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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.

We provide a novel methodological approach to the estimate of the change of the Quantum Vacuum electromagnetic energy density in a High critical Temperature superconducting metal bulk sample, when it undergoes the transition in temperature, from the superconducting to the normal phase. The various contributions to the Casimir energy in the two phases are highlighted and compared. While the TM polarization of the vacuum mode allows for a macroscopic description of the superconducting transition, the changes in the TE vacuum mode induced by the superconductive correlations are introduced within a microscopic model, which does not explicitly take into account the anisotropic structure of the material., 20 pages, 13 figures, to be published in Phys.Rev. B

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 discuss the result of the fabrication with photolithography process of microstrips made of MoSi/Al superconducting films, with different widths and patterns suitable for single photon detection. The different designs presented are adopted to increase the sensitive area.

In this paper we study the Casimir energy of a sample made by N cavities, with $$N\gg 1$$ N ≫ 1 , across the transition from the metallic to the superconducting phase of the constituting plates. After having characterised the energy for the configuration in which the layers constituting the cavities are made by dielectric and for the configuration in which the layers are made by plasma sheets, we concentrate our analysis on the latter. It represents the final step towards the macroscopical characterisation of a “multi-cavity” (with N large) necessary to fully understand the behaviour of the Casimir energy of a YBCO (or a GdBCO) sample across the transition. Our analysis is especially useful to the Archimedes experiment, aimed at measuring the interaction of the electromagnetic vacuum energy with a gravitational field. To this purpose, we aim at modulating the Casimir energy of a layered structure, the multi-cavity, by inducing a transition from the metallic to the superconducting phase. After having characterised the Casimir energy of such a structure for both the metallic and the superconducting phase, we give an estimate of the modulation of the energy across the transition.

A new class of graphene-related materials (GRMs) obtained as water suspensions through a two-step oxidation/reduction of a nanostructured carbon black, namely graphene-like (GL) materials, has recently emerged. GL materials undergo self-assembly in thin amorphous films after drying upon drop-casting deposition on different surfaces. The GL films, with thicknesses of less than a micron, were composed of clusters of nanoparticles each around 40 nm in size. The exploitation of the GL films for different options (e.g., bioelectronic, sensoristic, functional filler in composite) requires a deep characterization of the material in terms of their electric transport properties and their possible interaction with the surface on which they are deposited. In this work, a careful electrical characterization of GL films was performed at room temperature and the results were compared with those achieved on films of benchmark graphenic materials, namely graphene oxide (GO) materials, obtained by the exfoliation of graphite oxide, which differ both in morphology and in oxidation degree. The results indicate a non-linear current–voltage relationship for all the investigated films. The extrapolated dielectric constant (ε) values of the investigated GRMs (GL and GO materials) agree with the experimental and theoretically predicted values reported in the literature (ε~2–15). Because similar conductance values were obtained for the GL materials deposited on glass and silicon oxide substrates, no significant interactions of GL materials with the two different substrates were highlighted. These results are the starting point for boosting a feasible use of GL materials in a wide spectrum of applications, ranging from electronics to optics, sensors, membranes, functional coatings, and biodevices.

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 this work the results of two Lidar measurements in the infrared range with a Superconducting Nanowire Single Photon Detector (SNSPD) are presented. In the first measurement, performed at the wavelength of 1550 nm, we determined the distance of two hard targets. This result was used to calibrate the experimental setup and to test the readout electronics. Then, we combined the SNSPD with the Lidar setup MALIA (Multiwavelength Lidar Apparatus) to measure the atmospheric aerosol profile at 1064 nm during a cloudy day. For the last measurement the backscattering coefficient was also calculated.

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.

Herein, smart coatings based on photo-responsive polymer nanocapsules (NC) and deposited by laser evaporation are presented. These systems combine remotely controllable release and high encapsulation efficiency of nanoparticles with the easy handling and safety of macroscopic substrates. In particular, azobenzene-based NC loaded with active molecules (thyme oil and coumarin 6) were deposited through Matrix-Assisted Pulsed Laser Evaporation (MAPLE) on flat inorganic (KBr) and organic (polyethylene, PE) and 3D (acrylate-based micro-needle array) substrates. SEM analyses highlighted the versatility and performance of MAPLE in the fabrication of the designed smart coatings. DLS analyses, performed on both MAPLE- and drop casting-deposited NC, demonstrated the remarkable adhesion achieved with MAPLE. Finally, thyme oil and coumarin 6 release experiments further demonstrated that MAPLE is a promising technique for the realization of photo-responsive coatings on various substrates.

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.

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.

International audience; We present the status of the art of the Archimedes experiment, devoted to measuring the debated interaction of quantum vacuum fluctuations and gravity. The method is essentially the weighing of the transition energy of a layered superconductor where the contribution of vacuum energy to the transition energy is expected to be relevant. The transition is obtained by modulating the temperature of the superconducting sample at a frequency of about 10 mHz and the expected change of weight is measured with a suitably designed high sensitivity cryogenic beam balance. In this paper, we present an overview of the experiment, discussing the expected signal to be measured, and presenting in particular the result of a prototype balance operated in our present laboratory. In the frequency range of the measurement, the sensitivity is affected mainly by seismic, thermal, sensor, and control noise. We discuss these points showing in particular the design of the cryogenic apparatus, the final balance, and the quiet seismic site that will host the final measurement.

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.

The magnetic properties of nanoparticles make them ideal for use in various applications, especially in biomedical applications. Herein, we describe the fabrication of iron oxide nanoparticles encapsulated in polystyrene (PS) using two methods: a conventional batch and microfluidic synthesis. In particular, we present a simple synthesis method of magnetic composite nanoparticles, based on the use of a microfluidic elongational flow method in a continuous-flow apparatus where magnetite particles are embedded in a polystyrene matrix. Compared to conventional batch synthesis, microfluidics-based synthesis enables precise reaction control, enhanced mixing and rapid chemical reactions, allowing flow synthesis of particles in a controllable, sustainable, and cost-saving manner that is attractive to industry. The composite particles show a high encapsulation of magnetite nanoparticles, but with an inhomogeneous size distribution; instead, the sample obtained with microfluidic approach shows a homogenous composite particle size distribution although the magnetite content is lower compared to the miniemulsion batch methods.

β-glucosidase (BG) plays a key role in determining the efficiency of the enzymatic complex cellulase for the degradation of cellulose into sugars. It hydrolyses the cellobiose, an inhibitor of the enzymatic complex. Therefore, the immobilization of BG is a great challenge for the industrial application of cellulases. Cellulases usually contains a BG amount insufficient to avoid inhibition by cellobiose. Here the BG was immobilized by matrix assisted pulsed laser evaporation (MAPLE) technique. The frozen matrix was composed of water, water/m-DOPA and water/m-DOPA/quinone. The effect of the excipients on the final conformation of the enzyme after the MAPLE processing was determined. The enzyme secondary structure was studied by FTIR analysis. The catalytic performances of the deposited films were tested in the cellobiose hydrolysis reaction. The results demonstrate that the presence of the oxidized form of m-DOPA, the O-quinone form, can protect the protein native structure, with the laser inducing little or no damage. In fact, only the samples deposited from this target preserved the secondary structure of the polypeptide chain and allowed a complete hydrolysis of cellobiose for four consecutive runs, showing a high operational stability of the biocatalyst.

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.

In this work we present a study of the dark counts rate in a NbTiN Superconducting Nanowire Single Photon Detectors (SNSPD). We measure the distribution of the time intervals elapsed between two consecutive dark pulses at the fixed temperature of 4,2K. Due to the stochastic nature of the dark counts, the distribution is expected to have a Poisson shape but what we observe is a combination of two Poisson-like processes. A further analysis of the distributions dependence on the bias current highlights that the weight of the two process is not constant. In the scenario presented by Ejrnaes et al. [1], this result can confirm that, in this temperature regime, dark counts are generated mainly by multiple consecutive fluctuation events and the contribution to the dark counts rate coming from single fluctuations increases in the high bias current region.

Josephson junctions (JJs) containing ferromagnetic (F) materials are being considered for applications as cryogenic random access memories (RAM). In this work, we report on the fabrication and characterization of tunnel JJs, based on Nb technology with a strong ferromagnetic interlayer Ni80Fe20 alloy (Permalloy), which is suitable for the realization of devices with reduced area and guarantees relative low saturation and coercive fields in the use of JJs as RAM elements. We have successfully realized Josephson memory elements that work well down to 7 mu m(2) preserving high values of the characteristic voltage. We have also investigated the role of the F layer thickness, and by measuring the critical current dependence on the external applied magnetic field, we have optimized our devices as memory elements using thin ferromagnetic layers with thickness down to 3nm. We have experimentally proved their functioning as memory elements by applying magnetic field pulses in opposite directions that can change the F layer magnetization.

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 report on the magnetotransport properties of a spin-polarized two-dimensional electron system (2DES) formed in LaAlO3(LAO)/EuTiO3/SrTiO3 (STO) heterostructures. We show that, at low temperature, the 2DES magnetoconductance exhibits weak antilocalization corrections related to Rashba spin-orbit scattering, in analogy with the LAO/STO 2DES. However, the characteristic spin-orbit scattering field decreases substantially for carrier density higher than 1.9×1013cm-2. We attribute this behavior to the masking effect of ferromagnetism, which sets in at the same carrier density and at a temperature below 10 K. Our work shows that, while weak antilocalization corrections to the magnetoconductance are strongly reduced by the emergence of ferromagnetism, they persist in a large part of the phase diagram of a spin-polarized oxide 2DES.

We report on measurements of the switching current distributions on two-dimensional superconducting NbTiN strips that are 5 nm thick and 80 nm wide. We observe that the width of the switching current distributions has a non-monotonous temperature dependence, where it is constant at the lowest temperatures up to about 1.5 K, after which it increases with temperature until 2.2 K. Above 2.5 K any increase in temperature decreases the distribution width which at 4.0 K is smaller than half the width observed at 0.3 K. By using a careful analysis of the higher order moments of the switching distribution, we show that this temperature dependence is caused by switching due to multiple fluctuations. We also find that the onset of switching by multiple events causes the current dependence of the switching rate to develop a characteristic deviation from a pure exponential increase, that becomes more pronounced at higher temperatures, due to the inclusion of higher order terms.

We test the effect of an external RF field on the switching processes of magnetic Josephson junctions (MJJs) suitable for the realization of fast, scalable cryogenic memories compatible with Single Flux Quantum logic. We show that the combined application of microwaves and magnetic field pulses can improve the performances of the device, increasing the separation between the critical current levels corresponding to logical '0' and '1'. The enhancement of the current level separation can be as high as 80% using an optimal set of parameters. We demonstrate that external RF fields can be used as an additional tool to manipulate the memory states, and we expect that this approach may lead to the development of new methods of selecting MJJs and manipulating their states in memory arrays for various applications., Comment: 7 pages, 3 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.

The phenomenon of dark counts in nanostripes of different superconductor systems such as high-temperature superconducting YBa 2 Cu 3 O 7-x and superconductor/ferromagnet hybrids consisting of either NbN/NiCu or YBa 2 Cu 3 O 7-x /L 0.7 Sr 0.3 MnO 3 bilayers have been investigated. For NbN/NiCu the rate of dark-count transients have been reduced with respect to pure NbN nanostripes and the events were dominated by a single vortex entry from the edge of the stripe. In the case of nanostripes based on YBa 2 Cu 3 O 7-x , we have found that thermal activation of vortices was also, apparently, responsible for triggering dark-count signals.

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