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 circuit at frequencies 200 MHz and 1 GHz was utilized. The penetration depth lambda(T) and the complex conductivity sigma*(T) can be evaluated from these measurements. The temperature dependence of the penetration depth is well described by the Gorter-Casimir two-fluid model, yielding the surface reactance X(0) at 0K. The complex conductivity is compared 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).