Investigation of the magnetic field-induced microwave absorption across T_c in single crystals of YBa₂Cu₃O_7-δ and BiSrCaCuO. Anisotropy and magnetic field dependence

The magnetic field-modulated microwave absorption near T_c has been studied in high-quality single crystals of YBa₂Cu₃O_7-x (T_c ≈ 90 K) and in BiSrCaCuO (T_c ≈ 85 K) as a function of magnetic field B < 1.3 T and orientation. Sharp microwave absorption peaks were observed when sweeping the temperature through T_c from above. The analysis of the peaks using Tinkham's model for fluxon losses gives values for the pinning strength of A ≈ 1000 T and A ≈ 3 T for the YBaCuO and BSCCO crystals, respectively. The anisotropy values obtained from the orientation dependence of the peaks were ε{lunate} = 5 and ε{lunate} = 40-60 for these two compounds, respectively. The results are also discussed with respect to the B-T vortex phase diagram. A marked difference between the YBaCuO and BSCCO compounds is the presence of a low-temperature absorption signal with a different orientation dependence, observed only in the BSCCO compound, in addition to the signals just below T_c observed in both compounds.