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MM wave Josephson radiation in high-Tc bicrystal junction arrays

K. I. Konstantinian, A.D. Mashtakov, G. A. Ovsyannikov, Victor K. Kornev, N.A. Scherbakov, Marian Darula, N. F. Pedersen, J. Mygind

Published Jan 1, 2000• 2 citations
OpenAlex

Abstract

High-Tc Josephson junction arrays with coupling by superconducting short- circuited (5 µm wide and 0.1 µm thick) coupled microstrip lines, provided dc bias in parallel, were fabricated on bicrystal sapphire substrates for experimental investigation of the phase- locking phenomenon. Josephson self radiation, emitted by the array was measured by an external 90 GHz receiver for different applied dc magnetic fields. The numerical simulation of array shows that strong interaction between Josephson junctions and the standing waves within structure require small enough damping α≤ 10 -3 per coupling section for reduction of Josephson oscillation linewidth.

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High-Tc Josephson junction arrays with coupling by superconducting short- circuited (5 µm wide and 0.1 µm thick) coupled microstrip lines, provided dc bias in parallel, were fabricated on bicrystal sapphire substrates for experimental investigation of the phase- locking phenomenon. Josephson self radiation, emitted by the array was measured by an external 90 GHz receiver for different applied dc magnetic fields. The numerical simulation of array shows that strong interaction between Josephson junctions and the standing waves within structure require small enough damping α≤ 10 -3 per coupling section for reduction of Josephson oscillation linewidth.
We report the strong microwave Josephson radiation from an array of high-T/sub c/ junctions on a MgO bicrystal substrate from centimeter- to millimeter-wave ranges. The dc bias current was fed to the junction array having parallel geometry with the pair of junctions shunted by superconducting loops. The configuration of bias leads was a series of interlocking dc SQUID's geometry which guaranteed the oscillation of all junctions at the same frequency. The phase differences between adjacent junctions with different I/sub c/R/sub n/ products could be controlled by an external small magnetic field. For a five-junctions array, we observed the coherent output power of about 13 pW without external magnetic flux, which was nearly five times higher than that of a single bicrystal junction. We observed the Josephson linewidth of the self-radiation in the coherent state to be less than 1 GHz by adjustment of the external flux.
We have investigated the mutual phase interaction between a single detector junction and a phase-locked three-junction array of high-T/sub c/ Josephson junctions. The YBCO Josephson junctions were fabricated on a MgO bicrystal substrate. In the array junction structure, the dc bias current was fed to the junction array having parallel geometry with each pair of junctions shunted by a superconducting coplanar microstrip superconducting loop. The configuration of bias leads was a series connection of interlocking dc SQUIDs geometry which enables all junctions to oscillate at the same frequency. We measured the radiation power of a Josephson-junction array using an off-chip coupled to a radiometer and on-chip coupled to a single junction from centimeter- to millimeter-wave ranges. Clear self-induced Shapiro steps were observed in the current-voltage curves of the detector junction. The power transfered to the single detector junction was estimated from the modulation of Shapiro step height. The maximum detected power level was about 7 nW at frequency 150-250 GHz. The Josephson oscillation power excited the resonant mode coupled to the coplanar microstrip loops.
High effective autoregressive method has been applied to the calculation of the oscillation line width for the promising phase-locked Josephson-junction arrays. It has been found, that the oscillation linewidth decreases with the number N of Josephson junctions proportionally to N for one-dimensional arrays or even more fast for two-dimensional arrays until number N exceeds the coupling radius. It has been shown that in the case of parallel array inserted into superconducting microstrip line, the phase-locked oscillation state in-phase could be provided by the strong interaction between Josephson junctions and electromagnetic standing wave regardless to applied magnetic field. The necessary amplitude of the standing wave is easy attained at McCumber parameter value /spl beta/>1, when the high-frequency Josephson-junction impedance is small in comparison with the strip-line wave impedance. An extra reduction in the linewidth caused by electromagnetic standing wave impact has also been studied. The results are discussed from viewpoint of the possible applications of the structures as submillimeter wave generators.
The high frequency properties of the one-dimensional transmission line consisting of a parallel array of resistively shunted Josephson tunnel junctions have been studied in the limit of relatively low damping where this nonlinear system exhibits new and interesting phenomena. Here we report on experimental and numerical investigations of a resonant step observed at a voltage corresponding to 600 GHz in the dc current-voltage characteristic of a parallel array of 20 identical small NbAl2O3Nb Josephson junctions interconnected by short sections of superconducting microstrip line. The junctions are mutually phase locked due to collective interaction with the line sections excited close to the half wavelength resonance. The phase locking range can be adjusted by means of an external dc magnetic field and the step size varies periodically with the magnetic field. The largest step corresponds to a superconducting phase difference of π between neighboring junctions.
We fabricated a Josephson junction array with an additional inductive coupling line to detect the high frequency and power of Josephson self-radiation. The YBa2Cu3O y Josephson junctions are fabricated by a pulsed laser deposition method on bicrystal MgO substrates. We measured the submillimeter wave and the Josephson self-radiation power in an external waveguide system. The Josephson junction arrays showed the resistively shunted junction (RSJ)-like current–voltage characteristics and good frequency-locking of Josephson self-radiation. As the number of Josephson junctions increased, the maximum detected power was increased. We observed the maximum frequency of above 6 THz for Josephson self-radiation in the array with 11 Josephson junctions from the Shapiro steps under 22 GHz irradiation.