Magnetic Field Dependence of the Microwave Properties of Proximity Effect Nb/Al Bilayers Close to the Gap-frequency

High critical current density tunnel barriers of AlN have two effects on the operation of niobium-based SIS mixers close to the upper frequency limit of superconducting niobium: 1. the bandwidth is no longer limited by the RC time constant of the tunnel-junction, but the physical processes in the superconductors at high frequencies; 2. the devices have smaller lateral dimensions in order to provide impedance matching given the low RnA value, which requires the application of a higher magnetic field to quench the Josephson-effect through the Fraunhofer pattern. Consequently, under operating conditions the superconducting properties are no longer the equilibrium properties as realized by the fabrication conditions, but are deteriorated by the applied magnetic field. We report the measurements of I-V curves and Fourier transmission spectrum to determine and correlate the evolution of the superconducting properties in a high magnetic field. The results are compared with a calculation of the proximity effect based on the Keldysh Greens' function technique, leading to a magnetic field dependent density of states in the Al toplayer of the commonly used Nb-Al bilayer to fabricate tunnel barriers as well as forming one part of the superconducting stripline.