A study of hysteretic absorption in polycrystalline high-T c superconductor YBCO

The low-field microwave absorption in polycrystalline specimens of the high-T_c superconductor YBCO is studied. The low-field absorption derivative signal as measured using EPR techniques in the X-band at low fields (H<100 Oe) is nonresonant in nature. The peak value occurs at H=2-30 Oe depending on sample composition, temperature, and geometry with respect to the applied magnetic field. The microwave absorption signal level shows hysteresis if the field is swept back and forth through zero. At temperature close to T_c (T=76 K, T _c=85±2 K) the ratio of the signal height at the peak value (H=Hc₁~10 Oe) to the signal level at H=1000 Oe is approximately 70 as compared to 20 at T=35 K. This ratio is related to the K value of the material, where K is the ratio λ/ξ. Below T=35 K the drop-off in the ratio is almost constant, indicating that K is almost constant. For polycrystalline YBCO (T _c~85 K) K evaluated using this technique is approximately 20 at T=35 K