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Applied Physics Letters

Coherent effects in large arrays of superconducting bridges

David William Palmer, James E. Mercereau

Published Oct 15, 1974• 28 citations
Publisher

Abstract

Series arrays of nearly identical superconducting proximity effect bridges have been fabricated in Nb/Ta thin films. Arrays with up to 500 bridges respond cooperatively to incident radiation and produce voltage steps in response to incident 30–20 000-MHz radiation at V =(number of bridges in array)×(hν/2e).
DOI
10.1063/1.1655552

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Coherent effects in series arrays of proximity effect superconducting bridges

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Planar series arrays of up to 500 proximity effect superconducting bridges have been fabricated with sufficient uniformity to respond coherently to incident radiation (10-20,000MHz) without separate biasing. By varying the geometry, resistance for the arrays can range from .1-50Ω. Despite this high total impedance, low frequency coherent response to external radiation is limited only by single bridge voltage noise. Low frequency (30-500MHz) coherent internal voltage oscillations of current biased arrays have been detected and confirm that the oscillating voltage signal grows linearly in bridge number while the voltage noise increases only as the square root. An array of n elements responds to incident high frequency radiation in first approximation as though it were a simple Josephson junction but quantized in units of n(h/2e). Coincident operation to high frequency depends strongly on small (\leq 1\mu) bridge separation. High resistance, coincident operation, and exponential dependence of critical current on temperature make these arrays good candidates for voltage standards, coherent and bolometric radiation detectors, and cryogenic rf sources.

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IEEE Transactions on Magnetics•Mar 1, 1975•16 citations
Planar series arrays of up to 500 proximity effect superconducting bridges have been fabricated with sufficient uniformity to respond coherently to incident radiation (10-20,000MHz) without separate biasing. By varying the geometry, resistance for the arrays can range from .1-50Ω. Despite this high total impedance, low frequency coherent response to external radiation is limited only by single bridge voltage noise. Low frequency (30-500MHz) coherent internal voltage oscillations of current biased arrays have been detected and confirm that the oscillating voltage signal grows linearly in bridge number while the voltage noise increases only as the square root. An array of n elements responds to incident high frequency radiation in first approximation as though it were a simple Josephson junction but quantized in units of n(h/2e). Coincident operation to high frequency depends strongly on small (\leq 1\mu) bridge separation. High resistance, coincident operation, and exponential dependence of critical current on temperature make these arrays good candidates for voltage standards, coherent and bolometric radiation detectors, and cryogenic rf sources.
Techniques have been developed with which it is possible to fabricate superconducting thin-film structures with a wide variety of electrical and superconducting parameters which show Josephson-like phenomena. These bridges-based on the proximity effect-are made in layered thin-film ``substrates'' using a large variety of superconducting and nonsuperconducting materials. In this paper we detail the fabrication techniques and the electrical and superconducting characteristics for these proximity effect bridges including a simple low-frequency (<10 GHz) equivalent circuit based on analysis and experiment which closely describes the quantum phase coherence operation of the bridges in their voltage sustaining state. Extension of these fabrication techniques to entire thin-film electrical circuits including resistors, capacitors, and inductors with these bridges and leading to an entire superconducting microcircuit technology is discussed.
The effect of microwave radiation on the dc current-voltage characteristic of proximity effect bridges has been studied at frequencies of 10, 35 and 90 GHz. We report the properties of bridges fabricated in layered films of Nb and Ta. The fraction ε of the incident radiant power absorbed by the bridge is measured calorimetrically, permitting determination of the intrinsic responsivity (volts per watt of power absorbed when operated in a current-biased mode). In our test geometry ε is typically ˜ 10⁻⁴ for a 0.1 Ω Nb-Ta bridge, and increases roughly linearly with dc junction resistance R. Broad-band responsivitv is generally independent of incident power. increases with increasing critical current Ic (decreasing temperature), and decreases with increasing frequency, varying approximately as Ω-3 where Ω is the normalized frequency ħω/2 eRIc. Peak responsivities achieved to date are ˜ 105 V/W (absorbed) at 10 GHz and ˜ 102 V/W (absorbed) at 90 GHz. By mounting junctions in vacuum and exposing them to near-infrared radiation, we have found that their purely bolometric (thermal) response is much weaker than the microwave response at 90 GHz.
The current‐voltage characteristics and Josephson radiation have been studied for a number of indium bridges as a function of temperatures. Our results show strong deviations from the predictions of the RSJ model, even in the temperature regime where the bridge size is smaller than the coherence length. We ascribe this to non‐equilibrium effects in the around the microbridge.In order to improve the applicability of the Josephson effect, we have investigated coherent arrays of 2, 4 and 10 microbridges. The 10 microbridge array is a novel design which allows the bridges to be parallel‐biased at dc (equal voltages) and series‐biased at 9 GHz (high output impedance).
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.