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Journal of Applied Physics

Vortex structure and cavity modes in stacked double Nb/AlOx/Nb Josephson junctions

Shenian N. Song, Stevan R. Maglic, C. D. Thomas, Melville P. Ulmer, John B. Ketterson

Published Sep 1, 1996• 1 citation
Publisher

Abstract

We have studied the static I–V characteristics and cavity modes in stacked double Nb/AlOx/Nb Josephson junctions. In junction stacks consisting of two junctions with identical critical currents Ic, the Ic vs H characteristics have been observed to deviate from the usual Fraunhofer pattern in the small junction limit; the data are consistent with a model involving a structural phase transformation to a triangular vortex lattice with increasing H. In a finite voltage state interlayer coupling leads to splitting of the Swihart mode, which manifests itself as Fiske steps with different voltage spacings. The results provide clear evidence that the two junctions in the stack do phase lock.

DOI
10.1063/1.363151

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We have studied the static I–V characteristics and cavity modes in stacked double Nb/AlOx/Nb Josephson junctions. In junction stacks consisting of two junctions with identical critical currents Ic, the Ic vs H characteristics have been observed to deviate from the usual Fraunhofer pattern in the small junction limit; the data are consistent with a model involving a structural phase transformation to a triangular vortex lattice with increasing H. In a finite voltage state interlayer coupling leads to splitting of the Swihart mode, which manifests itself as Fiske steps with different voltage spacings. The results provide clear evidence that the two junctions in the stack do phase lock.
A system of two vertically stacked Nb/Al-AlOx/Nb long Josephson junctions is studied experimentally. The magnetic coupling between the junctions is provided by their common electrode of the thickness d∼λL. The current-voltage characteristics of the stack displays resonant steps which are understood to be the phase-locked flux-flow resonances of the two junctions. These modes can be tuned by the magnetic field in a broad voltage range which corresponds to the Josephson frequencies between 150 and 600 GHz. Our observations suggest a possibility of application of the stacked long Josephson junctions as local oscillators for millimeter and submillimeter wave circuits.
We present characteristics of overdamped Josephson junctions consisting of Nb/AlOx/Al/ AlOx/Nb structures. The junctions were fabricated using a well-developed Nb/AlOx/Nb-junction technology and showed well-defined Josephson characteristics at 4.2 K. The characteristic voltage Vc [the product of the critical current Ic and the effective normal resistance Rn(eff)] of junctions, which determines high-frequency performance of the junction, was in the range of 0.1–0.5 mV, and the critical current density Jc in the range of 102–104 A/cm2. Maximum-to-minimum variations in Ic over a wafer were ±4% for junctions with Vc=0.15 mV and ±13% for junctions with Vc=0.5 mV.
Vertical stacks of two Nb/Al-AlO/sub x//Nb long Josephson junctions with nearly identical parameters have been studied experimentally. The magnetic coupling between the junctions was provided by their common electrode of thickness d/spl sime//spl lambda//sub L/. The I-V characteristics displayed flux-flow resonances of the two junctions. Two different mutually phase-locked regimes were found: the lower velocity anti-phase mode, and the high velocity in-phase mode. These modes, while staying locked, could be tuned by the magnetic field in a broad voltage range which corresponds to the Josephson frequencies of 150-600 GHz for the lower mode and 280-400 GHz for the upper mode. Our observations suggest applications of the stacked long Josephson junctions as local oscillators for millimeter and sub-millimeter wave circuits.>
We demonstrate a double barrier long Josephson tunnel junction device of overlap geometry with an external electric contact to the middle electrode. The junctions were fabricated with a standard Nb/Al-AlO/sub x//Nb process. To exhibit fluxon coupling effects in stacked junctions, the intermediate Nb layer thickness between two barriers has to be of the order of the London penetration depth, i.e. 90 nm for sputtered Nb films. By using a planarization process, we were able to avoid steep steps on the wafer and so to reduce the intermediate layer thickness to 90 nm. Double-junction stacks with areas from 10/spl times/10 /spl mu/m/sup 2/ to 20/spl times/600 /spl mu/m/sup 2/ were fabricated. In the best stacks, the difference in the critical currents of two junctions was less than 3%. Clear voltage locking effects were observed in current-voltage (I-V) characteristics.>
A tuning technique for mutual phase locking of two vertically stacked long Josephson junctions is suggested and successfully tested. The technique is based on passing a current through the middle electrode between two tunnel barriers. Mutually synchronized oscillation modes in the Nb/Al–AlOx/Nb stack are found to be tunable in the frequency range from 180 to 420 GHz. Presented results extend the possibility of using stacked long Josephson junctions as coherently operating oscillators for millimeter and submillimeter wave bands.
We present Nb/AlOx/Nb–Al/AlOx/Nb stacked Josephson junctions which we propose as three-terminal elements to control the quantum behavior of Josephson complex systems. A bias current can be independently injected in the “top” junction, providing a fine control of the critical current of the “bottom” junction. The reported characterization of the device refers to data in the classical limit including measurements of the switching dynamics between metastable states of the system at different temperatures and bias conditions. At low temperature, the effective dissipation of the bottom junction remains substantially unchanged when different injection currents flow in the top junction. These results are interesting to project new configurations of macroscopic quantum experiments.