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Applied Physics Letters

Collective synchronization of arrays of superconducting thin-film microbridges

Terrence D. Clark, P. E. Lindelöf

Published Dec 1, 1976• 15 citations
Publisher

Abstract

Square networks (arrays) of superconducting thin-film ’’Dayem’’ microbridges have been investigated. Due to the uniform properties of our bridges, it was particularly easy to collectively synchronize these networks to a microwave field. Analogue computer calculations indicate that to obtain a free-running single-frequency 2×2 network, the diagonal pairs of junctions must be matched approximately to each other and be different from the other pair. We expect this to be a necessary condition for frequency and phase locking in larger networks, but as yet we have not realized this situation experimentally.

DOI
10.1063/1.88935

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Square networks (arrays) of superconducting thin-film ’’Dayem’’ microbridges have been investigated. Due to the uniform properties of our bridges, it was particularly easy to collectively synchronize these networks to a microwave field. Analogue computer calculations indicate that to obtain a free-running single-frequency 2×2 network, the diagonal pairs of junctions must be matched approximately to each other and be different from the other pair. We expect this to be a necessary condition for frequency and phase locking in larger networks, but as yet we have not realized this situation experimentally.
We have investigated two different methods of coupling between the Josephson oscillations in arrays of superconducting microbridges made of Sn and In in order to establish a single free-running coherent state. We report on experiments involving a broadband inductive coupling between pairs of microbridges, each pair constituting a dc-SQUID; and a narrow hand coupling between microbridges via resonant structures in transmission lines consisting of microstrip or slot lines. We discuss the properties of these two ways of long range coupling of Josephson weak links and evaluate their potential for high-frequency applications.
We report on the characteristics of thin film microbridge Josephson junctions fabricated using electron beam lithography and discuss the techniques used to fabricate these junctions, which are typically 0.3μ square. Extensive measurements have been made on both individual junctions of tin and indium and on small arrays of these junctions. Synchronization of current biased series arrays by an applied microwave field has been achieved. This synchronization appears as a zero differential resistance across the entire array at a voltage of MVJwhere M is the number of elements in series and VJis the Josephson voltage hv/2e. Furthermore we have observed a systematic variation with microbridge dimension of the microwave enhancement of the critical current.
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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.