Microwave absorption of the high‐Tc superconductor YBa2Cu3O7

Using a resonant‐cavity perturbation technique, the microwave absorption at 16.8 GHz of the high‐Tc compound YBa2Cu3O7 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 Hc2(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 Hc2 ; for H<200 Oe, the absorption increases very rapidly with field and it is believed to be correlated to Hc1 and to the anisotropic character of the structure.