Overdamped Nb/Al–AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. Stable and reproducible non-hysteretic current–voltage characteristics have been obtained with a proper choice of the fabrication parameters, featuring critical current densities Jc up to 25kA/cm2 and characteristic voltages up to 450μV. While these values make the junctions interesting for RSFQ electronic circuits, their response to an RF signal at 70GHz has demonstrated their suitability for both programmable and ac voltage standard. In these work we analyse the temperature behavior of these junctions up to T/Tc=1, Tc being the niobium critical temperature, which gives relevant information on the junction structure and, especially, on the oxide insulator/metallic film barrier, which is the key for the reproducible transition from an hysteretic to a non-hysteretic behavior. The results are also compared with other data of hysteretic and overdamped junctions.

We report the fabrication and characterization of overdamped Nb/Al-AlOx/Nb superconductor-insulator-superconductor Josephson junction whose fabrication process derives from that of the well-known hysteretic junctions. These junctions are an intermediate state between the superconductor-normal metal-superconductor and the superconductor-insulator-superconductor Josephson junctions. Stable and reproducible nonhysteretic current-voltage characteristics are obtained with a proper choice of the fabrication parameters. We have measured critical current densities J(C) from 10(3) up to 2x10(4) A/cm(2), with characteristic voltages from 80 to nearly 450 muV. The junctions are stable against time and repeated thermal cycling. (C) 2005 American Institute of Physics.

A new tape of nonhysteretic Josephson junction suitable for applications in voltage metrology has been developed. These junctions derive from the Nb/Al - AlOx/Nb SIS junctions using a relatively thick Al layer oxidized at a low value of oxygen exposure. This produces junctions with reproducible and spatially homogeneous I-V characteristics, having current densities ranging from 10(3) to more than 2 x 10(4) A/cm(2) and characteristic voltages up to 0.40 mV. The authors report here the rf response of these junctions at 70 GHz. The authors have measured the dependence of the rf-induced steps on the microwave power and the stability of the steps, in view of a future application of these junctions to an ac Josephson voltage standard.

We report on a double slit experiment where indistinguishable correlated photons generated through spontaneous parametric down-conversion, are each sent to a well-defined slit. An analysis of the results allows a test of de Broglie-Bohm mechanics against standard quantum mechanics.

Josephson devices for superconducting electronic applications, such as RSFQ logic circuits and programmable voltage standards, require nonhysteretic junctions with a high speed or a high voltage resolution and hence with characteristic voltages spanning over several orders of magnitude. We present here our recent results on Nb/Al/Nb junctions where, by changing some fabrication parameters such as the thickness and deposition rate of the Al barrier, it is possible to obtain junctions with very different electrical properties. These junctions have characteristic voltages varying from a few tens of /spl mu/V up to more than 1 mV, with critical current densities from 10/sup 3/ up to 10/sup 6/ A/cm/sup 2/.

Overdamped Josephson junctions are gaining interest in many fields of superconducting electronics. The characteristic voltage ICRN is the parameter limiting the highest possible speed and voltage resolution of the junctions. Nb/Al/Nb technology is a good candidate for these devices because of its wide range of characteristic voltages. In our laboratory we have so far fabricated Nb/Al/Nb SNS junctions having (a) high critical currents and smooth interfaces, (b) low critical currents and smooth interfaces, (c) low critical currents and rough interfaces. However, further improvements to this technology are still needed.

Overdamped Josephson junctions are gaining interest in many fields of superconducting electronics. The characteristic voltage ICRN is the parameter limiting the highest possible speed and voltage resolution of the junctions. Nb/Al/Nb technology is a good candidate for these devices because of its wide range of characteristic voltages. In our laboratory we have so far fabricated Nb/Al/Nb SNS junctions having (a) high critical currents and smooth interfaces, (b) low critical currents and smooth interfaces, (c) low critical currents and rough interfaces. However, further improvements to this technology are still needed.

A progress report of the MASTER project is presented here. MASTER is a system of three heterodyne receivers based on SIS (superconductor-insulator-superconductor) tunnel junction mixers. In our instrument these mixers will allow direct down-conversion from 94, 225, and 345 to 1.5 GHz, the IF where the signal will be detected. This instrument, coupled to a 2-4 m telescope like the one proposed for the DOME C base in Antarctica and in conjunction with an Acusto Optical Spectrometer, can be used to detect emission lines associated with molecular clouds in the interstellar medium. The current status of a 94 GHz receiver, the prototype of MASTER, will be presented too. The study of the optical coupling between the receiver and MITO telescope in the Italian Alps will also be described.

Superconductor–normal metal–superconductor (SNS) Josephson junctions are gaining interest in many fields, related to superconducting electronics. Their main advantages are the high critical current density and non-hysteretic I–V characteristic. We have studied Nb/Al/Nb SNS junctions, where the combined effects of the thickness and deposition rate of the Al barrier play a relevant role in determining the electrical behaviour of the junction. The increase of the Al deposition rate from 0.5 to 5 nm/s increases the surface roughness of the junction by a factor of two, with a corresponding variation of the current density and of the ICRN product from 106 to 103 A/cm2 and from 100 μV to 1 mV, respectively.

We will describe a system of three heterodyne receivers. Its mixers are based on SIS tunnel junctions which allow a very sensible downconversion of the detectable signal from 94 GHz, 225 GHz and 345 GHz to 1.5 GHz. This will allow the detection of molecular rotational transition lines from diffuse molecular clouds. Current status of a 94 GHz receiver, prototype of MASTER, will also be described. Technical design and cryogenic problems solution will be shown and we will focus our attention on the optical coupling technique based on gaussian beam analysis.

We report the current status of our SNS technology aimed at the development of programmable voltage standard devices. Using the simple Nb/Al/Nb trilayer process, with an Al barrier thickness of the order of 100 nm, we have fabricated SNS Josephson junctions whose electrical properties can be changed by varying the morphology of the Al film. The major role in determining the electrical behavior of the junctions is played by the roughness of the thick Al barrier. AFM analysis shows that the Al roughness is strongly reduced by increasing its deposition rate. The critical current density varies by two orders of magnitude, from 10(3) A/cm(2) up to 10(5) A/cm(2), with correspondingly normal resistances from 1 Omega down to few mn. The magnetic field dependence of the critical current is also affected by the barrier structure, while all the junctions show regular Shapiro-like rf-induced steps at 70 GHz.

In this work we report the current status in our development of sensitive heterodyne detectors for efficient down-conversion of submillimeter-wave signals to D-band. These detectors use the strong non-linearity of Nb/Al-AlO/sub x//Nb tunnel junctions and employ preferably quasi-optical techniques to couple local-oscillator and radio-frequency signals to the superconducting junctions via a log-periodic antenna and hyper-hemispherical lens combination. Several devices have been fabricated and tested.

We describe a system of three heterodyne receivers for astrophysical observation at 94G H z, 225G H z and 345G H z through the atmospheric windows available at dry, high altitude sites (Italian Alps, Antarctic Plateau).Three SIS mixers, fed by geometrically scaled corrugated horns, oriented in the same direction, form the core of our system. The four beams (HPBWs similar to 7 degrees) are optimized for matching the focus of MITO or others sub-mm wave telescopes. The intermediate frequency (IF) signal produced by each mixer is 1.5 G H z, with an instantaneous bandwidth of +/-0.4G H z. Each receiver is mechanically tunable over a total bandwidth of +/-1G H z at 94GHz, +/-2.5G H z at 225G H z and +/-4G H z at 345G H z.The SIS mixer and the first IF amplifier are cooled at 4K and the expected noise temperature of the system between 100K and 170K.The local oscillator and cold load reference signals are injected into the horns by a diplexer and mirror combinations. Rotation of a mirror modulates the system output for synchronous detection. An acousto-optical system at the IF output is used for spectroscopic observation.

We report here the status of development of our Nb/Al/Nb SNS Josephson junctions to be used as superconducting D/A converters. The electrical behaviour and the morphology of these Nb/Al/Nb junctions are strongly dependent on the deposition rate of the AL barrier. Junctions fabricated at a high rate of 5 nm/s have stable and reproducible properties. A possible interpretation of the effect of the Al morphology on the electrical behaviour of the junctions is given.

This paper reports the current status in our development of sensitive heterodyne detectors for the efficient down-conversion of submillimeter-wave signals to D-band. These detectors use the strong non-linearities of Nb/Al/AlOx/Nb tunnel junctions and employ waveguide or quasi-optical techniques to couple local-oscillator and radio-frequency signals to the junctions via a corrugated feed-horn or a log-periodic antenna and quartz hyper-hemispherical lens combination.The fabrication of Nb/Al/AlOx/Nb tunnel junctions is described and measured DC and RF characteristics are presented. Current densities of approximately 3000 A/m(2) and capacitances of 40fF/mu m(2) have been recorded - making such junctions applicable for down-conversion from submillimeter-wave frequencies with the aid of suitable distributed tuning structures. Proof of principle heterodyne mixing with series junctions integrated with planar antennae and planar intermediate frequency circuits has been demonstrated and preliminary results are described herein.Furthermore, the design of a broad-band open structure mixer for use at submillimeter-wave frequencies employing Nb/Al/AlOx/Nb junctions is also described. In particular, the design of optimised junction embedding circuits and planar log-periodic antennae is detailed. The latter has resulted in a planar antenna design which gives broad-band performance in terms of impedance and beam-pattern over the 90-400 GHz band. Junction characteristics are used to calculate the values of the junction embedding impedances needed to realise specific intrinsic cut-off frequencies and an optimised junction/antenna match.

Here we report the design and implementation of a low-noise heterodyne detector for use in a receiver for radio astronomy observations in the millimeter-wave band. Down-conversion is achieved using superconductor-insulator-superconductor (SIS) tunnel junctions as the non-linear mixing element; These junctions are integrated with a planar intermediate-frequency (IF) circuit which is housed in a reduced height rectangular waveguide block. Local-oscillator (LO) and radiofrequency (RF) signals are coupled to the junction via a bow-tie antenna and corrugated feed-horn and tuning is provided by a mechanical contacting back short. The detector is mounted in an InfraRed Laboratories HD-38 liquid helium cryostat which provides a 4.2 Kelvin operating environment for the SIS junctions and primary IF amplifier stage. LO and RF signals are diplexed externally using free-space techniques and applied to the horn antenna by means of polyethylene vacuum and quartz infra-red windows. Synchronous detection is achieved by means of alternative sampling of the sky signal and a reference provided by a cold load.

We measured, at liquid-helium temperature and in st weak magnetic field, the electrical properties of a thin-film superconducting and ferromagnetic structure. The structure is composed of a narrow (5 mu m wide, 100 mu m long and 100 nm thick) Nb strip, crossed by a Permalloy (Ni80Fe20) (30 mu m wide, 1 mm long and 140 nm thick) bridge, produced by sputtering and photolithography. A hysteretic dependence of the critical current of the Nb strip versus the applied magnetic field was observed. The current hysteresis loop showed a close resemblance to the magnetization hysteresis loop of the ferromagnetic Permalloy bridge in the same applied magnetic field, as measured at room temperature with magneto-optical ellipsometric techniques. The phenomenon is qualitatively discussed in terms of a magnetostatic interaction.

We report here our results on the development of Nb/Al/Nb Josephson junctions for a programmable voltage standard. The Al barrier is 5-120 nm thick. Its properties are quite different with respect to the single Al films because of diffusion between Al and the Nb counter-electrode. The resistivity of the Al barrier ranges from hundreds of microhm centimetres to a few milliohm centimetres, probably depending on the surface roughness of the Al layer, measured by atomic force microscopy (AFM). The junctions, with areas between 25 and 200 mu m(2) have critical currents I-C in the milliampere range, associated with normal resistances R-N of about 1 Omega. The very high ICRN product of these junctions allows us to use a microwave source of 70 GHz and above, as in present de voltage standards. Preliminary rf measurements on these devices are reported. Structural investigations by AFM and X-ray diffraction were also carried out to relate the electrical properties of the junctions to the Nb-Al interfaces.

Single and multijunctions thermal converters are devices widely utilized for AC measurements at room temperature. However, low sensitivity and high noise of these devices limits their applications at voltage level above 10 mV. A cryogenic thermal transfer sensor has been developed to realise AC measurements at 1 mV voltage level. This device consists of a superconducting-resistive-transition Nb thin film, which detects the AC power, dissipated on a Cr film resistor. To improve the sensitivity, the sensor has been fabricated on a 500 nm thick square silicon nitride membrane with size ranging from 1 × 1 mm 2 to 3.5 × 3.5 mm 2. The device has been tested at liquid helium and a thermal conductance lower than 10-6 W/K has been measured. From these data we expect an increasing of responsivity higher than three order of magnitude and a noise limited by intrinsic thermal noise.