MODULAR SYSTEM FOR MICROWAVE SURFACE IMPEDANCE MEASUREMENTS OF HIGH-TEMPERATURE SUPERCONDUCTORS

A novel modular cryogenic measurement system capable of determining th e surface impedance of high-temperature superconductors is described. The modules are interchangeable sections of the system, for example, s ections of the cavity and the vacuum interface, which can be optimized for each frequency band. It is expected that in a fully operational m ode the surface impedance of each specimen will be measured over the f requency range 9-100 GHz by interchanging the appropriate modules. The complete measurement will only take a few days, and therefore the spe cimen will remain under the same experimental conditions throughout, l eading to a more consistent characterization. The surface impedance of bulk polycrystalline specimens of YBa2Cu3O7-x have been measured with the first set of modules (9-18 GHz) over the temperature range 20-300 K. The results obtained show that in the superconducting state the ov erall temperature dependence of the surface resistance and the penetra tion depth cannot be described by a single-gap Bcs-type function; at l ower temperatures where the residual effects become dominant the data provide evidence for weak coupling while at higher temperatures strong coupling is indicated.