Surface Impedance Measurements On High-Tc Superconductors

An investigation has been made of the RF properties of two oxide superconductors, mainly Y1Ba2Cu3O7 and tentatively Bi2CaSr2Cu2Ox, between 4.2 and 300 K at S-, K-, and W-band frequencies. The surface resistance Rs and the magnetic-field penetration depth λ of ceramic pellets and of polycrystalline layers on silver substrates were measured in normal or superconducting host cavities. Improvement factors for Rs of up to three orders of magnitude at 3 GHz and one at 86 GHz have been achieved, but the temperature dependence of Rs deviates strongly from that of conventional superconductors. The main features are a slow fall-off near Tc and a remaining temperature dependence at 4.2 K, both depending on the preparation and the resulting homogeneity of the samples. From the frequency shift of the cavity near Tc the authors extrapolate λ(0) values between 200 and 1300 nm. Large fractions of the samples remain superconducting up to surface current densities on the order of 105 A/cm2