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IEEE Transactions on Applied Superconductivity

Quasiparticle mixing in Ba-K-Bi-O grain boundary junctions

Tetsuya Takami, Yuuichi Wada, K. Kuroda, T. Ozeki, Koichi Hamanaka

Published Jun 1, 1999• 0 citations
Publisher

Abstract

A Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) grain boundary junction formed on a MgO bicrystal substrate showed a superconductor-insulator-superconductor (SIS) behavior with a sharp gap structure and a Josephson current. A 270 GHz millimeter wave mixing in a BKBO junction was performed by using the quasi optical technique. A log-periodic antenna, a hyperhemispherical MgO lens and a Teflon lens were used in order to improve coupling for millimeter wave. The IF outputs have some peaks depending on the photon assisted tunneling steps. This is the first time that quasi-particle mixing could be done in an oxide superconductor.

DOI
10.1109/77.784010

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A Ba/sub 1-x/K/sub x/BiO/sub 3/ (BKBO) grain boundary junction formed on a MgO bicrystal substrate showed a superconductor-insulator-superconductor (SIS) behavior with a sharp gap structure and a Josephson current. A 270 GHz millimeter wave mixing in a BKBO junction was performed by using the quasi optical technique. A log-periodic antenna, a hyperhemispherical MgO lens and a Teflon lens were used in order to improve coupling for millimeter wave. The IF outputs have some peaks depending on the photon assisted tunneling steps. This is the first time that quasi-particle mixing could be done in an oxide superconductor.
We have designed, fabricated and tested a quasi-optical superconductor–insulator–superconductor (SIS) mixer employing distributed NbN/MgO/NbN tunnel junctions and NbN/MgO/NbN microstriplines at submillimetre-wave frequencies. These trilayers were fabricated by dc- and rf-magnetron sputtering on an MgO substrate at ambient temperature so that the NbN and MgO films were grown epitaxially. Our SIS mixer consists of an MgO hyperhemispherical lens with an antireflection cap and a self-complementary log-periodic antenna made of a single-crystal NbN film, on which the distributed SIS junctions and the two-section impedance transformers were mirror-symmetrically placed at the feed point of the antenna. As designed, the junctions are 0.6 μm wide and 15.5 μm long, which is sufficient to absorb the incoming signal along this lossy transmission line, assuming a current density of 10 kA cm −2 . The mixer showed good I – V characteristics, with subgap-to-normal resistance ratios of about 13, although weak-link breaks were observed above the gap voltage. The minimum double side band receiver noise temperature was 334 K at 678 GHz, including the input noise of about 200 K as estimated by the standard technique. The noise temperature gradually rose to 672 K at 820 GHz. This behaviour may be caused by RF losses from the weak-link parts, due to the steps between the distributed junctions and the microstriplines being less than 1 μm wide.
We report the millimeter wave mixing properties of high-T c superconductor Josephson junction arrays using a waveguide-to-microstrip converter in the 100 GHz frequency range. The junction arrays consist of 3 weak links fabricated from EuBa2Cu3O7-δ thin films grown on an MgO bicrystal substrate. In these junction arrays, Shapiro steps are observed in the presence of 100 GHz irradiation at intervals of 3 times the fundamental step voltage. Millimeter wave mixing with an input local oscillator frequency of 99.142 GHz and an input signal frequency of 99.880 GHz is detected as the IF output at 738 MHz. The IF output power peaks at voltages corresponding to those of the dynamic resistance dV/ dI. A mixing conversion efficiency of -24.1 dB is obtained for a 3-junction array.
Millimeter-wave detection in the heterodyne mixing mode has been carried out using the high-T/sub c/ superconducting GBJJ (grain boundary Josephson junction). The GBJJ was fabricated from high T/sub c/ superconducting Y-Ba-Cu-O films, prepared on YSZ and SrTiO/sub 3/ substrates using an RF sputtering technique. The junction was patterned by chemical wet etching with phosphoric acid solution. The authors carried out experiments in the heterodyne mixing mode for a signal of 35 GHz with local oscillation at 34.97 GHz. The detected IF (intermediate frequency) output power was obtained at a bias voltage lying midway between the zeroth and first Shapiro steps. A mixer conversion efficiency of -15 dB was obtained with a dynamic range of about 20 dB at 35 GHz. This result strongly implies that high-T/sub c/ superconducting GBJJ works well as a detector at millimeter-wave frequencies.>
A Josephson quasioptical detector comprising YBaCuO junction on bicrystal MgO substrate, integrated log-periodic Au antenna, MgO extended hyperhemisphere lens attached to the substrate and 1.4 GHz cold amplifier with cold circulator has been designed, fabricated and experimentally studied. We fabricated several integrated Josephson junctions 1-2 /spl mu/m wide which at 4.2 K demonstrate RSJ IV curves with critical currents 100-300 /spl mu/A and normal resistance of 10-20 /spl Omega/. The noise temperature (T/sub N/) at 4.2 K and 330 GHz measured by a hot/cold load method in fundamental mixing mode with external BWO local oscillator brings T/sub N/=1100 K for the junction shunted by low-inductance resistive shunt and T/sub N/=1400 K without shunting. At bath temperatures of 20-40 K the noise temperature was in the range 1500-3000 K, at 50 K the noise temperature was below 2000 K, and at 60-70 K increased over 3000 K in the frequency range 310-586 GHz. The noise temperatures measured in the self-pumping mode vary between 580-2300 K with dc bias in the range 0.1-0.5 mV. The linewidth of Josephson oscillations has been extracted from a selective detector response and IF noise voltage dependence. The measured excess noise can be attributed to shot noise and multiple Andreev reflection in pinholes with Ss'S structure. We observed small subharmonic steps in IV curve and noise dependencies, which are typical for SNS junctions.
We have designed, fabricated and evaluated a quasi-optical SIS receiver with NbN/AlN/NbN tunnel junctions in the submillimeter-wave region. The receiver consists of a planar self-complementary log-periodic antenna and MgO hyperhemispherical lens with an anti-reflection cap for RF optics, NbN SIS junctions and a radial short stub tuner to resonate out its capacitance for RF matching. The prepared NbN SIS junction has a current density of 20 kA/cm/sup 2/ and is about 1 /spl mu/m in diameter, supplying a small /spl omega/C/sub J/R/sub N/ product (/spl omega/C/sub J/R/sub N/=3 at 300 GHz). From 297 to 356 GHz the average double sideband (DSB) receiver noise temperature measured by using the standard Y-factor method was about 200 K. The lowest receiver noise temperature, 167 K (DSB), was obtained at around 303 GHz. These results suggest that our NbN SIS junctions can be used for terahertz mixer elements instead of Nb SIS junctions.
Single-particle (quasi-particle) tunneling through an insulating barrier between two superconductors or between a superconductor and a normal conductor is being used to make very-low-noise detectors and mixers for millimeter wavelengths. The nonlinearity of theI-Vcurve obtained from tunneling between two superconductors can be so strong that classical theory breaks down and photon-assisted tunneling theory must be used to understand device performance. Quantum theory predicts that a quasi-particle tunnel junction can be operated as a microwave photon detector with quantum efficiency close to unity or as a heterodyne mixer with conversion gain and with mixer noise temperature comparable with the quantum noise limitT_{M} =\bar{h}\omega/k. Both of these predictions have been experimentally realized at 36 GHz using superconductor-insulator-superconductor (SIS) junctions. It appears probable that these quasi-particle detectors and mixers will supercede the corresponding Josephson-effect devices at millimeter wavelengths.