quantization in weak links in melt-textured bulk YBa2Cu3O7-x

We have studied flux quantization in weak links of a-b- and a(b)-c-plane-oriented, melt-textured YBa2Cu3O7-x using a modulated-field microwave-absorption method. First derivative of the microwave absorption versus dc magnetic field was measured in a TE102 cavity at 9.5 GHz. We observed several series of well-resolved equally spaced absorption lines in the externally applied dc magnetic field with separation ranging from 6 μT to 2.3 mT. By analyzing the dependence of these series on microwave power, temperature, and orientation of the sample versus dc magnetic field we have identified the types and locations of Josephson-junction weak links in the samples. One of the series was found to be related to the microwave absorption in a strongly coupled superconductor-normal-metal-superconductor type of junction located in intragrain plane defects (complex twin boundaries). For this junction, the critical current Ic dependence on magnetic field B as well as lower critical field Bc1J were determined. For the location of the inter- and intragrain plane defects we have found C2 and C4 symmetry, respectively. A physical model of microwave absorption in nonuniform Josephson junctions is presented and sample microstructure is correlated with the measurement results.