Instrumentation for cryogenic microwave cavity resonance measurements

We describe instrumentation and measurement procedures for performing microwave measurements on small metallic samples at dilution refrigerator temperatures using the microwave cavity perturbation technique. The quality factor Q of the lead-plated cavity was of order of 10 ₅ at temperatures below the superconducting transition temperature of lead. Microwaves were coupled into and out of the resonator through two room-temperature positionable semirigid coaxial lines, each terminated in a small-loop antenna. We describe in detail the arrangement of the apparatus used to tune to, and lock onto, a cavity resonance and the strategy used to categorize various resonance modes. One of the main features of this microwave spectrometer is the application of the FM detection method to measure changes in both the frequency and the quality factor of the sample-loaded cavity. The procedures for converting these quantities to the real and imaginary components of the surface impedance are described. As an application of the methodology used, we present some results of measurements on the superconducting response of a heavy fermion superconductor, UBe ₁₃, which can be fitted with the Mattis-Bardeen theory.