A Sapphire Resonator for Microwave Characterization of Superconducting Thin Films

Abstract

A simple technique has been developed for the high frequency characterization of superconducting thin films. A microwave resonator is formed by sandwiching a high purity sapphire rod between a pair of superconducting thin films. A number of different sapphire resonators in the C-band and Ka-band have been designed, fabricated and tested. These resonators can be used to measure the surface resistance of a pair of thin films as a function of temperature, RF current density and RF magnetic field. This technique is especially useful since it has: no sample preparation; no calibration; great sensitivity; great accuracy; great repeatability; broad dynamic range; high internal power levels with only moderate input power levels; and broad temperature coverage (2 to over 150 K). For the TE011 resonant mode, a theoretical analysis of the EM field solutions allows for the surface resistance, the RF current density and the RF magnetic field to be calculated from the measured resonant frequency, Q-value and dissipated power. Only the physical dimensions of the sapphire rod are needed for this calculation and hence no calibration is required. In a Ka-band resonator, a round robin experiment using four HTS thin films was performed to deconvolute the films' surface resistance and also to provide a statistical analysis of the method's reproducibility. The standard error for a single surface resistance measurement was better than 2%. In addition, the surface resistance of a Nb thin film was measured as a verification of the technique.