Surface impedance measurements on high T/sub c/ superconductors

摘要

An investigation has been made of the RF properties of two oxide superconductors, mainly Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ and tentatively Bi/sub 2/CaSr/sub 2/Cu/sub 2/O/sub x/, between 4.2 and 300 K at S-, K-, and W-band frequencies. The surface resistance R/sub s/ and the magnetic-field penetration depth lambda of ceramic pellets and of polycrystalline layers on silver substrates were measured in normal or superconducting host cavities. Improvement factors for R/sub 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/sub s/ deviates strongly from that of conventional superconductors. The main features are a slow fall-off near T/sub 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/sub c/ the authors extrapolate lambda (0) values between 200 and 1300 nm. Large fractions of the samples remain superconducting up to surface current densities on the order of 10/sup 5/ A/cm/sup 2/.