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A flexible, fully organic detector for proton beams is presented here. The detector operates in the indirect mode and is composed of a polysiloxane-based scintillating layer coupled to an organic phototransistor, that is assessed for flexibility and low-voltage operation (V = −1 V), with a limit of detection of 0.026 Gy min−1. We present a kinetic model able to precisely reproduce the dynamic response of the device under irradiation and to provide further insight into the physical processes controlling it. This detector is designed to target real-time and in-situ dose monitoring during proton therapy and demonstrates mechanical flexibility and low power operation, assessing its potential employment as a personal dosimeter with high comfort and low risk for the patient. The results show how such a proton detector represents a promising tool for real-time particle detection over a large area and irregular surfaces, suitable for many applications, from experimental scientific research to innovative theranostics.

Molecular electronics represents the ultimate step of the miniaturization process of the integrated circuits. Including molecules in manmade devices may introduce novel functionalities in nanodevices, such as the possibility to interact with biological environments with tremendous implications in several fields. With these motivations, we present a Bogoliubov-de Gennes description of the transport properties of a Normal-metal/molecule/Superconductor (N/m/S) junction formed by contacting an annular molecule (m) (e.g., a benzene molecule) with a normal (N) and a superconducting electrode (S). Differently from N/m/N junctions, the superconducting correlations in S play a crucial role in determining the transparency of the system. We demonstrate that the zero-bias differential conductance of the device is strongly suppressed in presence of charge accumulation on the molecule, while a resonant transmission with maximum conductance is observed when the charge stored on the molecule is negligible. We explain these findings observing that the space distribution of molecular quantum states is strongly affected by the Andreev mechanism, which represents the dominant scattering process at the m/S interface when the energy of the incident quasi-particle is below the superconducting energy gap. The relevance of these findings is briefly discussed.

We have fabricated and characterized single junctions, dc Superconducting Quantum Interference Devices (SQUIDs), and nanostrips made by Fe(Te, Se) superconductive thin films. SrTiO 3 bicrystal substrates with different misorientation angles have been used for the fabrication of Josephson junctions and SQUIDs. Nanostrips have been realized growing thin films on CaF 2 substrates. The interaction between fluxons and antifluxons has been taken into account for the evaluation of the pinning energy values.

Single-photon avalanche diodes (SPADs) in the CMOS technology are very attractive solution for photon detection due to the excellent timing resolution achievable. Unfortunately, such devices suffer from large values of the dark count (DC) pedestals. In this paper, we analyzed a test chip containing SPADs with different layouts, implemented in the 150-nm CMOS technology. The behavior of such devices has been investigated after proton irradiation. It is observed that, after irradiation, the DC rate switches between two or more discrete levels, phenomenon known as random telegraph signal (RTS). The effect is related to the density and distribution of defects in the semiconductor lattice. RTS characteristics have been studied as the function of both temperature and bias voltage. Discussion of results and main hypotheses on defect types responsible for RTS are reported.

Among the various families of Fe-based superconductors, iron chalcogenides, while presenting a transition temperature not particularly high, show great advantages for potential applications at high fields, albeit at liquid helium temperature. In fact, the critical temperature can be increased by stress up to 21 K in thin films, moreover stress can push the critical field up to more than 50 T, and the irreversibility field up to of 45 T. Also critical current densities higher than 1 MA/cm 2 can be reached in self field and at 4.2 K, with a very weak dependence on the magnetic field. Interestingly, the J C anisotropy depends on the mode of growth and substrate used. These high values of J C can be achieved without appreciable anisotropy. In this work, we present an overview of Fe(Se 0.5 Te 0.5 ) thin film deposition by pulsed laser ablation, and a study of their superconducting properties. STM and TEM techniques have been used to characterise structural and morphological properties and, thus, pinning centres. On different substrates the films grow with different types of defects, hence giving totally different J C anisotropy. Our preliminary measurements on a Josephson junction show that in Fe(Se,Te), the J C at the grain boundary is not severely depressed, which makes this material even more interesting from the practical point of view.

We present, for the first time, Low Frequency Noise measurements in Vertical Organic Transistors. Investigated devices are p-type Metal-Base Organic Transistors employing Pentacene and Copper Phthalocyanine as active layers for the Emitter and Collector regions, respectively. The power spectral densities measured at the Base and Collector terminals follow a 1/fγ law, with γ≈1, suggesting that noise is generated, in both cases, by a continuous distribution of traps. Cross-correlation measurements show that Base current noise and Collector current noise are uncorrelated meaning that the physical origin of the noise measured at the Base and Collector terminals is different.

Vertical organic transistors are a promising candidate to overcome the scaling limits of conventional horizontal organic field-effect devices. In this paper, based on the results of direct current (dc) and low-frequency noise (LFN) measurements, we propose a picture for the carrier transport in metal-base organic transistors in which transmission across the base is due to the combined action of both hot-carriers (HCs) and high-conduction paths (pores) in the metal base. Investigated devices employ pentacene and copper phthalocyanine as active layers for the emitter and collector regions, and a bilayer consisting of Au/MoO3 as injecting electrode. The dc analysis highlights that the charge transport in the investigated devices is due to drift diffusion of the emitter-injected carriers in the highest occupied molecular orbital (HOMO) band and is dominated by HC injection, although conduction via pores increases and becomes not negligible at high base–emitter fields. The LFN analysis highlights the presence of two dominant and uncorrelated 1/f current noise sources, one located between base and emitter, due to the fluctuations of the not-transmitted HC current, and a second source located between collector and emitter, due to the fluctuations of the transmitted HC current and/or to the fluctuations of the current through the pores in the metallic base.

In this paper, the metal-semiconductor contacts in n-type bottom-contact bottom-gate Organic Field-Effect Transistors (OFET), based on evaporated films of a perylene diimide derivative (PDI8-CN2), have been investigated by Scanning Kelvin Probe Microscopy (SKPM). OFET were characterized for different thicknesses of the PDI8-CN2 film and in the light of the gold electrode functionalization with an aromatic thiol self-assembled monolayer. SKPM experiments reveal that aromatic thiol functionalization produces a 30% reduction in the contact resistance (Rc), while lowering the organic film thickness, the contact resistance (Rc) phenomenon tends to be magnified. The experimental observation that the voltage drops occurring close the drain electrode are even larger than those taking place at the source contact suggests that it is not possible, for these devices, to explain the overall Rc effects by simply referring to the presence of a reverse-biased Schottky junction limiting the electron injection process into the active channel.

In general, information about the structure of the barrier in YBa2Cu3O7(YBCO) grain boundaries Josephson junctions (GBJJs) has been mainly derived by means of destructive techniques. In this line, any structural data directly derived from the analysis of the electromagnetic and transport parameters would be very useful. For GBJJs based on three bicrystalline geometries (24deg [001] asymmetric, 45deg [100] asymmetric and 24deg [001]symmetric + 45deg [100] asymmetric) we present a structural study of the barrier based on the transport and electromagnetic parameters deduced from the current-voltage (I-V) curves. The dependence of the critical current (IC) on the junction resistance (RN) shows an approximately ICRN -1 power law type dependence. This result is in accordance with a SNINS or SIS description of the barrier. Concerning the electromagnetic parameters, we have calculated the capacitance (C) by means of Fiske resonance positions and hysteresis observed in the I-V curves. While C values calculated with both methods are similar in the case of YBCO 24deg [001] asymmetric GBJJs, the ones deduced from Fiske resonances are always higher in the case of the other two misorientation angles, where a [100] component is present. This discrepancy allows us to estimate the length of the normal N regions at both sides of the barrier.

We have studied the frequency dependence of the dielectric constant of YBa2Cu3O7 Josephson junctions fabricated on bicrystalline substrates with different angles tilted around alpha and c axis. The ratio of the dielectric constant to the thickness of the barrier, epsiv/t, can be deduced by measuring the voltage of Fiske steps Veta = etaOslash0c macr/ omega, where eta is the resonance number, omega the junction width and Oslash0 the magnetic flux quantum. Changing a technological parameter as omega we are modifying Veta, so the resonant frequency f n = V eta/Oslash0 for each fixed eta . This makes possible to generate experimentally a dispersion relation of the dielectric constant of the barrier, epsiv(f n) = epsiv(omega) . For all the bicrystalline geometries investigated, data can be fitted to the expression of epsiv(omega) that describes the behavior of the dielectric constant close to a resonance in a dielectric medium with losses. Consistent with the analysis of transport parameters, the values deduced for the resonance frequency and damping constant show a tendency to a more semiconductive behavior with the increase of the misorientation angle. In terms of the equivalent circuit RLC, we can obtain additional information on the inductive response of the barrier.

A theory describing Fiske resonance steps in high-temperature ''d-wave'' superconductive Josephson junctions has been developed. The model is an extension of the theory proposed by Kulik in 1965, which applies in the case of conventional low-temperature junctions (''s-wave'' superconductors). The theory allows to derive the magnetic field dependences of the n-th order Fiske step, also in the presence of ''0-p'' singularities in the junction phase difference. An analysis of Fiske steps in asymmetric 0-45 [001] ''d-wave'' Josephson junctions has been presented. Finally, in order to describe the phenomenology encountered in real grain boundary junctions, the presence of facets of different orientations and lengths has also been considered. 2005 Elsevier B.V. All rights reserved.

We present a comparative study of the electromagnetic parameters, mainly focused in the analysis of the capacitance (C) associated to the barrier, of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) grain boundary Josephson junctions (GBJJs) fabricated on 24/spl deg/ [001] symmetric and 45/spl deg/ [100] asymmetric tilt bicrystalline substrates. The Swihart velocity (c~) is slightly affected by the nature of the barrier and mainly fixed by the junction width. However, a notable influence of the barrier structure, so of the geometry of the bicrystal substrate, on the relative dielectric constant to the barrier thickness ratio (/spl epsiv//sub r//t) values, so on C, has been found. We have observed that the capacitance values deduced by means of Fiske steps in YBCO 24/spl deg/ [001] symmetric and 45/spl deg/ [100] asymmetric GBJJs scale with junction resistance in opposite direction. This result could reveal the presence of a tunnel barrier in the YBCO 45/spl deg/ [100] asymmetric GBJJs. On the other hand, different capacitance values have been obtained by means of Fiske steps and hysteresis observed in the current-voltage characteristics in 45/spl deg/ [100] asymmetric GBJJs. An interpretation of this result can be made taking into account the contribution of the depleted YBCO layers close to the crystallographic grain boundary.

We have fabricated and analyzed bicrystal grain boundary superconducting quantum interference devices made by Fe(Se,Te) superconductors, grown on [001] tilt SrTiO3 bicrystal substrates. We have realized several devices using the so-called hole-type geometry, with different loop inductances and different junction widths. Modulations of the voltage under an external applied magnetic field have been observed for three samples. Voltage amplitudes of the order of few microvolt have been measured. The maximum magnetic flux-to-voltage transfer parameter value that we have observed has been . The experimental data are analyzed in the famework of existing theoretical models.

Asymmetries between the two Josephson junctions of a dc-SQUID have always been considered undesirable spurious effects, responsible for the degradation of the device performance. However, it was recently demonstrated that a suitable choice of the asymmetric configuration can lead to magnetic flux noise values lower than symmetric ones. The numerical analysis was performed by using parameters typical of low-Tc SQUIDs, operating at the liquid helium temperature. In this paper, the analysis has been extended to high critical temperature dc SQUIDs, operating at the liquid nitrogen temperature. Also in this case, asymmetric SQUIDs show the best performance in terms of both flux to voltage transfer coefficient V/sub /spl Phi// and magnetic flux noise S/sub /spl Phi//. In order to optimize the device performance, the dependence of SQUID properties on damping resistance and normalized SQUID inductance has been computed for both symmetric and asymmetric configurations.

A voltage-noise analysis of FeTe0.5Se0.5 thin films has revealed the existence of nonlinear fluctuations above an electric field bias threshold. Below this threshold the fluctuations are standard and compatible with the presence of electron- and hole-type carriers, and it has been possible to evaluate the value of the Hooge noise parameter. Above the electric field bias threshold and at temperatures higher than 70 K, an increased nonlinear 1/f noise is found with a power exponent scaling with the temperature squared. Several possible theoretical interpretations are considered and discussed.

Electric transport and voltage-noise measurements on FeTe0.5Se0.5 epitaxial thin films are reported. The films were grown by pulsed laser ablation deposition on lanthanum aluminate single crystal substrates, and subsequently patterned by ion beam etching. Several configurations, with different voltage contacts distances and different strip widths, have been measured. At low temperature and current bias, the measured low frequency 1/f noise behaves according to the standard resistance fluctuations model. Conversely, an excess unconventional 1/f noise is observed in the high-temperature region (T >; 70 K). The experimental results indicate the existence of an electric field threshold, above which nonlinear fluctuation processes are activated. The results of noise measurements correlate well with the already reported change in sign of the Hall resistance in the same temperature range. Possible correlation between the hole contribution to the electrical transport and nonlinear noise effects observed is discussed.

We have fabricated and analyzed submicron YBa2Cu3O7−x grain boundary Josephson junctions grown on [100] tilt SrTiO3 bicrystal substrates. We present an experiment sensitive to the amplitude of the order parameter. To this aim, we have measured electrical properties of [100] tilt bicrystal YBa2Cu3O7−x grain boundary junctions with nominal widths of 700 nm and 300 nm. Junctions are fabricated so that positive lobes of the d-wave electrodes face one another (d0–d0 junction). We demonstrate that, in such devices, the temperature dependences of the critical current may be accounted for by very high-transparency junction barriers, in which the influence of nodes in the pair potential is an essential element. We based our analysis on a recent theoretical model that, starting from the Bogoliubov–de Gennes equations, takes into account the presence of Andreev bound states in layered superconductors, with Cu–O planes tilted with respect the substrate plane, as is the case of [100] tilt grain boundary junctions.

An improved procedure for fabricating step-edge Josephson junctions by YBCO on NdGaO 3 substrates is described. It is based on a reactive patterning of the superconducting film geometry by using a SiO x inhibit process. SQUID's have been fabricated following this technique. Preliminary measurements on the transport properties in single junctions and on the magnetic response in DC SQUID's are reported. Promising results have been obtained at T =4.2 K.

The wide variety of high T c Josephson junction configurations are reviewed. Properties and limits of the various structures are briefly discussed. The progress in view of future device applications appears to be quite promising.

We have fabricated and measured high-quality [100] Y-Ba-Cu-O grain boundary junctions in both symmetric and asymmetric configurations. High values of the critical current times normal resistance I/sub c/R/sub n/ products have been systematically observed. The high I/sub c/R/sub n/ values found in these junctions may be of interest for the fabrication of high-sensitive superconducting quantum interference devices.

YBa 2 C 3 O 7− δ artificial grain boundary Josephson junctions have been fabricated by a recently implemented biepitaxial technique, employing MgO and CeO 2 as seed layers. The Josephson properties of asymmetric grain boundaries (GBs) characterised by a misalignment of the c -axis (45° c -axis tilt or twist) are presented. A comparison with GBs characterised by 45° c -axis tilt or twist and by an additional rotation of 45° of the a – b planes of one electrode has been also performed. The differences observed in the transport properties have been interpreted in terms of d x 2 − y 2 wave symmetry of the order parameter and faceting of the grain boundary.

The effects of the order parameter symmetry and the interface grain boundary orientation on the transport properties of YBa/sub 2/Cu/sub 3/O/sub 7-x/ biepitaxial grain boundary junctions have been investigated. 45/spl deg/ [100] tilt and twist junctions on SrTiO/sub 3/ [110] and MgO [110] substrates by employing respectively MgO and SrTiO/sub 3/ as seed layers have been fabricated. More complex junctions by using CeO as seed layers have also been fabricated and analyzed. The analysis of experimental data points towards a predominant d-wave symmetry of the superconductive order parameter.

YBa/sub 2/Cu/sub 3/O/sub 7/-/sub x/ junctions and dc SQUIDs have been realized by employing 45/spl deg/ a-axis tilt grain boundaries through a fabrication procedure based on the biepitaxial technique. YBa/sub 2/Cu3O/sub 7-x/ grows mainly oriented [103] on SrTiO/sub 3/ and [001] on the MgO seed layer. A wide characterization of the tilt junctions and SQUID properties has been carried out. High values of the I/sub C/R/sub N/ quality factor and of the magnetic flux-to-voltage transfer parameter have been obtained, The observed phenomenology confirms differences from the one typical of 45/spl deg/ c-axis tilt grain boundary junctions and gives important information on the nature of the current steps appearing at finite voltages in the current-voltage characteristics. These devices could also have some impact on experiments designed to study the symmetry of the order parameter, exploiting their microstructure and anisotropic properties. A maximum speed of Josephson vortices as calculated from the voltage step values, of the order of 2 10/sup 6/ m/sec is obtained, which encourages high frequency applications.

YBa2Cu3O7-x artificial grain boundary Josephson junctions have been fabricated, employing a recently implemented biepitaxial technique. The grain boundaries can be obtained by controlling the orientation of the MgO seed layer and are characterized by a misalignment of the c-axes (45 degrees a-axis tilt or 45 degrees a-axis twist). These types of grain boundaries are still mostly unexplored. We carried out a complete characterization of their transport properties and microstructure. Junctions and de SQUIDs associated with these grain boundaries exhibit an excellent Josephson phenomenology and high values of the ICRN product and of the magnetic flux-to-voltage transfer parameter respectively. Remarkable differences in the transport parameters of tilt and twist junctions have been observed, which can be of interest for several applications. A maximum speed of Josephson vortices as calculated from the voltage step values of the order of 2 x 10(6) m s(-1) is obtained. These devices could also have some impact on experiments designed to study the symmetry of the order parameter, exploiting their microstructure and anisotropic properties. High-resolution electron microscopy showed the presence of perfect basal plane faced boundaries in the cross sections of tilt boundaries.

The use of a scanning magnetic microscope (SMM) with a high temperature superconducting quantum interference device (SQUID) for quantitative measurements in eddy current nondestructive analysis (NDA) is presented. The SQUID has been used to detect the weak magnetic field variations around a small defect, close to a structural part generating an intensive magnetic field. The experimental data for a deep crack close to a rivet in a multilayer conducting plate have been taken in a RF-shielded environment and discussed in the light of the theoretical predictions. The results show that eddy current NDA can distinguish subsurface crack signals from wider structural signals, with defects located 10 mm below the surface. Moreover, in order to visualize the structure of the probing current when a circular induction coil is used, the simulation of eddy currents in a thick unflawed conducting plate has been carried out.

The effect of the magnetic field on the current distribution in one-dimensional long Josephson tunnel junctions is investigated. Overlap and mixed overlap/in-line configurations are considered. The latter geometry corresponds to an experimental situation, since it takes into account nonuniformities of the bias current. A simple method to investigate the dependence of the Josephson critical current on the external magnetic field for any junction length is developed. The predictions, based on the phase space analysis, are compared with experimental results obtained on long junctions of different lengths. The possibility of using this method to evaluate relevant junction parameters from the experimental measurements is discussed.

The static configurations of the phase in a long Josephson junction are investigated by studying the phase space associated with the Static Perturbed Sine-Gordon Equation (SPSGE). Mixed overlap/in-line long Josephson junctions are considered. Analytical results concerning the dependence of the zero-field critical current on the junction length are reported.

Nitrogen ion implantation was used to induce local modifications of the superconductive properties of the niobium base layer in Nb-NbO/sub x/-Pb Josephson tunnel junctions. A local depression of the niobium critical temperature and energy gap turns into a nonuniform critical current density profile J/sub c/ in the sample. Different profiles have been obtained by modifying the implanted region geometry and the nitrogen ion concentration. The external magnetic field dependence of the maximum DC Josephson current has been used as a diagnostic tool to obtain a preliminary characterization of the shape of J/sub c/ in a small sample. Preliminary investigations on long junctions are also reported. >

The current-voltage characteristics of YBCO films were investigated on both c-axis and a/b axis oriented samples. The films, about 100 nm thick, were fabricated by inverted cylindrical dc magnetron sputtering on (100) and (110) SrTiO 3 substrates. In c-axis oriented films the observed behavior is consistent with a model based on two-dimensional pair vortex fluctuations in the CuO 2 planes. Preliminary measurements on the a/b axis oriented samples are reported, for different angles between incident bias current and c-axis planes.