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Journal of Physics D Applied Physics

Small-signal microwave impedance measurements of a superconducting thin-film microbridge

P W Forder, John Ryder Waldram

Published Sep 14, 1981• 3 citations
Publisher

Abstract

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.

DOI
10.1088/0022-3727/14/9/007

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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.
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.
We fabricated weak link rings using YBCO thin films deposited on the single crystal MgO substrate by laser ablation. The dimensions of the microbridges in the rings were between 0.5–1 μ in length and 500 Å–0.5 μ in width which were made by focused ion beam milling. We biased the ring by Faraday induction voltage while monitoring the voltage across the microbridge. The frequency of the input signal was varied between 10 kHz and 5 MHz. When we superimposed a microwave signal (2–18 GHz) to the ring, below the superconducting transition temperature (Tc), we observed strong nonlinearities in the voltage measured across the microbridge. The microwave frequency was chosen to be close to the ring natural electromagnetic resonance.
Three different schemes for coupling to low impedance Josephson devices have been investigated. They all employ superconducting thin-film microstrip circuit techniques. The schemes are: (i) a quarterwave stepped impedance transformer, (ii) a microstrip resonator, (iii) an adjustable impedance transformer in inverted microstrip. Using single microbridges to probe the performance we found that the most primising scheme in terms of coupling efficiency and useful bandwidth was the adjustable inverted microstrip transformer.
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.
Amplification, down-conversion and detection of the microwaves in thin-film Josephson variable thickness bridges of submicron dimensions have been investigated experimentally. The results for indium and tin bridges are discussed taking into account the influence of the nonequilibrium electrons in a weak link region. The same experiments were carried out with SNS variable thickness Nb-Al-Nb bridges. They showed that the properties of SNS bridges agreed better with the theoretical predictions of the resistively shunted junction model in a wide temperature range. Moreover the parameters of the Nb-Al-Nb bridges have possessed better stability and reliability.