J.B. Beyer

A model and equivalent circuit for a superconducting flux flow transistor

A three-terminal high-frequency active device made of a single film of a high T/sub c/ superconductor that is based on the magnetic control of flux flow is presented. The device is composed of parallel weak links with a nearby magnetic control line. A model has been developed that is based on solving the equation of motion of Abrikosov vortices subject to Lorentz viscous and pinning forces, as well as magnetic surface barriers. The model has been used to predict device transit time (computed from flux velocity) and device I-V curves. The predictions are compared to measured parameters with resulting very good agreement.<<ETX>>

S parameter measurements on single superconducting thin‐film three‐terminal devices made of high‐Tc and low‐Tc materials

We have investigated three‐terminal single‐layer thin‐film superconducting devices made of YBaCuO and Nb. The devices incorporate regions of weak superconductivity in multiple parallel links that are influenced by current in a separate control line. These experiments were designed to study the possible application of this device as an rf amplifier. With the device biased, rf power was applied to the control line and the transmission coefficient, S21, was measured. The reverse transmission coefficient, S12, was also measured for comparison. Upon biasing into a flux flow state, the S21 of the device at rf frequencies was found to increase 10–15 dB over the zero bias value and over the reverse feed (S12 ) value. The device behaved linearly up to power inputs of +5 dBm (1‐dB compression point). The bandwidth was limited only by the impedance transformer.

A highly directive, broadband, bidirectional distributed amplifier

Design considerations for developing a highly directive broadband bidirectional distributed amplifier are discussed. Through effective binomial scaling of the individual device transconductances, directivities on the order of -25 to -35 dB have been demonstrated using computer simulation and measured S-parameter data for the NEC 9000 transistor. The circuit is broadband, with the potential for operation over an octave or more in frequency. Finally, it is noted that the circuit is bidirectional in that it may be driven from both ends of the gate line simultaneously with high directivity exhibited at each respective port on the drain line.<<ETX>>

Microwave surface resistance of YBa2Cu3O6.9 superconducting films

The microwave surface resistance of an YBa2Cu3O6.9 superconducting thick film was measured over the range 7.0–16.7 GHz at 77 K. This was done by placing a sample in a TE01n wavemeter cavity and observing the change in selectivity of the cavity. The material’s surface resistance is of the same order of magnitude as that of silver at 77 K from 8 to 12 GHz and improves about another order at 4.2 K. The power‐law behavior of surface resistance with frequency is probably close to quadratic. This is similar to the behavior of low critical temperature superconductors.

Single superconducting thin film devices for applications in high T/sub c/ materials circuits

The authors investigated several different devices based on regions of weak superconductivity and multiple parallel links in thin films. Devices were fabricated with Nb and YBa/sub 2/Cu/sub 3/O/sub 7-x/ films. Hysteretic symmetric and asymmetric I-V (current-voltage) curves have been observed. Flux flow was indicated. Device switching properties and the dependence of the flux-flow signature in the I-V curve on applied magnetic field were explored. Contrary to vortex flow devices based on Josephson junctions, the devices described here do not possess a tunnelling barrier and are made of only a single superconducting layer. Hence they should be applicable to electronic circuits based on high-T/sub c/ superconducting materials without the need for tunnel junctions. >

Distributed amplifier using Josephson vortex flow transistors

A wide‐band traveling wave amplifier using vortex flow transistors is proposed. A vortex flow transistor is a long Josephson junction used as a current controlled voltage source. The dual nature of this device to the field effect transistor is exploited. A circuit model of this device is proposed and a distributed amplifier utilizing 50 vortex flow transistors is predicted to have useful gain to 100 GHz.

Characteristics of superconducting flux-flow transistors

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 Josephson junction to FET high speed line driver made of TlCaBaCuO

A Tl-Ca-Ba-Cu-O superconducting flux flow transistor (SFFT) was used as an active impedance converter between Josephson and FET circuitry. The input of the flux flow device is a control line of low impedance that can be driven by a tunnel junction. The output is the signal across the SFFT which is made of a parallel array of weak links. The output impedance is typically greater than 5 Omega , with a maximum voltage swing of over 100 mV into a 50- Omega system. The switching of an all-Nb junction induced a 90-mV voltage swing at the FET input and over 200 mV at the FET output. The line driver can operate anywhere between 4.2 K and 85 K with minor changes in speed (+or-5 ps) and output level (+or-10 mV). The switching time measured was about 100 ps and was fixture limited.

Optimization of circuit parameters for the vortex flow transistor

The Vortex Flow Transistor (VFT) has been proposed as a superconducting amplifying device. Useful circuits at microwave frequencies may be achievable using distributed amplifier concepts. However, realization of these circuits depends on obtaining a circuit model for this device and then characterizing and optimizing the elements that make up this model. In order to study these elements, a group of devices with various geometric differences were fabricated with Nb-Pb technology. Initial results of a study of the low and high frequency properties of the transresistance, output resistance, feedthrough capacitance, and input inductance of the VFT are presented.

A film transmission line resonator to measure the microwave surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7-x/

The authors constructed microstrip transmission line resonators with YBa/sub 2/Cu/sub 3/O/sub 6.9/ ground planes, SiO dielectric, and Nb top conductors. The film resonator is a low-impedance section n lambda /2 long, and the wave is launched from a coaxial line. Loose coupling is provided by the discontinuity between 50- Omega feed lines and the 0.05- Omega resonator section. After de-embedding the transmittance data of the structure, the surface resistance of the ceramic superconductor can be computed. The surface resistance of YBa/sub 2/Cu/sub 3/O/sub 6.9/ at 4.2 K was about a factor of 60 higher than the theoretical value for Nb at 4.2 K over the range 2.9-6 GHz. The power law behavior versus frequency of the surface resistance is roughly quadratic, which is consistent with two-fluid analysis. >