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AIP conference proceedings

Superconducting microbridges and arrays

P. E. Lindelof, Jørn Bindslev Hansen, P. Jespersen

Published Jan 1, 1978• 3 citations
Publisher

Abstract

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).

DOI
10.1063/1.31358

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IEEE Transactions on Magnetics•Jan 1, 1977•16 citations
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).
High resistance granular NbN microbridges with submicron dimensions have been fabricated and investigated. Devices with critical currents less than 5 microamps and bridge resistances of the order of 50 ohms have been studied as a function of temperature at both zero and finite voltages. The current‐voltage characteristics were measured with various levels of applied radiation in the frequency range 11–40 GHz. For these frequencies, constant voltage‐current steps were observed at the Josephson frequency, 2eV/h for low applied voltages, at higher voltages the radiation induced step spacing deviated from that of ideal Josephson behavior. Additional current steps were also observed at temperatures just above the bridge Tc where no finite critical current could be dected. These effects and other characteristics observed on the I‐V curves will be reported and discussed.
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.
We have studied the phase locking of two and eight microbridge arrays shunted by superconducting loops in a dc SQUID configuration. The difference in phase between adjacent bridges is controlled by the applied flux, and thus the radiated power is a periodic function of this flux. With adjustment of the external flux, the individual bridge voltages can be made to add in phase. The effect of additional normal shunts on the phase locking is also investigated. MICROBRIDGES COUPLED BY SUPERCONDUCTING LOOPS Experiment sly on two Josephson microbridges have shown that it is possible to obtain nearly complete in phase coherent radiation with narrowed linewidth by coupling the microbridges with external shunts. To obtain greater ouput power and narrower linewidth it is desirable to phase lock larger arrays of bridges. The narrow range of bias currents over which locking occurs, however, imposes strict tolerances on the uniformity of bridge characteristics if a single current bias is used. In this paper we discuss experiments which eliminate this difficulty by using a superconducting loop to couple the junctions, i.e., a dc SQUID configuration, so that the average frequency of each junction is always equal even for non-identical junctions. Also the phase difference between the junctions in the absence Of noise is set by the amount of externally applied flux due to the fluxoid quantization condition. With identical bridges and zero applied flux, the junctions oscillate in phase. In the geometry used in
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.
Voltage locking, defined as the production of an identical non-zero DC voltage in the absence of external microwave radiation across each Josephson junction of an array, has been observed in two microbridge Josephson junctions separated by a 2μm wide strip of superconductor. Voltage locking occurs when the bridges are biased with the current flowing in opposing directions thru the bridges and out the strip connecting them. The voltage across each bridge can be pulled over 1μV until each bridge displays an identical non-zero voltage, with the total voltage across both bridges equal to zero. Full locking, as defined above, is observed in excess of 40μV. The voltage and temperature dependence of the locking is described.