Microwave Admittance of Gold-Palladium Nanowires with Proximity-Induced Superconductivity

Quantitative electrical admittance measurements of diffusive superconductor-normal-metal-superconductor (SNS) junctions at gigahertz frequencies and millikelvin temperatures are reported. The gold-palladium-based SNS junctions are arranged into a chain of superconducting quantum interference devices. The chain is coupled strongly to a multimode microwave resonator with a mode spacing of approximately 0.6 GHz. By measuring the resonance frequencies and quality factors of the resonator modes, the dissipative and reactive parts of the admittance of the chain are extracted. The phase and temperature dependence of the admittance near 1 GHz are compared with theory based on the time-dependent Usadel equations. This comparison allows the identification of important discrepancies between theory and experiment that are not resolved by including inelastic scattering or elastic spin-flip scattering in the theory.