Search

We have fabricated and characterized at low temperatures Nb/Ni/(Al/Ni)14/Nb lateral junctions and Nb micro-SQUIDs involving (Al/Py)10 multilayers (here Py is permalloy: 80% Ni − 20% Fe). We have studied - curves and Josephson critical current as a function of an external magnetic field Ic(H) at various temperatures and orientations of the applied magnetic field. These studies revealed that, for some orientations of the externally applied magnetic field, the Ic(H) dependence of the lateral junctions has a component highly sensitive to magnetic field. In addition, we have observed a SQUID-like Ic(H) dependence for devices, in which an orthogonally oriented (with respect to the substrate) Nb loop that includes two nanoscopic Josephson junctions is filled with a (Al/Py)10 multilayer. We believe the devices presented here are promising as magnetic field sensors on nanoscale for various applications where high spatial resolution is required

In tunnel magnetic Josephson junctions (JJs), we have provided evidence of an inverse magnetic dependence of the critical current Ic(H), i.e., a shift of the Ic(H) curves in the opposite expected direction. We have established a self-consistent method to ascribe this unconventional feature to spin polarization phenomena at the S/F interface. Through a magnetic characterization of Superconductor (S) /Ferromagnet (F) /Superconductor (S) trilayers and a magnetostatic analysis of the devices, we show that the F-stray fields fail to determine the pattern inversion. The comparative analysis with the literature supports our thesis that this phenomenon can be explained in terms of inverse proximity effect and observed with a proper choice of materials and thickness of the ferromagnetic layer involved in the junction structure. More generally, our approach proves that a careful investigation of the magnetic field pattern in magnetic JJs can lead to the detection of novel effects arising at the S/F interface.

The strong requirement for high-performing quantum computing led to intensive research on novel quantum platforms in the last decades. The circuital nature of Josephson-based quantum superconducting systems powerfully supports massive circuital freedom, which allowed for the implementation of a wide range of qubit designs, and an easy interface with the quantum processing unit. However, this unavoidably introduces a coupling with the environment, and thus to extra decoherence sources. Moreover, at the time of writing, control and readout protocols mainly use analogue microwave electronics, which limit the otherwise reasonable scalability in superconducting quantum circuits. Within the future perspective to improve scalability by integrating novel control energy-efficient superconducting electronics at the quantum stage in a multi-chip module, we report on an all-microwave characterization of a planar two-transmon qubits device, which involves state-of-the-art control pulses optimization. We demonstrate that the single-qubit average gate fidelity is mainly limited by the gate pulse duration and the quality of the optimization, and thus does not preclude the integration in novel hybrid quantum-classical superconducting devices.

Ferromagnetic Josepshon junctions (SFS JJs) are the basic cell of an intense research activity that extends to spintronics. The simultaneous presence of low-dissipative behavior and spin-filtering properties in JJs composed of ferro-insulating barriers (SIfS JJs) suggests their possible implementation in dissipationless versions of spintronics devices. Here, we establish a direct correlation between the suppression of the critical current in the experimental I–V curves of SIfS JJs and the spin-polarization factor derived in terms of the tunneling amplitudes for spin up and down carriers. A modelling of the JJs in terms of the Tunnel Junction Microscopic model allows to provide the scaling of the suppression parameter as a function of the barrier thickness.

In this work we give a characterization of the RF effect of memory switching on Nb-Al/AlOx-(Nb)-Pd$_{0.99}$Fe$_{0.01}$-Nb Josephson junctions as a function of magnetic field pulse amplitude and duration, alongside with an electrodynamical characterization of such junctions, in comparison with standard Nb-Al/AlOx-Nb tunnel junctions. The use of microwaves to tune the switching parameters of magnetic Josephson junctions is a step in the development of novel addressing schemes aimed at improving the performances of superconducting memories., Comment: IEEE Trans. Appl. Supercond. Special Issue ISEC2017

Ultrathin superconducting/ferromagnet (S/F) nanostripes are very interesting systems to investigate both the physics involved on nanoscale size and as light sensitive elements for superconducting single photon detectors. The electrical transport properties of NbN(8 nm)/NiCu(10 nm) nanostripes are presented down to a temperature of 4.2 K. A number of voltage steps were observed on the current-voltage characteristics, and they were investigated at different temperatures. A possible explanation in terms of active phase-slip phenomena has been proposed on the basis of the time-dependent Ginzburg-Landau theory, leading to an estimation of the inelastic electron-phonon relaxation time around 0.8 ps. The latter value was found to be in good agreement with a relaxation time, independently measured by femtosecond transient optical reflectivity experiments performed on the same bilayer.

Energy efficiency is a key issue for modern high performance computing. Superconductive digital electronics has already demonstrated superior performances in terms of speed and energy dissipations. However, there is still the open issue of the realization of effective submicron scale superconductive memories. Superconducting nanowires represent the state-of-the-art of single-photon detectors. Their technology has also been used to realize three-terminal active devices, where the output response is triggered by a current pulse. The combination of the electrothermal mechanism of the nanowire and the magnetic coupling with a suitable material can be used for the realization of a nanowire-based memory device scalable to nanoscale. The principle of operation and material requirements are presented here. In particular, the feasibility of the proposed device using EuS as magnetic material and NbN as nanowire is discussed. By using a physical model of the nanowire dynamics, in terms of the spatial distribution of electron and phonon temperatures, the feasibility of the proposed device has been verified, through numerical simulations. The device configurations considered have the specific goal of realizing reliable and high-speed read and write operations, with the possibility of scalability to nanoscale.

During the last decade, research on nanowires as superconducting single-photon detectors (SNSPDs or SSPDs) has gained interest due to many experiments demonstrating their fascinating potentialities in several fields. The search for more appealing materials for SNSPDs is presently a hot topic. Several superconductors have been proposed as possible alternatives to NbN, which is presently the racehorse. Examples are WSi and MoSi, which have advantages such as amorphous morphology and narrow-gap and low quasi-particle diffusivity, which permits extending the application in the mid- and far-IR ranges. A strategy based on material hybridization of superconductors with ferromagnets can open a new direction in the development of SNSPD. We present results concerning the hybrid superconducting/ferromagnetic (S/F) single nanowires for SNSPDs. Experiments under laser illumination show a single-photon response of S/F-based nanowires. The ferromagnetic overlayer has the beneficial effect of an increase of the superconducting critical current density and a decrease of dark counts. We compare dark counts in hybrid S/F with pure S nanowires. The experimental results are explained in terms of the existing models based on the presence of magnetic vortices.

We propose a procedure for the growth of thin films of organic and bio-molecules, which is based on MAPLE (Matrix Assisted Pulsed Laser Evaporation) and inspired by the laser cleaning technique. This procedure is MAPLE-like, meaning that the “evaporation”, or better the ejection of the material to deposit, is not assisted by the matrix, but rather by the target-holder. The target-holder is basalt, a magmatic rock which is able to absorb the laser radiation used (1064 nm) thanks to its high content in iron oxides. A thin veil of solution that contains the molecule of interest is placed into the basalt cup, frozen and irradiated. The laser radiation reaches the bottom of the cup, where it is absorbed. The consequent increase in temperature generates the phase explosion of the overlying frozen matrix. This procedure can be used to obtain thin films from transparent matrices. Here we deposited the enzyme β-glucosidase highly preserving its native conformation. The laser power used was under the ablation threshold of the basalt, so that only few fragments of the rock reached the substrate.

Optical plastics filled by nano-sized magnetic particles may have relevant applications in magneto-optics. Here, an in-situ method for the preparation of magnetic nanoparticles embedded in polystyrene by thermal decomposition of it-on mercaptide is described. Transparent ferromagnetic and superparamagnetic nanocomposites have been produced using iron (II) and iron (III) mercaptides. (C) 2009 Wiley Periodicals, Inc. Microwave Opt Technol Lett 51: 2774-2777, 2009; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24733

A ferromagnetic Josephson junction (JJ) represents a special class of hybrid system where different ordered phases meet and generate novel physics. In this work we report on the transport measurements of underdamped ferromagnetic NbN/GdN/NbN JJs at low temperatures. In these junctions the ferromagnetic insulator gadolinium nitride barrier generates spin-filtering properties and a dominant second harmonic component in the current-phase relation. These features make spin filter junctions quite interesting also in terms of fundamental studies on phase dynamics and dissipation. We discuss the fingerprints of spin filter JJs, through complementary transport measurements, and their implications on the phase dynamics, through standard measurements of switching current distributions. NbN/GdN/NbN JJs, where spin filter properties can be controllably tuned along with the critical current density (J c ), turn to be a very relevant term of reference to understand phase dynamics and dissipation in an enlarged class of JJs, not necessarily falling in the standard tunnel limit characterized by low J c values.

An optically transparent nanographite coating has been developed on a poly-methyl methacrylate (PMMA) substrates. The adhesion to the PMMA surface combined with the shear stress allowed an uniform and continuous spreading of the graphite nanocrystals on the substrate surface with formation of a very uniform graphene multilayer coating. This coating is characterized by high piezoresitivity and it is suitable to work as sensible and reliable local strain sensor. Its piezoresistive features have been characterized by bending tests yielding a gauge factor (GF) which is of the order of 50. The structural and strain properties of the compound were studied under stress by infra-red thermography (IRT) and micro-Raman spectroscopy. The strain strength has been estimated as a function of the bending load. The electrical transport was investigated as a function of the applied stress. Features are consistent with an intergrain electrical transport mechanism among graphene platelets.

Thermal fluctuations in hybrid superconductor/ferromagnetic $\mathrm{NbN}/\mathrm{NiCu}$ bilayers, as well as in pure superconducting NbN, two-dimensional (2D), nanostripes, have been investigated in order to understand the origin of dark counts in superconducting nanostripes when operated as single-photon detectors in the temperature range from $4.2\phantom{\rule{0.28em}{0ex}}\mathrm{to}\phantom{\rule{0.28em}{0ex}}8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In 2D superconductors, the dynamics of vortex motion play a significant role in the formation of a transient normal state, leading to dark-count events in current-biased nanostripes. By introducing a weak ferromagnetic overlayer on top of pure NbN, we managed to control the vortex dynamics, which subsequently enabled us to differentiate between several proposed theoretical models. In particular, a $6\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{nm}\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{thick}$ NiCu film grown on top of $8\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{nm}\phantom{\rule{4.pt}{0ex}}\text{\ensuremath{-}}\phantom{\rule{4.pt}{0ex}}\text{thick}$ NbN nanostripes led to an enhanced critical current density in the resulting nanostructure, as well as significantly lowered fluctuation rates, as compared to pure NbN structures, leading to reduced dark counts. The enhancement of pinning in $\mathrm{NbN}/\mathrm{NiCu}$ bilayers provided evidence that thermal excitations of single vortices (vortex hopping) near the edge of a 2D nanostripe were the dominant mechanism of the observed dark-count transients. On the other hand, in pure NbN the leading source of thermal fluctuations was the current-assisted thermal unbinding of vortex-antivortex pairs.

Superconductors are highly suitable materials for radiation detection. Several detector types have been proposed, with properties of fast detection or high wavelength resolution over a wide range of optical frequencies. Their performances depend on the relaxation processes involving phonons, quasiparticles and Cooper pairs occurring during the energy cascade following the absorption of the radiation in the superconductor. The energy down-conversion processes are related to the electron–phonon scattering strength λe−ph which is usually expressed in terms of the electron–phonon coupling time, τ0, which is characteristic for each material. In this paper we estimate the value of τ0 for several classes of superconducting materials not yet investigated. It is calculated in the framework of the McMillan model for the superconducting critical temperature Tc within the Debye approximation. The values obtained for τ0 are discussed as regards new possibilities for unexplored materials in the field of superconducting detectors. In particular, we focus our attention on materials with τ0 values that could play a significant role in both hot electron photodetectors and superconducting tunnel junction devices.

We present experimental results concerning both the fabrication and the characterization of superconducting tunnel junctions based on Nb/NiCu superconductor/ferromagnet (S/F) bilayers. Josephson junctions have been characterized down to T=1.4 K in terms of current-voltage I-V characteristics, Josephson critical current vs magnetic field, role of external magnetic field. Proximity parameters of the bilayer, such as the coherence length and the exchange field energy of the F metal, have been estimated by means of a numerical deconvolution of the I-V data the electronic density of states on both sides of the S/F bilayer. Results have been compared with theoretical predictions from a proximity model for S/F bilayers in the dirty limit in the framework of Usadel equations for the S and F layer, respectively.

A theoretical approach for the calculation of the bimetallic nanoparticles absorption spectra has been developed as an extension of the Mie theory in which nanoparticle dielectric function is found by the weighted linear combination of the dielectric functions for particles made of the corresponding pure metals. In the frame work of the theoretical model an expression for the resonance light absorption frequency were derived taking into account the interband transitions in the dielectric functions. We propose a simple method for the on-line monitoring of the bimetallic nanoparticles composition based on the measurement of the absorption peak position. Elaborated theoretical approach was used to investigate the polymer embedded Ag/Au nanoparticles which were prepared by reducing gold and silver salts (HAuCl4 and AgNO3, respectively) by ethylene glycol in presence of poly(vinyl pyrrolidone) (PVP) at room temperature. Calculated absorption spectra for the Ag/Au nanoscopic systems showed good agreement with the experimental data. Temporal evolution of the Ag/Au nanoparticles has also been investigated by this approach.

We show that a two-dimensional 12-fold quasicrystal tiled with squares and triangles can be generated as a triangular periodic lattice in which the unit cell is replaced by a cluster of 19 elements defining an elementary supercell with a dodecagonal boundary. As a straightforward consequence, we obtain analytically the exact Fourier spectrum of the deodecagonal quasicrystal that can find interesting applications for modeling purposes. In perspective, since our spatially periodic assembling allows restoring Bloch-type periodic boundary conditions, photonic band gap calculations will be possible without approximating the quasicrystal geometry. We foresee extending the same basic idea to other quasiperiodic patterns.

Since many years, superconductive tunnel junction devices have been widely investigated in the context of energy spectroscopy. High-resolution detectors for energy spectroscopy have been realized based on the single particle tunneling. On the other hand, the Josephson effect offers a very powerful tool for the design of proper detector configurations and an intrinsic potential for fast-response detectors to the radiation interaction. The present paper deals with this last issue discussing the underlying physical aspects and potential performance of such devices. (C) 2002 Elsevier Science B.V. All rights reserved.

A new fabrication process for three-terminal superconducting devices consisting of two Josephson junctions in a stacked configuration is reported. The process is based on the deposition of the whole Nb/AlxOy/Nb-Al/AlxOy/Nb multilayer on a Si crystalline wafer without any vacuum breaking. Lift-off techniques, anodization processes and a SiO film deposition have been adopted for patterning and insulating the two tunnel stacked junctions. Devices have been characterized in terms of current-voltage (I-V) curves and Josephson critical current vs. the externally applied magnetic field. They show high quality factors (V m values up to 65 mV at 4.2 K), and good current uniformity.

A modification of the polyol process has enabled the direct synthesis of nm-sized Ag particles with narrow size distribution and controlled average dimension embedded in a polymeric matrix. Dispersion of colloidal silver was obtained by reduction of silver nitrate in ethylene-glycol in the presence of a polymeric protective agent (i.e., poly(N-vinylpyrrolidone)) and ultrasounds. The final particle size was controlled by removing the colloid from the reactive mixture by addition of acetone. The very strong plasmon resonance peak at 410 nm and a feature at 350 nm in the UV-visible spectra are a clear consequence of the nano-size of dilute Ag particles. The proposed process offers the possibility to effectively use these synthesised materials for the production of colour filters for advanced optical devices.

Coherent passage of Cooper pairs in a Josephson junction (JJ) above the liquid nitrogen temperature has been the first impressive revolutionary effect induced by high critical temperature superconductors (HTS) in the domain of the study of Josephson effect (JE). But this has been only the start. A d-wave order parameter has lead to significant novel insights in the physics of the JE turning into a device the notion of a ?-junction. Spontaneous currents in a frustrated geometry, Andreev bound states, long-range proximity effect have rapidly become standard terms in the study of the JE, standing as a reference bench for conventional systems based on low critical temperature superconductors (LTS) and inspiring analogies for junctions based on novel superconductors discovered in the meantime. The extreme richness of the physics of HTS JJs has not been adequately supported by the expected impact in the applications, the main reason lying in the complexity of these materials and in the consequent unsatisfactory yield and reproducibility of the performances of the JJs within the required limits. The continuous progress in material science, and specifically in the realization of oxide multi-layers, and in nanotechnologies applied to superconductors, accompanied by the advances in a better understanding of the properties of HTS and of HTS devices, has as a matter of fact opened possible novel scenarios and interest in the field. We intend to give a brief overview on interesting new problems concerning HTS JJs of inspiration also for other systems. We also review some ideas and experimental techniques on macroscopic quantum decay phenomena occurring in Josephson structures. The attention is mainly addressed to intermediate levels of dissipation, which characterize a large majority of low critical current Josephson devices and are therefore an unavoidable consequence of nanotechnology applied more and more to Josephson devices.

The moderately damped regime (MDR) in a Josephson junction (JJ) is quite distinct from the well-known case of underdamped systems and quite common in devices characterized by low critical currents. Measurements of switching current distribution (SCD) are a direct way of discriminating the phase dynamics and the transport also in nontrivial cases of moderate damping. MDR is going to be more and more common with advances in nanopatterning superconductors and in materials science finalized to build hybrid devices. We report on measurements of SCDs, both in the thermal and quantum regime, on moderately damped NbN/MgO/NbN JJs. A transition from the Thermal Activation (TA) regime to Macroscopic Quantum Tunnelling (MQT) occurs at about 90 mK, while above 1.6 K a phase diffusive dynamics is clearly observed. The combined analysis of these regimes through numerical simulations allowed us to extract the fundamental parameters, which completely characterize the phase dynamics of such moderately damped JJs.

Optothermal properties of noble metal nanoparticles can be used in a wide range of applications. This paper presents the results of a theoretical study on the utilization of laser-induced heating of a gold nanoparticle (GNP) to melt a region of a transparent material with sub-wavelength spatial resolution. The considered system consists of a 10 or 15 nm diameter GNP fixed inside a silica substrate. The silica surface is covered with a thick film of the transparent polymeric or crystalline material. The heating and melting processes are studied under a 7.5 ns pulsed laser illumination. Calculations are conducted under three temperature limits, on the maximum temperature of the free electrons, the maximum temperature of the GNP and the maximum temperature of the film layer. The temperature limits lead to the limiting of the average laser power. The maximum allowable value of the average laser power and the effect of various parameters on spatial characteristics of the molten region are considered.

We report on the occurrence of multiple hopping and retrapping of a Brownian particle in a tilted washboard potential. The escape dynamic has been studied experimentally by measuring the switching current distributions as a function of temperature in a moderately damped NbN/MgO/NbN Josephson junction. At low temperatures the second moment of the distribution increases in agreement with calculations based on Kramers thermal activation regime. After a turn-over temperature T*, the shape of the distributions starts changing and width decreases with temperature. We analyze the data through fit of the switching probability and Monte Carlo simulations and we find a good agreement with a model based on a multiple retrapping process.

Interactions of superconducting nanostructures with external optical radiation represent a very interesting topic in both the framework of nonequilibrium physics and photodetector applications. Heterogeneous nano-layers, such as proximized superconductor/normal metal and superconductor/ferromagnet (S/F) bilayers, are very promising since they exhibit the ultrafast Cooper-pair and quasiparticle dynamics. We have characterized Nb/NiCu, NbN/NiCu, YBaCuO/Au/NiCu, and YBaCuO/ LaSrMnO proximized S/F nano-bilayers, using time-resolved, all-optical, femtosecond pump-probe spectroscopy measurements down to 4 K. Our experiments demonstrate that these bilayers are very promising for novel, ultrafast photodetection and spintronics device applications. Moreover, our time-resolved studies of the carrier dynamics in oxide-based S/F structures open the way to novel basic-physics investigations of nonequilibrium effects in correlated systems.

The analysis of the electrical properties of polystyrene films containing gold nanoparticles capped with 1-dodecanethiol and 8-hydroxyquinoline molecules is reported. The conductivity of the nanocomposite as a function of the doping level has been investigated both in planar and stacked configurations. While the former configuration evidenced low field tunneling between nanoparticles in the polymer matrix, stacked devices allowed us to investigate the main phenomena ruling the transport properties when switching effects are present close to critical electric fields. In particular, through the analysis of current-voltage characteristics we studied the charge transport at different fillers concentrations and sketched a physical picture of conductivity in such nanocomposite systems.

We demonstrate that the physical mechanism behind electroresistive switching in oxide Schottky systems is electroformation, as in insulating oxides. Negative resistance shown by the hysteretic current-voltage curves proves that impact ionization is at the origin of the switching. Analyses of the capacitance-voltage and conductance-voltage curves through a simple model show that an atomic rearrangement is involved in the process. Switching in these systems is a bulk effect, not strictly confined at the interface but at the charge space region., 4 pages, 3 figures, accepted in PRB

Time-resolved optical pump-probe and photoimpedance studies of proximized ferromagnet/superconductor nanobilayers are presented. The weak ferromagnetic nature of an ultrathin ${\mathrm{Ni}}_{0.48}{\mathrm{Cu}}_{0.52}$ film makes it possible to observe the dynamics of the nonequilibrium superconductivity in $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ hybrids through time-resolved measurements of a near-surface optical reflectivity change, which is generated by femtosecond optical pump pulses and discussed within a nonequilibrium two-temperature electron-heating model. We observed that the NiCu overlay significantly reduced the slow bolometric contribution present in the photoresponse of a pure Nb film, resulting in a strong enhancement of the nonequilibrium kinetic-inductive component of the transient photoimpedance, measured as an $\ensuremath{\sim}700\text{\ensuremath{-}}\mathrm{ps}$-wide voltage waveform generated across an optically excited current-biased $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ bilayer microbridge. The sensitive picosecond photoresponse makes our $\mathrm{Ni}\mathrm{Cu}∕\mathrm{Nb}$ heterostructures suitable for ``engineered'' ultrafast superconducting photodetectors since the photoimpedance signals observed in plain Nb bridges were at least $10\phantom{\rule{0.3em}{0ex}}\mathrm{ns}$ long and were due to the light-induced simple-heating effect.

The thermal stability of the superconducting phase of nominal composition YBa2Cu3O7−x-sintered pellets has been studied with respect to different temperatures (ranging from 300 to 950° C), time (ranging from 1 to 72 h), oxygen partial pressure (from 4 Pa to 1 atm) and carbon dioxide partial pressure (from 10−4 Pa to 1 atm). Annealed samples were characterized by X-ray diffraction analysis, optical microscopy, and resistive measurements of the superconductive transition temperature. A stability field of the orthorhombic and tetragonal phases was obtained, showing a region of coexistance. The decomposition of the 1 2 3 phase is found to be strongly influenced by the presence of a small amount of CO2 (1 p.p.m.) in the sintering atmosphere. A sintering process is proposed to avoid the formation of by-products.

We present our femtosecond optical pump-probe studies of proximized ferromagnet-superconductor nanobilayers. The weak ferromagnetic nature of a thin NiCu film makes it possible to observe the dynamics of the nonequilibrium carriers through the near-surface optical reflectivity change measurements. The subpicosecond biexponential reflectivity decay has been identified as electron-phonon Debye and acoustic phonon relaxation times, and the decay of Debye phonons versus temperature dependence was used to evaluate the electron-phonon coupling constants for both the pure Nb and proximized Nb/NiCu heterostructures down to low temperatures. We have also demonstrated that the NiCu overlay on top of Nb dramatically reduced the slow, bolometric component of the photoresponse component, making such bilayers attractive for future radiation detector applications.

The origin of hysteresis effect in the voltage region of the I-V curves of a superconducting tunnel junction near the threshold value V=(E-1+E-2)/e, where E-1 and E-2 are the energy gap values in superconductors, is discussed from both a theoretical and experimental point of view. It is shown how the actual structure of the I-V curve can be determined by multiparticle tunneling (MPT) processes. This circumstance is related to the essential role played by a large parameter (Delta/Gamma)(2/3) (Gamma and Delta being the depairing factor and the order parameter value in the superconductor, respectively), which can compensate the low barrier transparency. As a consequence, the perturbation theory in the transparency coefficient does no longer hold in such a threshold region. The competition of proximity effect in producing hysteresis in the I-V curve is also discussed. Besides the underlying physical aspects of the phenomenon, the actual structure of I-V characteristics is an issue of obvious interest for Josephson-junctions based devices. Experiments have been performed on high-quality NbN-based 6x6 mu m(2) tunnel junctions at temperatures down to 300 mK. I-V curves clearly show the presence of hysteresis at the threshold voltages corresponding to both n=1 and n=2 MPT processes in agreement with the predictions of the presented theory.

We present experimental results concerning both the fabrication and characterization of superconducting tunnel junctions containing superconductor/ferromagnet (S/F) bilayers made by niobium (S) and a weak ferromagnetic Ni0.50Cu0.50 alloy. Josephson junctions have been characterized down to T=1.4 K in terms of current-voltage I-V characteristics and Josephson critical current versus magnetic field. By means of a numerical deconvolution of the I-V data the electronic density of states on both sides of the S/F bilayer has been evaluated at low temperatures. Results have been compared with theoretical predictions from a proximity model for S/F bilayers in the dirty limit in the framework of Usadel equations for the S and F layers, respectively. The main physical parameters characterizing the proximity effect in the Nb/NiCu bilayer, such as the coherence length and the exchange field energy of the F metal, and the S/F interface parameters have been also estimated.

Optical plastics filled by nanosized magnetic particles may have important applications in the optical field (e.g. magneto–optics, etc.). Amorphous polystyrene filled by acicular magnetite particles of nanosized dimension (ca. 10 nm × 50 nm) has been obtained by thermal decomposition (at 200°C) of iron(II) mercaptide dissolved in the polymer. This nanocomposite material is quite transparent to visible light and shows ferromagnetic characteristics at room temperature. In particular, magnetic properties behaves anisotropically in the material and are related to the magnetite content. © 2006 Wiley Periodicals, Inc. Microwave Opt Technol Lett 48: 2505–2508, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22012

The piezoresistivity in a heterogeneous material formed by conducting nickel particles uniformly dispersed into a silicone-insulating matrix has been studied as a function of the filler content. A proper experimental apparatus was realized to investigate the direct dependence of resistivity on an uniaxial strain. In particular, when the volume fraction of the conductive charge approaches the conduction percolation threshold, at a proper value of the volume strain a little increment (2%) of the relative deformation can induce a transition from the insulating to conducting state accompanied by a giant resistivity change (about nine orders of magnitude). Some evidence that elastomer matrix properties contribute to determine both the peculiar development of the elasto-resistive coupling and its reversibility has been also found. The obtained results indicate the opportunity to optimize the investigated composite materials for the application as core of novel sensor devices governed by a threshold strain. (c) 2005 Elsevier B.V. All rights reserved.

Decay of a zero-voltage state of a Josephson junction at low temperature occurs via quantum tunneling through an effective potential barrier. An extremely small probability of quantum tunneling may become not very small under the action of an ac component of the bias current. The tunneling rate has a peak as a function of a dc component of the bias current (Euclidean resonance). An analysis of this extremely multiphoton process is done on the basis of classical trajectories in imaginary time. The studied phenomenon does not involve transitions between energy levels and, thus, is distinctly different from the well-known process of photon-assisted tunneling.

We present preliminary experimental results concerning both single and stacked tunnel junctions in which one of the electrodes was formed by a superconductor/ferromagnet (S/F) bi-layer. In particular, in the stacked configuration a Nb/NiCu bi-layer was used as the intermediate electrode, and it was probed by tunneling on both sides. Tunnel junctions have been characterized in terms of current-voltage characteristics (IVC), and differential conductance. Preliminary steady-state injection-detection measurements performed in the stacked devices at T 4: 2 K are also presented and discussed. (C) 2003 Elsevier B.V. All rights reserved.

We present optical properties of nanowires consisting of both proximitized superconductor/ferromagnet bilayers of NbN/NiCu and pure superconducting NbN layers. Thin NbN/NiCu hetero-structures were deposited on MgO(100) substrates by in situ dc magnetron sputtering, while nanowires were patterned using standard electron beam lithography and reactive ion etching. Photo-response signals were recorded from 100-nm-wide, three-parallel and six-series NbN(8 nm)/NiCu nanowires by 850-nm pulsed laser irradiation (time width 400 ps, repetition rate ~ 100 MHz). For the latter configuration the fall time of voltage responses was found to be close to 1.7 ns, and it gradually increased with increasing the NiCu film thickness. Moreover, three discrete families of photo-response signals recorded for the three-parallel and six-series nanowires were also observed.

The behavior of a superconducting tunnel junction detector (STJ) in the regime of low excitation is described, and the characteristic times of the dynamical response are discussed. The effect of a proximity layer in the STJ is analyzed in terms of the McMillan model. A set of phenomenological equations for the proximized STJ driven out of equilibrium is obtained, showing the importance of the role of the proximity effect in the dynamics of the device.

Results concerning Nb/BiSCCO point contact junctions (PCJs) are presented. I-V (current voltage) and (dV/dI)-V measurements were performed at various temperatures and contact pressures. Differential resistance measurements performed by a standard modulation technique exhibits structures which can be related to the energy gap of BiSCCO. Features of the derivative plots are discussed in connection with models proposed previously for similar junctions. Nb-Nb PCJs based on sintered pellets of granular Nb have been investigated for comparison. >

We have experimentally investigated the effects of intense proton beam irradiation (up to 10/sup 15/ p/cm/sup 2/) on Josephson junctions and junction arrays. The devices we have studied were realized using state of the art full-Nb technology, employing same materials and thicknesses of common Josephson digital circuit designs. We have analysed in detail the magnetic field dependence of the junction critical current, and the quasiparticle tunneling current, in order to observe possible occurrence of permanent changes produced by the ionizing particles. No evidence of radiation induced damage on the properties of the junctions has been found.