COMPLEX CONDUCTIVITY OF YBA2CU3O7 SINGLE-CRYSTALS AT RADIOWAVE FREQUENCIES

The complex (surface) impedance of superconducting YBa2Cu3O7-d Single crystals have been measured as function of temperature in the vicinity of superconducting phase transition. A technique using a resonance ci rcuit at frequencies 200 MHz and 1 GHz was utilized. The penetration d epth lambda(T) and the complex conductivity sigma(T) can be evaluated from these measurements. The temperature dependence of the penetratio n depth is well described by the Gorter-Casimir two-fluid model, yield ing the surface reactance X(0) at 0K. The complex conductivity is comp ared with the predictions of the BCS theory. While the imaginary part can well be described using a dirty parameter xi(0)/l similar to 0.1, the real part of the conductivity does not follow the BCS predictions (even with the addition of the distribution of transition temperatures T-c's).