Surface impedance measurements on high Tc superconductors

An investigation has been made of the RF properties of two oxide superconductors, mainly Y₁Ba₂Cu₃O₇ and tentatively Bi₂CaSr₂Cu₂O_x, between 4.2 and 300 K at S-, K-, and W-band frequencies. The surface resistance R_s 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 R_s of up to three orders of magnitude at 3 GHz and one at 86 GHz have been achieved, but the temperature dependence of R_s deviates strongly from that of conventional superconductors. The main features are a slow fall-off near T_c 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 T_c 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