A STUDY OF HYSTERETIC ABSORPTION IN POLYCRYSTALLINE HIGH-TC SUPERCONDUCTOR YBCO

We have investigated in detail the low-field microwave absorption in polycrystalline specimens of the high-Tc superconductor YBCO. The low field absorption derivative signal as measured using EPR techniques in the X-Band at low fields (H<l00 oe) is non resonant in nature. The peak value occurs at H between 2 to 50 Oersted depending on sample composition, temperature, and geometry w.r.t the applied magnetic field. The microwave absorption signal level shows hysteresis if the field is swept back and forth through zero. At temperatures close to Tc (T- 76K, Tc- 85±2 K) the ratio of the signal height at the peak value (H-Hc1 - 10 Oe) to the signal level at H-l000 Oe is approximately 70 as compared to 20 at T-35 K. This ratio is related to the Xvalue of the material, where X is the ratio, λ/ξ. Below T-35 K the drop off in the ratio is almost constant indicating X is almost constant. For polycrystalline YBCO (Tc-85 K) X evaluated using this technique is approximately 20 at T=35 K. © 1990 IEEE