Conductivity anisotropy and pseudogap in the microwave response of high- T c superconductors

Results of investigating the temperature dependences of the surface impedance Z(T) = R(T) + iX(T) and complex conductivity σ(T) = σ'(T) – iσ''(T) in the ab-planes and along the c-axis of high-Tc superconductors (HTSCs) are reviewed and critically analyzed. An electrodynamic method is considered for extracting all components of (T) and (T) tensors from quantities measured in microwave experiments. The main attention is focused on the evolution of (T) and (T) dependences in a YBa2Cu3O7–x crystal with the oxygen content variation. Possible mechanisms of conductivity are discussed in the framework of models for normal, superconducting, and pseudogap states of HTSCs.