Microwave absorption of the high-T_c superconductor YBa ₂Cu₃O₇

Using a resonant-cavity perturbation technique, the microwave absorption at 16.8 GHz of the high-T_c compound YBa₂Cu₃O ₇ is measured as a function of temperature (4-300 K) and magnetic field (0-7 T). At zero field the microwave loss shows a rapid decrease below 93 K, the onset of the superconducting transition. An important absorption, which is a decreasing function of temperature, is observed in the superconducting state.When a magnetic field is applied, the transition is broadened and shifted toward lower temperatures: an upper-critical field H_c2(T=0)∼ 150 T is obtained via extrapolation of the data. In the superconducting state, the absorption is found to be highly dependent on the magnetic-field intensity. For magnetic fields above 500 Oe, the absorption is a slowly increasing function and it is understood in terms of the temperature behavior of H_c2; for H<200 Oe, the absorption increases very rapidly with field and it is believed to be correlated to H_c1 and to the anisotropic character of the structure.