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IEEE Transactions on Applied Superconductivity

Low-voltage negative-resistance mixers of nano-meter SNS junctions

Toshiaki Matsui, Hiroshi Ohta

Published Mar 1, 2001• 2 citations
Publisher

Abstract

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.

DOI
10.1109/77.919317

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IEEE Transactions on Applied Superconductivity•Jun 1, 1999•2 citations
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 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.
A Superconducting - Normal metal - Superconducting junction (SNS junction) may exhibit a Low Voltage Negative Differential Resistance (LVNDR) effect over part of its Current Voltage Characteristic (CVC). As the LVNDR effect is stable against a bias voltage at this CVC range, it should be possible to combine a SNS junction with conventional electronic circuits to obtain electronic devices such as mixers, amplifiers and oscillators. Making use of this remarkable effect, we show tliat an amplifier may be feasible by assembling a simple voltage divider made up of a SNS junction in series witli a resistor. The amplifier circuit includes an adjustable DC voltage supply (the bias voltage) and an AC signal source with a given voltage. The SNS junction is connected in series with a resistor R. Choosing values of the load resistance R approximately equal to the module of the negative differential resistance (dV/dI), at the bias voltage, we may obtain large gains in this amplifier device. In order to get an oscillator, the SNS junction should be connected to a RLC tank circuit with a bias voltage adjusted in the range of the LVNDR region of its CVC. A power output of the order of one microwatt may be easily obtained.
We report signal amplification and harmonic multiplication processes based on the nonlinear current-voltage characteristics (CVC) displayed by superconductor-normal metal-superconductor (SNS) junctions. Such junctions may exhibit a negative differential resistance at low voltages, the so called low-voltage-negative-differential-resistance (LVNDR) effect. This effect is due to the quantum mechanical multiple Andreev reflection mechanism in very clean systems and is stable against a bias voltage. Therefore, it is in principle possible to use these junctions as an active element in a variety of circuit applications such as oscillators, amplifiers or mixers, in the microwave and millimeter-wave range.
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 measured the current-voltage $(I\ensuremath{-}V)$ characteristics of small-capacitance single Josephson junctions at low temperatures (T=0.02−−0.6K),(T=0.02--0.6\mathrm{K}),(T=0.02−−0.6K), where the strength of the coupling between the single junction and the electromagnetic environment was controlled with one-dimensional arrays of dc superconducting quantum interence devices (SQUIDs). The single-junction $I\ensuremath{-}V$ curve is sensitive to the impedance of the environment, which can be tuned in situ. We have observed Coulomb blockade of Cooper-pair tunneling and even a region of negative differential resistance, where the zero-bias resistance ${R}_{0}^{\ensuremath{'}}$ of the SQUID arrays is much higher than the quantum resistance ${R}_{K}\ensuremath{\equiv}{h/e}^{2}\ensuremath{\approx}26\mathrm{k}\ensuremath{\Omega}.$ The negative differential resistance is evidence of the coherent single-Cooper-pair tunneling within the theory of current-biased single Josephson junctions. Based on this theory, we have calculated the $I\ensuremath{-}V$ curves numerically in order to compare then with the experimental ones at ${R}_{0}^{\ensuremath{'}}\ensuremath{\gg}{R}_{K}.$ The numerical calculation agrees with the experiments qualitatively. We also discuss the ${R}_{0}^{\ensuremath{'}}$ dependence of the single-Josephson-junction $I\ensuremath{-}V$ curve in terms of the superconductor-insulator transition driven by changing the coupling to the environment.
The conversion efficiency was calculated for some type of high-T/sub c/ superconducting junctions, which have negative derivative on current-voltage characteristics. This distinguishing feature can be explained by Andreev reflections in superconductor/normal metal boundaries. In this work it is analyzed a combination of the theoretical predictions of noise, based on noise correlation matrix of the mixer model, and theory of Likharev-Semenov for the thermal noise. The conversion gain is possible in SNS mixers in spite of noise in negative derivative region.