Effect of thermal expansion in microwave conductivity measurement

Systematic investigation was performed experimentally on the effect of the thermal expansion of the samples in the microwave-surface-impedance measurement by the resonant technique, using conventional superconductors, Pb and Nb, with various sample sizes. The measurement of the surface reactance, XS, was found to be very sensitive to the thermal expansion, whereas that of the surface resistance, RS, was insensitive. We propose a new relation between the measured experimental data and the surface impedance, to deal with this problem. We also propose a quantity which represents the degree of the thermal expansion in the microwave measurement.