Surface impedance of high T/sub c/ superconductors

The surface impedance of various high-temperature superconductors has been examined in the millimeter-wave spectral range. Experiments on YBa/sub 2/Cu/sub 3/O/sub 7/ ceramics and thin films and on (evidently fairly poor quality) Bi/sub 2/CaSr/sub 2/Cu/sub 2/O/sub 8/ single crystals give effective penetration lengths which exceed the London penetration depth and finite and significant surface impedance in the microwave and millimeter-wave spectral range. It is found that both R/sub S/ (surface resistance) and lambda /sub eff/ (effective wavelength) decrease with increasing sample quality, and the temperature dependence of lambda approaches the form expected for single pairing. These observations are in agreement with a model where the surface impedance is dominated by losses and effective magnetic field penetration caused by Josephson-type effects at grain boundaries. The surface resistance exceeds the Mattis-Bardeen limit in both ceramic and thin-film specimens.<>