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Applied Physics Letters

Direct detection of the Josephson current in Nb superconducting microbridges

Paul Crozat, G. Vernet, Robert Adde

Published Apr 15, 1978• 4 citations
Publisher

Abstract

The detection of the ac Josephson current in Nb microbridges with dimensions larger than the coherence length but smaller than the effective penetration depth is reported at frequencies between 1.7 and 180 GHz. Precise Josephson linewidth measurements are performed as a function of dc bias and frequency and the results are compared with the existing theories.

DOI
10.1063/1.90108

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Physical review. B, Condensed matter•Jul 1, 1990•6 citations
The detection of the ac Josephson current in Nb microbridges with dimensions larger than the coherence length but smaller than the effective penetration depth is reported at frequencies between 1.7 and 180 GHz. Precise Josephson linewidth measurements are performed as a function of dc bias and frequency and the results are compared with the existing theories.
The rf-power dependence of the ac-Josephson steps is measured at 720 GHz, using small area Nb tunnel junctions. This frequency is well above the gap frequency of Nb. The junction is placed in a waveguide, and connected to a superconducting stripline, which effectively tunes out the junction capacitance and facilitates the coupling of the radiation to the junction. We observe three Josephson steps, and the first step crosses the zero current axis over a considerable range of rf-power. This indicates the possible application of THz Josephson steps in voltage standards. The data are compared to the theory and we find clear evidence for the predicted intrinsic roll-off of the Josephson current amplitude above the gap frequency.
The Josephson frequency conversion in the millimeter and submillimeter wave region is studied with superconducting Nb-Nb point contact junctions at 4.2 K, using the contact both as the local oscillator and the mixer with intermediate frequencies of 0.1 and 9.2 GHz. Frequency conversion signals are only detected in the latter case with a microwave receiver at 9.2 GHz in a range of Josephson frequencies between 145 and 1 600 GHz. The experiments with intermediate frequency fi of 0.1 GHz indicate for this small value of f i that the observed signals are not related to frequency conversion but noise effects. An analysis of the frequency conversion mechanism in a Josephson junction operated as an oscillator-mixer is proposed which includes the noise effects and also the feedback effects induced in a junction driven by a constant current source. We have then investigated the noise properties up to 450 GHz of superconducting point-contacts using the junction as an oscillator-mixer where the Josephson current is mixed with harmonics of an externally applied signal. The emitted intermediate frequency is detected at 9.2 GHz and its linewidth is measured. In the very high frequency range the dc quasiparticle current dominates in a Josephson weak-link and we compare the corresponding theoretical linewidth calculated by Scalapino with our experimental results.
An experimental study of small superconducting microbridges in order to examine parameter conditions (i.e., junction resistance and junction inductance) necessary for these junctions to exhibit the Josephson effect is described. Nb variable thickness junctions with very narrow width (<50 nm) and short length (<100 nm) were fabricated using electron beam lithography. The junctions which showed the Josephson effect were thinned by ion beam etching, and consequent changes of junction characteristics were measured. The experimental results indicate that the Josephson effect disappears at some critical resistance. These parameter limits of the Josephson junction are compared with theory.
The output noise of a granular NbN microbridge has been measured at frequencies near 25 MHz as a function of bias voltage, temperature, and rf bias at 70 GHz. It is generally greater than predicted by the resistively shunted junction model, but is significantly lower than has previously been observed in clean superconducting microbridges with comparable RIc products. An estimate of the performance of the device as a Josephson mixer at 70 GHz is made on the basis of these measurements. At an operating temperature of 10 K a double sideband mixer noise temperature ∼200 K, with conversion loss of ∼3 dB should be attainable. Using the noise as a probe of the average temperature within the microbridge, it is found that simple self-heating is not a likely explanation for the disappearance of Josephson effects at high dc or microwave bias.