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Revue de Physique Appliquée

Interactions in small systems of coupled Josephson junctions at microwave frequencies

Thomas F. Finnegan, J. Wilson, J. Toots

Published Jan 1, 1974• 14 citations
Publisher

Abstract

A major factor in the practical utilization of any Josephson device for microwave and higher frequency applications (such as the generation and detection of electromagnetic radiation) is the success with which the junction device can be coupled to an external transmission line. As part of an experimental investigation of the electrodynamics of small arrays of Josephson tunnel junctions, the properties of the coherent radiation emitted both by individual junctions and by coupled junction systems have been studied at frequencies between 2 and 12 GHz. Near 9 GHz, more than 10-9 W of coherent radiation has been detected from a single Pb-Pb oxide-Pb junction coupled via waveguide. The development of thin film devices in which the tunnel junctions form part of a microstrip line have made it feasible to observe directly (in a single device) the radiation emitted both at the fundamental geometrical frequency and at the low order harmonics. The observed temperature dependence of the junction cavity Q indicate the low temperature losses are primarily due to cavity loading of the external transmission line.

DOI
10.1051/rphysap:0197400901019900

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Applied Physics Letters•Jun 16, 1997•8 citations
A major factor in the practical utilization of any Josephson device for microwave and higher frequency applications (such as the generation and detection of electromagnetic radiation) is the success with which the junction device can be coupled to an external transmission line. As part of an experimental investigation of the electrodynamics of small arrays of Josephson tunnel junctions, the properties of the coherent radiation emitted both by individual junctions and by coupled junction systems have been studied at frequencies between 2 and 12 GHz. Near 9 GHz, more than 10-9 W of coherent radiation has been detected from a single Pb-Pb oxide-Pb junction coupled via waveguide. The development of thin film devices in which the tunnel junctions form part of a microstrip line have made it feasible to observe directly (in a single device) the radiation emitted both at the fundamental geometrical frequency and at the low order harmonics. The observed temperature dependence of the junction cavity Q indicate the low temperature losses are primarily due to cavity loading of the external transmission line.
A promising method for microwave coupling to thin-film Josephson devices via microstripline techniques has been developed which has significant advantages over more traditional waveguide techniques. In particular, direct determination of intrinsic junction cavity parameters such as the geometrical resonance frequencies and Q values are made practical and compact cryogenic multi-octave microwave holders readily realized. The results of coupling experiments with Pb-Pb oxide-Pb and Nb-Nb oxide-Pb tunnel junctions are discussed and applications of these results to the design and construction of shielded single junction 2e/h devices and small multi-junction arrays are described.
This thesis is devoted to some selected applications of the Josephson junction oscillator. The initial work was performed using low-Tc Nb/PbBi tunnel junctions, while the later parts were done using the recently developed high-Tc YBa2Cu3O7 (YBCO) Josephson junctions. The latter junctions have raised the hope for Josephson oscillator applications with substantially increased operation frequencies and temperatures. In the first part of this thesis the use of inductor-resistor (LR) shunted tunnel junctions in arrays of phase locked Josephson junction oscillators was investigated. The LR shunt is designed to resonate out the intrinsic tunnel junction capacitance at the operation frequency. We show that LR shunted tunnel junctions have important advantages compared to the commonly used resistively shunted tunnel junctions; i.e. narrower linewidths, higher impedance of the junctions and that they can be optimized also at frequencies higher than about 100 GHz. Design rules for stable, non-chaotic oscillations with optimized output power were developed. Measurements on a non- optimized circuit show that oscillator arrays of LR shunted tunnel junctions can generate substantial power, ~10 nW to a 300 .OMEGA. load at 390 GHz from a 10 junction array. In the second part the phase locking of YBCO Josephson junctions was clearly demonstrated for the first time. Mutual phase locking of two junctions was observed at frequencies between 0.2 and 1 THz, and was only limited by the bandwidth of the coupling circuit. The locking was in good agreement with simulations based on the resistively shunted junction model. The interaction of a single Josephson junction with a 5 junction oscillator array was also measured at frequencies up to about 350 GHz, but was substantially reduced by the high losses of the coupling circuit. The YBCO Josephson junctions were found to be interesting candidates for microwave generation at frequencies approaching 1 THz and oscillator arrays to be mainly limited by the high losses of the coupling circuits at these frequencies. In the third part the interaction of YBCO Josephson junctions with integrated resonators were studied for the first time. Using a novel method, Josephson Broadband Spectroscopy (JOBS), the high frequency properties of the resonators were measured with a bandwidth of up to 1 THz and in the temperature range 4-77 K. The YBCO surface resistance (9 m.OMEGA. at 4 K and 100 GHz) was found to scale as f2 and had a temperature dependence in good agreement with the two-fluid model, modified with 18% non-pairing charge carriers. The YBCO London penetration depth was frequency independent (460 nm at 4 K) and had a temperature dependence in agreement with the two-fluid model.
A high temperature superconducting YBa2Cu3Oy array of five Josephson junctions designed with additional coupling lines has been developed to demonstrate the effects of frequency locking and impedance matching for applications such as oscillators, mixers, and detectors. The Josephson self-radiation power was directly detected by a superheterodyne receiver, and Shapiro steps were also measured. The Josephson self-radiation properties reveal good quality of phase locking and microwave coupling with external circuits. The maximum self-radiation power of our array is about 50 pW which is several ten times higher than that of a single Josephson junction, and its peak point exactly satisfies the Josephson current-voltage relation. The Shapiro-step measurements show that the behavior of current-voltage curve depends on the effective inductance of coupling lines which affects the total impedance of Josephson junction array and microwave coupling. The Josephson oscillation frequency was obtained up to about 880 GHz which is 73% of the maximum available frequency calculated from the characteristic voltage of the Josephson junctions. Experimental results show that this type of Josephson junction array can improve the Josephson self-radiation power and increase the maximum detectable frequency.