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IEEE Transactions on Magnetics

Coupled tunnel junction experiments at the gap frequency

R. P. Robertazzi, B. Hunt, Robert A. Buhrman

Published Mar 1, 1987• 9 citations
Publisher

Abstract

By using a capacitively coupled set of Josephson tunnel junctions we have performed experiments on the feasibility of using a Josephson junction as a local oscillator at frequencies up to and in excess of the gap sum of the junction. One junction was biased to serve as a local oscillator, whose AC power output could be monitored by observing the constant voltage Josephson steps on the current-voltage characteristic of the second junction. Using this coupling scheme we have coupled large oscillator signal levels (∼ 12.5nW) into the detector junction. For frequencies below the gap sum frequency our measurements are consistent with the Werthamer theory. For bias voltages above the gap sum we observe an anomalously fast roll off of the detected power from the oscillator junction.

DOI
10.1109/tmag.1987.1065060

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By using a capacitively coupled set of Josephson tunnel junctions we have performed experiments on the feasibility of using a Josephson junction as a local oscillator at frequencies up to and in excess of the gap sum of the junction. One junction was biased to serve as a local oscillator, whose AC power output could be monitored by observing the constant voltage Josephson steps on the current-voltage characteristic of the second junction. Using this coupling scheme we have coupled large oscillator signal levels (∼ 12.5nW) into the detector junction. For frequencies below the gap sum frequency our measurements are consistent with the Werthamer theory. For bias voltages above the gap sum we observe an anomalously fast roll off of the detected power from the oscillator junction.
Recent coupled Josephson junction experiments in our laboratory have demonstrated that high critical current density tunnel junctions can serve as effective local oscillators at frequencies up to and in excess of the gap sum frequency of the junction, i.e. well above 1 Terahertz for a niobium or niobium compound tunnel junction. While the details of the behavior of such a THz. oscillator were found not to be in accord with the predictions of the accepted theory of the A.C. Josephson effect in the gap region significant radiation could be capacitively coupled from the oscillator junction to an adjacent junction, sufficient for SIS mixer experiments at Terahertz frequencies. Research efforts are now under way to further extend and ex-pand these studies. A high critical current density all NbN tunnel junction system is now under development for Terahertz applications and a new set of coupled Josephson oscillator - SIS detector experiments is being initiated using NbN tunnel junc-tions. In this paper we will review the original coupled junction high frequency experiments and report on the recent progress of the current NbN tunnel junction experiments.
The rf-power dependence of the ac-Josephson steps is measured at 720 GHz, using small area Nb tunnel junctions. This frequency is well above the gap frequency of Nb. The junction is placed in a waveguide, and connected to a superconducting stripline, which effectively tunes out the junction capacitance and facilitates the coupling of the radiation to the junction. We observe three Josephson steps, and the first step crosses the zero current axis over a considerable range of rf-power. This indicates the possible application of THz Josephson steps in voltage standards. The data are compared to the theory and we find clear evidence for the predicted intrinsic roll-off of the Josephson current amplitude above the gap frequency.
Rugged, high current density NbN1−xCx/MgO/NḃN1−xCx tunnel junctions have been fabricated and tested as voltage tunable Josephson junction terahertz oscillators. The emitted radiation from these junctions is detected on chip by a second junction which is capacitively coupled to the first. For oscillator junctions with a critical current density of Jc∼3.5×104 A/cm2 we find that the junction oscillates with a voltage amplitude of ∼1.5 mV. The detected rf voltage level remains essentially constant from 300 GHz to above 1 THz. The oscillator junction produces 0.5 μW of terahertz radiation of which, due to impedance mismatch, 0.01 μW is coupled into the detector junction.
A small-area Josephson tunnel junction is pumped by the radiation emitted by a long Josephson junction dc biased on a zero-field singularity. Josephson constant voltage steps are observed in the current-voltage characteristic of the small junction at voltages of ±150 μV corresponding to the fundamental frequency of emission of the oscillator. The radiation also gives rise to quasiparticle steps around the gap-sum voltage of the small junction. From the measured current amplitude of the steps we estimate that the maximum power coupled to the small junction is about 90 nW.
The authors report on results of phase-locking experiments in a system composed of two long Josephson junctions oscillating at millimeter-wave frequencies. The effects of the coherent behavior of the two junctions are visible on the current-voltage characteristic of the long junctions and in the power enhancement effects observed on the current-voltage characteristic of a detector junction. It is shown that is possible to couple two long junctions in such a way that the enhanced radiation can be transmitted to other devices. Results from another mode of operation of the long Josephson junction as an oscillator are also presented. This mode is based on magnetic-field-induced singularities in long junctions (Fiske steps) generated by the interaction of the cavity modes of the junction seen as an open-ended transmission line and the Josephson oscillations. This mode has given very promising results above 200 GHz, where a power of 20 nW was coupled in a 30- Omega normal state resistance tunnel junction.>
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.