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Final Report

Vortices in long Josephson junctions

J.E. Nordman, James B. Beyer

Published Feb 1, 1989• 0 citations
OpenAlex

Abstract

Research during the initial year of this grant concentrated on fabrication and modelling of long Josephson junction structures. A particular effort was made to understand the basic operating mechanisms of a type of superconducting transistor, the vortex flow transistor, and to optimize its circuit performance. Fabrication techniques were developed using niobium and lead superconductors. These techniques allow rapid production of a variety of device shapes. An optimization of geometric configuration was made to produce the most desirable device characteristics. Amplification was observed at low frequencies and initial coupling circuit designs were made to enable high frequency measurement. Development of potentially more versatile fabrication techniques using niobium and niobium nitride was also begun, as was a simulation study aimed primarily at understanding spatially nonuniform long junction structures.

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IEEE Transactions on Applied Superconductivity•Mar 1, 2004•7 citations
Research during the initial year of this grant concentrated on fabrication and modelling of long Josephson junction structures. A particular effort was made to understand the basic operating mechanisms of a type of superconducting transistor, the vortex flow transistor, and to optimize its circuit performance. Fabrication techniques were developed using niobium and lead superconductors. These techniques allow rapid production of a variety of device shapes. An optimization of geometric configuration was made to produce the most desirable device characteristics. Amplification was observed at low frequencies and initial coupling circuit designs were made to enable high frequency measurement. Development of potentially more versatile fabrication techniques using niobium and niobium nitride was also begun, as was a simulation study aimed primarily at understanding spatially nonuniform long junction structures.
Research during the initial year of this grant concentrated on fabrication and modeling of long Josephson-junction structures. A particular effort was made to understand the basic operating mechanisms of a type of superconducting transistor, the vortex-flow transistor, and to optimize its circuit performance. Fabrication techniques were developed using niobium and lead superconductors. These techniques allow rapid production of a variety of device shapes. An optimization of geometric configuration was made to produce the most desirable device characteristics. Amplification was observed at low frequencies and initial-coupling circuit designs were made to enable high-frequency measurement. Development of potentially more versatile fabrication techniques using niobium and niobium nitride was also begun, as was a simulation study aimed primarily at understanding spatially nonuniform long junction structures.
The operational characteristics and physics of three superconducting thin-film-based transistor structures are compared. The devices are based on the motion of quantized vortices, either Josephson fluxons in a long tunnel junction of Abrikosov fluxons in a superconducting film. The transistor amplification mechanism is accomplished by controlling magnetic field at the boundaries of the structures. An overview of the present understanding of device mechanisms and measured characteristics, including voltampere relations and small and large signal circuit parameters is provided. Demonstrated applications and anticipated limitations are discussed.
A niobium-based superconducting amplifier consisting of a series array of 60 vortex flow transistors (VFT) was fabricated and tested. Each VFT consisted of a long Josephson junction biased in the flux flow state, magnetically coupled to a current based on a distributed amplifier configuration. The transresistance of the amplifier in a 50- Omega environment for frequencies up to 1 MHz was measured. The transresistance of the array was less than the expected value calculated from measurements obtained on a single VFT. The difference was probably due to nonuniformities in the fabrication process, which caused some of the junctions not to be biased in the flux flow state or receive the same amount of magnetic field from the control line. However, measurements of the array showed that a large number of VFTs could be connected in series and simultaneously be biased into the flux flow state.>
Over the last few years, considerable progress have been made in the development of techniques for fabricating high-quality superconducting circuits, and this success, together with major advances in the theoretical understanding of quantum detection and mixing at millimeter and submillimeter wavelengths, has made the development of CAD techniques for superconducting nonlinear circuits an important new enterprise. For example, arrays of quasioptical mixers are now being manufactured, where the antennas, matching networks, filters and superconducting tunnel junctions are all fabricated by depositing niobium and a variety of oxides on a single quartz substrate. There are no adjustable tuning elements on these integrated circuits, and therefore, one must be able to predict their electrical behavior precisely. This requirement, together with a general interest in the generic behavior of devices such as direct detectors and harmonic mixers, has lead us to develop a range of CAD tools for simulating the large-signal, small-signal, and noise behavior of superconducting tunnel junction circuits.
Josephson fluxonic bipolar junction transistor (JFBJT) has been proposed as a superconducting amplifying element. The results on a simple realization of this device are provided to demonstrate its working principle. These experiments demonstrate that JFBJT provides current gain without suffering from the problems associated with the regular long junction vortex flow transistors. In particular, no resonance structure is present in its I-V characteristics and its impedance can be matched to any load without degradation of its characteristics. Suggestion is made on how a practical device can be fabricated.