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This paper presents a mathematical model for determining the stable phase-lock areas of microwave-biased superconductor-normal metal- superconductor Josephson junctions with low normal state resistance and small junction capacitance. The calculations are based on the resistively shunted junction model. The theoretically determined phase-lock areas are in accordance with the experimental results on Nb/HfTi/Nb junctions. [ABSTRACT FROM AUTHOR]

The paper describes the theory and the experimental results for the synthesis of precision waveforms by using the pulse-width modulation of the power of the microwave, which drives the superconductor–insulator–normal metal–insulator–superconductor Josephson array. The synthesized sine waveform has a high precision. Its harmonic content was determined to be less than 0.003%. This degree of harmonic content is in agreement with the theory. [ABSTRACT FROM AUTHOR]

The paper discusses the dynamics of different types of overdamped Josephson junctions biased by a sinusoidal microwave current. The results clarify the theoretical background for optimizing the single junction and the junction array parameters for their use as alternating current (ac) wave form synthesizers. The determined phase lock areas demonstrate the degree of overlap for Shapiro steps [S. Shapiro, Phys. Rev. Lett. 11, 80 (1963)] of different orders as a function of the normalized frequency μ=2πfωc-1, where f is the microwave frequency and ωc is the Josephson junction characteristic frequency. For μ<=0.7, which is a typical operation range for superconductor-insulator-superconductor Josephson junctions in the conventional direct current (dc) voltage standard at the bias frequency range from 10 to 100 GHz, the Shapiro steps overlap completely. For μ>=1.4 [typical for superconductor-normal-superconductor junctions at the same frequency range], the steps do not overlap. This allows for ac voltage synthesis by means of binary arrays and pulse-driven systems. For 0.7<=μ<=1.4 there is a range of partially overlapping Shapiro steps, which allows a rapid switching of the Josephson dc voltages for ac synthesis by microwave power modulation. Experimentally we have achieved this range through properly designed superconductor-insulator-normal-insulator-superconductor junctions. [ABSTRACT FROM AUTHOR]

We have investigated the dependence of the critical current I[sub C] on the value and orientation of an externally applied magnetic field H for interface-engineered YBa[sub 2]Cu[sub 3]O[sub 7-x] ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The I[sub C] versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the I[sub C](H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We demonstrate that a large area Josephson junction oscillating in the fluxon oscillator mode can be synchronized to other large junctions and simultaneously pump, by emitted radiation, a small area junction. We study the synchronization of the oscillations of the long junctions as a function of relevant experimental parameters such as bias current and stripline coupling characteristics. Experimental results obtained on coupled series arrays of Josephson junctions designed on the basis of our calculations are presented. We have fabricated coupled arrays containing each up to 1500 junctions in order to estimate the usefulness of our calculations for voltage standard devices. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents information on a study which evaluated the voltage standards of Josephson tunneling junctions in niobium (Nb)/Al[sub2]O[sub3]/Nb technology. Description of the design; Replacement of the original quarter wavelength transformers with arcs and tapered stripline transitions; Termination of each junction stripline configuration in the designs.

Provides information on an experiment examining two-dimensional arrays of overdamped Josephson junction under microwave irradiation. Importance on the absence of a voltage signal when an array with inhomogeneities is biased near its critical current; Influence of the current spread; Results of the experiment.

Investigates the higher order zero field steps of long inline and overlap Josephson junctions by means of low temperature scanning electron microscopy. Dynamics of the Josephson tunnel junction; Expression for the exact analytical solution of the unperturbed system corresponding to the presence of a fluxon.

Superconductor/insulator/normal conductor/insulator/superconductor (SINIS) junctions are damped by internal shunts. Avoiding space consuming external shunts, this junction type in principle allows a high density of integration. To make use of this advantage, the preparation of high quality SINIS junctions with small parameter spread and a small suppression parameter is required. Results of this development and their implementation in digital circuits and devices for precision voltage metrology are presented. The standard circuits consist of up to 70,000 SINIS junctions. [Copyright &y& Elsevier]

We consider the applicability of the established rf readout technique, which allows to obtain the supercurrent-phase relation of a Josephson element from impedance measurements in the phase-biased regime. In experiments on Nb-based single and double tunnel junctions, we demonstrate that this method holds even if the Josephson coupling energy is smaller than the thermal energy. Compared with conventional current-voltage measurements, we evaluate the rf technique to be favorable in particular for investigations of small supercurrents in low-capacitance Josephson elements. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated two-dimensional arrays of overdamped Josephson junctions that have been fabricated in Nb/Al/sub 2/O/sub 3//Nb-technology, together with microwave coupling structures. Different arrays have shown to emit coherent microwave radiation by using microwave-coupled on-chip detectors. Low-temperature scanning electron microscopy is used to study the coupling of the microwave radiation to an on-chip load. From the appearance of Shapiro steps in the current vs. voltage curves of detector junctions for different degrees of disorder in the array-junctions, margins of the spread in the critical currents of the Josephson junctions for coherent oscillation can be drawn. [ABSTRACT FROM PUBLISHER]

1 V and 10 V Josephson voltage standard arrays are fabricated on the basis of an improved design, using a modified technology. The Nb/Al/sub 2/O/sub 3//Nb trilayer is deposited directly on the Si wafer. The edges of the base electrodes are insulated by an anodization process. The maximum step voltage increases linearly with the external microwave voltage. Switching between different voltage levels is possible by simple switching of the dc voltage bias. A laboratory-made, oversized circular waveguide reduces the attenuation of the experimental set-up. [ABSTRACT FROM PUBLISHER]

We developed a virtual model of a Josephson junction series array based on a rigorous dynamic simulation and investigated coupling effect of Josephson oscillations. Our virtual model for a superconductor-insulator-normal metal-insulator-superconductor junction array reveals that a time-lag junction instability should be avoided for global synchronization in a long array with a large number of junctions. We found that the coupling between the self-generated Josephson oscillations through a microwave transmission line plays an important role in the collective synchronization of the Josephson array. [ABSTRACT FROM AUTHOR]

Quantum voltages of an array of YBa[sub 2]Cu[sub 3]O[sub 7] bicrystal junctions were calibrated against a programmable Josephson array voltage standard. We demonstrated that steps of the current-voltage characteristic of an array of bicrystal junctions at voltages of about 9 mV were flat over the current range of about 80 µA to within six parts in 10[sup 8]. The coincidence of quantum voltages on the array of high-temperature superconductor junctions at 64 K and the reference voltage on the array of niobium junctions at 4.2 K was measured with an uncertainty of two parts in 10[sup 8]. With the same uncertainty, we revealed the coincidence of the Josephson constant K[sub J]≡h/2e in YBa[sub 2]Cu[sub 3]O[sub 7] and in metallic superconductors. [ABSTRACT FROM AUTHOR]

It is demonstrated that a series array of 1474 Josephson junctions can produce quantized voltages up to 1.2 V when driven by microwaves at 90 GHz in the absence of a dc bias. This result brings closer the possibility of a practical Josephson voltage standard at the 1-V level. [ABSTRACT FROM AUTHOR]

The first application of low-temperature scanning electron microscopy for investigating the homogeneity of an array of Josephson tunnel junctions is reported. The experiments were performed on a series configuration of 166 junctions. The junction fabrication process was identical to that developed recently for the fabrication of a frequency-based Josephson voltage standard. The shorted junctions of the array could be easily detected. The quasiparticle imaging method, used previously for investigating spatial structures within individual tunnel junctions, can be extended in a straightforward way also to junction arrays. [ABSTRACT FROM AUTHOR]

A new series array of 1440 Josephson tunnel junctions has been developed and tested as a reference voltage standard. It yields microwave induced quantized voltage steps up to 1.3 V. The steps are usually stable for more than 5 h with a microwave driving frequency of either 70 or 90 GHz. A high-resolution comparison of a constant voltage step at the 1-V level with the electromotive force of a saturated Weston cell is described. The comparison shows that the step voltage is constant to within ±1 nV over the full step width. [ABSTRACT FROM AUTHOR]

For a study of the supercurrent-phase relationship, we have fabricated Nb/Al/AlO[sub x]/Al/AlO[sub x]/Al/Nb-based Josephson junctions of low critical current density. At 4.2 K, junctions of a nominal area of 10x10 μm[sup 2] showed a nonhysteretic dc current-voltage characteristic with a critical current of about 10 μA. Similar junctions were part of washer-type rf superconductor quantum interference devices. These samples were inductively coupled to high-quality tank circuits for measurements at 4.2 K or at higher temperatures up to values close to the critical temperature of niobium. In this range, the current-phase relation, as deduced from impedance measurements of the phase-biased junction, did not deviate significantly from the sinusoidal shape. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We present a study of the microwave characteristics of Josephson junctions based on a superconductor–insulator–normal–insulator–superconductor sandwich, fabricated in Nb/Al/AlO[sub x] technology. With the nonhysteretic Shapiro steps and the small parameter spread observed, the junctions are suitable for programmable Josephson voltage standards. Their characteristic voltage V[sub c]approx. 100 μV enables operation at microwave frequencies up to 100 GHz. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Executive Summary: For four decades semiconductor electronics has followed Moore’s law: with each generation of integration the circuit features became smaller, more complex and faster. This development is now reaching a wall so that smaller is no longer any faster. The clock rate has saturated at about 3–5GHz and the parallel processor approach will soon reach its limit. The prime reason for the limitation the semiconductor electronics experiences is not the switching speed of the individual transistor, but its power dissipation and thus heat. Digital superconductive electronics is a circuit- and device-technology that is inherently faster at much less power dissipation than semiconductor electronics. It makes use of superconductors and Josephson junctions as circuit elements, which can provide extremely fast digital devices in a frequency range – dependent on the material – of hundreds of GHz: for example a flip-flop has been demonstrated that operated at 750GHz. This digital technique is scalable and follows similar design rules as semiconductor devices. Its very low power dissipation of only 0.1μW per gate at 100GHz opens the possibility of three-dimensional integration. Circuits like microprocessors and analogue-to-digital converters for commercial and military applications have been demonstrated. In contrast to semiconductor circuits, the operation of superconducting circuits is based on naturally standardized digital pulses the area of which is exactly the flux quantum Φ 0. The flux quantum is also the natural quantization unit for digital-to-analogue and analogue-to-digital converters. The latter application is so precise, that it is being used as voltage standard and that the physical unit ‘Volt’ is defined by means of this standard. Apart from its outstanding features for digital electronics, superconductive electronics provides also the most sensitive sensor for magnetic fields: the Superconducting Quantum Interference Device (SQUID). Amongst many other applications SQUIDs are used as sensors for magnetic heart and brain signals in medical applications, as sensor for geological surveying and food-processing and for non-destructive testing. As amplifiers of electrical signals, SQUIDs can nearly reach the theoretical limit given by Quantum Mechanics. A further important field of application is the detection of very weak signals by ‘transition-edge’ bolometers, superconducting nanowire single-photon detectors, and superconductive tunnel junctions. Their application as radiation detectors in a wide frequency range, from microwaves to X-rays is now standard. The very low losses of superconductors have led to commercial microwave filter designs that are now widely used in the USA in base stations for cellular phones and in military communication applications. The number of demonstrated applications is continuously increasing and there is no area in professional electronics, in which superconductive electronics cannot be applied and surpasses the performance of classical devices. Superconductive electronics has to be cooled to very low temperatures. Whereas this was a bottleneck in the past, cooling techniques have made a huge step forward in recent years: very compact systems with high reliability and a wide range of cooling power are available commercially, from microcoolers of match-box size with milli-Watt cooling power to high-reliability coolers of many Watts of cooling power for satellite applications. Superconductive electronics will not replace semiconductor electronics and similar room-temperature techniques in standard applications, but for those applications which require very high speed, low-power consumption, extreme sensitivity or extremely high precision, superconductive electronics is superior to all other available techniques. To strengthen the European competitiveness in superconductor electronics research projects have to be set-up in the following field: [–] Ultra-sensitive sensing and imaging. [–] Quantum measurement instrumentation. [–] Advanced analogue-to-digital converters. [–] Superconductive electronics technology. [Copyright &y& Elsevier]

Abstract: The potentials of LTS junction technology and electronics offer innovative solutions for the processing of quantum information in RSFQ and qubit circuits. We discuss forthcoming approaches based on standard SIS technology and addressed to the development of new superconducting device concepts. The challenging problem of reducing back action noise of the RSFQ circuits deteriorating coherent properties of the qubit is currently solved by implementing Josephson junctions with non-linear shunts based on LTS SIS-SIN technology. Upgraded NbAlO x trilayer technology enables the fabrication of high-quality mesoscopic Josephson junction transistors down to the nanometer range suitable for a qubit-operation regime. As applications, circuit concepts are presented which combine superconducting devices of different nature. [Copyright &y& Elsevier]

We report on the results that we have obtained on systems consisting of two arrays of Josephson junctions coupled by means of stripline techniques. One array, formed by a series connection of long junctions is operated on magnetic field-induced singularities of the current–voltage characteristics (Fiske steps) and emits radiation at 250 GHz. The other array detects the emitted radiation and exhibits Shapiro step sum voltages generated by the coherent operation of several hundred junctions. [Copyright &y& Elsevier]

The fabrication process in superconductor–normal metal–superconductor (SNS) technology has been improved to raise the characteristic voltage of SNS active elements. Josephson junctions with normal metal interlayers made from HfTi were realized in ramp-type design with dimensions down to the deep sub-micron range. To investigate the influence of the interfaces, junctions with different sequences of interim layers were fabricated, according to Nb/HfTi/Nb trilayer and Nb/HfTi–Nb–HfTi/Nb multilayer structures. Typical values of the characteristic voltage and the critical current density are Vc=107 μV and jc=550 kA/cm2 for Nb/HfTi/Nb junctions and Vc=460 μV and jc=210 kA/cm2 for Nb/HfTi–Nb–HfTi/Nb junctions, each for areas of A=0.2 μm2. The junctions were experimentally investigated for the temperature dependence of critical currents, the influence of externally applied magnetic fields, and the application of microwave power. [Copyright &y& Elsevier]

A programmable voltage standard of fundamental accuracy has been realized with a binary sequence of nonhysteretic junctions. The series arrays of intrinsically shunted superconductor–insulator–normal conductor–insulator–superconductor (SINIS) Josephson junctions have been fabricated using the reliable Nb–Al/AlOx technology. The arrays of 8192 junctions (14 bit) are arranged in a new, improved design allowing a large Shapiro step width of 1 mA to be achieved. [Copyright &y& Elsevier]

The paper reports on the development of ring-shaped rapid single flux quantum (RSFQ) circuits fabricated in superconductor–insulator–normal metal–insulator–superconductor Nb/AlxOy/Al/AlxOy/Nb technology. The circuits enable the generation and maintenance of permanent SFQ pulse circulation and the bit error rate (BER) of test circuits to be determined experimentally. Various ring structures comprising switching circuits of several stages have been realized. A single switching stage consists of an escape junction and is fed by a separate bias current to adjust the threshold level of this junction. The reliability of circuit operation has been experimentally proven by BER<10−15. The critical current densities of the junctions are in the range jC≅450–750 A/cm2 and the characteristic voltage is about VC=170 μV. [Copyright &y& Elsevier]