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International audience; Nature has engineered complex designs to achieve advanced properties and functionalities through millions of years of evolution. Many organisms have adapted to their living environments by producing extremely efficient materials and structures exhibiting optimized mechanical, thermal, and optical properties, which current technology is often unable to reproduce. These properties are often achieved using hierarchical structures spanning macro-, meso-, micro-, and nanoscales, widely observed in many natural materials like wood, bone, spider silk, and sponges. Thus far, bioinspired approaches have been successful in identifying optimized structures in terms of quasistatic mechanical properties, such as strength, toughness, and adhesion, but comparatively little work has been done as far as dynamic ones are concerned (e.g., vibration damping, noise insulation, sound amplification). In particular, relatively limited knowledge currently exists on how hierarchical structure can play a role in the optimization of natural structures, although concurrent length scales no doubt allow multiple frequency ranges to be addressed. Here, we review the main work that has been done to analyze structural optimization for dynamic mechanical properties, highlighting some common traits and strategies in different biological systems. We also discuss the relevance to bioinspired materials, in particular in the field of phononic crystals and metamaterials, and the potential of exploiting natural designs for technological applications.

Thermally conductive nanopapers fabricated from graphene and related materials are currently showing a great potential in thermal management applications. However, thermal contacts between conductive plates represent the bottleneck for thermal conductivity of nanopapers prepared in the absence of a high temperature step for graphitization. In this work, the problem of ineffective thermal contacts is addressed by the use of bifunctional polyaromatic molecules designed to drive self-assembly of graphite nanoplates (GnP) and establish thermal bridges between them. To preserve the high conductivity associated to defect-free sp2 structure, non-covalent functionalization with bispyrene compounds, synthesised on purpose with variable tethering chain length, was exploited. Pyrene terminal groups granted for a strong {\pi}-{\pi} interaction with graphene surface, as demonstrated by UV-Vis, fluorescence and Raman spectroscopies. Bispyrene molecular junctions between GnP were found to control GnP organization and orientation within the nanopaper, delivering significant enhancement in both in-plane and cross-plane thermal diffusivity. Finally, nanopapers were validated as heat spreader devices for electronic components, evidencing comparable or better thermal dissipation performance than conventional Cu foil, while delivering over 90% weight reduction.

The extreme miniaturization in NEMS resonators offers the possibility to reach an unprecedented resolution in high-performance mass sensing. These very low limits of detection are related to the combination of two factors: a small resonator mass and a high quality factor. The main drawback of NEMS is represented by the highly complex, multi-steps, and expensive fabrication processes. Several alternatives fabrication processes have been exploited, but they are still limited to MEMS range and very low-quality factor. Here we report the fabrication of rigid NEMS resonators with high-quality factors by a 3D printing approach. After a thermal step, we reach complex geometry printed devices composed of ceramic structures with high Young’s modulus and low damping showing performances in line with silicon-based NEMS resonators ones. We demonstrate the possibility of rapid fabrication of NEMS devices that present an effective alternative to semiconducting resonators as highly sensitive mass and force sensors., NEMS devices, nano-electro-mechanical systems, by virtue of their minute size, offer ultra-high sensitivity, though at the expense of manufacturing complexity. Here, Stassi et al succeed in manufacturing high quality factor NEMS devices using high resolution 3D printing.

In this work, we performed a systematic study on the effect of the geometry of pre-patterned templates and spin-coating conditions on the self-assembling process of colloidal nanospheres. To achieve this goal, large-scale templates, with different size and shape, were generated by direct laser-writer lithography over square millimetre areas. When deposited over patterned templates, the ordering dynamics of the self-assembled nanospheres exhibits an inverse trend with respect to that observed for the maximisation of the correlation length &xi, on a flat surface. Furthermore, the self-assembly process was found to be strongly dependent on the height (H) of the template sidewalls. In particular, we observed that, when H is 0.6 times the nanospheres diameter and spinning speed 2500 rpm, the formation of a confined and well ordered monolayer is promoted. To unveil the defects generation inside the templates, a systematic assessment of the directed self-assembly quality was performed by a novel method based on Delaunay triangulation. As a result of this study, we found that, in the best deposition conditions, the self-assembly process leads to well-ordered monolayer that extended for tens of micrometres within the linear templates, where 96.2% of them is aligned with the template sidewalls.

International audience; Nonlinear mesoscopic materials exhibit anomalous elastic nonlinearity in which fast and slow dynamics effects are mixed up. The former is an instantaneous nonlinear phenomenon due to the explicit strain dependence of velocity and damping. Slow dynamics is a non-equilibrium effect, governed by the dependence of the linear modulus and Q-factor on the dynamic strain level: when excited at constant strain, the sample properties vary in time (over minutes or more) until they reach a new equilibrium state. When excitation is removed, again slowly in time, the system recovers its original viscoelastic properties. The goal of this contribution is to show how slow dynamics might affect fast dynamics measurements and thus point out that care should be given to all time scales of the experiment, e.g. lag time between amplitude change and acquisition, duration of the acquisition and relaxation time between successive measurements.

We fabricate ion-gated field-effect transistors (iFET) on mechanically exfoliated multilayer MoS$_2$. We encapsulate the flake by Al$_2$O$_3$, leaving the device channel exposed at the edges only. A stable Li$^+$ intercalation in the MoS$_2$ lattice is induced by gating the samples with a Li-based polymeric electrolyte above $\sim$ 330 K and the doping state is fixed by quenching the device to $\sim$ 300 K. This intercalation process induces the emergence of anomalies in the temperature dependence of the sheet resistance and its first derivative, which are typically associated with structural/electronic/magnetic phase transitions. We suggest that these anomalies in the resistivity of MoS$_2$ can be naturally interpreted as the signature of a transition to a charge-density-wave phase induced by lithiation, in accordance with recent theoretical calculations., Comment: 8 pages, 4 figures

We have irradiated with Au ions (having an energy of 250 MeV) epitaxial thin films of BaFe2(As(1-x)P(x))2 with x @ 0.2 (optimal doping), grown by MBE on MgO single-crystal substrates. We have studied the effects of irradiation (for three different fluences, F=2.4×10^11 cm^-2, 4.8×10^11 cm^-2 and 7.3×10^11 cm^-2) on the surface morphology, on the resistivity and on the critical temperature. We have found that irradiation progressively destroys the very clear and interconnected growth terraces typical of the pristine surface, leading first to their smoothing - accompanied by the appearance of localized defects and cracks - and then to a completely disordered surface. However, the resistivity increases only slightly and the critical temperature decreases very little (i.e. by about 2%) going from the pristine film to the most-irradiated one. The latter result is confirmed by local measurements of the critical temperature carried out by point-contact Andreev reflection spectroscopy.

The nonlinear elastic response of consolidated granular or damaged materials is the result of the combination of nonlinear attenuation and velocity, coupled with hysteresis, which is linked to non equilibrium effects (often termed conditioning). Thus, a preliminary step towards the comprehension of the physical mechanisms responsible of the nonlinear elastic behaviours consists in quantifying and separating the various contributions. To this purpose, an approach based on a semi-analytical treatment of signals resulting from a monochromatic continuous wave excitation can be successfully implemented. Its validation is discussed here, applying the proposed approach to the analysis of numerical data obtained by using a finite difference spring model code. The accuracy, sensibility and robustness of the protocol are verified in different nonlinear conditions.

International audience; Consolidated granular materials and materials with damage at the microstructural level exhibit anomalous elastic behavior even when excited by low-amplitude elastic waves. Their response is given by a combination of slow- and fast-dynamics effects which, as their definition implies, act on very different time scales. In particular, conditioning (a transition to an elastic state dependent on the strain amplitude) and relaxation (full recovery of the elastic properties when the strain is removed) have been observed in different materials and under different dynamic excitations. An experimental parametric analysis of the phenomenon, aiming to establish correlations between the effects on different elastic physical properties (wave velocity and attenuation coefficient) and between the evolution of conditioning and relaxation is proposed here, with the goal of better characterizing slow dynamics and allowing one to go beyond the phenomenological description of elastic hysteresis currently available. At the same time, by studying different materials, we wish to highlight the possibility of using slow dynamics as an additional tool for materials characterization.

The identification and the analysis of the non-linear behavior of a structure under proper excitation is a powerful tool to obtain information on its mechanical properties and for early-detecting possible internal damages. Among the techniques proposed for this purpose, Pulse-Compression-based techniques and the Scaling Subtraction Method seem to be among the most efficient ones for both damage detection and classification. In this work, the two methods have been compared under the same experimental conditions to highlight their respective advantages and/or complementarity. A laboratory test experiment has been performed on mortar samples, to analyze the efficiency of the two approaches in detecting early damage in samples which already exhibit a nonlinear response in their intact state.

When excited by elastic waves, nonlinear mesoscopic elastic materials (for example, consolidated granular materials) show, at the same time, fast and slow dynamics effects. Here we introduce suitable parameters for quantifying them and for studying their correlations. We found that significant differences between intact and damaged samples can be observed by studying slow dynamics effects. The longitudinal profiles of the surface deformations induced by a sinusoidal excitation of a cracked sample were also investigated during different measurement configurations, showing that a considerable distortion of the original profile is induced by conditioning. These results can be useful for studying the behavior of cracks and defects in the nonclassical nonlinear regime, in view of their modeling at the microstructural level and for applications in innovative nondestructive test and evaluation techniques.

Even if soft tissue adhesion on titanium surfaces is an issue of great interest for transmucosal dental implants and percutaneous devices, few specific surface modification strategies have been approached so far to face it. Topographical surface modification is an interesting strategy because quality of soft tissues can be effectively improved by contact guidance effect on fibroblasts. Conversely, biofilm formation is encouraged in presence of rough or porous surfaces, as well as of surface patterns, if the roughness overcome the threshold of 0.2 μm. This issue becomes particularly critical in medical devices which cross different biological tissues and, through soft tissues, establish a direct contact with the external environment. In the present research, three different grooved patterns (5, 10 and 30 μm wide) were produced on commercially pure titanium by means of Electron Beam (EB) structuring maintaining the average surface roughness lower than 0.2 μm. The EB surface structured samples were characterized by means of optical and atomic force microscopy, contact and non-contact profilometry, X-Ray Diffraction, residual stress measurements as well as bacteria and cell adhesion tests. The 10 μm wide grooves resulted able to support fibroblast alignment onto the metallic substrate. Unexpectedly, all the EB surface structured surfaces not only did not enhance, but even reduced bacterial adhesion up to 48 h culture.

[Image: see text] We performed scanning thermal microscopy measurements on single layers of chemical-vapor-deposited (CVD) graphene supported by different substrates, namely, SiO(2), Al(2)O(3), and PET using a double-scan technique to remove the contribution to the heat flux through the air and the cantilever. Then, by adopting a simple lumped-elements model, we developed a new method that allows determining, through a multistep numerical analysis, the equivalent thermal properties of thermally conductive coatings of nanometric thickness. In this specific case we found that our CVD graphene is “thermally equivalent”, for heat injection perpendicular to the graphene planes, to a coating material of conductivity k(eff) = 2.5 ± 0.3 W/m K and thickness t(eff) = 3.5 ± 0.3 nm in perfect contact with the substrate. For the SiO(2) substrate, we also measured stacks made of 2- and 4-CVD monolayers, and we found that the effective thermal conductivity increases with increasing number of layers and, with a technologically achievable number of layers, is expected to be comparable to that of 1 order of magnitude-thicker metallic thin films. This study provides a powerful method for characterizing the thermal properties of graphene in view of several thermal management applications.

Nonlinear ultrasound measurements are very sensitive to the presence of small changes in the microstructure of diverse materials. Specifically, in nonlinear mesoscopic materials, fast- and slow-dynamics effects are usually occurring simultaneously. We used an approach which allowed us to quantify fast- and slow-dynamics (during both conditioning and relaxation) almost in real time, i.e. also close to the very beginning of the conditioning process (early stages). Nonlinear and non-equilibrium parameters were extracted from the measurements and studied as a function of conditioning amplitude and conditioning time. Moreover, the functional dependence of velocity and damping was investigated from the early stages to the equilibrium state.

A well-organized immobilization of bio-receptors is a crucial goal in biosensing, especially to achieve high reproducibility, sensitivity and specificity. These requirements are usually attained with a controlled chemical/biochemical functionalization that creates a stable layer on a sensor surface. In this work, a chemical modification protocol for silicon-based surfaces to be applied in biosensing devices is presented. An anhydrous silanization step through 3-aminopropylsilane (APTES), followed by a further derivatization with succinic anhydride (SA), is optimized to generate an ordered flat layer of carboxylic groups. The properties of APTES/SA modified surface were compared with a functionalization in which glutaraldehyde (GA) is used as crosslinker instead of SA, in order to have a comparison with an established and largely applied procedure. Moreover, a functionalization based on the controlled deposition of a plasma polymerized acrylic acid (PPAA) thin film was used as a reference for carboxylic reactivity. Advantages and drawbacks of the considered methods are highlighted, through physico-chemical characterizations (OCA, XPS, and AFM) and by means of a functional Protein G/Antibody immunoassay. These analyses reveal that the most homogeneous, reproducible and active surface is achieved by using the optimized APTES/SA coupling.

Optical fibers have been employed for several years in biomedicine and mainly used for light delivery and collection with high efficiency and selectivity. So far most of the research effort has been devoted to the optical configurations and the functions of the fibers rather than on the materials employed. Indeed, this aspect has been mainly considered to assure biocompatibility with tissues and non-toxic behavior limiting the choice mostly to silicate fibers. We report on the recent advances in engineering inorganic glass optical and hollow fibers fabricated with optical glasses which are also resorbable in body fluids. Suitable phosphate glass compositions were designed to combine resorbability and optical transparency. Glasses were fabricated by melt-quenching inside a chamber furnace under a flux of dried air and cast into preheated brass molds. The fibers were obtained by preform drawing, with the core rod fabricated by melt quenching whilst the cladding tube by rotational casting. Extrusion techniques were also applied to obtain more complex cross sections with increased functionalities. Glasses and fibers were characterized in their physical and optical properties. The materials showed high stability toward crystallization and a wide optical transmission window, ranging from the ultraviolet to the near infrared wavelength regions. Hollow fibers were employed to demonstrate multifunctional fiber probes, able to provide drug delivery and light excitation in the prospect of developing resorbable endoscopes for intravital monitoring and therapy, such as photodynamic therapy. Optimization of drug delivery was carried out using functionalization procedures on the surface of both bulk glasses and hollow fibers, aiming to modify the release kinetics: a silanization protocol was developed and successfully tested using different organic compounds. The modification of the surface roughness was monitored using atomic force microscopy, while surface energy changes verified using contact angle measurements. The possibility of performing drug excitation was assessed by guiding light through the capillary using optical beams produced by different wavelength sources covering the visible spectrum. Finally, mechanical characterization of the prepared optical fibers and hollow fibers was carried out to measure the elastic moduli, the tensile strength and the minimum radius of curvature attainable. The overall results allowed to demonstrate the reliability of the proposed optical fibers and hollow fibers for biomedical applications.

In this work, the preparation of nanocomposites based on poly(l-lactide) PLLA and graphite nanoplatelets (GNP) was assessed by applying, for the first time, the reactive extrusion (REX) polymerization approach, which is considered a low environmental impact method to prepare polymer systems and which allows an easy scalability. In particular, ad hoc synthesized molecules, constituted by a pyrene end group and a poly(d-lactide) (PDLA) chain (Pyr-d), capable of interacting with the surface of GNP layers as well as forming stereoblocks during the ring-opening polymerization (ROP) of l-lactide, were used. The nanocomposites were synthesized by adding to l-lactide the GNP/initiator system, prepared by dispersing the graphite in the acetone/Pyr-d solution, which was dried after the sonication process. DSC and X-ray diffraction measurements evidenced the stereocomplexation of the systems synthesized by using the pyrene-based initiators, whose extent turned out to depend on the PDLA chain length. All the prepared nanocomposites, including those synthesized starting from a classical initiator, that is, 1-dodecanol, retained similar electrical conductivity, whereas the thermal conductivity was found to increase in the stereocomplexed samples. Preferential localization of stereocomplexed PLA close to the interface with GNP was demonstrated by scanning probe microscopy (SPM) techniques, supporting an important role of local crystallinity in the thermal conductivity of the nanocomposites., Ad hoc synthesized pyrene-based molecules allow enhancement of thermal conductivity of PLA/GNP nanocomposites prepared by environmentally friendly reactive extrusion.

In a recent paper, we presented the first directional point-contact Andreev reflection spectroscopy (PCARS) measurements on CaFe2As2 crystals under quasi-hydrostatic pressure and discussed the pressure dependence of the gaps and the critical temperature. While T c exhibits a well-known smooth dependence and a broad maximum at about 0.6 GPa, both gaps increase very sharply in a small pressure range between 0.5 and 0.6 GPa, leading to a doubling of the ratio 2Δ2/k B T c and a quadruplication of the ratio 2Δ1/k B T c , Δ1 and Δ2 being the small and large gap, respectively. This peculiar behavior is likely to be related to a sharp change in the lattice structure that, in turn, produces a 2D-3D topological transition in the hole-like Fermi surface sheet. In this work, we show that, within an effective three-band Eliashberg theory for the electron-spin fluctuation coupling (s± symmetry), these results can be rationalized as being due to a large increase of the electron-boson coupling, which mimics the effects of a boost mechanism for the coupling related to the underlying Lifshitz transition.

We investigated the Josephson effect between the conventional superconductor Pb0.7In0.3 and Ba0.4K0.6 (FeAs)2 single crystals by performing directional point-contact experiments. The junctions were irradiated with microwaves and the behavior of the Shapiro steps as a function of the square root of the microwave power was studied by means of the RSJ model extended to the nonautonomous case with an rf current-source term. The analysis reveals that, for current injection along the ab plane, the current-phase relation deviates from being simply described by the standard sin(ϕ) (as we indeed observed along the c axis) and a dominant sin(2ϕ) component is observed. The results are compared with a theory of the dc Josephson effect, formulated by using a method based on the calculation of temperature Green’s function in the junction within the tight-binding model (Burmistrova et al., Phys. Rev. B 91, 214501, 2015). The comparison shows that the experimental results here presented strongly favor an s ±-wave symmetry of the order parameter in this compound.

We fabricate electric double-layer field-effect transistor (EDL-FET) devices on mechanically exfoliated few-layer graphene. We exploit the large capacitance of a polymeric electrolyte to study the transport properties of three, four and five-layer samples under a large induced surface charge density both above and below the glass transition temperature of the polymer. We find that the carrier mobility shows a strong asymmetry between the hole and electron doping regime. We then employ ab-initio density functional theory (DFT) calculations to determine the average scattering lifetime from the experimental data. We explain its peculiar dependence on the carrier density in terms of the specific properties of the electrolyte we used in our experiments., 6 pages, 3 figures

The design and realization of a cryogenic rig for point-contact spectroscopy measurements in the needle-anvil configuration is presented. Thanks to the use of two piezoelectric nano-positioners, the tip can move along the vertical ($z$) and horizontal ($x$) direction and thus the rig is suitable to probe different regions of a sample \textit{in situ}. Moreover, it can also form double point-contacts on different facets of a single crystal for achieving, e.g., an interferometer configuration for phase-sensitive measurements. For the latter purpose, the sample holder can also host a Helmholtz coil for applying a small transverse magnetic field to the junction. A semi-rigid coaxial cable can be easily added for studying the behavior of Josephson junctions under microwave irradiation. The rig can be detached from the probe and thus used with different cryostats. The performance of this new probe has been tested in a Quantum Design PPMS system by conducting point-contact Andreev reflection measurements on Nb thin films over large areas as a function of temperature and magnetic field., Comment: 7 pages, 7 figures, published in Rev. Sci. Instrum

This paper reports on the thermal properties of reduced graphite oxide (RGO) flakes, studied by means of scanning thermal microscopy (SThM). This technique was demonstrated to allow thermal characterization of the flakes with a spatial resolution of the order of a few tens of nanometers, while recording nanoscale topography at the same time. Several individual RGO flakes were analyzed by SThM, both as obtained after conventional thermal reduction and after a subsequent annealing at 1700 °C. Significant differences in the thermal maps were observed between pristine and annealed flakes, reflecting higher heat dissipation on annealed RGO flakes compared with pristine ones. This result was correlated with the reduction of RGO structure defectiveness. In particular, a substantial reduction of oxidized groups and sp3 carbons upon annealing was proven by X-ray photoelectron and Raman spectroscopies, while the increase of crystalline order was demonstrated by X-ray diffraction, in terms of higher correlation lengths both along and perpendicular to the graphene planes. Results presented in this paper provide experimental evidence for the qualitative correlation between the defectiveness of graphene-related materials and their thermal conductivity, which is clearly crucial for the exploitation of these materials into thermally conductive nanocomposites.

The interest in clean energy is of increasing importance nowadays. In this regard, hydrogen production from sun-assisted water splitting is a field of intense research in the scientific community. In the design of artificial innovative devices devoted to this task, the simultaneous transport of protons and electrons from an anode (where they are generated by water oxidation) to a cathode (where they have to recombine in order to form molecular hydrogen) is essential. Nafion, a sulfonated fluoropolymer, is of great interest because of its proven ability as a proton-conducting membrane in fuel cells, and also for its good thermal and mechanical stability. Carbon nanotubes (CNTs), rolls of graphene sheets, possess a high thermal and mechanical stability together with excellent electron conductivity. Nafion/CNTs nanocomposites with CNT amounts above the percolation threshold have been recently obtained which are capable of simultaneously conducting protons and electrons. Here we report on the fabrication of nafion/CNTs membranes in an alternative configuration, namely vertically-aligned carbon nanotubes columns embedded in a nafion matrix. Nanocomposite films of nafion embedded with an array of CNT columns were prepared by solvent casting. Films of thickness around 100 μm were obtained. They were characterized by scanning electron microscopy, electrochemical impedance spectroscopy, electronic conductivity tests, water uptake and water contact angle. Electronic and protonic conductivities were measured in ambient and wet conditions and turned out to be higher than those previously reported for nafion/CNTs nanocomposites in the conditions where such devices are expected to operate.

Point-contact Andreev-reflection (PCAR) experiments were performed in the Fe-1111 layered superconductor LaFeAsO0.9F0.1 with resistive T con ∼ 27 K. The observation of two pairs of peaks in the low-temperature Andreev-reflection spectra clearly indicates the presence of two order parameters. The behavior of the two gaps as a function of temperature, obtained by fitting the conductance curves by means of the generalized two-band Blonder-Tinkham-Klapwijk model, shows some anomalies. A theoretical analysis performed within the two-band Eliashberg theory with a generic electron-boson coupling can reproduce the low-temperature value of the two gaps but generally fails in giving a satisfactory fit of their overall temperature dependence, indicating the rich and complex physics of these newly discovered superconductors.

We discuss the results of point-contact Andreev-reflection spectroscopy (PCAR) measurements in SmFeAsO0.8F0.2 state-of-the-art polycrystals, with critical temperature Tc≈53 K. The low-temperature conductance curves show clear peaks at about 7 meV and additional shoulders at 16–20 meV. Their shape is similar to that of PCAR spectra in MgB2 and suggests the presence of two superconducting energy gaps even in this Fe-based superconductor. The fit of the conductance curves with a two-band BTK model, up to Tc, further supports this indication in spite of a marked asymmetry in the conductance curves for positive/negative bias. The gaps obtained from the fit are Δ1=6.15±0.45 meV and Δ2=18±3 meV, and they follow a nice BCS-like temperature dependence, closing both at the same Tc. Our results are discussed in comparison with experimental and theoretical results in this and other Fe-based superconductors.

We summarize the results of point-contact Andreev-reflection (PCAR) spectroscopy in MgB2 doped by chemical substitutions, either magnetic (Mn) or non-magnetic (Al, C), obtained by us and by other groups in the last four years. Despite the variety of samples used (crystals and polycrystals of various origin) and some minor differences in the experimental techniques, these measurements have directly provided a complete and consistent picture of the effects of chemical substitutions on the gaps of MgB2 shedding light on other relevant parameters (scattering rates, DOSs) affected by doping. In Al-doped crystals and polycrystals, the gap amplitudes Δσ and Δπ – obtained through a two-band Blonder-Tinkham-Klapwijk (BTK) fit of the Andreev-reflection conductance curves – decrease on increasing the Al content x (i.e. on decreasing the critical temperature of the contacts T c A ), but remain clearly distinct with no evidence of gap merging even when T c A is of the order of 10 K. The case of C-substituted MgB2 is less clear: the merging of the two gaps at x ≃ 0.13 was observed in single crystals grown at ETH, while the persistence of two gaps up to x = 0.10 – with apparently no tendency to merging at higher x – was found in other crystals and polycrystals. Finally, in Mn-doped single crystals the effects of pair-breaking magnetic scattering strongly suppress the critical temperature and the gaps, which however remain clearly distinct from each other down to Tc ≈ 10 K. In all cases, the analysis of the Δσ and Δπ data as a function of T c A (or of the doping content x) within the two-band Eliashberg theory gives interesting information about the effects of substitutions on the different scattering channels (interband and intraband, magnetic or non-magnetic).

We present theory of dc Josephson effect in contacts between Fe-based and spin-singlet $s$-wave superconductors. The method is based on the calculation of temperature Green's function in the junction within the tight-binding model. We calculate the phase dependencies of the Josephson current for different orientations of the junction relative to the crystallographic axes of Fe-based superconductor. Further, we consider the dependence of the Josephson current on the thickness of an insulating layer and on temperature. Experimental data for PbIn/Ba$_{1-x}$K$_{x}$(FeAs)$_2$ point-contact Josephson junctions are consistent with theoretical predictions for $s_{\pm}$ symmetry of an order parameter in this material. The proposed method can be further applied to calculations of the dc Josephson current in contacts with other new unconventional multiorbital superconductors, such as $Sr_2RuO_4$ and superconducting topological insulator $Cu_xBi_2Se_3$., 16 pages, 14 figures

The temperature dependence of electric transport properties of single-layer and few-layer graphene at large charge doping is of great interest both for the study of the scattering processes dominating the conductivity at different temperatures and in view of the theoretically predicted possibility to reach the superconducting state in such extreme conditions. Here we present the results obtained in 3-, 4- and 5-layer graphene devices down to 3.5 K, where a large surface charge density up to about 6.8x10^14 cm^(-2) has been reached by employing a novel polymer electrolyte solution for the electrochemical gating. In contrast with recent results obtained in single-layer graphene, the temperature dependence of the sheet resistance between 20 K and 280 K shows a low-temperature dominance of a T^2 component - that can be associated with electron-electron scattering - and, at about 100 K, a crossover to the classic electron-phonon regime. Unexpectedly this crossover does not show any dependence on the induced charge density, i.e. on the large tuning of the Fermi energy., Comment: 13 pages, 6 color figures

Electrodes with novel architectures may enable the construction of high power density batteries with ultrafast charge/discharge performances, which are fundamental for todays power tools, medical devices and transportation systems. Hereby, we demonstrate the application of vertically arranged multilayered-graphene, namely carbon nanowalls (CNW), as a potential high power anode to be used in Li-ion secondary batteries. CNWs are deposited onto Cu foils using microwave plasma enhanced chemical vapor deposition (MPE-CVD) technique. MPE-CVD is a simple and rapid technique, which is effectively proven to produce self-standing layered anodes by single step processing. The direct deposition over Cu avoids the addition of foreign ingredients such as binders and conductive agents. This opens up the possibility to reduce the number of processing steps and processing time, which are key aspects when upscaling is sought. The superior cycling behaviour of the newly elaborated CNWs is demonstrated in lab-scale Li-metal and Li-ion cells (LiFePO4 cathode) capable of very stable and prolonged reversible cycling with excellent durability (>1000 cycles), good specific capacity and capacity retention when subjected to ultrafast current regimes.

In the first part of the present paper we discuss the fabrication and the characterization of an MgB 2 -based SQUID magnetometer with a directly coupled large-area pick-up loop, made on an MgB 2 film deposited by an all in situ technique. The coarse structure of the SQUID was defined by optical lithography and Ar-ion milling, while the two nanobridges acting as weak links in the superconducting loop were made by focused ion beam (FIB) milling. The device was characterized at different temperatures and showed Josephson quantum interference up to 20 K as well as a noise level already compatible with the recording of an adult magnetocardiogram. In the second part, concerning the fundamental physics of MgB 2 , we present the results of very recent point-contact measurements on Mg 1− x Mn x B 2 single crystals with 34.1 ⩾ T c ⩾ 13.3 K (i.e. 0.37% ⩽ x ⩽ 1.5%). The experimental conductance curves were fitted with the generalized two-band BTK model and their behaviour in magnetic fields was studied to check if both the order parameters (OPs) of the σ and π bands were present in the whole doping range. The dependence of the OPs (evaluated through the fit) on the Andreev critical temperature of the junctions is analyzed in the framework of the two-band Eliashberg theory by including the effects of magnetic impurities. The results give an evidence of a dominant effect of the magnetic impurities on the σ-band channel.

We report the results of directional point-contact measurements in Mg(B\(_{1-x}\)C\(_{x})_{2}\) single crystals. The amplitudes of the gaps, \(\Delta_{\pi}\) and \(\Delta_{\sigma}\), were determined for each C content by fitting the experimental low-temperature normalized conductance curves of our “soft” point contacts with the BTK model generalized to the two-band case. We found that, on increasing the carbon content, \(\Delta_{\sigma}\) decreases almost linearly with \(T_{c}\) and \(\Delta_{\pi}\) slightly increases until, at \(x=0.132\) (where \(T_{c}=19\) K), they assume the same value \(\Delta =3.2 \pm 0.9\) meV. This result is confirmed by the temperature and magnetic-field dependence of the conductance curves at this C content, which do not show any evidence of two distinct gap values. In particular, the Δ versus T curve follows very well a standard BCS curve, with a gap ratio \(2\Delta /k_{B} T_{c}=3.9\). These experimental findings are compared to the theoretical predictions of the two-band model in the Eliashberg formulation.

In this study, we report a different strategy to prepare graphene nanopapers from direct vacuum filtration. Instead of the conventional method, i.e. thermal annealing nanopapers at extremely high temperatures prepared from graphene oxide (GO) or partially reduced GO, we fabricate our graphene nanopapers directly from suspensions of fully reduced graphene oxide (RGO), obtained after RGO and thermally annealing at 1700 {\deg}C in vacuum. By using this approach, we studied the effect of thermal annealing on the physical properties of the macroscopic graphene-based papers. Indeed, we demonstrated that the enhancement of the thermal and electrical properties of graphene nanopapers prepared from annealed RGO is strongly influenced by the absence of oxygen functionalities and the morphology of the nanoflakes. Hence, our methodology can be considered as a valid alternative to the classical approach.

A new-designed transparent, conductive and porous electrode was developed for application in a compact laboratory-scale proton exchange membrane (PEM) photo-electrolyzer. The electrode is made of a thin transparent quartz sheet covered with fluorine-doped tin oxide (FTO), in which an array of holes is laser-drilled to allow water and gas permeation. The electrical, morphological, optical and electrochemical characterization of the drilled electrodes is presented in comparison with a non-drilled one. The drilled electrode exhibits, in the visible region, a good transmittance (average value of 62%), a noticeable reflectance due to the light scattering effect of the hole-drilled internal region, and a higher effective surface area than the non-drilled electrode. The proof-of-concept of the applicability of the drilled electrode was achieved by using it as a support for a traditional photocatalyst (i.e. commercial TiO 2 nanoparticles). The latter, coupled with a polymeric electrolyte membrane (i.e.Nafion 117) and a Pt counter electrode, forms a transparent membrane electrode assembly (MEA), with a good conductivity, wettability and porosity. Electrochemical impedance spectroscopy (EIS) was used as a very powerful tool to gain information on the real active surface of the new drilled electrode and the main electrochemical parameters driving the charge transfer reactions on it. This new electrode architecture is demonstrated to be an ideal support for testing new anodic and cathodic photoactive materials working in tandem configuration for solar fuels production by water photo-electrolysis.

We performed point-contact Andreev reflection spectroscopy measurements in Ca(Fe_{1-x}Co_x)2As2 single crystals with effective x=0.060 +- 0.005. The spectra of ab-plane contacts show a zero-bias maximum and broad shoulders at about 5-6 meV. Their fit with the three-dimensional Blonder-Tinkham-Klapwijk model (making use of a analytical expression for the Fermi surface that mimics the one calculated from first principles) shows that this compound presents a large isotropic gap on the quasi-2D electronlike Fermi surface sheets and a smaller anisotropic (possibly nodal) gap on the 3D holelike Fermi surface pockets centered at the Z point in the Brillouin zone. These results nicely fit into the theoretical picture for the appearance of nodal superconductivity in 122 compounds., Comment: 11 pages, 3 figures

The field-effect technique, popular thanks to its application in common field-effect transistors, is here applied to metallic thin films by using as a dielectric a novel polymer electrolyte solution. The maximum injected surface charge, determined by a suitable modification of a classic method of electrochemistry called double-step chronocoulometry, reached some units in 10^15 charges/cm^2. At room temperature, relative variations of resistance up to 8%, 1.9% and 1.6% were observed in the case of gold, silver and copper, respectively and, if the films are thick enough (> 25 nm), results can be nicely explained within a free-electron model with parallel resistive channels. The huge charge injections achieved make this particular field-effect technique very promising for a vast variety of materials such as unconventional superconductors, graphene and 2D-like materials., 6 pages, 6 figures. Applied Surface Science, in press

We report on the results of directional point-contact Andreev-reflection (PCAR) measurements in Ba(Fe_{1-x}Co_x)2As2 single crystals and epitaxial c-axis oriented films with x = 0.08 as well as in Ca(Fe_{1-x}Co_x)2As2 single crystals with x = 0.06. The PCAR spectra are analyzed within the two-band 3D version of the Blonder-Tinkham-Klapwijk model for Andreev reflection we recently developed, and that makes use of an analytical expression for the Fermi surface that mimics the one calculated within the density-functional theory (DFT). The spectra in Ca(Fe_{0.94}Co_{0.06})2As2 unambiguously demonstrate the presence of nodes or zeros in the small gap. In Ba(Fe_{0.92}Co_{0.08})2As2, the ab-plane spectra in single crystals can be fitted by assuming two nodeless gaps, but this model fails to fit the c-axis ones in epitaxial films. All these results are discussed in comparison with recent theoretical predictions about the occurrence of accidental 3D nodes and the presence of "hot spots" in the gaps of 122 compounds., 7 pages, 4 figures. Journal of Superconductivity and Novel Magnetism, in press

In this paper we propose a model to reproduce superconductive and normal properties of the iron pnictide LiFeAs in the framework of the four-band spm wave Eliashberg theory. A confirmation of the multiband nature of the system rises from the experimental measurements of the superconductive gaps and resistivity as function of temperature. We found that the most plausible mechanism is the antiferromagnetic spin fluctuation and the estimated values of the total antiferromagnetic spin fluctuation coupling constant in the superconductive and normal state are lambda{tot}=2.00 and lambda{tot,tr}=0.77., Comment: 5 pages, 3 figures

A century on from its discovery, a complete fundamental understanding of superconductivity is still missing. Considerable research efforts are currently devoted to elucidating mechanisms by which pairs of electrons can bind together through the mediation of a boson field different than the one associated to the vibrations of a crystal lattice. PuCoGa_5, a 5f-electron heavy-fermion superconductor with a record critical temperature T_c=18.5 K, is one of the many compounds for which the short-range, isotropic attraction provided by simple electron-phonon coupling does not appear as an adequate glue for electron pairing. Here, we report the results of point-contact spectroscopy measurements in single crystals of PuCoGa_5. Andreev reflection structures are clearly observed in the low-temperature spectra, and unambiguously prove that the paired superconducting electrons have wavefunction with the d-wave symmetry of a four-leaf clover. A straightforward analysis of the spectra provide the amplitude of the gap and its temperature dependence, \Delta(T). We obtain \Delta(T -> 0) = 5.1 \pm 0.3 meV and a gap ratio, 2\Delta/k_B T_c = 6.5 \pm 0.3, indicating that the compound is in the regime of strong electron-boson coupling. The gap value and its temperature dependence can be well reproduced within the Eliashberg theory for superconductivity if the spectral function of the mediating bosons has a spin-fluctuations-like shape, with a peak energy of 6.5 meV. Electronic structure calculations, combining the local density approximation with an exact diagonalization of the Anderson impurity model, provide a hint about the possible origin of the fluctuations., Comment: Main article with 4 figures, and supplementary info with 7 figures

By using an electrochemical gating technique with a new combination of polymer and electrolyte, we were able to inject surface charge densities n_2D as high as 3.5 \times 10^15 e/cm^2 in gold films and to observe large relative variations in the film resistance, DeltaR/R', up to 10% at low temperature. DeltaR/R' is a linear function of n_2D - as expected within a free-electron model - if the film is thick enough (> 25 nm), otherwise a tendency to saturation due to size effects is observed. The application of this technique to 2D materials will allow extending the field-effect experiments to a range of charge doping where giant conductance modulations and, in some cases, even the occurrence of superconductivity are expected., 5 pages, 5 figures, RevTeX

Polycrystalline samples and single crystals of SmFeAs1−xPxO1−y were synthesized and grown employing different synthesis methods and annealing conditions. Depending on the phosphorus and oxygen content, the samples are either magnetic or superconducting. In the fully oxygenated compounds, the main impacts of phosphorus substitution are to suppress the Neel temperature TN of the spin density wave (SDW) state and to strongly reduce the local magnetic field in the SDW state, as deduced from muon spin rotation measurements. On the other hand, the superconducting state is observed in the oxygen-deficient samples only after heat treatment under high pressure. Oxygen deficiency as a result of synthesis at high pressure brings the Sm-O layer closer to the superconducting As/P-Fe-As/P block and provides additional electron transfer. Interestingly, the structural modifications in response to this variation of the electron count are significantly different when phosphorus is partly substituting arsenic. Point contact spectra are well described with two superconducting gaps. Magnetic and resistance measurements on single crystals indicate an in-plane magnetic penetration depth of ∼200 nm and an anisotropy of the upper critical field slope of ∼4–5.