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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]

Josephson junction transistors and 50-junction arrays with linear junction dimensions from 200 nm down to 70 nm were fabricated from standard Nb/AlOx/Nb trilayers. The fabrication process includes electron beam lithography, dry etching, anodization, and planarization by chemical-mechanical polishing. The samples were characterized at temperatures down to 25 mK. In general, all junctions are of high quality and their I-U characteristics show low leakage currents and high superconducting energy gap values of Δ≈1.35 meV. The characteristics of the transistors and arrays exhibit some features in the subgap area, associated with tunneling of Cooper pairs, quasiparticles, and their combinations due to the redistribution of the bias voltage between the junctions. Total island capacitances of the transistor samples ranged from 1.5 fF to 4 fF, depending on the junction sizes. Devices made of junctions with linear dimensions below 100 nm×100 nm demonstrate a remarkable single-electron behavior in both superconducting and normal state. We also investigated the area dependence of the junction capacitances for transistor and array samples. [ABSTRACT FROM AUTHOR]

We investigate low-temperature and low-voltage-bias charge transport in a superconducting Al single electron transistor in a dissipating environment, realized as on-chip high-ohmic Cr microstrips. In samples with relatively large charging energy Ec>EJ, where EJ is the Josephson coupling energy, two transport mechanisms were found to be dominant, both based on discrete tunneling of individual Cooper pairs: Depending on the gate voltage Vg, either sequential tunneling of pairs via the transistor island [in the conducting state of the transistor around the points Qg≡CgVg=e mod (2e), where Cg is the gate capacitance] or their cotunneling through the transistor (for Qg away of these points) was found to prevail in the net current. As the conducting state of our transistors had been found to be subject to quasiparticle poisoning, high-frequency gate cycling (at f∼1 MHz) was applied to study the sequential tunneling mechanism. A simple model based on the master equation was found to be in a good agreement with the experimental data. © 2004 American Institute of Physics. [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]

Investigates Al single electron structures equipped with miniature on-chip Cr resistors of R>R[sub k]=h/e[sup 2] 26k omega. Evaluation of the self-capacitance of the resistors per unit length; Development of the concept of the three-junction single electron pump with on-chip resistors.

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.

Presents a study on an advanced cryoelectronic detector concept combining superconducting circuit and microfabrication technology. Advantages of the membrane configuration; Details on the fabrication of a superconducting tunnel junction detector on silicon membranes; Heat diffusion in two and three dimensions.

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.

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 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]

We demonstrate that shunting of superconductor-insulator-superconductor (S-I-S) Josephson junctions by superconductor-insulator-normal metal (S-I-N) structures having pronounced nonlinear I–V characteristics can remarkably modify the Josephson dynamics. In the regime of Josephson generation the phase behaves as an overdamped coordinate, while in the superconducting state the damping and current noise are strikingly small, that is vitally important for application of such junctions for readout and control of Josephson qubits. Superconducting Nb/AlOx/Nb junction shunted by Nb/AlOx/AuPd junction of S-I-N type was fabricated and, in agreement with our model, exhibited nonhysteretic I–V characteristics at temperatures down to at least 1.4 K. [ABSTRACT FROM AUTHOR]

Deep submicron Al/AlO[sub x]/Nb tunnel junctions and single electron transistors with niobium islands were fabricated by electron beam gun shadow evaporation. Using stencil masks consisting of the thermostable polymer polyethersulfone and germanium, high quality niobium structures with good superconducting properties and a gap energy of up to 2Δ[sub Nb]=2.5 meV were achieved. The I(U) characteristics of the transistors show special features due to tunneling of single Cooper pairs and significant gate modulation in both the superconducting and the normal state. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have prepared integrated oscillators consisting of niobium-made pancake coils and plate capacitors with sputter-deposited silicon dioxide as the dielectric. In combination with a GaAs-based preamplifier, samples with different layout parameters taken from the same wafer were operated in the liquid helium bath. Resonant frequencies in the range from 50 to 150 MHz were found. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the operation of a single-electron pump (comprising three Al/AlO[sub x]/Al tunnel junctions and two gates) connected to the bias electrodes through the compact on-chip Cr resistors, Rapprox. 60 kΩ>R[sub k]=h/e[sup 2]approx. 26 kΩ. The function of the resistors in this so-called R pump was to suppress electron cotunneling, the process which otherwise severely deteriorates the performance of few-junction single-electron devices. When a harmonic ac drive of frequency f of several MHz was applied to the gates, the current-voltage curve of the R pump exhibited remarkably horizontal current steps at I=ef. We show that the use of the resistors is capable of substantially increasing the accuracy of the pump in comparison to operation of the pump without resistors. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Examines a single-electron transistor (SET) comprised of highly resistive chromium thin-film strips. Observations in the Coulomb-blockade region; Measurement of modulation curves with appreciable amplitude; Background-charge noise of SET; Details about the electron transport mechanism.

The effect of thermal annealing on the properties of Al–AlO[sub x]–Al single electron tunneling transistors is reported. After treatment of the devices by annealing processes in forming gas atmosphere at different temperatures and for different times, distinct and reproducible changes of their resistance and capacitance values were found. According to the temperature regime, we observed different behaviors with regard to the resistance changes, namely the tendency to decrease the resistance by annealing at T=200 °C, but to increase the resistance by annealing at T=400 °C. We attribute this behavior to changes in the aluminum oxide barriers of the tunnel junctions. The good reproducibility of these effects with respect to the changes observed allows the proper annealing treatment to be used for postprocess tuning of tunnel junction parameters. Also, the influence of the annealing treatment on the noise properties of the transistors at low frequency was investigated. In no case did the noise figures in the 1/f regime show significant changes. © 2001 American Institute of Physics. [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]

Presents a method for the imaging of the standing-wave patterns inside superconducting tunnel junctions coupled to an external microwave source. Basis of the method; Confirmation of established models for the propagation of microwaves inside the tunnel junctions.

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]