MICROWAVE CONDUCTIVITY OF YBA2CU3O6.9 SINGLE-CRYSTALS NEAR T(C)

Measurements of the temperature dependencies of the surface resistance and the resonance frequency shift of some YBa2Cu3O6.9 single crystals have been performed at 3 GHz. A narrow peak in the real part of the conductivity sigma1 was observed near T(c). The experimental data fit to calculations based on the strong coupling theory assuming a narrow T(c) distribution in the sample. We conclude that the conductivity peak in our measurements, as well as in earlier experiments on high-T(c) materials in the microwave range, is not the so-called coherence peak.