A. Davidson

Chaos and noise rise in Josephson junctions

Digital computer simultations have been used to map parameters of the transition to chaos in an rf current driven Josephson junction. Our results are qualitatively like those reported by others using analog techniques, but differ quantitatively. Our calculations show that the parameters for the onset of chaos are the same as those required for high parametric gain. This leads to the conclusion that the ’’noise rise’’ in Josephson junction parametric amplifiers is due to chaos.

A new superconducting-base transistor

The continuing search for a good cryogenic transistor has led to a new proposal, the superconducting-base semiconductor-isolated transistor (SUB-SIT). This three-terminal device is expected to have characteristics very similar to those of bipolar transistors, but at millivolt operating levels. We present discussions of the concepts involved in the SUBSIT, proposed fabrication techniques, and theoretical results for its DC and high frequency characteristics.

Effect of surface losses on soliton propagation in Josephson junctions

We have explored numerically the effects on soliton propagation of a third order damping term in the modified sine‐Gordon equation. In Josephson tunnel junctions such a term corresponds physically to quasiparticle losses within the metal electrodes of the junction. We find that this loss term plays the dominant role in determining the shape and stability of the soliton at high velocity.

In situ resistance of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub x/ films during anneal

Measurements of electrical resistance of thin films of the high‐temperature superconductor Y1Ba2Cu3Ox have been carried out between 100 and 800 °C, and in the range between roughly 0.1 and 1 atm of ambient oxygen. Results are reported for both quick and slow heating, and also for sudden changes in oxygen partial pressure. At constant temperature we observe that the response to the oxygen partial pressure is both quick and reversible. We show evidence that quick heating is preferable, and we also observe features in the resistance similar to those reported in bulk samples and interpreted as an order–disorder transition of oxygen in the one‐dimensional Cu‐O chains.

In situ resistance of Y1Ba2Cu3Ox films during anneal

Measurements of electrical resistance of thin films of the high‐temperature superconductor Y1Ba2Cu3Ox have been carried out between 100 and 800 °C, and in the range between roughly 0.1 and 1 atm of ambient oxygen. Results are reported for both quick and slow heating, and also for sudden changes in oxygen partial pressure. At constant temperature we observe that the response to the oxygen partial pressure is both quick and reversible. We show evidence that quick heating is preferable, and we also observe features in the resistance similar to those reported in bulk samples and interpreted as an order–disorder transition of oxygen in the one‐dimensional Cu‐O chains.

IV curves of long annular Josephson junctions

IV curves of long annular Josephson junctions (circumference much larger than the Josephson penetration depth) have been investigated numerically. It is concluded that an experiment on such a system is preferable to the usual linear geometries and will allow a study both of the undisturbed motion of a fluxon, and the fluxon‐antifluxon collision process. The simulations show that the fluxon‐antifluxon collisions generate long wavelength plasma oscillations which may give rise to fine structure in the IV curves.

Role of quasiparticle scattering in Gray’s superconducting transistor

We report the results of detailed numerical simulations of Gray’s transistor mechanism [Appl. Phys. Lett. 32, 392 (1978)]. These calculations, carried out using two different approaches, show that the energy scattering of quasiparticles is very important in achieving gain such as Gray reported. When this scattering is included, we find that gain is possible, not only in aluminum, but also in niobium and tantalum.

Noise temperature contours for high‐sensitivity SQUID’s

A simple expression is developed for the noise temperature T N of SQUIDamplifiers. The expression is in terms of equivalent current and voltage noise generators in the input circuit, and is shown to contain the spectral energy resolution, which is the currently accepted figure of merit for SQUID’s. It is also shown that a SQUID optimized for the best energy resolution is not necessarily optimized for the lowest noise temperature.

Experiments on the interaction between long Josephson junctions and a coplanar strip resonator

Experiments are reported on a new geometry designed to couple long Josephson junction fluxon oscillators to a resonant cavity. The junctions were made with a niobium‐aluminum oxide‐niobium trilayer process with a critical‐current density of around 1000 A/cm2. Various numbers of such junctions were incorporated directly as part of a coplanar strip half‐wave resonator, with fundamental mode of about 34 GHz. Both the current density and oscillation frequency are higher than in previous experiments. Evidence for phase locking of multiple junctions is presented.

Analytical and numerical results for a long Josephson junction with surface losses

In this paper we show several analytical approaches to study the effect of dissipation on fluxon motion; our attention is mainly devoted to the surface impedance term which is the main quantity responsible for qualitative dynamical changes in the junction.