Surface Impedance Measurements Of Superconducting Films By Means Of Sapphire Dielectric Resonators

Microwave surface impedance (Zs=Rs+iXs ωμ0λ) of superconducting film is an important characteristic parameter, which Rs and Xs are surface resistance and surface reactance, respectively. Rs represents the loss of superconducting film, which is determined by quasi-particle density. Xs is proportional to the magnetic penetration depth, which is determined by the density of Cooper pair. From its relationship with the temperature, we can get information about the superconducting energy gap. Therefore, the precise measurement on Zs is significant for the better understanding on the physical and electrical characteristics of the material, as well as the practical applications of superconducting film.We use the sapphire dielectric resonator technique to measure the microwave surface impedance (Zs) of superconducting film. The superconductive film is placed at the end of the sapphire puck to form a dielectric resonator with a supercoducting wall. Due to the very low dielectric loss at low temperature, relative high dielectric constant and low radiation loss result in a high quality factor of resonator. In addition, the temperature stability of dielectric constant is very high. So, we can think the change of resonant frequency with temperature mainly comes from the temperature dependence of superconducting film penetration depth. Therefore this technique can accurately measure the surface impedance of superconducting film.The resonator is working at 18.3 K (TE01δmode) with a temperature range from 3 K to room temperature. Zs can be determined by measuring the temperature dependence of the unloaded quality factors and the resonator frequency. We measured the surface impedance of two Nb films, and the highest quality factor is up to 130900. The films can be tested without damage and the surface impedance can be measured accurately. Through the measurement and calculation of surface impedance, penetration depth, conductivity, etc. of two Nb superconducting films and two NbN superconducting films, we can verify the accuracy of the test system. At the same time it can also reflect the problems in our test system from the data measured, and how to solve these problems in the future.