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International Journal of Modern Physics B

PHASE LOCKING AND AC AMPLIFICATION OF SMALL JOSEPHSON JUNCTIONS

P. Caputo, Giovanni Carapella, G. Costabile, Giovanni Filatrella, Rossella Latempa, Nadia Martucciello, S. Pagano

Published Oct 30, 2000• 0 citations
Publisher

Abstract

We have addressed the problem of the phase-locking mechanism of a single Josephson junction irradiated with microwaves, and have numerically and theoretically explored the possibility to achieve ac gain in this system. By investigating the capability of Josephson junctions to enhance the signal received form an external rf source we obtain useful information of the possibility of mutual phase-locking of large arrays of junctions.

DOI
10.1142/s021797920000337x

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•Nov 9, 2004•0 citations
We have addressed the problem of the phase-locking mechanism of a single Josephson junction irradiated with microwaves, and have numerically and theoretically explored the possibility to achieve ac gain in this system. By investigating the capability of Josephson junctions to enhance the signal received form an external rf source we obtain useful information of the possibility of mutual phase-locking of large arrays of junctions.
We have studied a new type of Josephson junction amplifier which is based on a two-junction array; the junctions are mutually phase locked at the Josephson self-oscillation frequency. With this frequency at 82 GHz, the voltages of the junctions remain equal (locked) for a bias current range as large as 60% of the critical current. Over a much smaller bias range, with an applied signal frequency of 1 kHz, a small-signal power gain of 19 dB was measured, accompanied by a negative resistance input impedance. This performance is consistent with a quasistatic theory of the amplifier.
We analyze experimentally and theoretically mutual phase locking and electromagnetic interaction between two linear arrays with a large number of Josephson junctions. Arrays with different separation, either on the same chip or on two separate substrates are studied. We observe a large coherent gain, up to a factor of three, of emitted power from two simultaneously biased arrays, compared to the sum of powers from two individually biased arrays. The phenomenon is attributed to the phase locking of junctions in different arrays via a common electromagnetic field. Remarkably, the gain can exceed the factor of two expected for a simple constructive interference of two oscillators. The larger gain is explained by an additional consequence of mutual interaction between two large arrays. Mutual phase locking of large arrays does not only result in constructive interference outside the arrays, but also improved synchronization of junctions inside each array. Our conclusion is supported by numerical modelling.
We discuss the dynamics of two-dimensional arrays of Josephson junctions in the overdamped regime. Our discussion is restricted to “classical” arrays, where the Josephson coupling energy is much larger than the charging energy. Such arrays represent a highly controllable system to study the dynamics of vortices. Furthermore, they are promising for applications like, e.g., a coherent microwave source. The experiments have been performed with arrays of Nb/AlOx/Nb-tunnel junctions allowing a high degree of junction uniformity across the array. The typical array size is 10×10 junctions. We show spatially resolved measurement results obtained by low-temperature scanning electron microscopy, and demonstrate the merit of these investigations for the clarification of complex regimes of vortex dynamics as well as for the optimization of coherent microwave sources. In the case of external microwave irradiation with increasing power, a continuous transition from a non-locked to a completely locked oscillation state is presented. In addition, we report on the quantitative measurement of the emitted microwave power from the arrays.
Properties of weak links between two superconductors, or Josephson junctions, make them interesting for fundamental physics research. Since their discovery over four decades ago, they have provided a unique way to study the behavior of the superconducting quantum phase. More recently, ultra small, or mesoscopic, Josephson junctions with substantial single Cooper pair charging energy have gained interest due to their behavior as macroscopic quantum objects. In addition to the theoretical interest, Josephson junctions can be used as active elements in circuit applications. Particularly, in this Thesis we study two different devices. We develop the required theoretical treatments, derive device properties, and compare the results with experimental data. The first application is a Josephson voltage standard based on externally damped Superconductor - Insulator - Superconductor junctions. It consists of an array of large Josephson junctions connected in series and irradiated with a 70 GHz microwave signal. Phase locking the Josephson dynamics into the signal leads to the quantization of the voltage. This is utilized in metrology. We introduce a new circuit solution based on frequency dependent damping of the junctions. Optimization and some designs for practical arrays are presented. The purpose is to find such a design that the array is fast, has low power consumption and is as stable as possible. Arrays able to generate DC voltages of order 1 volt with metrological accuracy are demonstrated experimentally and their applicability in AC voltage calibrations is analyzed. The second application is the Bloch Oscillating Transistor (BOT). The BOT is based on controlling the Cooper pair current in an ultra small Josephson junction by means of quasiparticles tunneling through a normal junction. As part of the thesis work, the principle of operation is first demonstrated computationally. The model is then refined to yield quantitative predictions of the characteristics. Finally, an analytic theory for the device is developed and the properties as an amplifier are derived.