Large-area Josephson Junctions Research Articles

Rotationally symmetric sG oscillator with tunable frequency

The equations governing the dynamics of the kink-like pulson in the pertubed rotationally symmetric sine-Gordon equation with inhomogeneities are derived and integrated. The dependence of the pulson oscillation frequency on amplitude and inhomgeneity parameters is analyzed. The obtained results are applied to the description of ring fluxons in an inhomogeneous large-area Josephson junction.

Hysteresis in rf-driven large-area Josephson junctions.

We have studied the effect of an applied rf signal on the radiation emitted from a large-area Josephson junction by means of a model based on the sine-Gordon equation. The rms value of the voltage of the emitted signal has been calculated and a hysteresis loop found. An analysis shows that the hysteresis is due to the nonlinearity in the system, i.e., the dynamics of the lower branch can be described by a solution to the linearized system while the upper branch is described by a breather mode. These solutions are frequency locked to the driving signal. Various characteristics of the loop such as threshold value and level of the branch are predicted analytically.

Period doubling and chaos in large area Josephson junctions induced by rf signals

The influence of an applied rf signal on the emitted radiation from a large area Josephson junction is examined. A model of the system is presented in the framework of the one-dimensional sine-Gordon equation. The model linearizes for small and large values of the amplitude of the applied signal. In the intermediate regime lower and upper threshold values (for the amplitude) for transition to chaos are found. Feigenbaum period doubling (period-doubling bifurcation cascade) appears when chaos is approached for increasing amplitude. The findings are supported experimentally.

Dynamics of rotationally symmetric solitons in near-SG field model with applications to large-area Josephson junctions and ferromagnets

The ( n + l)-dimensional rotationally symmetric sine-Gordon solitons of large radius under potential perturbations are considered by means of perturbation theory. All kinds of dynamical regimes which correspond to various initial conditions and soliton energies are studied in detail. In particular, the conditions of the so-called return effect as well as the main features of the latter are derived and analyzed. As an application ring waves in a large-area Josephson junction and also in a weak two-sublattice ferromagnet are considered.

All-refractory Josephson logic circuits

A process for the fabrication of Josephson integrated circuits is described which uses only refractory materials. The Josephson devices are Nb-Si-Nb tunnel junctions which are formed in the initial phase of the process. After depositing a Nb-Si-Nb `trilayer' over the entire substrate, the individual devices are isolated by the selective niobium anodization process (SNAP). Other materials used are molybdenum for the normal resistors and bias-sputtered SiO/SUB 2/ for additional insulator layers. The process uses only five photolithographic steps to produce circuits of the direct-coupled isolation type. This simplicity is achieved by using some layers for multiple purposes and by fabricating components with different functional purposes in a single step. For example, the lower electrode of the Josephson devices also functions as the ground plane and the contacts to the ground plane are actually large-area Josephson junctions formed simultaneously with the active devices. Low capacitance junctions (~0.025 pF//spl mu/m/SUP 2/) are produced with good uniformity.

On vortex configurations in two-dimensional sine-Gordon systems with applications to phase transitions of the Kosterlitz-Thouless type and to Josephson junctions

Vortex solutions of the two-dimensional sine-Gordon equation are found by a new method. Their properties and connections to the Kosterlitz-Thouless type of phase transitions are investigated. An interpretation of the single vortex configuration in large area Josephson junctions is given.

Numerical study of 2+1 dimensional sine-Gordon solitons

Numerical studies of the dynamics of sine-Gordon solitons in two spatial dimensions are presented. New results like 4π-break up, “soliton-on-soliton effect”, and scattering at a circular inhomogeneity for line solitons on large-area Josephson junctions are reported. For elliptic ring solitons we find oscillating eccentricity between the major axes in the pulson phase. Colliding non-concentric ring solitons are shown to form larger oval ring solitons or to repel each other. This new effect is qualitatively explained in terms of the 4π-break up for line solitons.

Regulated AC power for Josephson interferometer latching logic circuits

We have developed an ac power distribution scheme which provides switched regulated power of alternating polarity (no rectification necessary) for Josephson interferometer latching logic circuits. Distribution from the power source relies on a tree system of thin film transformers having single primaries and multiple secondaries to maintain low current levels and small phase skews throughout the system. Power signals are clipped on chip to the desired voltage supply level by series connected large area Josephson junctions (regulator junctions) and distributed across the chip by voltage busses. Supply resistors between the regulated voltage bus and the logic circuits define the operating current level. A detailed design is presented for a specific interferometer latching circuit family. We report experimental data on the two major components of the power system design - the regulators and the transformers. A series string of four regulator junctions provides a trapezoidal waveform of 11 mV amplitude at > 66% logic duty cycle (that portion of the machine cycle in which logic operations can be performed). Thin film transformers have their designed inductance (350 pH) and coupling coefficient ( > .90).