Microwave absorption in high-T c superconductors at high magnetic fields

In the mixed state of superconductors (H _c1?H?H _c2) the penetration of microwaves is governed by both, complex conductivity σ(T) and driven oscillation of vortices. In this paper, we show that an effective microwave conductivity can be derived and used to fit the field dependences of the surface resistanceR _s. The fit parameter is the upper critical field. Measurements on single crystals YBa₂Cu₃O_7-δ were made in magnetic fields (H∥c) up to 2.25 T, and in the temperature range from 70–100 K. The critical temperature for mean field superconductivity appears to be 89.3 K, while the apparent onset in the curve ofR _s(T) appears at about 92 K. The magnetic dependences ofR _s clearly demonstrate that one can separate the regions of mean field superconductivity from the region of fluctuations.