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Results from tests of several Nb thin film microstrip superconducting detectors are reported. A preliminary measurement of critical radius of the hot spot generated by 5 MeV alpha -particles is compared with simple model predictions. An effort is made to understand and measure the propagation characteristics of the superconducting to normal transition along the strip. For this purpose, fast current pulses are injected into the strip, and the voltage drop is observed with a digital scope. >

Niobium superconductive films for metrological applications (refractory Josephson tunnel junctions, superconductive bolometers and radiation sensors), have been fabricated via r.f. magnetron sputtering. Their electrical and structural properties have been measured on a wide range of film thicknesses (10–1000 nm). The data indicate the growth of a first layer at the substrate interface, which has a different behaviour with respect to the remaining of the film. The electrical data of resistivity at various temperatures show the relevant role of this layer at thicknesses

Thermal annealing of sputtered Nb films has been carried out in vacuum between 200 and 600 °C. Electrical properties have been seen to worsen over 200 °C while grain size improve until 400 °C and decrease over 600 °C.Possible applications of annealing process on Nb film have been evaluated. Resistivity and grain size data have been used to calculate electron reflectivity at the grain boundaries through Mayadas-Shatzkes theory; this has indicated that conducibility degradation can be due to oxygen diffusion into the grains.

NbCl(5)center dot(N,N '-dicyclohexylurea) 1a owns a distorted octahedral structure due to intramolecular NH center dot center dot center dot Cl bonding. The unit cell contains four units which are intermolecularly NH center dot center dot center dot Cl and NH center dot center dot center dot N bonded. An extended intramolecular network of H-bonding (N-H center dot center dot center dot Cl, CH center dot center dot center dot Cl, CH center dot center dot center dot N) causes the 3D self assembling of the units. Upon addition of base, the HCl release from 1a is observed with the transfer to Nb of the O-atom of the carbonylic function of the starting urea which is converted into the relevant carbodiimide CyN=C=NCy 4. The latter is quantitatively released by adding an excess of NEt(3) at 308 K (py and DBU are less efficient) with formation of the known NbOCl(3)(NEt(3))(2), isolated in quantitative yield. Increasing the temperature leads to a loss in selectivity as the formed DCC undergoes further reactions. At 350 K, the isocyanate CyN=C=O has been isolated in 60% yield besides a mixture of Nb-complexes. DFT calculations have been coupled to IR and NMR experiments for characterizing possible reaction intermediates and the behaviour of 1a. Several other MCl(x) species (ScCl(3), YCl(3), LaCl(3), TiCl(4), TaCl(5), AlCl(3), SnCl(4)) have been shown to be able to co-ordinate DCU but not all of them promote the conversion of urea into DCC.

Fourier syntheses providing electron density maps always show series-termination errors due to the limited experimental resolution. The worse the resolution, the worse the quality of the map: peaks are misplaced with respect to the true positions, positive and negative peaks occur, the peak profile is deformed. Two new algorithms have been developed aimed at reducing the resolution bias. The first one works in direct space [1,2], the second one in reciprocal space [3]. They are able to improve both peak positions and peak form in such a way that the map better fits the real atomic electron density. The new procedures have been introduced in EXPO2009, the strengthened and updated version of EXPO [4] and applied to the electron density maps provided by Direct Methods. Organic as well as metallorganic powder structures with non-atomic resolution experimental data have been tested. In spite of the difficulties in structure solution by powder data, the new approaches succeeded in several cases which could not be ,solved without the resolution bias correction. The theory of the resolution bias correction and its application by EXPO2009 will be presented.

Fourier syntheses are always affected by series-termination errors, which generate sets of positive and negative ripples around each main peak in the map. The interaction among the ripples distorts the profile of the map and moves peaks away from their correct positions. In a previous paper [Altomare et al. (2008). Acta Cryst. A64, 326-336] an algorithm was described which reduces the resolution bias by removing the effects of the ripples in direct space. In this paper the correction is performed in reciprocal space: the effect of the ripples on the atomic scattering factors is calculated and subtracted from the usual atomic scattering factors. The modified scattering factors are used to calculate new structure factors, from which more accurate electron-density maps may be obtained. The experimental tests show that the procedure minimizes the effects of the resolution bias and provides atomic positions that are more accurate than those provided by traditional approaches.

Overdamped Nb/Al-AlOx/Nb Josephson junctions are an intermediate state between the SIS and SNS Josephson junctions. With a proper choice of the fabrication parameters, stable and reproducible nonhysteretic current-voltage characteristics have been obtained Featuring critical current densities up to 70 kA/cm2 and characteristic voltages up to 450 muV at 4.2 K, overdamped junctions are suitable for many applications ranging from RSFQ electronic circuits to programmable and ac voltage standards. Tests on single junctions and arrays of series-connected junctions have shown a suitable behaviour when driven by a microwave with frequencies around 75 GHz. The quantized steps have proven to be stable and flat at nV level. These junctions have also shown an improved temperature dependence, which can be interesting in view of the development instrumentation with an integrated quantum voltage reference operated in a closed cycle refrigerator.

Superconducting tunnel junctions are promising as radiation detectors because of their larger energy resolution with respect to conventional Si-based detectors. We have developed a fabrication process for Nb-based STJ devices which allows to fabricate junctions with leakage currents well below 50 pA at 300 mK. a factor of 10~6 less than the leakage current at 4.2 K. The junctions are diamond-shaped with areas between 20times20 and 100times100 mum2. We report the details of the fabrication process, together with an analysis of the structure of films by AFM, SEM and X-ray spectroscopy. We show the measured I-V curves at 300 mK, together with preliminary results on x-ray measurements at 300 mK with a 5 mC 55Fe source.

Superconducting tunnel junctions (STJ) are promising high-resolution radiation detectors because of their larger energy resolution with respect to conventional Si-based detectors. We have developed a fabrication process for Nb/Al–AlO x /Nb STJ devices which produces junctions with subgap currents below 50 pA at 300 mK, a factor of 10 −6 less than the subgap current at 4.2 K. We report the details of the fabrication process and preliminary results on X-ray measurements at 300 mK with a 5 mC 55 Fe source.

We have developed a fabrication process for Nb-based STJ detectors which allows to fabricate junctions with subgap currents close to 20 pA at 300 mK, more than six orders of magnitude less than the subgap current at 4.2 K. The junctions are diamond-shaped devices, with areas between 20×20 and 100×100 µm2. We report the details of the fabrication process, together with an analysis of the grain structure of the films by AFM and XRD. We show the measured I - V curves at 300 mK and the temperature dependence of the subgap current between 300 and 1100 mK. We also report preliminary results on x-ray measurements at 300 mK with a 5 mC 55Fe source.

A novel approach to a Josephson voltage standard is presented. It uses as the drive signal the combination of a 10-GHz signal and its second harmonic. The effect of this biharmonic drive is to enhance the constant-voltage step amplitudes with respect to those obtained with a single-frequency drive. The optimum operating conditions are discussed, and the first experimental results obtained with Josephson devices containing both single junctions and small arrays are reported. >

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. The 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 have developed superconducting Nb/Al microstrip detectors. These devices appear to be very attractive due to their low sensitivity to radiation damage and aging, and to their high energy resolution. The samples are fabricated by superposing a 2 mm/spl times/2 mm Al area on a 5 /spl mu/m wide, 100 /spl mu/m long Nb strip. The thickness of each film is between 50 and 200 nm. We present the results of both steady and dynamical tests on Nb/Al bilayers, after irradiation with a /sup 241/Am alpha-source. We show that the performance of Nb/Al bilayers is better than that of single Nb strip devices.

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.

Preliminary results on the sensitivity to α-particles of Nb thin film microstrip detectors are presented.

We report some numerical results regarding the characteristics of rf-induced constant voltage steps in a voltage-biased Josephson junction irradiated by a biharmonic drive, namely a rf signal obtained by combining two microwaves of frequency ω and 2ω. The maximization of step amplitudes depends strongly on a careful choice of biharmonic drive parameters. We have made experiments on single, large, Josephson junctions, using a biharmonic signal with a fundamental frequency in the range 8–12 GHz. Under these conditions, only a small enhancement of the amplitude of the quantized steps with respect to the single drive case has been observed.

The amplitude of the steps at the reference voltages V n = nf(h/2e) in the I–V characteristic of a Josephson junction RF biased at a frequency f is an important parameter in designing devices for voltage standards. In fact the stability of the lock between the junction phase and the applied microwave field, which is a crucial problem for voltage standard application, is strongly dependent on the step amplitude1. It is well-known that in the simplified model of a voltage biased junction, where the spatial dependence of the phase is neglected, the expected amplitude △I n of the nth step is a fraction of the critical current I c given by the Bessel function of the first kind of integer order n, as in the equation2

Optimization of the performance of Nb-based overdamped Josephson junctions for applications to superconducting electronics is a major issue today. The use of cryocoolers, thus avoiding liquid helium refrigeration, with junctions working at temperatures higher than 4.2 K, could greatly increase the use of these devices in applications such as fundamental metrology and measurement sensors. The question we address here is how to maximize the characteristic voltage and minimize the temperature influence on the current-voltage curves of overdamped junctions enabling their use at T > 4.2K. To optimize the properties of a single Josephson heterostructure, we propose a type of overdamped junction that consists of a relatively thin insulating layer and a thick (tens of nm) normal-metal film between two superconducting electrodes. Measurements of the dependence of the critical current and characteristic voltage as function of temperature for dierent electrodes configurations show how it is possible to improve the design of these junctions.

The temperature dependence of the quantized voltage steps near the superconducting transition temperature of Nb∕Al–AlOx∕Nb overdamped Josephson junctions irradiated at 75GHz has been studied. These junctions can switch between the hysteretic and nonhysteretic regime either by varying a fabrication parameter or, as shown in the present paper, by changing the junction operating temperature. Due to the high values of JC and VC up to 75kA∕cm2 and 0.5mV at 4.2K, respectively, and to the moderate temperature dependence of these parameters, the height of the first rf-induced step was larger than 1mA at T=6–8K, corresponding to 0.7–0.9T∕TC on a junction of 5×5μm2. The flatness at the nanovolt level of this step demonstrates the feasibility of programable or ac voltage standards operating in a cryocooler setup.

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 JC from 103 up to 2×104A∕cm2, with characteristic voltages from 80 to nearly 450μV. The junctions are stable against time and repeated thermal cycling.

Scanning tunneling microscopy (STM) was used to analyze the surface topography of rf sputtered Nb films, 10–1000 nm thick. Surface crystalline sizes within 100 nm have been observed. Grain sizes deduced from STM have been compared to those evaluated from x‐ray diffractometry. The effect of both deposition rate and substrate temperature on surface roughness has also been studied on 100 nm thick Nb samples. All the results show a behavior typical of thin films grown under low mobility conditions, where a columnar structure prevails.

At IEN, two types of superconductive microwave sensors have been developed. Nb microbridge bolometers, working in the frequency range between 26 and 40 GHz, have been fabricated on polyimide substrates. For these devices we have measured a responsivity of 7 X 103 V/W. With a careful choice of device parameters, it is possible to achieve a noise equivalent power (NEP) of about 10-12\(W/\sqrt {Hz} \), 3 orders of magnitude lower than that obtained with conventional bolometers. SIS mixers for radioastronomical observations at 94 GHz based on Nb/Al-AlOx/Nb Josephson tunnel junctions have been also realized. Josephson junctions, both single and two junctions stacks, have been fabricated using a SNAP process where liquid anodization is used not only for insulation and area patterning, but also for trimming the junction dimensions at a micrometer level. In order to use these junctions as SIS mixers for astrophysical observations at 94 GHz and possibly at higher frequencies, we have fabricated junctions with an area of a few square micrometer and current densities up to 104 A/cm2. The junctions are integrated with a low-pass filter made with a stripline technique, designed by means of a 3D electromagnetic simulator. The reflection coefficient of the filter has been evaluated to be 0.2 in the best case.

This paper presents a general methodology for processing bioclimatic data in the temporal domain. Two different methods are used to assess the presence of temporal trends in the time-series of bioclimatic indices at each mea- surement station. A preliminary stage checks for the statistical homogeneity in the data set and for the presence of serial autocorrelation in the data, applying the proper methods to remove these effects. The methodology has been applied to a case study in Apulia, Italy, using the popular De Martonne index as a bioclimatic indicator.

We report here the current development status of our Nb/Al/Nb SNS Josephson junctions for a programmable voltage standard. The morphology and the electrical parameters of these junctions are strongly dependent on the Al deposition rate. The RF and magnetic field behaviour is also reported, together with preliminary data about the uniformity and reproducibility of the series arrays made with this technology.

We report first measurements of the magnetic field dependence of the critical currents of vertical stacks of two Nb/Al-AlO/sub x//Nb Josephson junctions with large idle regions. We kept constant the width of the lateral idle region and changed the ratio of the longitudinal idle part to the junction length. To provide a close coupling of the two junctions of the stack, the thickness of the intermediate Nb electrode was smaller than the London penetration depth. The effect of the idle region on the magnetic behavior of the stack is discussed. In same experiments, a clear and nearly periodic current locking effect has been observed, that was independent of the dimensions of the idle region.

The gap voltage shift of Nb Josephson junctions fabricated on different substrates is measured under optical illumination at several conditions of light intensity and chopping frequency. The signal dependence on the chopping frequency for junctions realized on glass substrates is bolometer-like with a strong thermal coupling between film and substrate. The responsivity of a junction of 50 μ m × 20 μ m is 50 V/W in vacuum and at liquid helium temperature, and its time constant is lower than 10 μ s . The linearity of the response on the optical power extends over five order of magnitude. The response of junctions fabricated on silicon substrates is flat versus the chopping frequency and about 100 times lower than a junction on glass with the same incident optical power. The shift of the gap at temperatures between 1.2 K and 4.2 K has been measured to separate the nonequilibrium effects from the heating effects.

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.

The variogram incorporates the basic information for the geostatistical analysis of spatial data, making it one of the most popular and widespread approaches for the evaluation and assessment of environmental data. Fitting an experimental variogram with a proper model, capable of catching the main characteristics of the sampled data, is a critical stage of spatial analysis. This stage is generally carried out using a semi-Automatic approach, with different levels of user's contribution. Once a model and the related starting parameter values have been defined, they can be refined using a trial-And-error strategy and the impact of such changes can be evaluated through several calibration indices. In this paper, we present an overview of the main indices used for the refinement of the variogram model parameters, highlighting the specific information provided by each of them together with a few heuristic strategies for driving the trial-And-error refinement process.