High temperature superconductors in the radiofrequency field

The characterization of high T_c superconducting materials by a superregenerative radio frequency oscillator is considered. The physical principles of measurements performed on different YBa₂Cu₃O_7-δ samples are presented. A strong, reproducible and measurable effect on the superregenerative oscillator output was noticed when the sample in the tank circuit coil subjected to the low magnetic modulation field (about 4 mT, 25 Hz) passed through the superconductive transition in either direction. The signal is roughly independent of the spectrometer frequency from 2 MHz to 350 MHz. The results of measurements of the signal shape, the frequency shift, the line splitting and the influence of applied DC magnetic field are discussed