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Japanese Journal of Applied Physics

Comparison of Nb Thin-Film Point-Contact Josephson Junction Characteristics with the Stewart-McCumber Model

Seiichi Naitô, Yasushi Higashino

Published Jul 1, 1984• 7 citations
Publisher

Abstract

This paper compares the measured characteristics of a three-dimensional Nb thin-film point-contact junction with those predicted by the Stewart-McCumber model. The erratic microwave-induced I-V characteristic we observed at low microwave power levels was also evident in a digital simulation –based on the Stewart-McCumber model–of the time dependence of the junction voltage and the phase space trajectory. Subharmonic steps V=(n/m)(h f/2e) were observed when a 70 GHz signal was applied, but the usual condition ωC R n≃1 for substeps was not fulfilled. Substeps were also observed in the digital simulation even when the current-phase relationship was purely sinusoidal.

DOI
10.1143/jjap.23.861

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This paper compares the measured characteristics of a three-dimensional Nb thin-film point-contact junction with those predicted by the Stewart-McCumber model. The erratic microwave-induced I-V characteristic we observed at low microwave power levels was also evident in a digital simulation –based on the Stewart-McCumber model–of the time dependence of the junction voltage and the phase space trajectory. Subharmonic steps V=(n/m)(h f/2e) were observed when a 70 GHz signal was applied, but the usual condition ωC R n≃1 for substeps was not fulfilled. Substeps were also observed in the digital simulation even when the current-phase relationship was purely sinusoidal.
For a current-driven Josephson junction shunted by an Ohmic resistance, the dc voltage response and impedance to external high-frequency currents is calculated with a second-order perturbation method based on the unperturbed solution for the time evolution of the voltage as given by Aslamazov and Larkin. The response is proportional to signal power and has three characteristic-frequency regions depending on whether the internal self-generated Josephson frequency, ω0, is larger than, equal to, or smaller than the signal frequency, ω. For ω0< or >ω the response could be expressed by easily observable parameters of the dc voltage-current characteristics. This permits comparison of the predictions with experimental results obtained on point-contact junctions whose dc characteristics were only approximately represented by the simple model chosen for analysis. For ω0≪ω the response is found to be proportional to (i) the slope of the dc characteristic, (ii) the inverse square of the applied frequency, (iii) the inverse battery current in terms of the critical current, and (iv) the voltage amplitude of the inherent Josephson oscillations. For ω0≫ω ``classical'' detection proportional to curvature is obtained. For ω0∼ω resonance detection occurs, which depends to a considerable degree on fluctuations of the average contact voltage. These predictions are in reasonable agreement with experiments in which point-contact junctions were illuminated with klystron radiation at 10 and 90 GHz. Evidence is presented that the voltage fluctuations limiting the resonance response are the result of noise generated in the junction. Comparison of the noise-equivalent power expected from the simple model with measured results suggests that parasitic elements, such as shunt capacity, are not negligible at 90 GHz.
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.
The Shapiro steps in the submillimeter wave (SMMW) region were measured in point-contact Josephson junctions made of a Nb strip with a sharpened edge and a YBaCuO thin film. The thin film was prepared by the chemical vapor deposition (CVD) method. The dependence of the Shapiro step width on the SMMW power was measured for the wavelength of 570.6 µm (525.4 GHz). The dependence was similar to that found in microbridges made of low T c superconductors. The first-order Shapiro step was observed for the wavelength of 232.9 µm (1.287 THz).
Microwave superconducting mixers operating at 10–13 GHz utilizing Josephson junctions have been investigated analytically and experimentally. Measurements on mixer sensitivity were carried out at 13 GHz using waveguide-mounted experimental point-contact Josephson junctions at 4.2 K. Results showed sensitivities of −90 dBm/MHz (10−18 W/Hz) which included mismatch losses encountered in the waveguide structure. The Josephson junction mixing mechanism was analyzed on the basis of small-signal modulation of the staircase like dc I-V characteristics. It was shown that a small-signal microwave excitation results in an ac lateral shift of the regions between steps, such that: (a) Optimum Josephson mixing occurs halfway between steps. (b) Mixing occurs between all steps with maximum conversion efficiency between the zero and first step. (c) Predicted low conversion loss and possibly internal gain for a matched mixer. The analytical model was simulated by a digital computer and the behavior of the Josephson mixer predicted for various conditions of excitation. The model chosen for the analysis and the computer simulation is based on the simplest assumptions concerning bulk-formed Josephson junctions. In the laboratory an external local oscillator set 60 MHz below the signal frequency was used to induce the well-known step structure in the dc I-V characteristic of the point-contact Josephson junction. Mixing of the Lo and signal was found to occur between constant voltage steps in agreement with both analysis and computer simulation. The correctness of the model is demonstrated by the agreement between the theory and experiment.
We observe several sharp changes in the slope of the current-voltage characteristics (CVCs) of thin-film ramp-edge Josephson junctions between ${\text{YBa}}_{2}{\text{Cu}}_{3}{\text{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ and Nb when applying strong microwave fields. Such behavior indicates an intriguing Josephson dynamics associated with the switching from a parametric excitation regime induced by the magnetic field of the microwave via oscillations of the Josephson critical current to an ac-current-excitation regime triggered by the electric field of the microwave. We propose a model, which describes the observed features on the CVC in terms of microwave-induced multiple switching between running and locked solutions of sine-Gordon equation.
We report on measurements of dc electron transport and microwave dynamics of thin film hybrid Josephson junctions with an oxide antiferromagnetic interlayer. The base superconducting electrode YBaCuO and the antiferromagnetic (AF) interlayer CaSrCuO (with thickness d = 20 – 70 nm) were grown by laser ablation on NdGaO 3 substrates. I-V curves were well fitted to RSJ model and had no excess current, I C R N products were of order 0.2 mV at T =4.2 K. We did not observe any noticeable reduction of I C R N with increasing d . Such "tunnellike" behaviour also resulted in appearance of singularities on I-V curve when magnetic field was applied. Oscillating with microwave power integer and half-integer Shapiro steps were registered along with sub-harmonic detector response. Moreover, for some of junctions a "devil" staircase structure was observed on I-V curves and giant noise-like signals were measured in 1-2 GHz band at the certain levels of microwave power. Observed features, noise performance and the impact of the second harmonic in current-phase relation on junction dynamics are discussed taking into account data for structures without AF interlayer.