Millimeter wave surface resistance measurement on high temperature superconductors using a liquid nitrogen cooled cavity

An accurate, relatively convenient and economical method of evaluating surface resistance R_s of high-temperature superconductors has been developed. This method is based on a technique of replacing one end wall of a copper cylindrical cavity which resonates at a frequency of 50.9 GHz in TE₀₁₁ circular mode. Measurement at 50.9 GHz allows evaluation of high-quality samples whose R_s value at 10 GHz is even one order of magnitude lower than that of copper. The proper diameter of the cavity is suitable for measuring samples with an area of 1 cm², the usual area used in thin-film growth and device fabrication. The cavity and the sample to be measured were cooled by liquid nitrogen, whose vapor pressure could be reduced to obtain temperatures down to 65 K. The temperature dependence of the R_s value of RF-magnetron-sputtered TBCCO thick film on Ag substrate and RF-magnetron-sputtered YBCO thin film on LaAlO₃ substrate has been measured in the temperature range of 65-250 K. The R_s value of the thick film was 0.17 Ω at 77 K. The R_s value of the thin film dramatically decreases below 90 K and reaches 18 mΩ at 77 K, which is almost two times lower than that of oxygen-free, high-conductivity (OFHC) copper