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IEEE Transactions on Applied Superconductivity

Discrete underdamped vortex flow devices

Amy Duwel, Herre S. J. van der Zant, Terry P. Orlando

Published Jun 1, 1995• 6 citations
Publisher

Abstract

Vortex flow devices consisting of discrete arrays of underdamped Josephson junctions have been fabricated and measured. These devices and their long, continuous junction counterparts are based on the ability to magnetically control the Eck voltage in an underdamped system. We find that both transistor and oscillator parameters are improved by placing the arrays above a superconducting ground plane and by connecting two arrays in series. We developed models for the device behavior which numerically account for the measured parameters. Our models also suggest that junctions with smaller capacitances will further improve the device parameters.>

DOI
10.1109/77.403311

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Vortex flow devices consisting of discrete arrays of underdamped Josephson junctions have been fabricated and measured. These devices and their long, continuous junction counterparts are based on the ability to magnetically control the Eck voltage in an underdamped system. We find that both transistor and oscillator parameters are improved by placing the arrays above a superconducting ground plane and by connecting two arrays in series. We developed models for the device behavior which numerically account for the measured parameters. Our models also suggest that junctions with smaller capacitances will further improve the device parameters.>
We have measured vortex motion in a long one-dimensional array of small underdamped niobium tunnel junctions which are coupled in parallel by superconducting wires. In the current-voltage characteristic, a resonant step occurs similar to the Eck peak in long continuous junctions. Due to the discrete nature of our samples, the voltage of this resonant step follows a sinusoidal dependence on the magnetic field rather than the linear dependence found in long continuous junctions. With a control current applied parallel to the array of junctions, the device can be operated as a vortex-flow transistor or as a tunable vortex-flow oscillator. Compared to long continuous junctions, the performance of discrete transistors is better with regard to the threshold current and transresistance, but worse with regard to the output resistance. Discrete oscillators have the advantage over their continuous counterparts in that the output impedance is of the order of 10 Ω. Numerical simulations indicate power levels around 0.5 μW and frequencies above 500 GHz.
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.>
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.
We study experimentally and numerically the zero-voltage supercurrent vs. magnetic field of non-uniform arrays of Josephson junctions parallel-connected by a superconducting stripline. The measured curves are complex, unique and in excellent agreement with numerical simulations using a specially developed model. Using this, we can optimize the arrays to have any desired interference pattern. Such new devices could find applications in magnetometry, quasiparticle mixers and detectors, flux-flow oscillators and superconducting electronics.
We fabricated Josephson vortex flow transistors (JVFTs) with a parallel array of Josephson junctions that were prepared using c-axis-oriented 100-nm-thick YBa 2 Cu 3 O 7-δ (YBCO) thin films grown on 24° bicrystal MgO (100) substrates. We observed clear modulations of the critical current and the flow voltage with DC current input to the control line that was inductively coupled to the array of junctions. From the results, we estimated the parameters of the device, e.g., the mutual inductance and the self-inductance, and calculated the operation frequency at which the device potentially exhibited these parameters. Moreover, the current gain and the transresistance were evaluated and found to be 0.5 and 0.15 Ω, respectively. In addition, we observed the high-frequency responses of the JVFT to the input AC current of the sine wave or the square pulse wave. A clear oscillation of the output voltage could be observed with a 1 MHz sine wave and 250 kHz square pulse wave. We also discussed the feasibility of higher frequency operation by using it as an input interface for a single flux quantum (SFQ) logic circuit.
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.