We have studied flux quantization in weak links of a-b- and a(b)-c-plane-oriented, melt-textured ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ using a modulated-field microwave-absorption method. First derivative of the microwave absorption versus dc magnetic field was measured in a ${\mathrm{TE}}_{102}$ 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 ${\mathit{I}}_{\mathit{c}}$ dependence on magnetic field B as well as lower critical field ${\mathit{B}}_{\mathit{c}1\mathit{J}}$ were determined. For the location of the inter- and intragrain plane defects we have found ${\mathit{C}}_2$ and ${\mathit{C}}_4$ symmetry, respectively. A physical model of microwave absorption in nonuniform Josephson junctions is presented and sample microstructure is correlated with the measurement results.

• Received 16 January 1995

DOI:https://doi.org/10.1103/PhysRevB.51.16289