Search

We have fabricated microstrips of Nb thin films by dc magnetron sputtering and lift-off. Focused ion beam (FIB) has been used to mill the surface of the films in a nanometer-sized region across the width of the strip. FIB has the side effect of implanting Ga+ ions on the exposed surface of Nb, damaging the film and modifying its superconducting properties. To overcome this problem, the surface of the FIB-milled Nb films has been passivated by anodization, including the damaged region of the strip into the oxidized layer. Anodization also allows for finely tuning the thickness of the nanoconstrictions after the milling process. We report here the effect of FIB exposure and anodization voltage on the superconducting transition temperature and on the temperature dependence of the resistance of the Nb nanoconstric- tions. We also show some preliminary results suggesting that these devices behave as overdamped SNS-like Josephson junctions.

A novel and convenient approach to functionalized pyrroles is presented, based on Pd-catalyzed oxidative heterocyclization-alkoxycarbonylation of readily available N-Boc-1-amino-3-yn-2-ols. Reactions were carried out in alcoholic solvents at 80-100 °C and under 20 atm (at 25 °C) of a 4:1 mixture of CO-air, in the presence of the PdI(2)-KI catalytic system (2-5 mol % of PdI(2), KI/PdI(2) molar ratio = 10). In the case of N-Boc-1-amino-3-yn-2-ols 3, bearing alkyl or aryl substituents, the carbonylation reaction led to a mixture of Boc-protected and N-unsubstituted pyrrole-3-carboxylic esters 4 and 5, respectively. This mixture could be conveniently and quantitatively converted into deprotected pyrrole-3-carboxylic esters 5 by a simple basic treatment. In the case of diastereomeric (3RS,4RS)- and (3RS,4SR)-N-Boc-3-amino-2-methyldec-5-yn-4-ol (syn-3f and anti-3f, respectively, whose relative configuration was determined by X-ray crystallographic analysis), no particular difference was observed in the reactivity of the two diastereomers between them and with respect to the diastereomeric mixture (3S,4S) + (3S,4R). Interestingly, N-Boc-2-alkynyl-1-amino-3-yn-2-ols 6, bearing an additional alkynyl substituent α to the hydroxyl group, spontaneously underwent N-deprotection under the reaction conditions and regioselective water addition to the alkynyl group at C-3 of the corresponding pyrrole-3-carboxylic ester derivative, thus directly affording highly functionalized pyrrole derivatives 7 in one step. In a similar manner, a novel functionalized dihydropyrrolizine derivative 9 was directly synthesized starting from (S)-7-(pyrrolidin-2-yl)trideca-5,8-diyn-7-ol 8.

Calcium hydroxyapatite (CaHAp) was prepared in the presence of two alkylphosphonates, the tert-butylphosphonic acid TBPOH and the 2-carboxyletylphosphonic acid 2-CEPA, by hydrothermal method at120 ◦ C for 15h. The modification of hydroxyapatite by grafting organic moieties is confirmed by IR andNMR MAS ( 1 H and 31 P) spectroscopy and chemical analysis. X-ray powder diffraction patterns show thatthe incorporation of organic moieties induces a significant loss of the material crystallinity and a clearincrease of the unit cell lattice parameter a as function of 2-CEPA grafting rate. The specific surface area(SSA) increases with increasing phosphonate amount especially for 2-CEPA. All techniques show thelower reactivity of TBPOH due to the steric effects of tert-butyl, whereas the 2-CEPA with a linear chainand double acidic functions is more reactive and can replace the OH − groups of the apatitic structure.© 2010 Elsevier B.V. All rights reserved. 1. IntroductionCalciumhydroxyapatiteCa

The temperature stability is one of the most important factors determining the successful application of the Josephson effect to the ac voltage standard or to analog and digital electronics. Whereas SIS hysteretic junctions with an Ambegaokar-Baratoff (A&B) IC(T) behavior exhibit a reduced drift around the working temperature of the liquid helium, metallic-barrier SNS junctions follow a Kulik-Omelianchuck (K&O) behavior with a sharp temperature dependence of Ic in this region. The objective of our research is to study this aspect for overdamped Nb/Al-AlOx/Nb heterostructures with current densities of 10-75 kA/cm2 and characteristic voltages from 100 to more than 500 muV, that have been fabricated at INRiM. The IC(T) characteristics, measured for Josephson heterostructures with different thickness, s, and exposure, E, essentially deviate from A&B and K&O curves, because of proximity effect caused by the comparatively high value of s (up to 100 nm). We study theoretically two extreme limits: the clean and the dirty limit for the interlayer between the superconducting electrodes and discuss the temperature stability of the junctions characterizing it with the temperature derivative dIC/dT. The combined experimental and theoretical analysis of the problem provides a way for understanding, controlling and improving the design of the Nb/Al-AlOx/Nb junctions in order to enhance their reliability.

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 25 kA/cm2 and characteristic voltages up to 450 μV. While these values make the junctions interesting for RSFQ electronic circuits, their response to an RF signal at 70 GHz 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.

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/.

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 I C R N product from 10 6 to 10 3 A/cm 2 and from 100 μV to 1 mV, respectively.

The water retention curve has a key role for the hydraulic characterization of porous media. Its experimental determination requires the accurate and detailed measurement of the matric potential and water content from saturation to oven dryness. The experimental data are fitted with a parametric model to calculate the analytic curve describing the hydraulic properties of the sample. We present a new method, based on differential evolution, for the determination of the characteristic parameters of the water retention models. This method allows to reduce the number of measurements necessary for obtaining accurate model parameters, decreasing the time needed to adequately characterize the sample. The method is applied to the calculation of water retention curves of sedimentary carbonatic rock of marine origin.

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/sup 3/ A/cm/sup 2/ up to 10/sup 5/ A/cm/sup 2/, with correspondingly normal resistances from 1 /spl Omega/ down to few m/spl Omega/. 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.

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 atomie force microscopy (AFM). The junctions, with areas between 25 and 200 μm2, have critical currents I C in the milliampere range, associated with normal resistances R N of about 1 Ω. The very high I C R N product of these junctions allows us to use a microwave source of 70 GHz and above, as in present dc 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.

We studied electron localization effects in superconducting Nb/Al bilayers. These effects are characterized by a decrease in resistivity with increasing temperature typical of a semiconductor and by an enhancement of the critical temperature Tc. The samples, made by superposition of Nb and Al films, each 50–200 nm thick, show a sharp superconducting transition between 8 and 9K. The electron localization effects are detectable only when the resistivity at 10 K is larger than about 100μΩcm. We discuss the fabrication conditions leading to the observed electron localization. We also present some measurements of the current-voItage characteristics in localized and metallic Nb/Al bilayers and we analyse them in terms of a simple thermal picture.

We have measured the temperature dependence of resistivity in relatively thick Nb/Al bilayers fabricated at room temperature, observing the decrease of ρ for increasing T typical of Anderson localization in disordered systems. We report the experimental conditions which determine this behavior and compare it to theoretical models for localization in 3D systems.

We report results achieved in the fabrication of two different Nb-based SNS devices which can be used for the development of a programmable Josephson voltage standard. Junctions with an Al metal barrier, 5 to 100 nm thick, have been fabricated obtaining devices with I/sub C/R/sub N//spl ap/0.5 mV. A second kind of junction has been fabricated using 10 nm thick non-stabilized TaO/sub x/ as a barrier, deposited by bias sputtering. For these devices, using the bias voltage as the main process parameter, I/sub C/R/sub N//spl ap/0.1 mV was obtained. Measurements of the electrical properties of the Nb/Al/Nb and the Nb/TaO/sub x//Nb junctions are reported.

We report here the experimental evidence of semiconductor-superconductor transitions in relatively thick Nb/Al structures. The temperature dependence of resistivity ρ(T) shows a sharp superconducting transition followed by either a normal metallic behavior in low-resistivity samples, or a semiconducting behavior when the sample resistivity at 10 K is above 100 μΩ cm. We discuss here the fabrication conditions and the electron localization regime associated with the measured ρ(T) dependence.

We report recent results on the development of two different kinds of SNS Josephson junctions which can be used as quantum D/A converters. We have fabricated these devices using as a barrier either a thick 50-100 aluminum layer or a not-stabilized tantalum oxide layer. For both technologies we have determined the dependence of the main electrical properties of the junctions on the fabrication process. Measurements of the rf properties for the Nb/Al/Nb junctions and of the magnetical properties for the Nb/TaO x/ Nb junctions are also reported.

The surface properties of vertical stacks of Nb/(Al-AlO/sub x//Nb)/sub n/ Josephson junctions have been investigated by Atomic Force Microscopy (AFM). The results indicate that the roughness of the AlO/sub x/ layers is nearly constant and independent of the surface features of the underlaying Nb films. The superposition of several Nb and Al-AlO/sub x/ films has an overall smoothing effect on their surfaces, as indicated by the strong reduction of the rms roughness in a stack of junctions with respect to that of the base Nb film. The height-height correlation function shows that the rms roughness increases on small sampling lengths l(l

A new type of nonhysteretic Josephson junction suitable for applications in voltage metrology has been developed. These junctions derive from the Nb/Al-AlO/sub x//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/sup 3/ to more than 2/spl times/10/sup 4/ A/cm/sup 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.

A detailed analysis of the properties of Nb/Al-AlO/sub x//Nb Josephson tunnel junctions fabricated with a modified SNAP process is presented. Selective niobium anodization has been used not only as on-line monitoring of the junction quality, through depth profiling anodization spectroscopy, but also to achieve a controlled reduction of the junction area at micrometer level by a proper choice of the anodization current. The dependence of the most relevant junction electrical parameters on fabrication variables has been investigated. With this fabrication process, junctions with V/sub m/ as high as 60 mV and current densities J/sub c//spl les/2000 A/cm/sup 2/ at 4.2 K are routinely obtained, thus demonstrating its feasibility for high frequency mixer and voltage standard applications.

A new and convenient method to prepare a number of pyrrole-3-carboxylic esters is reported.

C27H36ClN2O9PS, monoclinic, C12/c1 (no. 15), a = 36.495(3) A?, b = 8.5574(4) A?, c = 20.487(4) A?, b = 101.18(2)°, V = 6276.8 A?3, Z = 8, Rgt(F) = 0.052, wRref(F2) = 0.125, T = 295 K.

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.

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 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.

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.

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.