Measurement of the absolute penetration depth and surface resistance of superconductors and normal metals with the variable spacing parallel plate resonator

The variable spacing parallel plate resonator (VSPPR) is a microwave transm ission line resonator with a continuously variable thickness of the dielect ric spacer between the superconducting or metallic plates, filled by cryoge nic liquid or vacuum. We measure the dielectric spacer thickness dependenci es of the resonator frequency and quality factor, and fit them to theoretic al forms, in order to extract the absolute values of penetration depth, lam bda, and surface resistance, R-s. A cryogenic micropositioning setup is dev eloped to vary the spacer thickness from 0 to 100 mu m with a resolution of 8.5 nm, and to maintain parallelism of the resonator plates. Measurement o f ac capacitance between the plates is utilized to directly determine the s eparation between the resonator plates and to reduce the effect of their ti lt and nonflatness on the accuracy of the measured R-s and lambda. Because the operating temperature is fixed (77 K), the result for a superconductor is independent of an a priori model for the penetration depth versus temper ature. This technique can also be employed as a surface impedance standard for characterization of high temperature superconducting films for microwav e applications. (C) 2000 American Institute of Physics. [S0034-6748(00)0360 5-4].