J. E. Lukens

Mutual phase-locking in Josephson junction arrays

Abstract We discuss mutual phase locking of Josephson oscillations in an array of Josephson junctions. The discussion is focussed on phase locking due to electromagnetic coupling. The theoretical analysis is based on a secular-term free perturbative solution of the resistively shunted junction model. Detailed experimental results on phase locking of two coupled micro-bridges are presented. The perturbation analysis provides a complete qualitative description of the experimental results, including the effects of fluctuations on phase locking and the oscillation linewidth. Measurements have also been made on linear arrays of up to 100 junctions. The observed increase in the radiated power (as N2) and the decrease in the radiation linewidth (as N−1) are all in agreement with the theory. Optimized arrays suitable for use as either microwave and millimeter wave radiation sources or as mixers are discussed. The properties of the arrays are found to solve various problems associated with the use of single junctions as mixers, such as excess noise temperature, limited dynamic range and low sensitivity for narrow band signals.

Rapid single flux quantum T-flip flop operating up to 770 GHz

Rapid Single Flux Quantum (RSFQ) T-flip flops (TFFs) operating up to 770 GHz have been demonstrated at 4.2 K. The devices, consisting of either resistively shunted or unshunted Josephson junctions, are fabricated using a planarized Nb/AlO/sub x//Nb trilayer process. Electron beam lithography is used to pattern all levels with a minimum junction area less than 0.1 /spl mu/m/sup 2/. Critical current densities of 0.5 mA//spl mu/m/sup 2/ and 2.5 mA//spl mu/m/sup 2/ are used for the shunted (tested at 1.8 K) and unshunted devices (tested at 4.2 K) respectively. The input and output frequencies of the TFFs are obtained from the input and output voltages by the Josephson relation. The output voltage is exactly half of the input voltage when the divide-by-two operation is correct.

Superconductor digital frequency divider operating up to 750 GHz

A superconductor frequency divider based on rapid single flux quantum (RSFQ) logic has been demonstrated to operate from dc to 750 GHz with an error rate less than the measurement limit (25 MHz). This high operating frequency is made possible by the use of small (0.25 μm2), high critical current density Josephson junctions along with an optimized design having parameter margins of ±30%. Simulations based on the Werthamer model are in reasonable agreement with the data and give a SFQ pulse width of 1 ps and a maximum power dissipation of 1.5 μW.

Self-shunted Nb/AlO/sub x//Nb Josephson junctions

We describe the fabrication and properties of high critical current density (J/sub c/) Nb/AlO/sub x//Nb Josephson junctions with deep-submicron dimensions. The junctions are fabricated using a planarized process in which all levels are patterned using a combination of optical and electron-beam lithography. The base and counter electrodes are defined by reactive ion etching using quartz etch masks to give a minimum feature size of 0.2 microns. For J/sub c/=2.1 mA//spl mu/m/sup 2/ and junction area less than 0.1 /spl mu/m/sup 2/ the devices are self-shunted and exhibit nonhysteretic I-V characteristics. A small hysteresis in the larger junctions is caused by heating in the electrodes.

Demonstration of Josephson effect submillimeter wave sources with increased power

A submillimeter wave source based on a new design using Josephson junction arrays has been developed and tested. The maximum rf power, delivered to a 68Ω load and detected on chip, was 47 μW at 394 GHz. Significant power was detected at a number of frequencies from 300 to 500 GHz where the power was 10 μW. The observed power at the designed operating frequency near 400 GHz is consistent with all 500 junctions in the series biased array delivering current in phase to the loads. This is in agreement with simulations of smaller arrays of the same design. The linewidth, inferred from the measured resistance at the point of maximum power, with T=4.2 K, is less than 1 MHz. The minimum inferred linewidth near 400 GHz, at somewhat lower power, is about 100 kHz.

Submillimeter wave generation using Josephson junction arrays

Arrays of phase‐locked Josephson junctions have been developed and tested which deliver over 1 μW of power, via superconducting microstrip, to loads with resistances ranging from 20 to 60 Ω. The arrays, designed to operate at 350 GHz, typically are continuously tunable from 350 to 450 GHz or higher, the upper limit being set by the onset of loss in the lead alloy microstrip.

Aluminum Oxide Layers as Possible Components for Layered Tunnel Barriers

We have studied transport properties of Nb/Al/AlOx/Nb tunnel junctions with ultrathin aluminum oxide layers formed by (i) thermal oxidation and (ii) plasma oxidation, before and after rapid thermal postannealing of the completed structures at temperatures up to 550 °C. Postannealing at temperatures above 300 °C results in a significant decrease of the tunneling conductance of thermally grown barriers, while plasma-grown barriers start to change only at annealing temperatures above 450 °C. Fitting the experimental I-V curves of the junctions using the results of the microscopic theory of direct tunneling shows that the annealing of thermally grown oxides at temperatures above 300 °C results in a substantial increase of their average tunnel barriers height, from ∼1.8 eV to ∼2.45 eV, versus the practically unchanged height of ∼2.0 eV for plasma-grown layers. This difference, together with high endurance of annealed barriers under electric stress (breakdown field above 10 MV/cm) may enable all-AlOx and SiO2/A...

Universal distribution of transparencies in highly conductive Nb/AlO(x)/Nb junctions.

We report the observation of the universal distribution of transparencies, predicted by Schep and Bauer [Phys. Rev. Lett. 78, 3015 (1997)] for dirty sharp interfaces, in uniform Nb/AlO(x)/Nb junctions with high specific conductance (10(8) ohm(-1) cm(-2)). Experiments used the BCS density of states in superconducting niobium for transparency distribution probing. Experimental results for both the dc I-V curves at magnetic-field-suppressed supercurrent and the Josephson critical current in zero magnetic field coincide remarkably well with calculations based on the multimode theory of multiple Andreev reflections and the Schep-Bauer distribution.

HIGH-SPEED SINGLE-FLUX-QUANTUM CIRCUIT USING PLANARIZED NIOBIUM-TRILAYER JOSEPHSON JUNCTION TECHNOLOGY

A simple test circuit of the rapid single‐flux‐quantum (RSFQ) logic family has been implemented in a planarized all‐refractory technology using only two superconducting layers and small Nb/AlOx/Nb Josephson junctions (nominally 1.5×1.5 μm2) with critical current density jc≊6 kA/cm2. A special layout design of the integrated circuit (including a SFQ pulse generator, Josephson transmission line, and T flip–flop) was used to reduce 3D inductances resulting from absence of the ground plane. For samples with various junction parameters, the maximum frequency of the SFQ pulse train, which could be divided by 2 by the flip–flop, ranged from 200 to 370 GHz.

Flux amplification using stochastic superconducting quantum interference devices

The flux change δ Φ through a bistable superconducting quantum interference device has been measured in the presence of thermally induced switching (with rate Γ) versus δ Φx, the change in the applied flux. For small δ Φx, δ Φ is proportional to δ Φx with a measured flux gain g, depending on the temperature, barrier height, and frequency Ω, with a maximum of about 16. In agreement with theories of periodically driven stochastic bistable systems, g(Ω) is nearly frequency independent up to Γ and is proportional to Ω−1 for Ω≫Γ. For larger amplitude signals, harmonic generation has been measured in the adiabatic limit (Ω≪Γ) and found to be in good agreement with theory. Possible applications of this system for flux measurement are discussed.