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Electronics and Communications in Japan (Part I Communications)

Coherent characteristics of superconducting closed loop composed of Josephson junctions

Tadayuki Kobayashi, Nobuyoshi Kondo, Katsuyoshi Hamasaki, Tsutomu Yamashita

Published Jan 1, 1983• 1 citation
Publisher

Abstract

Abstract When the impedance of a Josephson junction is too low, the elements are connected in series. We connected two arrays with N series elements of Josephson junctions each (denoted 2N‐JJ loop) and tried to find conditions for it to operate coherently by investigating voltage‐current curves. the computer simulation showed that coherent operation is easier with this circuit than with a series connection of the elements when the elements have different junction parameters. Pb‐alloy tunnel‐type junctions were produced which have N = 2 and 3, and we found that voltage steps generated by application of rf current with frequency F have a voltige interval corresponding to NF. This agrees with the simulation results and indicates coherent operation.

DOI
10.1002/ecja.4400660914

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Abstract When the impedance of a Josephson junction is too low, the elements are connected in series. We connected two arrays with N series elements of Josephson junctions each (denoted 2N‐JJ loop) and tried to find conditions for it to operate coherently by investigating voltage‐current curves. the computer simulation showed that coherent operation is easier with this circuit than with a series connection of the elements when the elements have different junction parameters. Pb‐alloy tunnel‐type junctions were produced which have N = 2 and 3, and we found that voltage steps generated by application of rf current with frequency F have a voltige interval corresponding to NF. This agrees with the simulation results and indicates coherent operation.
The use of unshunted junctions to close the superconducting loop of two shunted junctions in series is shown to improve the voltage locking range compared to the more common use of passive elements such as resistors or capacitors. Stacking pairs of junctions in a ladder-like form with one junction common for each two steps makes it possible to phase lock more than two junctions. Simulations show that four junctions with 5% matching can be locked from 0 to 55μV. The device can also be used as a voltage amplifier by driving one of the junctions. Simulations show that an array of four series junctions operates as an amplifier between 12μV to 33μV input. Input impendance was 6 ohms and output impedance was 12 ohms.
Frequency locking effects in two interacting Josephson junctions are investigated. The junctions are described in the usual resistively-shunted-junction (RSJ) model and coupling between junctions is provided by a resonant circuit. The system can be used to simulate interactions between superconducting weak links in a resonant cavity or via a general impedance shunt. Various types of v-i curves in the voltage locking region are obtained in this calculation by using a first harmonic approximation in the high-frequency regime. Stability of order parameter phase oscillations for two identical junctions in the voltage-locked region is examined. We also show that dissimilar junctions can be synchronized in a resonant cavity using a common series bias current.
Both resonant and microwave-induced steps in the V-I characteristics of four-Josephson-junction loop with a small junction-parameter spread have been investigated. An rf current was superimposed onto a dc current and the radio frequency was tuned to the resonance frequency of the loop. In the high-Q region, the rf-power dependence of the step amplitude was similar to that of a single junction. However, we obtained V-I characteristics which indicated that the behavior of the junction in the loop was coherent in the low-Q region.
We have studied via computer simulations the dynamical properties of a circuit consisting of two arrays of Josephson junctions closed into a superconducting loop. Due to an internal circuit resonance the phase locking of Josephson junctions is enhanced, which makes possible the application of such a circuit as a source of coherent microwave radiation. The optimal circuit parameters and the influence of thermal noise are discussed. It is shown that the critical spread in junction parameters is about twice that of linear Josephson junction arrays. The possibility of tuning such a device is demonstrated.
Two-dimensional arrays of Josephson junctions have been shown to operate in a state in which the microwave oscillations of the junctions are phase-coherent. However, the performance of an array is strongly dependent on both the details of the load-coupling circuit and the characteristic parameters of the junctions. We have measured two-dimensional arrays of resistively shunted Josephson tunnel junctions which are capacitively coupled to a nearby detector junction. Changing the critical current of the junctions in our arrays, while keeping the junction parasitics and array coupling roughly the same, results in an unexpected change in device operation. We have also investigated variations of the coupling circuit. Numerical simulations of the detection circuit enable us to estimate the power output from the array and to optimize the parameters of the system.