Surface impedance of HTSC single crystals in the microwave band

Currently available data on the real, R_s, and imaginary, X_s, parts of the microwave frequency surface impedance Z_s = R_s + i X_s are presented for the high-quality singlecrystal high-temperature superconductors YBa₂Cu₃O_6.95, Ba_0.6K_0.4BiO₃, Tl₂Ba₂CaCu₂O_8-δ, Tl₂Ba₂CuO_{6 + δ}, and Bi₂Sr₂CaCu₂O₈. A high-precision technique for measuring the temperature dependences R_s(T) and X_s(T) in the range 4.2 ≤ T ≤ 150 K is described. Surface impedance and complex conductivity features common to single-crystal high-temperature superconductors are formulated and the temperature variation of these properties is analyzed. To explain the experimental data, a modified two-fluid model is used, which includes quasi-particle scattering and accounts for the characteristic change in the density of superconducting carriers at low and near-critical temperatures. Prospects for the microscopic theory of the high-frequency response of high-temperature superconductors are discussed.