Development of Low-Noise Terahertz SIS Mixers with High Current Density NbN / AlN / NbN Tunnel Junctions(Special Issue on Superconductive Devices and Systems)

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We report on progress in the development of high current density NbN / AlN / NbN tunnel junctions for application as submillimeter wave SIS mixers.A ultra-high current density up to 120kA / cm^2, roughly two orders of magnitude larger than any reported results for all-NbN tunnel junctions, was achieved in the junctions.The magnetic field dependence and temperature dependence of critical supercurrents were measured to investigate the Josephson tunneling behaviour of critical supercurrents in the high-J_c junctions.We have developed a low-noise quasi-optical SIS mixer with the high-current density NbN / AlN / NbN junctions and two-junction tuning circuits which employ Al / SiO / NbN microstriplines.The tuning characteristics of the mixer were investigated by measuring the response in the direct detection mode by using the Fourier Transform Spectrometer(FTS)and measuring the response in the heterodyne detection mode with the standard Y-factor method at frequencies from 670 to 1082GHz.An uncorrected double sideband receiver noise temperature of 457K(12hv / k_B)was obtained at 783 GHz.
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2000-01-25