Microwave measurements on high Tc superconducting single crystals and films

The authors have studied the microwave properties of a single crystal of ErBa₂Cu₃O_7-x. The crystal was placed in the bottom of a rectangular TE 103 copper cavity whose resonant frequency was 9.2 GHz. The Q of the system, cavity and sample, as well as the frequency shift, was monitored as the system was cooled. Below about 50 K the measurements became limited by the copper cavity. Close to the superconducting transition, about 90 K, the real surface impedance is at least one order of magnitude higher than that calculated from Mattis and Bardeen´s (1958) formula. The authors have also studied using both resonant and nonresonant methods the microwave properties of an oriented film of YBa₂Cu₃O_7-x grown on a MgO substrate. For both types of measurements they have studied the effects of small magnetic fields, less than 100 G, on the microwave losses at 9.2 and 10 GHz in a microwave cavity and by a nonresonant technique. The results are discussed in terms of flux entry and pinning at grain boundaries