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Applied Physics Letters

Mutual phase-locking of ten YBa2Cu3O7 step-edge Josephson junctions up to 45 K

Gerhard Kunkel, Ronald H. Ono

Published Sep 23, 1996• 13 citations
Publisher

Abstract

We have developed a microwave circuit using ten parallel-biased high-temperature superconductor Josephson junctions to demonstrate phase-locking for applications such as oscillators, mixers, and detectors. The basic cell consists of two Josephson junctions enclosed in a microstrip resonator, which provides voltage-locking and low dynamic resistance in the current–voltage characteristic. Partial synchronization of the ten junctions was observed up to 45 K and up to 1 THz. The implications of this design for power and radiation linewidth will be discussed.

DOI
10.1063/1.117635

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Microwave oscillator using arrays of long Josephson junctions

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The authors report on measurements performed on integrated superconducting devices based on arrays of long Josephson tunnel junctions operating in the resonant fluxon oscillation regime (i.e. biased on the zero field steps). The electromagnetic coupling among the junctions causes a mutual phase-locking of the fluxon oscillations with a corresponding increase of the emitted power and a decrease of the signal linewidth. This phase-locked state can be controlled by means of an external DC bias current and magnetic field. The effect of the generated microwave signal has been observed on a small Josephson tunnel junction coupled to the array via a microstrip transmission line. The feasibility of the reported devices as local oscillators in an integrated microwave Josephson receiver is discussed.>

IEEE Transactions on Magnetics•Mar 1, 1989•46 citations
We have developed a microwave circuit using ten parallel-biased high-temperature superconductor Josephson junctions to demonstrate phase-locking for applications such as oscillators, mixers, and detectors. The basic cell consists of two Josephson junctions enclosed in a microstrip resonator, which provides voltage-locking and low dynamic resistance in the current–voltage characteristic. Partial synchronization of the ten junctions was observed up to 45 K and up to 1 THz. The implications of this design for power and radiation linewidth will be discussed.
We have demonstrated mutual phase locking of two YBa2Cu3O7 Josephson junction oscillators in the frequency range from 0.2 to 1 THz. A maximum power of 0.2 μW was delivered to a distributed 8 Ω load at 0.7 THz. The corresponding dc locking current, ‖IL‖, was 43 μA. A maximum locking current of 75 μA at 0.44 THz or 7% of the critical current agrees well with a calculated maximum value of 6% for this circuit. The excellent high frequency operation is due to the use of a lumped microstrip circuit and a high characteristic voltage of the grain boundary junctions (1.7 mV at 4 K). This circuit was continuously tunable for all frequencies and was operated from 4 to 60 K.
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.
Interaction between two Josephson junctions and a microstrip resonator is investigated using a resonator shunted junction (RLCSJ) model. A sharp current step resulting from coupling with the resonator is observed in simulated I-V characteristics. There is strong phase-locking and decreasing of dynamic resistance at the current step. We design and fabricate two types of Josephson array oscillators using these effects to obtain stable phase locking and narrow linewidth for the oscillation. The fabricated oscillators are formed with nine shunted Nb/AlOx/Nb tunnel junctions and Nb microstrip resonators. The power of the oscillator delivered to the load resistor was estimated to be about 3.6 /spl mu/W at 330 GHz. A composite linewidth of the oscillators is also measured at about 19 MHz at 4.2 K using two oscillators with a log-periodic antenna and a SIS detector.
Josephson junctions made from high temperature superconductors (HTS) are active high frequency devices which can be used as oscillators or frequency mixers in the submillimeter-wave region. The frequencies of these electromagnetic waves range from 300 GHz up to 3 THz. Up to now, this wave band is a relatively unexploited region of the electromagnetic spectrum because it is very difficult to produce and detect submillimeter-waves. However, submillimeter-wave technologies are becoming important in many practical scientific applications such as radio astronomy, atmospheric research, plasma diagnostics and THz-imaging, recently. In this work, the behaviour HTS Josephson junctions have been investigated experimentally and theoretically with the aim to THz-applications. In the first part the behaviour of intrinsic Josephson junction stacks are measured, and the possibility of phase locking is discussed by the theoretical analysis of two different shunt technologies depending on the parameter spread of the junction and the shunt parameters. It could be shown that stacks made from sputtered BSCCO layers does not act as intrinsic Josephson junction arrays because of their many micro shorts resulting from the layer growth. However, the stacks made from sputtered TBCCO show clear intrinsic Josephson junction behaviour. But their quasiparticle branches in the current-voltage characteristic deviate from the behaviour predicted by the common used RCSJ model. These branches can be discussed within a tunneling model using d-wave superconductor density of states. A temperature dependent current distribution and a zero bias anomaly was found. Furthermore, the coherent behaviour is studied for intrinsic arrays with additional side-wall shunts. The existence of thresholds of phase locking at especial junction and shunt parameters are demonstrated and discussed in regard to THz-oscillator applications. It was found that the Josephson junctions in a stack can be phase locked even if they have a parameter spread in the critical current up to 15% using a resonant shunt. In the second part of this work the mixing properties of HTS bicrystal Josephson junctions as wave guide mixers for 115 and 345 GHz have been investigated. (orig.)
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.
The authors report on measurements performed on integrated superconducting devices based on arrays of long Josephson tunnel junctions operating in the resonant fluxon oscillation regime (i.e. biased on the zero field steps). The electromagnetic coupling among the junctions causes a mutual phase-locking of the fluxon oscillations with a corresponding increase of the emitted power and a decrease of the signal linewidth. This phase-locked state can be controlled by means of an external DC bias current and magnetic field. The effect of the generated microwave signal has been observed on a small Josephson tunnel junction coupled to the array via a microstrip transmission line. The feasibility of the reported devices as local oscillators in an integrated microwave Josephson receiver is discussed.>