Search

The Josephson Arbitrary Waveform Synthesizer (JAWS) allows AC voltages to be generated with fundamental ac- curacy. Lumped arrays of Superconductor/Normal conductor/Superconductor (SNS) Josephson junctions were developed and investigated for this application. Combining electron-beam lithography and chemical-mechanical polishing, series arrays of sub-micrometer SNS junctions consisting of Nb/HfTi/Nb were fabricated. The junction size varied from 0.04 μm2 to 4 μm2 in series arrays up to 2560 junctions in different microwave designs. Stable Shapiro steps are generated in a frequency range from 5 GHz to 20 GHz using continuous microwave irradiation. Using a simple pulse pattern at different pulse repetition frequencies, the arrays show current-voltage characteristics with flat Shapiro steps. Sinusoidal AC voltages were synthesized using a commercial pulse pattern generator. Peak-to-zero voltages of some mV for frequencies below 1 kHz were achieved. Higher harmonics were measured to be more than 80 dB below the fundamental tone. The behavior of the arrays was investigated in more detail by measuring the amplitude-frequency spectra. [ABSTRACT FROM AUTHOR]

Quantized values of magnetic flux trapped in a superconducting loop enable a new type of passive phase shifting elements. These elements can be incorporated into digital Josephson circuits making their design compact. We have proven the functionality of such phase shifters fabricated in conventional Nb/Al trilayer technology. We report on the successful low speed operation of a Rapid Single Flux Quantum toggle flip-flop circuit with the integrated passive π-phase shifting element. [ABSTRACT FROM AUTHOR]

The Rapid Single flux Quantum (RSFQ) technique is regarded as one of the most promising electronics due to its extremely high operating speed and low power consumption. Middle-scale RSFQ applications operating at multigigahertz frequency have recently been reported on. At such high operating frequencies, the on-chip interconnects start to play a limiting rote for the performance of the densely packaged digital circuits. Our previous studies have theoretically investigated the parasitic coupling effects between superconductive microstrip transmission lines and conclusions have been drawn about the critical coupling levels of several typical microstrip geometries. Here, basic RSFQ structures are investigated experimentally, which contain a Josephson transmission line stage with large inductance and can either trap or pass SFQ pulses. These structures are incorporated in critical microstrip layouts, thus being subjected to parasitic coupling. The bias levels are monitored at which the structures start to pass the SFQ pulses with and without coupling influences. Thus, the conclusions of our previous theoretical investigations are verified experimentally. [ABSTRACT FROM AUTHOR]

At the typical operating temperature of 4.2 K, the theoretical upper limit of the clock frequency of the Low-Temperature Superconductive (LTS) Nb based Rapid Single Flux Quantum (RSFQ) digital circuits is several hundred 6Hz. Nevertheless, the few middle-scale RSFQ circuits reported on up to now operate at only some tens of 6Hz. An important performance-limiting factor is in this case the clock signal distribution, so nowadays asynchronous RSFQ designs are often considered. Our previous studies have shown that it is an important advantage of a given asynchronous RSFQ cell library to contain gates with tunable delays. We have already shown by simulations that the RSFQ gate delays are best adjusted by changing the Stewart-McCumber parameter βc of the Josephson junctions-this method tunes the delay within a large interval of values while scarcely affecting the margins and the fabrication yield of the RSFQ gates. In order to prove this statement experimentally, we have designed, fabricated, and tested several ring-shaped oscillators with identical topologies but different Stewart-McCumber parameters of the Joseph son junctions. Their operating speeds and margins are measured and compared. [ABSTRACT FROM AUTHOR]

Long superconducting niobium coplanar waveguides (CPW) on silicon wafers have been implemented with inner conductor width, thickness and length of about 50 nm × 250 nm × 3.2 mm, respectively. CPW tapers yield the transition from small to large cross sections for 50 Ω coaxial SMA connectors. Microwave properties are measured at 4.2 K via λ/2-resonators where the inner conductor width w[sub i] is about half the London penetration depth. Due to the dominant kinetic inductance the measured quality factor is close to an evaluated minimum value for w[sub i] < 50 nm. The quality factors of such nanolines with niobium are approximately 150 at 10 GHz, i.e., more than two orders of magnitude larger than with aluminum at the same temperature. An unexpected power dependence of the intermodulation amplitudes and the corresponding dc properties are described. [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]