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Physical review. B, Condensed matter

Imaging of the dynamic magnetic structure in a parallel array of shunted Josephson junctions

T. Doderer, V. K. Kaplunenko, J. Mygind, N. F. Pedersen

Published Sep 1, 1994• 5 citations
Publisher

Abstract

A one-dimensional (1D) parallel array of shunted Josephson junctions is one of the basic elements in the family of rapid single-flux quantum logic circuits. It was found recently that current steps always show up in the current-voltage curve of the generator junction when an additional bias current is applied to the edge junction of the array. This effect was found to be due to the self-induced magnetic field produced by the edge current. This nonuniform field divides the array into domains each spanning several unit cells and each containing the same number of flux quanta. We report on experimental results obtained by low-temperature scanning electron microscopy (LTSEM) on the 1D array. The (1--3)-\ensuremath{\mu}m spatial resolution achieved by LTSEM enables us to image these domains in scanned measurements where the junctions in the array are heated sequentially. Computer simulations confirm the mechanism of the obtained images and the number of observed domains corresponds to the step position as predicted numerically.

DOI
10.1103/physrevb.50.7211

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A one-dimensional (1D) parallel array of shunted Josephson junctions is one of the basic elements in the family of rapid single-flux quantum logic circuits. It was found recently that current steps always show up in the current-voltage curve of the generator junction when an additional bias current is applied to the edge junction of the array. This effect was found to be due to the self-induced magnetic field produced by the edge current. This nonuniform field divides the array into domains each spanning several unit cells and each containing the same number of flux quanta. We report on experimental results obtained by low-temperature scanning electron microscopy (LTSEM) on the 1D array. The (1--3)-\ensuremath{\mu}m spatial resolution achieved by LTSEM enables us to image these domains in scanned measurements where the junctions in the array are heated sequentially. Computer simulations confirm the mechanism of the obtained images and the number of observed domains corresponds to the step position as predicted numerically.
A transmission line consisting of a parallel connection of shunted Josephson junctions is one of the base elements in the new Rapid Single Flux Quantum (RSFQ) logic. When an additional bias current is applied to the edge junction of the transmission line it generates and injects series of flux quanta on the line. This kind of pulse generators have important applications for testing RSFQ devices at high frequencies. In such experiments unwanted steps always show up in the I-V curve of the generator junction. This problem has been experimentally and numerically investigated for parallel arrays of identical junctions. Steps with extremely low differential resistance are found to be due to the self-induced magnetic field produced by the edge current. The underlying mechanism is that the non-uniform field divides the array into domains consisting of several (unit) cells each containing the same number of flux quanta. The influence of an applied homogeneous external magnetic field and a uniformly distributed bias current is also considered.
Mutual locking due to the internal coupling in two-dimensional arrays of Josephson junctions was investigated. The appearance of Shapiro steps in the current versus voltage curve of a coupled on-chip detector junction is used to indicate coherent oscillations in the array. A highly coherent state is observed for some range of the array bias current. By scanning the array with a low-power electron beam, mutually locked junctions remain locked while the unlocked junctions generate a beam-induced additional voltage drop at the array. This imaging technique allows the detection of the nonlocked or weakly locked Josephson junctions in a (partially) locked array state.
We have studied the self-field effect on a parallel array of 21 Josephson junctions based on an ion-damaged barrier technology [1]. The device is made of 2 chips: a superconducting chip integrating the Josephson junction array, and another chip with single layer of normal metal (gold) patterned with a grid of parallel stripes. These chips are assembled in a piggy-back configuration, and wire bonded in such a way that the bias current flowing through the array can be drained in different positions with respect to the Josephson array. As expected, we observed that the antipeak of the voltage response to an applied magnetic field was shifted by the proximity of the drained current. In addition, we observed a change in the shape of the response which we attribute to a change of the supercurrent distribution in the array. This geometry will allow proper biasing of large parallel and 2D arrays of Josephson junctions.
Low temperature scanning electron microscopy (LTSEM) offers the possibility for spatially resolved investigations of microwave properties of single Josephson junctions, superconducting striplines, resonators, and complex cryoelectric circuits during their operation. The spatial resolution of about 1 μm is at least two orders of magnitude better than the typical wavelength (≳100 μm in the elements) in the frequency range of interest (50–500 GHz). We demonstrate that LTSEM is a well suited tool for the experimental investigation of superconducting microwave devices and cryoelectronic circuits.
Abstract Designing superconducting electronic devices involves a careful study of all the elements in the circuit, including the superconducting bias leads. In this work, we introduce a theoretical model for two-dimensional (2D) superconducting quantum interference device (SQUID) arrays capable of simulating the voltage-to-magnetic flux response of devices with different bias current configurations. First, we compare uniformly biased and centre biased SQUID arrays by investigating the voltage versus magnetic flux response, maximum transfer function and voltage modulation depth. Then, we calculate the time-averaged fluxoid distributions for one-dimensional (1D) and 2D centre biased arrays. Finally, we study the performance of the two bias current configurations depending on array size, screening parameter, thermal noise strength and kinetic self-inductance fraction. Our calculations reveal: (i) centre biased 1D parallel SQUID arrays present an unusual voltage response caused by the asymmetric fluxoid distribution; (ii) the optimal transfer function of centre biased arrays strongly depends on the number of junctions in parallel; (iii) the performance of centre biased arrays approaches the uniform biased ones when the number of junctions in series exceeds those in parallel; (iv) while the screening parameter and the thermal noise strength clearly affect the device performance, the kinetic to self-inductance fraction seems to play only a secondary role.
One-dimensional parallel arrays of Josephson junctions have the ability to perform as oscillators tunable by magnetic flux. We have designed and tested an array impedance matched to a detector junction load. The amplitude and frequency of the array are controlled by the independent variables of magnetic field and current bias. We report on the array's characteristics and compare them to the results predicted by nonlinear simulations and a linear circuit model. This confirms the accuracy of the circuit model, both for impedance matching and in describing the dynamics of the array, even in a multiple-frequency regime.