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IEEE Transactions on Magnetics

Flux modulated coherent radiation from arrays of Josephson microbridges coupled by superconducting loops

Robert D. Sandell, C. Varmazis, Amit Jain, James E. Lukens

Published Jan 1, 1979• 13 citations
Publisher

Abstract

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.

DOI
10.1109/tmag.1979.1060130

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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.
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 synchronous operation of series arrays of inductively coupled superconducting quantum interference devices (SQUIDs). Each array consisted of N=3 or 11 dc SQUIDs with common inductances providing a strong interaction between neighboring cells. Externally shunted (βc≊1) trilayer Nb—AlOx—Nb Josephson junctions were used. Coherent radiation was detected in the frequency range f=67–73 GHz while the array voltage was VN=NfΦ0 (Φ0=2.07×10−15 V s). The stability of the coherent state was influenced by the external dc magnetic field. The linewidth depended on the external loading. For small loading it was N times smaller than expected for a single cell. The influence of the inductive coupling mechanism on the operation of discrete Josephson junction circuits and the similarity to the coupling in layered structures of long Josephson junctions is discussed.
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).
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.
We report the strong microwave Josephson radiation from an array of high-T/sub c/ junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUID's geometry which guaranteed the oscillation of all junctions at the same frequency. The phase differences between adjacent junctions with different I/sub c/R/sub n/ products could be controlled by an external small magnetic field. For a five-junctions array, we observed the coherent output power of about 13 pW without external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the self-radiation in the coherent state to be less than 1 GHz by adjustment of the external flux.