Computer-controlled system for surface resistance measurements of HTc superconducting films

A measurement system capable of determining the microwave surface resistance of high-temperature superconducting films is described. The measurement is based on evaluating the resonant curve of a circular cylindrical waveguide transmission resonator. The cylindrical wall, and the top plane of the resonator are made of copper. The test sample is the ground plane of the resonator. The surface resistance of the test sample can be directly evaluated from the quality factor of the resonator when the surface resistance of copper is used as a reference. The computer-controlled measurement system consists of a backward-wave oscillator, the cylindrical resonator, a spectrum analyzer for the detection of power and frequency, and a refrigerator for cooling the resonator and the test sample. Several measurements have been made. Surface resistances of different thin-film samples have been determined. The lowest values measured so far approximate 25 mΩ at 66.8 GHz and 77 K