The microwave surface impedance of high-Tc superconductors

The available data on the microwave surface impedance Z_s of high-T_c superconductors from more than 30 laboratories around the world are reviewed to show the frequency, temperature, and field dependence of the surface resistance R_s and the temperature dependence of the field penetration depth λ. The data are discussed on the basis of a two-fluid model and are compared with results from the classical superconductors Nb and Nb₃Sn. At temperatures above 0.8 T_c, Z_s values of classical and high-T_c superconductors show strong similarities and are in close agreement with the Mattis-Bardeen theory. The comparatively very high R_s of the cuprates at 4.2 K, however, is indicative of a high density of unpaired charge carriers. The lowest values for R_s and λ obtained until now with epitaxially grown YBa₂Cu₃O_7-δ films are 16 μΩ at 4.2 K and 10 GHz and 140 nm, respectively. Such films are required for high RF field applications. Polycrystalline samples show a strong field dependence of R_s. The highest magnetic surface field without R_s deterioration achieved so far on single crystals of YBa₂Cu₃O_7-δ is about 100 G