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 is capable of evaluating high quality samples whose R(s) value at 10 GHz is even one order of magnitude lower than that of copper. Furthermore, the proper diameter of the cavity is suitable to measure 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 65K. A straightforward analysis has been made to obtain the R(s) value from Q measurement. Geometric factor has been determined by both the theoretical calculation and the experimental acessment. It was found that they were in accordance. The temperature dependence of the R(s) value of one rf magnetron sputtered TBCCO thick film on Ag substrate and one rf magnetron sputtered YBCO thin film on LaAlO3 substrate has been measured in the temperature range of 65 - 250 K. It turned out that the R(s) value of the thick film is 0.17 OMEGA at 77 K. The R(s) value of the thin film dramatically decreases below 90K and reaches 18 m-OMEGA at 77 K which is almost two times lower than that of OFHC copper. Finally, the measurable range of our equipment and the possible origin of the measurement error have been discussed briefly.