THE MICROWAVE SURFACE IMPEDANCE OF HIGH-TC SUPERCONDUCTORS

Passive microwave components are among the first practical devices fabricated from high-T(c) superconductors. The surface impedance Z(s) determines the quality of superconducting material for such applications. In this report the available data on Z(s) 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 lambda. 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 for a high density of unpaired charge carriers. The lowest values for R(s) and lambda obtained until now with epitaxially grown YBa2Cu3O7-delta films are 16-mu-OMEGA 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 YBa2Cu3O7-delta is about 100 G. At lower fields and at 77 K, epitaxial films of YBa2Cu3O7-delta are superior to copper surfaces for frequencies below 500 GHz and ready for applications.