A Simple GHz Resonator for Superconducting Materials Characterization

This work examines the design and operation of a longitudinal resonant cavity, paired with monopole send and reciprocal patch receive antennae, that couples radio-frequency energy to a superconducting thin film carrying high current densities (~10⁵ A/cm²). The dielectric substrate supporting the film penetrates the waveguide, which operates in an evanescent mode below the design cutoff frequency of 18 GHz. Oscillatory vortex motion in the thin film is found to produce a small (~0.1 mV) dc voltage. When the niobium film is patterned to form an aperture that permits resonant conditions within the waveguide volume, the measured voltage increases by an order of magnitude. The increase is explained in the framework of the Larkin-Ovchinnikov model for quasiparticle behavior inside a moving normal vortex core. Operated near the superconducting transition, this device is useful for materials characterization, including the possibility to extract parameters including the pinning force. The authors suggest that the device could be used to characterize the pinning potential or to explore quasiparticle dynamics in superconducting thin films.