Low-field microwave absorption in single-crystal superconducting YBa₂Cu₃O_7-δ

The low-field microwave absorption line spectrum of a single crystal of superconducting YBa2Cu3O7-δ has been studied as a function of the external magnetic field. The threshold microwave power necessary to nucleate fluxons is found to vary with field in such a way that only about one thousandth of the junction length is active in interacting with the microwaves to create fluxons.