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IEEE Transactions on Applied Superconductivity

Mixer of short weak link using a resonance effect due to multiple Andreev reflections at subgap voltages

Toshiaki Matsui, Hiroshi Ohta

Published Jun 1, 1995• 10 citations
Publisher

Abstract

Short metallic weak links of Nb are used for millimeter-wave mixer experiments at 100-110 GHz. When the weak links are connected to a voltage bias circuit with zero output impedance, subharmonic gap structures are observed as a large dynamic resistance region or a negative differential resistance region at voltages V/sub u/=2/spl Delta//ne on the I-V curve, where n=1, 2, 3, ... . These strong structures are caused by a resonance effect between the energy gap and the multiple Andreev reflections that are present whenever there is a spatial inhomogeneity of the pair potential. Enhancement of IF signal peaks is observed at the subgap voltages. These experimental results suggest the appearance of the density of states predicted theoretically by R. Kummel et al. The subharmonic gap structure and details of the SNS mixer experiment will be described.>

DOI
10.1109/77.403297

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IEEE Transactions on Magnetics•Mar 1, 1989•3 citations
Short metallic weak links of Nb are used for millimeter-wave mixer experiments at 100-110 GHz. When the weak links are connected to a voltage bias circuit with zero output impedance, subharmonic gap structures are observed as a large dynamic resistance region or a negative differential resistance region at voltages V/sub u/=2/spl Delta//ne on the I-V curve, where n=1, 2, 3, ... . These strong structures are caused by a resonance effect between the energy gap and the multiple Andreev reflections that are present whenever there is a spatial inhomogeneity of the pair potential. Enhancement of IF signal peaks is observed at the subgap voltages. These experimental results suggest the appearance of the density of states predicted theoretically by R. Kummel et al. The subharmonic gap structure and details of the SNS mixer experiment will be described.>
We have studied negative-resistance mixers both theoretically and experimentally. The current carried by the bound quasi-particles in the N-region of a mesoscopic SNS weak-link has a dc and cosine components. Current transport in the SNS weak-links is affected by bias-voltages in many experiments. The I-V curves often show complex structures including negative differential resistance which cannot be understood by the simple RSJ model. Supercurrent is directly affected both by the bias voltage and the density of states at the energy gap of superconductor. The supercurrent by the Multiple-Andreev-Reflection is considerably changed across subharmonic gap voltages because of the singularity of the density of state of superconductor near the energy gap. We have done mixer experiment of SNS weak-links and have observed IF peaks at subgap voltages which are caused by the negative differential resistance.
The current carried by the bound quasi-particles in the N region of a mesoscopic SNS junction has dc and cosine components. The dc current component is carried by the pair charge (-2e) transferred by a couple of Andreev reflections at both of the NS interfaces. The pair-charge transfers decrease due to reduction of the allowed number, 2/spl Delta//V, of Andreev reflections when the voltage, V, increases. Therefore, the low-voltage negative-differential-resistance is observed on the I-V curves of mesoscopic SNS junction, when the junction is driven by the low impedance voltage-bias source. The supercurrent carried by the quasi-particle in the N-region is sensitive to the external high-frequency fields. In a mixer experiment using the nano-meter SNS junctions of NbN, prominent IF signal peaks are observed at low bias voltage. Each IF signal peak corresponds to the negative differential resistance region at low bias voltage.
We investigated subharmonic gap structures in the current-voltage (I-V) characteristics of NbN tunnel junctions by observing frequency response using a quasioptical mixer configuration. We chose a 10.5-/spl mu/m-long by 0.6-/spl mu/m-wide junction for the mixer so that the junction behaved as a distributed element at submillimeter-wave frequencies in wideband observations. In the presence of electromagnetic waves, subharmonic photon-assisted tunneling steps with a voltage width of hf/2e could clearly be observed below and above the half-gap voltage in the I-V curves of junctions at measured frequencies from 600 to 950 GHz, where h is Planck's constant, f is frequency, and e is electron charge. This evidence is consistent with a consequence of multiple Andreev reflection at low transmissions or multiparticle tunneling.
The subharmonic gap structure (SGS) in long-overlap Nb-AlOx-Nb Josephson tunnel junctions has been investigated. The experimental results show peaks in the differential conductance at both odd and even integer fractions of the gap voltage, VG. Furthermore, the conductance peaks at VG/2 has been observed to split into two peaks with different characteristics. At high magnetic fields, the I-V characteristics approach a single curve, while retaining the SGS conductance peaks. The gap structure and the SGS show the same temperature dependence. The SGS can be explained by a Josephson self-coupling effect or multiple Andreev reflections. The results are important for the development of the flux flow oscillator used a local oscillator in integrated all-superconducting 200-800 GHz receivers.
Millimeterwave detectors have been made of a series-connected array of Josephson junctions. Each junction of the array responds in phase to radiation and therefore the series-connected array as a whole behaves as if it were a single Josephson junction. When the Josephson junctions have a high critical current density, the IF outputs have sharp peaks at the voltages V = 2NΔ/ne where N is the number of the Josephson junctions in series connection and n is an integer. The subharmonic energy gap structures in voltage-current curves of Josephson junctions come from the Andreev-reflection-assisted-tunneling according to the recent Arnold theories.
A new type of harmonic mixer using a series array of short superconducting weak-links is proposed for mixer applications at far-infrared and submillimeter-wave frequencies. The problem of limited mixer dynamic range can be eliminated by the use of a Josephson junction series array structure. Using a eleven-element series array, a preliminary experiment on harmonic mixing was carried out for two signals at 105.9 GHz and 5.4-12.6 GHz. IF (intermediate frequency) output was measured for each harmonic order from 9th to 19th by optimizing local power and bias voltage. In this experiment, 5-dB heightening of the signal-to-noise ratio was obtained at the IF output when the harmonic mixing order n was equal to the series element number, N=11.>