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Applied Physics Letters

Photon-assisted tunneling in double-barrier superconducting tunnel junctions

M. M. T. M. Dierichs, Pieter Dieleman, J. J. Wezelman, Cornelia E. Honingh, T. M. Klapwijk

Published Feb 14, 1994• 5 citations
Publisher

Abstract

Double-barrier Nb/Al2O3/Al/Al2O3/Nb tunnel junctions are used as mixing elements in a 345 GHz waveguide mixer. Noise temperatures (double side band) down to 720 K at 3.0 K are obtained without the need to apply a magnetic field to suppress the Josephson current. It is shown that the composite barrier acts as a single barrier for photon-assisted tunneling. Surprisingly, at these frequencies the capacitance of two stacked tunnel barriers is only determined by one barrier.

DOI
10.1063/1.110996

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Journal of Applied Physics•Oct 1, 1993•9 citations
Double-barrier Nb/Al2O3/Al/Al2O3/Nb tunnel junctions are used as mixing elements in a 345 GHz waveguide mixer. Noise temperatures (double side band) down to 720 K at 3.0 K are obtained without the need to apply a magnetic field to suppress the Josephson current. It is shown that the composite barrier acts as a single barrier for photon-assisted tunneling. Surprisingly, at these frequencies the capacitance of two stacked tunnel barriers is only determined by one barrier.
The rapid onset of quasiparitcle tunneling current in superconductor-insulator-superconductor (Josephson) junctions at voltages above 2Δ/e is being used for millimeter-wave heterodyne mixing. Junctions with a 2-μm diameter and RN=50 Ω have little capacitive shunting at the signal frequency of 36 GHz. Because there is no series resistance, residual capacitance can be tuned out. Double sideband conversion efficiencies of 0.32 and mixer noise temperatures as low as TM⩽7 K=4hν/k have been observed. The results are compared with shot-noise-limited mixer theory. Photon-assisted tunneling effects are seen which indicate the approach to photon-noise-limited operation.
The noise and gain of a heterodyne waveguide mixer employing Nb/Al2O3/Nb superconducting tunnel junctions with an on-chip integrated tuning element are measured and analyzed at 680–750 GHz and at 840 GHz. The lowest receiver noise temperatures are 400 K (double side band) at 720 GHz and 1500 K (3000 K including the beam splitter loss) at 840 GHz. We compare data of the pumped I–V curves with the quantum theory of mixing and demonstrate good agreement at frequencies well above the gap frequency.
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.
A fabrication technique for Nb-Al2O3-Nb Josephson tunnel junctions is described that is an alternative to the trilayer method generally used. At 4.2 K the magnitude of the low-frequency noise in the critical current I0 of four junctions with areas 𝒜 ranging from 7.8 to 115 μm2 was characterized by S1/2I0𝒜1/2/I0 =(14±6)pA μm/μA Hz1/2, where SI0 is the spectral density of the excess noise at 1 Hz. The noise power spectrum of one 3×3 μm2 junction exhibited a Lorentzian feature associated with the emptying and filling of a single trap in the barrier. The low level of noise makes these junctions attractive for use in superconducting quantum interference devices.
The rapid onset of quasiparticle tunneling current in superconductor-insulator-superconductor (Josephson) junctions at voltages above 2 delta/e is being used for millimeter wave heterodyne mixing. Junctions with 2 micron diameter and RN = 50 ohms have little capacitive shunting at the signal frequency of 36 GHz. Because there is no series resistance, residual capacitance can be tuned out. Single sideband conversion efficiencies of 0.18 and mixer noise temperatures as low as TM approximately 14K have been observed. Both results agree with shot noise limited mixer theory. Photon assisted tunneling effects are seen which indicate the approach to photon noise limited operation. (Author)
Predictions of the three-port model of the quantum theory of mixing are compared with measured results on 345 GHz superconductor-insulator-superconductor waveguide mixers. Single Nb-Al2O3-Nb tunnel junctions or two or four identical junctions in series are used as mixing elements. Two different waveguide mixerblocks, one with two tuners and another with one tuner, are used. In addition a single junction with integrated tuning stub is analyzed. Embedding impedances are obtained from fits to the pumped I-V curves for all three types of mixing elements. In all cases the dependence of mixer conversion and mixer noise on bias voltage, pump power, and embedding impedance is well described by the three-port model. The measured mixer gain is lower than the calculated gain by a factor of 0.35–0.65, independent of the type of mixer. The use of an additional integrated tuning element does not change this factor. It is concluded that an excess noise power equivalent with a blackbody source of 40–65 K must be added to the mixer noise to account for the absolute value of the observed noise power.