J.E. Nordman

Evidence for channel conduction in low misorientation angle [001] tilt YBa2Cu3O7−x bicrystal films

Extended voltage–current (V–I) characteristics of [001] tilt YBa2Cu3O7−x bicrystal films having misorientation angles, θ, of 3° to 20° showed a substantial and progressive change with θ, when measured in large fields. Transmission electron microscopy of the 10° bicrystal showed the grain boundary to contain edge dislocations separated by channels of relatively undisturbed lattice. A large drop in the intergrain irreversibility field, H*, occurred between 10° and 15°, and the characteristics became qualitatively different by 20°. Both the microscopy and the electromagnetic data support a heterogeneous description of the grain boundary, consisting of strongly coupled channels that close at a misorientation angle of around 15°.

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.

JOSEPHSON VORTEX FLOW IN SUPERCONDUCTING SINGLE-CRYSTAL BI2SR2CACU2OX

Using various size rectangular mesas formed by photolithographically patterning and etching on single‐crystal Bi2Sr2Ca1Cu2Ox superconductors, we have obtained c‐axis volt‐ampere characteristics as a function of magnetic field applied parallel to the a‐b planes. Enhanced sensitivity with field perpendicular to the long side was observed even in mesas with dimensions smaller than the magnetic penetration depth λc. This can be explained in terms of viscous flow of Josephson vortices. The measurements are in good quantitative agreement with theoretical models for Josephson vortex motion in layered superconductors. Vortex flow coexists with the multiple hysteretic structure previously presented as evidence that this material behaves as a stack of underdamped intrinsic Josephson junctions.

Superconductor‐normal‐superconductor behavior of Josephson junctions scribed in Y1Ba2Cu3O7−δ by a high‐brightness electron source

We report a substantial improvement in the behavior of Josephson junctions scribed in Y1Ba2Cu3O7−δ films using a high‐brightness field‐emission electron gun source instead of a lower‐brightness thermionic source. These junctions exhibit resistively shunted junction behavior over the entire temperature range from the coupling temperature to at least 4 K, a temperature window which can be larger than 55 K. Superconductor‐normal‐superconductor character is indicated by the exponential dependence of the critical current on temperature for all temperatures below 90 K. The data demonstrate that electron irradiation under these conditions produces a modified region which is completely normal above 4 K and is narrower in width than previously obtained.

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. >

Superconducting Bi-Ca-Sr-Cu oxide thin films by spray pyrolysis of metal acetates

Superconducting Bi‐Ca‐Sr‐Cu oxide thin films have been prepared on ZrO2‐coated silicon (111) wafers by spray pyrolysis of metal acetate precursors followed by rapid annealing to 850 °C in air. Resistivity measurements indicate a broad superconducting transition with Tc onset near 90 K and zero resistivity below 60 K. The films are highly oriented with the c axis normal to the substrate surface and can be indexed to a tetragonal structure with lattice parameters of a=3.832(1) A and c=30.78(5) A. Both x‐ray photoelectron spectroscopy and x‐ray diffraction measurements indicate the loss of Ca to the ZrO2 buffer layer.

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.

Au diffusion in amorphous and polycrystalline Ni0.55 Nb0.45

Diffusion of Au in amorphous and polycrystalline Ni0.55 Nb0.45 has been studied by Rutherford backscattering spectroscopy of heat‐treated Au/NiNb couples. The diffusion process is characterized by interdiffusion of Nb and Au with NiNb showing a Au miscibility of ∼20 at. %. The Au diffusion coefficient in polycrystalline NiNb at 400 °C was found to be 1.6×10−15 cm2/s. The diffusion coefficient for Au in amorphous NiNb was found to be substantially less, ∼10−21 cm2/s at 400 °C. These results support the concept of using amorphous films as metallizations or diffusion barriers in high‐temperature solid‐state device applications.

Niobium thin-film Josephson junctions using a semiconductor barrier

Thin‐film superconductive tunnel junctions were fabricated using rf‐sputtered germanium or indium antimonide films as barrier layers on photoetched rf‐sputtered niobium patterns with evaporated lead counter electrodes. Tunneling characteristics similar in quality and shape to those of niobium‐oxide‐lead junctions were obtained, large Josephson currents occurring in the higher‐conductance devices. Stability of the characteristics with time was also found to be comparable to oxide junctions. Reducing the thickness of the semiconductor film appears to produce an increase in the effective tunneling barrier energy, and a model is proposed to explain this. This behavior is of importance when considering the relative merits of semiconductor barriers compared to oxide barriers for Josephson junctions.