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This paper focuses on the understanding of the Random Telegraph Signal (RTS) in Single-Photon Avalanche Diodes (SPAD). We studied the RTS of two different SPAD layouts, designed and implemented in a 150-nm CMOS process, after proton irradiation. The two structures are characterized by different junction types: the first structure is constituted by a P+/Nwell junction, while the second is formed by a Pwell/Niso junction. RTS occurrence has been measured in about one thousand SPAD pixels and the differences addressed in two layouts are motivated and discussed. Hypotheses on the RTS origin are drawn by analyzing the RTS time constants and the RTS occurrence evolution as a function of the annealing temperature.

Nanostructured films of carbon and TiO2 nanoparticles have been produced by means of a simple two-step procedure based on flame synthesis and thermophoretic deposition. At first, a granular carbon film is produced on silicon substrates by the self-assembling of thermophoretically sampled carbon nanoparticles (CNPs) with diameters of the order of 15 nm. Then, the composite film is obtained by the subsequent thermophoretic deposition of smaller TiO2 nanoparticles (diameters of the order of 2.5 nm), which deposit on the surface and intercalate between the carbon grains by diffusion within the pores. A bipolar resistive switching behavior is observed in the composite film of CNP-TiO2. A pinched hysteresis loop is measured with SET and RESET between low resistance and high resistance states occurring for the electric field of 1.35 × 104 V/cm and 1.5 × 104 V/cm, respectively. CNP-TiO2 film produced by flame synthesis is initially in the low resistive state and it does not require an electroforming step. The resistance switching phenomenon is attributed to the formation/rupture of conductive filaments through space charge mechanism in the TiO2 nanoparticles, which facilitate/hinder the electrical conduction between carbon grains. Our findings demonstrate that films made of flame-formed CNP-TiO2 nanoparticles are promising candidates for resistive switching components.

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.

Incipient carbon nanoparticles, CNP, formed by incomplete hydrocarbon combustion are composed of polycyclic aromatic hydrocarbons [1,2] and exhibit a quantum dot behavior [3], which make them ideal for a wide range of applications. Flame synthesis offers a continuous and low-cost process for the production of inorganic nanoparticles, such as TiO2, with fine control of cristallinity and phase. Likewise, carbon nanoparticles, CNPs, with tailored size and optical and electronic properties can be produced by optimizing the flame process. The assembly of nanoparticles into a uniform thin film with precise control over chemical and physical properties poses a significant challenge. Thermophoretic sampling relies on the thermophoretic forces driving the particles in the hot flame towards a cold substrate inserted in the flame and can be used to produce nanostructured, self-assembled films that can lead to novel applications. In this work, CNP and CNP/TiO2 nanoparticles have been produced in premixed ethylene-air flames and thin films have been realized by thermophoretic deposition on silicon substrates with gold interdigitated electrodes. Scanning mobility particle sizer furnished the particle size distribution and Raman and optical spectroscopy chemico-physical and structural information. The electrical characterization of the films was conducted by measuring the IV characteristics which shows a good linearity in the interval [-5 V, 5 V] whereas non-linear phenomena appear for wider voltage ranges and hysteresis phenomena are also observed in carbon/TiO2 films. Impedance spectroscopy was used to investigate the films with respect to their microstructure. This method is indeed particularly useful for nanostructured films with inter-grain and grain-edge impedances and capacities. Finally, noise spectral density at low and very low frequency was measured to investigate the electron transport. The main source of noise is of thermal type for the carbon material, whereas carbon/TiO2 film shows a Fliker type, 1/f noise, consistent to a larger amount of localized states responsible for trapping/detrapping processes of charge carriers and charge fluctuations during the conduction process. [1] Schulz, F., Commodo, M., Kaiser, K., De Falco, G., Minutolo, P., Meyer, G., D`Anna, A., Gross, L., "Insights into incipient soot formation by atomic force microscopy" Proc. Combust. Inst. 37: 885-892 (2019). [2] Commodo, M., Kaiser, K., De Falco, G., Minutolo, P., Schulz, F., D`Anna, A., Gross, L., "On the early stages of soot formation: molecular structure elucidation by high-resolution atomic force microscopy" Combust. Flame 205, pp. 154-164. [3] Liu, C, Singh, A.V., Saggese, C., Tang, Q., Chen, D., Wan, K., Vinciguerra, M., Commodo, M., De Falco, G., Minutolo, P., D'Anna, A., Wang H. , Flame-formed carbon nanoparticles exhibit quantum dot behaviors, 2019, DOI:10.1073/pnas.1900205116

This paper investigates the Random Telegraph Signal (RTS) in Single Photon Avalanche Diodes (SPADs). The test-chip features SPADs with different architectures implemented in 150 nm CMOS technology. The test-chip has been irradiated with 21 MeV proton beam. RTS occurrence probability has been investigated in two different architectures. RTS measurements allowed to investigate the defect responsible for RTS phenomenon.

The electronic properties of an infinite row of freestanding, aligned side-by-side, pentacene molecules are derived as a function of the intermolecular overlap integral and the chemical potential shift. We use a semiclassical approximation and a first principles tight binding method to obtain conductance and mobility of this one-dimensional crystal as a function of temperature and gate voltage. For two values of the intermolecular overlap, energy bands show a metallic behavior. For all the other values, a bandgap is present and evolves with the intermolecular overlap following a typical modulation. The magnitude of the scattering parameters estimated by the observed conductivity is coherent with the existing literature values. These findings could be relevant for the implementation of organic-based sensors.

The effects induced by radiation on an innovative photo-sensor consisting of Single-Photon Avalanche Diodes arrays, implemented in a 150-nm CMOS technology, are presented. Samples were irradiated with a proton beam at different doses, and the impact of radiation-induced damage on the performance of the devices in terms of Dark Count Rate has been estimated. Furthermore, possible methods to reduce the impact of radiation-induced damage are discussed.

In this paper, we report on the fabrication of n-type bottom-gate bottom-contact transistors, based on evaporated films of a perylene diimide derivative (PDI8-CN2), displaying electrical performances comparable to their top-contact counterparts. By combining very thin (20 nm) electrodes and a cleaning process with oxygen plasma, indeed, bottom-contact devices with maximum mobility approaching 0.2 cm2/V s and a contact resistance lower than 35 kΩ cm at low VDS values have been achieved. AFM analyses reveal that the improved electrical responses are accompanied by the optimized PDI8-CN2 film morphology which, very significantly, exhibits similar features on the gold and dielectric (i.e., SiO2) surfaces.

We investigated the radiation tolerance of a Single-Photon Avalanche Diode (SPAD) device manufactured using a 150-nm CMOS process. CMOS SPADs have been studied in view of utilization as photo-detectors in future space missions. An irradiation campaign has been carried out on several test chips containing SPAD arrays with different dimensions. Samples have been exposed to different displacement damage dose. The dark count rate (DCR) characterization as a function of the delivered proton fluence has showed a significant increase in mean DCR, providing the limits of operability of such devices in a space environment. Annealing and cooling have been investigated as possible damage mitigation approaches.

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.

Single molecules can be coupled to metallic electrodes when the latter are in the superconducting state. In such emerging hybrid molecular devices, the possibility of the Josephson effect, that is, the dissipation-less transport of Cooper-paired electrons from one electrode to the other, arises. In this theoretical study, we demonstrate that a junction formed by two superconductors linked by an annular molecule, of which benzene (phenylene group) is a prototype, can sustain indeed a supercurrent and work as a “single molecule superconducting field-effect transistor (SMoSFET)”. In this device, Cooper-paired electrons are transmitted via the molecule in the presence of quantum interference. Through the presented model, we show that the resonant nontrivial modulation of the critical current with an external gate voltage may be strongly modified by choosing a paracoupled, a metacoupled, or an orthocoupled molecular ring. These features are directly related to the possibility of obtaining a SMoSFET controlled ...

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.

The Josephson effect in a molecular structure mimicking a benzene molecule (C6H6) placed between two nanometric superconducting electrodes is theoretically studied. In the tight bindingmodel, the Bogoliubov-de Gennes equations are solved to derive the Andreev reflection coefficient and hence the dc Josephson current. Two effects are pointed out: the quantum interference effect on the supercurrent due to the presence of two branches in the molecule and a on-resonance off -resonance state operation in the presence of an applied gate voltage. The results provide an insight on the molecular electronics devices in the case of superconducting nano-electrodes.

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, nanoscale organic two-terminal devices, based on evaporated films of a perylene diimide derivative (PDI8-CN2) and with channel lengths ranging from 1 μm down to 50 nm, have been fabricated by using Focused Ion Beam technique. Devices have been characterized in terms of I–V curves at several temperatures and perylene diimide nanochannels displayed a measurable electrical conduction even at temperatures lower than 50 K. The recorded experimental data stressed the electrical response according to the channel size. Investigation of nanoscale devices allowed us to study the nature of the charge injection and to estimate the energy barrier and the factors influencing it.

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.

We formulate a quantum waveguide theory of the Josephson effect in multiband superconductors, with special emphasis on iron-based materials. By generalizing the boundary conditions of the scattering problem, we first determine the Andreev levels spectrum and then derive an explicit expression for the Josephson current which generalizes the formula of the single band case. In deriving the results, we provide a second quantization field theory allowing to evaluate the current-phase relation and the Josephson current fluctuations in multiband systems. We present results for two different order parameter symmetries,namely $s_\pm$ and $s_{++}$, which are relevant in multiband systems. The obtained results show that the $s_\pm$ symmetry can support $\pi$ states which are absent in the $s_{++}$ case. We also argue a certain fragility of the Josephson current against phase fluctuations in the $s_{++}$ case. The temperature dependence of the Josephson critical current is also analyzed and we find, for both the order parameter symmetries, remarkable violations of the Ambegaokar-Baratoff relation. The results are relevant in view of possible experiments aimed at investigating the order parameter symmetry of multiband superconductors using mesoscopic Josephson junctions., Comment: 10 pages, 5 figures

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.

Recent experimental results on the physics of high temperature superconductivity (HTS) related to coherent phenomena and unconventional pairing symmetry are discussed. Some consequences of the d-wave order parameter symmetry on the phenomenology of HTS junctions of different type are investigated. The attention is focused on recent experiments based on biepitaxial YBa 2 Cu 3 O 7-δ Josephson structures where junctions with a π shift in the phase have been reproducibly obtained. Anisotropy measurements confirm the relevance of effects of d-wave order parameter symmetry on the properties of the junctions, and prove that intrinsic d-wave effects can be predominant over extrinsic effects. The possibility to obtain some kind of tunnel-like barriers and to tune d-wave effects through a suitable geometry has been demonstrated opening some perspectives towards novel device concepts within the framework of π circuitry and quiet qubit.

We have designed an experiment that could allow to investigate, on the same sample, the interplay between the morphological anisotropy of high critical temperature superconductors and the effects related to the order parameter symmetry. To this aim we have employed c-axis tilt biepitaxial grain boundary YBa2Cu3O 7 - δ Josephson junctions and measured the junction properties for different junction interface orientations. The results give evidence of the possibility to tune normal state resistance and critical current density of a Josephson junction almost independently of each other. We interpret the results of this experiment as an indication that Cooper pairs and quasi-particles transport properties may be ruled by different mechanisms. Measurements apparently give evidence of the possibility to determine to some extent the presence of “π-loops” and “0” loops along the same grain boundary line.

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.

An important step forward for the understanding of high-temperature superconductivity has been the discovery of iron-based superconductors. Among these compounds, iron pnictides could be used for high-field magnet applications, resulting more advantageous over conventional superconductors, due to a high upper critical field as well as its low anisotropy at low temperatures. However, the principal obstacle in fabricating high quality superconducting wires and tapes is given by grain boundaries. In order to study these effects, the dc transport and voltage-noise properties of Co-doped BaFe₂As₂ superconducting films with artificial grain boundary junctions have been investigated. A specific procedure allows the separation of the film noise from that of the junction. While the former shows a standard 1/f behaviour, the latter is characterized by an unconventional temperature-dependent multi-Lorentzian voltage-spectral density. Moreover, below the film superconducting critical temperature, a peculiar noise spectrum is found for the grain boundary junction. Possible theoretical interpretation of these phenomena is proposed.

We study the Josephson effect in S±IS± junctions made by a two band superconductor with s± wave symmetry. We derive the Andreev coefficients for the scattering problem at the junction interface and the temperature dependence of the critical current. We predict various features of the Josephson current for certain values of the second band strengths and tunnel barrier amplitude among which a high temperature π state coupling, and a π to 0 transition as the temperature lowers.

A detailed characterization of the voltage-noise properties has been performed in FeTe0.5Se0.5 epitaxial thin films and Co-doped BaFe2As2 bilayers, deposited by pulsed laser deposition. In all the samples analyzed, the experimental voltage-spectral density has a 1/f noise component. Different behaviors are observed for the bias current and temperature dependencies of this 1/f noise, and are related to specific structural and electric transport properties of the two materials.

We have fabricated submicron YBa2Cu3O7−x grain boundary junctions using [100] tilt bicrystal substrates. In these substrates, crystal planes are tilted around the b-axis, parallel to the bicrystal line. We have obtained high-quality junctions and fitted experimental data using a recently theory accounting for d-wave symmetry of the order parameter in layered superconductors with arbitrary orientation. In our experiment, sensitive to the amplitude of the pairing state, lobes of different signs do not influence the electrical transport in the junction. Nevertheless, the effects of a non-conventional symmetry are clearly observed on the temperature dependence of the critical current.

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.

Concepts and experimental procedures to obtain different types of YBa 2 Cu 3 O 7 grain boundaries Josephson junction through the biepitaxial technique have been developed. Various seed layers (MgO, CeO 2 and SrTiO 3) and substrates (MgO, SrTiO 3) have been used providing, for the biepitaxial technique, additional flexibility in circuit design. A detailed investigation on transport properties evidenced good performances for both junctions and superconducting quantum interference devices. Experimental evidence is given of a conventional magnetic field behavior in 45° tilt Artificial Grain Boundary (AGB) for which no π-loops are predicted along the interface.

SQUID configurations with an asymmetry in both the critical current and the normal resistance of the two Josephson junctions have been numerically analyzed. The analysis has been carried out on a high performance computer, by choosing parameters typical of low critical temperature SQUIDs. In particular the dependence of both the magnetic flux noise S Φ and the flux to voltage transfer function V Φ on the bias current and the normalized SQUID inductance β have been investigated. Thermal noise effects, including contributions of both junction and damping resistance have been introduced in the calculations. The results show that an increase of the asymmetry leads to higher V Φ and lower S Φ values. This suggests to use asymmetric dc-SQUIDs to improve the device performance for both small and large inductance values.

High sensitivity HTc dc-SQUID magnetometers, of bicrystal junction type, have been designed, fabricated and characterized. The magnetometers are characterized by a direct-coupling planar design and their parameters and geometrical dimensions have been optimized in order to achieve the lowest possible noise level. The IV characteristics are RSJ-like and show no excess current. The measured SQUID normalized inductances and magnetic field modulation periods are in good agreement with the expected values. The magnetic field noise spectral densities, measured with a flux locked loop electronic setup, exhibit typical white noise levels of about 150 fT/Hz 1/2 at frequencies above 2 KHz. At lower frequencies a 1/f noise is observed. By employing a bias-reversal electronic readout scheme, the 1/f corner frequency can be reduced to values less than 100 Hz.

An increasing interest in the behaviour of silicon detectors at cryogenic temperatures has been awakened by the discovery of the so-called Lazarus effect, namely the recovery of charge collection efficiency (CCE) by means of cryogenic cooling. We measured the CCEs of three single diodes previously irradiated with different neutron fluences. The current-voltage characteristic were measured at 300 and 77 K, showing that the low-temperature operation considerably decreases the steady-state current. This is also the case when a forward voltage bias is applied, which then becomes a suitable option. At 77 K, in the case of samples irradiated with 5 x 10 14 neutrons cm -2 , the CCE is completely recovered. A third sample irradiated with 2 x 10 15 neutrons cm -2 shows a 60% CCE at 250V forward bias.

Electron beam irradiation has been used to induce controllable variations in the properties of YBa 2 Cu 3 O 7-x biepitaxial grain boundary Josephson junctions. Electron irradiation decreased the critical current of the junctions IC and increased the normal state specific resistivities. The junction parameters can be adjusted controllably by applying an appropriate dose. The original properties of the junctions could be partly restored by isothermal annealing A correlation between the transport properties and the microstructure was obtained by determining the ratio of a barrier thickness to the dielectric constant of the junctions with different barriers. The experiment also demonstrates frequency tunability in a resonant soliton oscillator. Preliminar results of irradiation on (103) YBCO films have been given demonstrating differences from data on c -axis films, as partly expected from the anisotropy of these materials.

In order to optimize the magnetic field sensitivity [Formula: see text] we have followed two different approaches. In the former we have analyzed the influence of the quality of YBCO films on the superconducting properties and noise characteristics of bicrystal grain boundary junctions and dc-SQUIDs. For this purpose we have studied and compared three different YBCO deposition processes. In the latter we have reported the different criteria for the design of SQUID magnetometers usable for applicalions in shielded and unshielded environment.

Electron-beam irradiation has been used to induce controllable variations in the properties of YBa2Cu3O72x biepitaxial grain-boundary Josephson junctions. A correlation between the transport properties and the microstructure was obtained by determining the ratio of a barrier thickness to the dielectric constant of the junctions with different barriers. These results give evidence of the role of the oxygen content and the dielectric constant of the interface region in transport phenomena. The experiment also demonstrates frequency tunability in a resonant soliton oscillator. @S0163-1829~98!51322-2#

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.

We present a structural analysis of YBCO superconducting thin films fabricatedin situ by Inverted Cylindrical Magnetron (ICM) sputtering on commercial SrTiO3 single-crystal and bicrystal substrates. A detailed structural characterization of the superconductor films was performed by using single-crystal X-ray diffractometry confirming that YBCO films have a strongc-axis orientation of the grains with a small mosaic spread. In the films grown on bicrystal substrates we observed a strong correlation with the lattice structure of the substrate. In addition, a surface analysis of the region across the grain boundary edge has been performed by using scanning electron microscopy.

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.