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International Journal of Electronics

An analysis of the inflexion point structure of Josephson absorption effect current steps

Ranko Mutabžija

Published Mar 1, 1977• 0 citations
Publisher

Abstract

The inner structure of the Josephson absorption effect IV characteristic in the vicinity of the inflexion points is analysed for stationary finite bandwidth irradiation. The zero crossings and the slopes of the higher-order derivatives of the IV characteristic offer further possibilities for more precise Josephon voltage determination. The results of the analysis indicate that the properties of the inner structure of the IV characteristic affect applications such as frequency multiplication and frequency mixing.

DOI
10.1080/00207217708900637

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The inner structure of the Josephson absorption effect IV characteristic in the vicinity of the inflexion points is analysed for stationary finite bandwidth irradiation. The zero crossings and the slopes of the higher-order derivatives of the IV characteristic offer further possibilities for more precise Josephon voltage determination. The results of the analysis indicate that the properties of the inner structure of the IV characteristic affect applications such as frequency multiplication and frequency mixing.
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Current-voltage characteristics of NbN-NbN Josephson point contacts have been examined. The point contacts were formed by NbN thin films deposited on Nb or W whiskers and Nb posts. The current-voltage characteristics of those contacts were investigated with and without irradiation of microwaves and far-infrared light. High values of the I c R n product and the energy gap structure were observed, which corresponded to the high-energy gap of NbN. Shapiro steps for far-infrared light up to 170 µm were observed and the possibility of replacing Nb Josephson point contacts with NbN ones is indicated.
We consider a current biased, resistively shunted josephson junction irradiated at two frequencies. The perturbation technique introduced by Aslamasov and Larkin is used in the calculations. Both signals are treated as perturbations. The second order calculation yields the size of the mixing steps atV_{\pm}=h(\omega_{1}\pm\omega_{2})/2e. As in the case of a single frequency we show that subharmonic mixing steps are absent. The amplitude of the voltage oscillation at the difference and sum frequencies is shown to be non-zero at all voltages. We calculate the microwave resistance for one frequency ω2to third order in the perturbation. There are negative resistance regions nearV\pm(as well as nearV_{2}= h\omega_{2}/2e). Near V, the negative resistance region appears for bias voltage V just aboveV_{-}, while near V the region appears for V just belowV_{+}. This means that when an incident frequency mixes with a cavity mode the mixing step atV_{-}will be inverted compared to the cavity step itself.
The electrical equations of a resonant circuit fed with a radio-frequency current and inductively coupled to a weakly superconducting loop have been derived. The current-voltage characteristic of the rf superconducting quantum interference device can then be calculated and studied as a function of the junction resistance or of the resonance frequency, enabling its behavior to be better understood. As shown by Kurkijärvi, the large positive slope of the steps and the rounding of the characteristic result from thermal noise. However, in order to explain quantitatively the experimental results concerning, namely, point contact weak links which are generally oxidized or which have several contact points, it is useful to remark that the connection between the two superconductors constituting the Josephson junction may be made from n resistive paths. The mean thermal energy of the fluctuating flux is then close to nkbT instead of kbT if the inductances linking the contact points are large enough, that is, on the order of 10−11 H.