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We propose to exploit currently available tunnel ferromagnetic Josephson junctions to realize a hybrid superconducting qubit. We show that the characteristic hysteretic behavior of the ferromagnetic barrier provides an alternative and intrinsically digital tuning of the qubit frequency by means of magnetic field pulses. To illustrate functionalities and limitation of the device, we discuss the coupling to a read-out resonator and the effect of magnetic fluctuations. The possibility to use the qubit as a noise detector and its relevance to investigate the subtle interplay of magnetism and superconductivity is envisaged.

We propose a picture for the magnetic properties of superconductor/ferromagnet (S/F) heterostructures based on Nb and permalloy $(\mathrm{Py}:{\mathrm{Fe}}_{20}{\mathrm{Ni}}_{80})$. By measuring the magnetic moment as a function of the temperature in S/F/S trilayers for different thicknesses of the middle F layer, we give evidence of the presence of a magnetic stray field of the F layer. For values of F-layer thickness below a threshold, we establish a correlation between the magnetic measurements of the S/F/S trilayers and the anomalous magnetic dependence of the critical current in S/Insulator/thin superconducting film/F/S (SIsFS) Josephson junctions (JJs). These complementary investigations provide a self-consistent method to fully characterize S/F heterostructures and possibly demonstrate effects arising from the mutual interactions between ferromagnetism and superconductivity. A shift in the Fraunhofer critical current oscillations has been observed in the opposite direction to the one commonly observed in JJs with F barriers, as it has been recently predicted by inverse and electromagnetic proximity theories. This inverse memory effect is relevant for the design of these heterostructures as memory cells and spintronic devices.

In Magnetic Josephson Junctions (MJJs) based on Superconductor-Insulator-Superconductor-Ferromagnet-Superconductor (SIS’FS), we provide evidence of an unconventional magnetic field behavior of the critical current characterized by an inverted magnetic hysteresis, i.e., an inverted shift of the whole magnetic field pattern when sweeping the external field. By thermoremanence measurements of S/F/S trilayers, we have ruled out that this uncommon behavior could be related to the F-stray fields. In principle, this finding could have a crucial role in the design and proper functioning of scalable cryogenic memories.

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.

We show that infrared radiation impinging onto a one-dimensional array of grooves drilled in the superconductor electrode of a long overlap junction can improve matching between fluxon oscillations at THz frequencies and a spoof plasmon of comparable wavelength. This example proves that metamaterials can be very helpful in integrating superconductive and subwavelength optical circuits with optimized matching, bridging the gap between infrared and microwave radiation.

Josephson junctions drive the operation of superconducting qubits and they are the key for the coupling and the interfacing of superconducting qubit components with other quantum platforms. They are the only means to introduce non linearity in a superconducting circuit and offer direct solutions to tune the properties of a superconducting qubit, thus enlarging the possible qubit layouts. Junctions performances and tunability can take advantage of using a large variety of barriers and their special functionalities. We mention pertinent results on the advances in understanding the properties of ferromagnetic junctions, which make possible the use of these devices either as memory elements and as core circuit elements.

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.

The integration of superconductive nanowire logic memories and energy efficient computing Josephson logic is explored. Nanowire memories are based on the integration of switchable superconducting nanowires with a suitable magnetic material. These memories exploit the electro-thermal operation of the nanowires to efficiently store and read a magnetic state. In order to achieve proper memory operation a careful design of the nanowire assembly is necessary, as well as a proper choice of the magnetic material to be employed. At present several new superconducting logic families have been proposed, all tending to minimize the effect of losses in the digital Josephson circuits replacing resistors by adequate combinations of inductors in the logic gates. Among those, the nSQUID concept relies on the propagation of vortices along underdamped Josephson transmission lines. While, in principle, this logic family provides very low dissipation, several issues, such as speed and stability, have yet to be addressed. The current status of design and testing of n-SQUID logic gates and circuits as well as of the design and implementation of the nanowire memories is reported

We suggest a single-photon thermal detector based on the abrupt jump of the critical current of a temperature-biased tunnel Josephson junction formed by different superconductors, working in the dissipationless regime. The electrode with the lower critical temperature is used as radiation sensing element, so it is kept thermally floating and is connected to an antenna. The warming up resulting from the absorption of a photon can induce a drastic measurable enhancement of the critical current of the junction. We propose a detection scheme based on a threshold mechanism for single- or multi-photon detection. This Josephson threshold detector has indeed calorimetric capabilities being able to discriminate the energy of the incident photon. So, for the realistic setup that we discuss, our detector can efficiently work as a calorimeter for photons from the mid infrared, through the optical, into the ultraviolet, specifically, for photons with frequencies in the range $[30-9\times10^4]\;\text{THz}$. In the whole range of detectable frequencies a resolving power significantly larger than one results. In order to reveal the signal, we suggest the fast measurement of the Josephson kinetic inductance. Indeed, the photon-induced change in the critical current affects the Josephson kinetic inductance of the junction, which can be non-invasively read through a LC tank circuit, inductively coupled to the junction. Finally, this readout scheme shows remarkable multiplexing capabilities., Comment: 15 pages, 11 figures

We study the superconducting state of the two-dimensional electron gas (2DEG) at the ${\mathrm{LaAlO}}_{3}$/${\mathrm{SrTiO}}_{3}$ interface using Josephson junctions as spectroscopic probes. The transport properties of these devices reveal the presence of two superconducting gap structures and of an unconventional superconducting $\ensuremath{\pi}$ channel. These features provide evidence of an unconventional superconducting ground state, possibly related to the interplay between superconductivity and the large Rashba spin-orbit coupling in the 2DEG.

The interplay between superconducting and ferromagnetic order pa¬rameters in S/F interfaces gives rise to a wide range of peculiar properties with applications in high-efficiency computation and in the emerging field of super¬conducting spintronics. In NbN/GdN/NbN Josephson junctions, GdN barriers give unique properties due to the double insulting and ferromagnetic nature of the material, as demonstrated in previous works. Here we focus on tunneling spectroscopy of these junctions down to 0.3 K when changing the barrier thick¬ness, which contributes to complete a consistent picture on the physics of these junctions and supports the previous indications of equal-spin Cooper pairs con¬tributing to the total supercurrent of the devices.

We propose a single photon detector based on a superconducting quantum interference device (SQUID) with superconductor-normal metal-superconductor Josephson weak links. One of the two Josephson junctions is connected to an antenna, and is heated when a photon is absorbed. The increase of the weak link temperature exponentially suppresses the Josephson critical current thereby inducing an asymmetry in the SQUID. This generates a voltage pulse across the SQUID that can be measured with a threshold detector. Realized with realistic parameters the device can be used as a single photon detector, and as a calorimeter since it is able to discriminate photons frequency above $5~$THz with a signal-to-noise ratio larger than $20$. The detector performance are robust with respect to working temperatures between $0.1~$ K and $0.5~$K, and thermal noise perturbation., Comment: 8 pages, 6 figures. Updated version

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.

We report on femtosecond optical pump-probe studies of proximized ferromagnet/superconductor (F/S) hybrids, consisting of Ni0.5Cu0.5 layers deposited on top of Nb films. The weak ferromagnetic nature of the completely proximised Ni0.5Cu0.5 film makes possible to observe the dynamics of the nonequilibrium superconductivity through the near-surface optical reflectivity change measurements. The time-resolved photoresponse transient of the NiCu(21 nm)/Nb bilayer in the superconducting state shows strongly suppressed slow bolometric component. The fast relaxation time is also discussed accordingly to current theories on S/F heterostructures. The proposed S/F nanobilayers represent a new, artificially designed superconductor with the features (sub-picosecond photoresponse with suppressed bolometric component) very desirable for superconducting photodetectors and photon counters. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

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 novel class of sensors is constituted by radiation detectors based on superconducting tunnel junctions, which could lead in the next future to an advanced instrumentation with an interesting fall out for applications. So far, this area is restricted to LTc superconductors. Among them, niobium has the highest critical temperature and is therefore less demanding in terms of operational temperature. The best performances achieved so far in pure Nb-based junction are far away from the expected theoretical limit. The reason of this discrepancy could be attributed to various problems. Quasiparticle diffusion and edge losses seem to be responsible for the broadening of the signal amplitude spectra so often observed and reported in literature. In this paper we report on efforts to investigate the response to X-Ray of Nb-based superconducting tunnel junctions in terms of a model which includes the quasiparticle diffusion and the edge losses. Our model takes into account the contribution from both the electrodes of the junction and well reproduces the experimental data.

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.

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.

High quality epitaxial NbN/MgO/NbN Josephson junctions have been realized with MgO barriers up to a thickness of d = 1 nm. The junction properties coherently scale with the size of barrier, and low critical current densities down to 3 A/cm(2) have been achieved for larger barriers. In this limit, junctions exhibit macroscopic quantum phenomena for temperatures lower than 90 mK. Measurements and junction parameters support the notion of a possible use of these devices for multiphoton quantum experiments, taking advantage of the fast non equilibrium electron-phonon relaxation times of NbN. (C) 2011 American Institute of Physics. [doi:10.1063/1.3624471]

Nb-based superconducting tunnel junctions have been proposed as detectors in nuclear physics. A discussion in terms of the achieved junction quality concerning the energy resolution and the limit performances will be presented.

Weak ferromagnet/superconductor (F/S) proximity bilayers have been fabricated and characterized for basic physics studies concerning the ultrafast carrier dynamics in layered materials. The normalized reflectivity change (Δ R / R ) as a function of the time delay between the pump and the probe laser beams has been measured in F/S heterostructures formed by a low critical temperature superconductor (Nb) with a NiCu overlayer, and a high T c superconductor (YBCO) covered by Au/NiCu and manganite (LSMO) overlayers. The attention is mainly focused to the investigation of nonequilibrium excitation dynamics inside different bilayers in the low temperature region. The presence of the weak ferromagnetic overlayer produces faster optical relaxation times with respect to sole superconducting films. The results are promising in view of potential applications as ultrafast kinetic inductance superconducting photodetectors as confirmed by preliminary time-resolved photoimpedance experiments on both Nb and NiCu(21 nm)/Nb(70 nm) samples.

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.

We propose a mesoscopic kinetic-inductance radiation detector based on a long superconductor--normal metal--superconductor Josephson junction. The operation of this proximity Josephson sensor (PJS) relies on large kinetic inductance variations under irradiation due to the exponential temperature dependence of the critical current. Coupled with a dc SQUID readout, the PJS is able to provide a signal to noise (S/N) ratio up to ~10^3 in the THz regime if operated as calorimeter, while electrical noise equivalent power (NEP) as low as ~7x10^{-20} W(Hz)^(-1/2) at 200 mK can be achieved in the bolometer operation. The high performance together with the ease of fabrication make this structure attractive as an ultrasensitive cryogenic detector of THz electromagnetic radiation., 4 pages, 3 figures

We have integrated on oxidized silicon wafers superconductive films and Josephson junctions along with sol-gel optical channel waveguides. The fabrication process is carried out in two steps that result to be solid and non-invasive. It is demonstrated that 660 nm light, coupled from an optical fibre into the channel sol-gel waveguide, can be directed toward superconducting tunnel junctions whose current-voltage characteristics are affected by the presence of the radiation. The dependence of the change in the superconducting energy gap under optical pumping is discussed in terms of a non-equilibrium superconductivity model., Comment: Document composed of 7 pages of text and 3 figures

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.

The interest of superconductor materials for the realization of radiation detectors lies in a variety of concrete achievements and stimulating proposals. In this paper, recent aspects of superconductive radiation detectors are reviewed wich refer to two important issues. The former deals with results concerning detectors based on superconductive junctions in a configuration which provide both energy and time discrimination of single photons. The latter concerns materials and device aspects of hot-electron photodetectors based on nanosized heterostructured superconducting strips. Both aspects offer a new potential in the field of radiation detection. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.

Interest in superconducting transistors is due to some potential benefits of these devices, such as the possibility to have large current gain and bandwidth at low temperatures. In this work we present experimental results concerning with a new superconducting 3-terminal device showing true transistor-like properties at T=4.2K. It is a stacked double tunnel junction structure where the intermediate film is a bilayer of superconducting Nb with an Al quasiparticle trap which can be either in the superconducting or in the normal metal state. Current amplification factors of up to 2.0 are observed at a temperature of 4.2 K when the Al is superconducting. When the Al is in the normal state, large current gains of more than 50 are observed. The operation is highly directional, i.e. practically no effect is observed when the roles of the injector and detector junctions are reversed. The results are explained according to the recently proposed QUAsiparticle TRApping TRANnsistor model.

Tunnel quasi-particle injection into a superconducting film provides useful information on the nonequilibrium state inside the perturbed superconductor as well as on the potential application to electronic devices. Three terminal injector-detector superconducting devices have a. long history in non-equilibrium superconductivity. In the recent past non-equilibrium phenomena have attracted again considerable attention because of many superconducting based detectors involve processes substantially non-equilibrium in nature. The possibility of using a stacked double tunnel junction to study the influence of Iron-equilibrium superconductivity on the Josephson critical current is now considered. All experimental study of the effect of quasi-particle injection on the Josephson current both in steady-state and pulsed experiments down to T=1.2 K is presented using 3 terminal Nb-based stacked double tunnel devices. The feasibility of a new class of particle detectors based on the direct measurement of the change in the Josephson current following the absorption Of 3 X-my quantum is also discussed in terms of non-equilibrium theories.

Josephson tunnel junctions are well known as low temperature electronics devices. However, they are also important as very powerful sensors. A novel class of sensors is constituted by radiation detectors based on superconducting tunnel junctions, which could lead in next future to an advanced instrumentation with an interesting applicative future. So far this area is restricted to low-temperature superconductors. Several superconducting materials have been considered. Among them, Niobium is very attractive because of its highest critical temperature T c We have measured detailed X-ray detection characteristics for Nb-based tunnel junction made with different fabrication processes. We will discuss the difference in the X-ray response in terms of the relevant junction's parameters, underlying the problems of the niobium as suitable material for this kind of device.

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.

Superconducting Tunnel Junctions are considered for their use as new nuclear radiation detectors with high energy resolution, for radioastronomy. In fact they have potentiality for 0.05% resolution, more than one order of magnitude better than conventional detectors. The reasons and the advantages of using this tunnel junctions are briefly introduced. A description of the working principle and the basic experimental set up are also outlined. The attention is focused on Nb/AlOx/Nb junction which is presently the state of art of superconductive tunnel junction technology with respect to stability and cyclability. Junctions with high dynamical resistance in the biasing region are in fact necessary to optimize the signal to noise ratio and then improve the high energy resolution. The characterization at low temperatures (down to 1.2 K) of samples fabricated with a special <> geometry is presented to demonstrate the feasibility of their use because they satisfy the rather stringent quality requirements. Finally, preliminary results concerning the junction responsivity to nuclear radiation at relatively high temperature (T=1.5 K) are reported. Although the obtained resolution of 700 eV with 6 keV X-rays is far from the theoretical limit, better results can be expected working with the devices at lower temperatures and improving the structure configuration.

Time-resolved transient pump-probe spectroscopy measurements on proximized ferromagnet/superconductor (F/S) hybrid structures are presented. We focused our attention on both low and high critical temperature superconductors such as Nb and YBCO, while for F the weak-ferromagnetic alloy Ni0.48Cu0.52 has been used. Dynamics of the electron-phonon relaxation process has been investigated as a function of both the temperature and the F-film thickness. In the case of NiCu/Nb bilayers the presence of a thin F overlayer reduces the slow bolometric component of the photoresponse, while in YBCO-based structures faster relaxation times were measured. F/S nanobilayers are very attractive toward the development of novel hybrid superconducting photodetectors.

Superconducting Tunnel Junctions are promising devices in next generation nuclear and optical detectors. Unfortunately, a number of relaxation processes in the number of initially produced excitations degrades the performances of such devices. In order to investigate the role of charge loss mechanisms in Nb and/or at the Nb/Si interface we have fabricated three terminal in-plane double tunnel junctions, in which quasi-particles are injected into a Nb common strip and detected at different distances by tunnel junctions. The detection efficiency η decreases of about one order of magnitude from T=4.2K to T=1.2K for any injector-detector distance. The presence of charge trapping centers, activated at low temperatures, can qualitatively explain such a result. The estimation of the quasi-particle diffusion constant D=28±5 cm2/s at T=1.2 K is in agreement with other similar values reported in literature.

We report measurements of the switching induced by a rapidly varying magnetic field in a single, small, fully hysteretic Josephson junction. We demonstrate that the variation of the magnetic field, generated by a current step in a control line deposited on the top of the junction, can control the transitions from Josephson branch to the resistive branch and vice versa. The transition from resistive branch to Josephson branch is not trivial for an hysteretic junction. With a suitable choice of the magnetic pulse shape, a "deterministic flip-flop regime" can be achieved in this system.

Ultrathin microstrips based on polycrystalline NbRe films were investigated in order to preliminarily test the suitability of this material for the realization of superconducting single-photon detectors. The voltage-current characteristics measured on these samples show clear hysteresis. This is a fundamental ingredient for investigating single-photon detection as well as single vortex fluctuation phenomena in 2D NbRe-based devices.

Eddy currents non-destructive evaluation (NDE) consists in the use of electromagnetic techniques through which the inspection is carried out by detecting magnetic anomalies in the material under examination. The exploitation of the very high sensitivity of SQUID superconducting magnetometers to magnetic flux variations makes it an interesting and promising candidate for magnetic field detection. SQUID (superconducting quantum interference device) magnetometry can allow to overcome the traditional limits due to the lack of sensitivity in conventional electromagnetic NDE techniques. In this paper we present a new project concerning the design of a prototype of a high-temperature SQUID-based NDE instrumentation and its mains applications.

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.