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Properties of superconductor‐insulator‐normal conductor tunnel junctions made from Nb‐Al/ Aloxide‐Al trilayers are described. These devices were successfully employed as quasiparticle mixers. The lowest double sideband receiver noise temperature in the 290–320 GHz band was about 140 K. The Nb wiring, which also defines the tuning circuit of the mixer, induces a small energy gap in the Al counter electrode at a physical temperature of 2.2 K.

Double‐barrier niobium tunnel junctions with the middle electrode thinner than the London penetration depth were fabricated. Their I‐V curves, annealing behavior, response to external magnetic field, and response to 230 and 350 GHz irradiation in mixer experiments is discussed. Junctions with an integrated tuning circuit designed for the 230 GHz frequency range gave double‐sideband receiver noise temperatures of 100 K. Vertically stack arrays can be attractive for various applications. Compared to single‐barrier devices possible drawbacks were noted. The I‐V curve indicates heating and/or nonequilibrium effects. Double‐barrier devices became nonuniform after thermal annealing at 200 °C. The response to the external magnetic field is more complex and considerably larger flux densities are needed to suppress Josephson effects.

We present a theoretical and experimental study of electrodynamics of a planar spiral superconducting resonator of a finite length. The resonator is made in the form of a monofilar Archimedean spiral. By making use of a general model of inhomogeneous alternating current flowing along the resonator and specific boundary conditions on the surface of the strip, we obtain analytically the frequencies fn of resonances which can be excited in such system. We also calculate corresponding inhomogeneous RF current distributions ψ n ( r ) , where r is the coordinate across a spiral. We show that the resonant frequencies and current distributions are well described by simple relationships f n = f 1 n and ψ n ( r ) ≃ sin [ π n ( r / R e ) 2 ] , where n = 1 , 2... and Re is the external radius of the spiral. Our analysis of electrodynamic properties of spiral resonators' is in good agreement with direct numerical simulations and measurements made using specifically designed magnetic probe and laser scanning microscope.

We present analytical, numerical and experimental investigations of electromagnetic resonant modes of a compact monofilar Archimedean spiral resonator shaped in a ring, with no central part. Planar spiral resonators are interesting as components of metamaterials for their compact deep-subwavelength size. Such resonators couple primarily to the magnetic field component of the incident electromagnetic wave, offering properties suitable for magnetic meta-atoms. Surprisingly, the relative frequencies of the resonant modes follow the sequence of the odd numbers, despite the nearly identical boundary conditions for electromagnetic fields at the extremities of the resonator. In order to explain the observed spectrum of resonant modes, we show that the current distribution inside the spiral satisfies a particular Carleman type singular integral equation. By solving this equation, we obtain a set of resonant frequencies. The analytically calculated resonance frequencies and the current distributions are in good agreement with experimental data and the results of numerical simulations. By using low-temperature laser scanning microscopy of a superconducting spiral resonator, we compare the experimentally visualized ac current distributions over the spiral with the calculated ones. Theory and experiment agree well with each other. Our analytical model allows for calculation of a detailed three-dimensional magnetic field structure of the resonators., 25 pages, 8 figures