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European Microwave Conference (EuMC)

Small Dayem Bridges as Active Components in Microwave Circuits

P. E. Lindelöf, Mogens T. Levinsen, N. F. Pedersen, O.H. Sorensen, Terrence D. Clark, J. Mygind, M. Danielsen

Published Sep 1, 1976• 0 citations

Abstract

We report X-band microwave measurements on superconducting thin film micro-bridges (Dayem bridges), mounted in a half wavelength cavity with an adjustable coupling to match the bridge properly. Power emission from the bridge of 10-13W in a 10 MHz bandwidth was measured. Current noise fluctuations caused a total voltage line width corresponding to 150-200 MHz, when the voltage across the bridge was swept through the resonance; from this we estimate a total power emission of 1-2·10-12W into this frequency interval. Transmission measurements and comparison with analogue computer calculations have revealed that an equivalent diagram consisting of an inductance in series with the resistively shunted Josephson junction model can account for the experimental results. A conclusion from these measurements is that there is a possibility for amplification in the device. Moreover, the nonlinearity of the device was used in a mixing experiment with the Josephson oscillation as local oscillator.

DOI: 10.1109/euma.1976.332240

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We report X-band microwave measurements on superconducting thin film micro-bridges (Dayem bridges), mounted in a half wavelength cavity with an adjustable coupling to match the bridge properly. Power emission from the bridge of 10-13W in a 10 MHz bandwidth was measured. Current noise fluctuations caused a total voltage line width corresponding to 150-200 MHz, when the voltage across the bridge was swept through the resonance; from this we estimate a total power emission of 1-2·10-12W into this frequency interval. Transmission measurements and comparison with analogue computer calculations have revealed that an equivalent diagram consisting of an inductance in series with the resistively shunted Josephson junction model can account for the experimental results. A conclusion from these measurements is that there is a possibility for amplification in the device. Moreover, the nonlinearity of the device was used in a mixing experiment with the Josephson oscillation as local oscillator.

Measurements at 3 GHz of the microwave properties of a superconducting thin-film microbridge coupled to a narrowly tuned strip-line resonator (coupled Q approximately 10 3 ) indicate that such a device displays the same variations in small-signal impedance as predicted by the resistively shunted junction model of a Josephson weak-link. Whenever this impedance becomes negative the microbridge is found to generate microwave radiation whose frequency is determined not by the junction bias voltage but by the natural resonance of the strip-line.

The first measurements on the Dayem bridge showed that the behaviour in an applied microwave field was quite different from that of the Josephson tunnel junction. Measurements on bridges one magnitude smaller later showed this difference to result from the bridge size. A very simple equivalent circuit was furthermore shown to characterize these small bridges. Analog computer calculations on this circuit will be compared to experimental results. At high temperatures a rounding of steps and supercurrent presumably due to noise is observed. A comparison with the theory of Ambegaokar and Halperin will be presented. At low temperatures a hysteresis develops. This may result from the bridge capacitance which also may give rise to subharmonic steps. However, the magnitude (1 pF) inferred from the hysteresis is too small to account for the subharmonic steps actually observed. In contrast to other types of junctions the bridge is mechanically very robust. We have therefore been interested in the applicability of the bridge. An upper limit to the sensitivity to 10 GHz radiation has been found in terms of NEP to 9 × 10-14 W/√Hz. Furthermore, we have demonstrated that mixing via the Josephson effect is possible. The power dependence of the sum and difference steps is in fair agreement with analog computer calculations on the equivalent circuit. It has been debated whether the bridge radiates at all. By the observation of a cavity induced step we have now established that bridges do radiate.

Josephson junctions using Dayem bridges made from unusually thin films have been operated as mixers. The thickness of the film was made small to increase the r.f. impedance of the device and so to improve r.f. matching, but such thin films often fail to act as Josephson junctions because of surface roughness. In the device reported here, attempts were made to obtain specular reflection of the electron wave functions from the surface. Our devices behaved as good Josephson junctions and gave an internal mixing efficiency of 4.5% at 32 GHz.

Current-voltage characteristics and microwave mixing properties of step-edge microbridge Josephson junctions (SEJJs) made of YBaCuO thin films were systematically investigated. The step-edge was formed on the surface of a MgO(100) substrate employing the common photolithograph and wet-etching techniques. YBaCuO thin film was deposited by the off-axis sputtering method, and the microbridge junctions were fabricated so as to cross the step-edge. The SEJJs showed typical DC and AC characteristics which were explained by the resistively shunted junction (RSJ) model. Mixing experiments were carried out in the K-band microwave region. An intermediate frequency (IF) output with the frequency range of 20 MHz~2.0 GHz was successfully obtained.

Measurements are reported of the Josephson radiation from microbridges coupled to an X-band receiver via a nonresonant microwave transformer. Although closely coupled to the bridge, the transformer does not modify the bridge properties. We find that near the transition temperature the radiation linewidth is proportional to the square of the dynamic resistance, which is also predicted by the resistively shunted junction model. The effective noise temperature determined by the linewidth is 19±3 K and the measured maximum integral power is 5×10−12 W.