In this paper, two procedures for the growth of both binary and ternary single crystals of bismuth chalcogenides by optical floating zone technique are described. Detailed characterization has been carried out on a series of samples, i.e. Bi2SexTe3-x, with x=0 and 0.9, and Bi2-xSbxSe3, with x=0 and 0.15, to isolate high quality single crystals, essential for an accurate study of the physical properties of the materials. Systematic compositional and structural analysis on the samples grown by the two different procedures have been compared to infer the optimal growth parameters to obtain the largest possible single crystals. The c-axis lattice parameter of Bi1.85Sb0.15Se3 is reported here for the first time.

We report the analysis of the magnetic response detected on the cuprate superconductor Nd1.85Ce0.15CuO4. In particular the magnetic behavior of the sample has been studied by means of DC magnetization measurements as a function of the temperature (T) and DC magnetic field (H). The superconducting critical temperature T-c has been obtained by analyzing the m(T) curve performed in Zero Field Cooling-Field Cooling conditions. Moreover, the m(T) curve shows the presence of a magnetic background for temperatures above T-c. By considering the superconducting m(H) hysteresis loop at different temperatures, it can be noted that the width of the curves appears narrow corresponding to a weak superconductivity. This is confirmed by the field dependence of the critical current densities J(c) extracted from the superconducting hysteresis loops m(H) at different temperatures within the Bean critical state model. In fact, at the lowest measurement temperature, J(c) is close to zero already at low magnetic fields. Nevertheless, by means of the temperature dependence of J(c), the sample shows a strong pinning behavior that can open perspectives for future improvement in the fabrication of this material.

We present the fabrication of FeSe 0.5 Te 0.5 polycrystalline samples by self-flux method, showing the presence of the peak-effect in the vortex lattice configuration. To improve the performances at high magnetic fields for this iron-based superconductor of the 11-family, the two-step solid-state reaction process has been successfully modified by increasing the temperature of the heat treatment and by slowing down the cooling rate. The magnetic field-temperature phase diagram has been investigated by magneto-resistance, magnetization, and heat capacity measurements in applied magnetic fields up to 18 T. The magnetization curves exhibit an enhancement of the peak-effect whose position shifts by varying the temperature, following a similar dependence than that previously reported in high-temperature superconducting materials such as YBa 2 Cu 3 O 7-δ . The presence of the peak-effect can be correlated to the sample manufacture, since by tuning a proper heat treatment it becomes observable ever more in the magnetic field-temperature phase diagram. This fabrication route paves the way to a systematic increase in the critical current density thus becoming relevant for applications.

The study of unconventional materials with peculiar properties is a fundamental step for the design and advancement of superconducting photon detectors. Nd2−xCexCuO4±δ is a non-traditional cuprate superconductor exhibiting n-type conduction which properties can be changed by modifying its cerium and oxygen content. Ultra-thin films of this compound have been deposited by dc sputtering technique, and systematically characterized by using X-ray diffraction and electrical measurements. Design and patterning of sample geometries have been performed by optical and electron beam lithography in order to obtain sub-micron wide strips for measurements of photon detection.

Vortex dynamics is strongly connected with the mechanisms responsible for the photon detection of superconducting devices. Indeed, the local suppression of superconductivity by photon absorption may trigger vortex nucleation and motion effects, which can make the superconducting state unstable. In addition, scaling down the thickness of the superconducting films and/or the width of the bridge geometry can strongly influence the transport properties of superconducting films, e.g. affecting its critical current as well as its switching current into the normal state. Understanding such instability can boost the performances of those superconducting devices based on nanowire geometries. We present an experimental study on the resistive switching in NbN and NbTiN ultra-thin films with a thickness of few nanometers. Despite both films were patterned with the same microbridge geometry, the two superconducting materials show different behaviors at very low applied magnetic fields. A comparison with other low temperature superconducting materials outlines the influence of geometry effects on the superconducting transport properties of these materials particularly useful for devices applications.

The stability against quench is one of the main issues to be pursued in a superconducting material, which should be able to perform at very high levels of current densities. Here we focus on the connection between the critical current I-c and the quenching current I* associated to the so-called Flux-Flow Instability phenomenon, which sets-in as an abrupt transition from the flux flow state to the normal state. To this purpose, we analyze several current-voltage characteristics of three types of Iron-Based thin films, acquired at different temperature and applied magnetic field values. For these samples, we discuss the impact of a possible coexistence of intrinsic electronic mechanisms and extrinsic thermal effects on the quenching current dependence upon the applied magnetic field. The differences between the quenching current and the critical current are also reported in the case of predominant intrinsic mechanisms. Carrying out a comparison with the HTS case, we suggest, which material can be the best tradeoff between maximum operating temperature, higher upper critical field, and stability under high current bias.

Epitaxial ultra-thin films of the electron-doped compound Nd 2-x Ce x CuO 4±δ (NCCO) have been fabricated by dc sputtering technique. The NCCO parent compound, namely Nd 2 CuO 4 , has been used as buffer layer to improve the crystalline properties because of the very small lattice parameter mismatch, as well as compatible depositions conditions, compared to NCCO. A deep morphological and structural characterization has been carried out on several samples, by means of surface analysis techniques and X-ray diffraction, in order to optimize the growth procedure.

The development of novel superconducting photon detectors requires the investigation of non-traditional materials to exploit their specific properties and realize novel/enhanced devices. Nd 2−x Ce x Cu O4±5 is a moderate high transition temperature cuprate superconductor exhibiting n-type conduction which properties can be changed by modifying its cerium and oxygen content. Ultra-thin films of this compound have been deposited by dc sputtering technique, and systematically characterized by using X-ray diffraction and electrical measurements. Design and patterning of sample geometries have been performed by optical and electron beam lithography in order to obtain sub-micron wide strips for measurements of photon detection.

High-temperature superconductivity in cuprate materials is achieved by hole or electron doping of the parent Mott insulator. In this paper, electron-doped Nd2−xCexCuO4±δ films with different content of cerium have been grown on (010) SrTiO3(TiO2) substrates through a dc sputtering technique. Since the normal conducting films can be obtained by controlling the oxygen content inside the crystalline structure by means of appropriate annealing procedures (that allow a reduction process), different as-grown nonsuperconducting films have been obtained. The samples have been thermally treated ex-situ at different temperatures and atmospheres in order to investigate the effects of different content of oxygen on the films properties. The results of the composition analysis, of the structural characterization and of the electric transport measurements on samples with a Ce fraction of 0 and 0.15 will be discussed.

The improvement in the fabrication techniques of iron-based superconductors has made these materials real competitors of high-temperature superconductors and MgB 2 . In particular, iron chalcogenides have proved to be the most promising for the realization of high current-carrying tapes. However, their use on a large scale cannot be achieved without the understanding of the current stability mechanisms in these compounds. Indeed, we have recently observed the presence of flux-flow instabilities features in Fe(Se,Te) thin films grown on CaF 2 . Here, we present the results of current-voltage characterizations at different temperatures and applied magnetic fields on Fe(Se,Te) microbridges grown on CaF 2 . These results will be analyzed from the point of view of the most validated models with the aim to identify the nature of the flux-flow instabilities features (i.e., thermal or electronic), in order to advance further to the high current-carrying capability of iron-chalcogenide superconductors.

Cu 2p core levels spectra measured by X-ray photoemission spectroscopy of selected as-grown Nd2-xCexCuO4+delta samples are presented and discussed. The presence of a satellite peak in the 2p core level of Nd2-xCexCuO4+delta single crystal by hard X-ray photoemission is confirmed in all non-superconducting samples, films and single crystals investigated in this work. The comparison of the spectral features of the different samples suggests that the presence and the intensity of this satellite peak is not related to the electric transport properties, but to the texture characteristics. (C) 2016 Elsevier B.V. All rights reserved.

Nd$_{2-x}$Ce$_x$CuO$_{4\pm\delta}$ (NCCO) epitaxial thin films have been deposited on (100) SrTiO$_3$ substrates by DC sputtering technique in different atmosphere. The as-grown samples show different dependence of the in-plane resistivity at low temperature, when they are grown in pure argon atmosphere or in oxygen. Moreover, an unusual behaviour is also found when transport takes place in the presence of an external magnetic field. It is commonly accepted that the higher anisotropic properties of NCCO crystalline cell with respect to the hole doped YBCO and LSCO and the electric conduction mainly confined in the CuO$_2$ plane, strongly support the two-dimensional (2D) character of the current transport in this system. Results on the temperature dependence of the resistance, as well as on the magnetoresistance and the Hall coefficient, obtained on epitaxial NCCO thin films in the over-doped region ($x\ge0.15$) of the phase diagram are presented and discussed.

Two-dimensional materials, such as graphene, topological insulators, and two-dimensional electron gases, represent a technological playground to develop coherent electronics. In these systems, quantum interference effects, and in particular weak localization, are likely to occur. These coherence effects are usually characterized by well-defined features in dc electrical transport, such as a resistivity increase and negative magnetoresistance below a crossover temperature. Recently, it has been shown that in magnetic and superconducting compounds, undergoing a weak-localization transition, a specific low-frequency 1/f noise occurs. An interpretation in terms of nonequilibrium universal conductance fluctuations has been given. The universality of this unusual electric noise mechanism has been here verified by detailed voltage-spectral density investigations on ultrathin copper films. The reported experimental results validate the proposed theoretical framework, and also provide an alternative methodology to detect weak-localization effects by using electric noise spectroscopy.

Among the families of iron-based superconductors, the 11-family is one of the most attractive for high field applications at low temperatures. Optimization of the fabrication processes for bulk, crystalline and/or thin film samples is the first step in producing wires and/or tapes for practical high power conductors. Here we present the results of a comparative study of pinning properties in iron-chalcogenides, investigating the flux pinning mechanisms in optimized Fe(Se1-xTex) and FeSe samples by current-voltage characterization, magneto-resistance and magnetization measurements. In particular, from Arrhenius plots in magnetic fields up to 9 T, the activation energy is derived as a function of the magnetic field, U-0(H), whereas the activation energy as a function of temperature, U(T), is derived from relaxation magnetization curves. The high pinning energies, high upper critical field versus temperature slopes near critical temperatures, and highly isotropic pinning properties make iron-chalcogenide superconductors a technological material which could be a real competitor to cuprate high temperature superconductors for high field applications.

The electrical transport properties of c-axis oriented Nd1.85Ce0.15CuO4-delta superconducting films have been investigated to analyze the pinning mechanism in this material. The samples were grown on SrTiO3 substrates using the dc sputtering high-pressure technique, whereas a detailed analysis of the structure and local composition of the films has been achieved using highresolution electron microscopy and x-ray microanalysis. Magneto-resistance and current-voltage measurements, in the temperature range from 1.6 to 300 K and in magnetic field up to 9 T, have been reported. In particular, the anisotropic coefficient defined as the ratio between the parallel upper critical field, H-c2 parallel to ab, and the perpendicular one, H-c2 parallel to c, has been evaluated, pointing out the high anisotropy of this compound. Furthermore, the vortex activation energy as a function of the applied magnetic field, parallel and perpendicular to the CuO2 planes, has been derived and compared with the flux-pinning forces to enlighten the peculiar nature of pinning centers in this material.

Among the families of iron-based superconductors, we investigate flux pinning mechanisms in the Fe(Se1-xTex) compound. We perform magneto-resistance and currentvoltage measurements on single-crystals, as well as on several epitaxial thin films grown on different substrates (CaF2, LaAlO3). The activation energy is derived as a function of magnetic field, U(H). The influence of magnetic field orientation on the pinning energy activation mechanism is also studied, leading to the anisotropy analysis which reveals low anisotropy in thin films grown on CaF2 substrate with respect to single crystals and films grown on LaAlO3. Concerning the dominant pinning regime, the exponents of the power law dependence U-0(H) proportional to H-proportional to have been evaluated, confirm that weak pinning is a general characteristic of this compound. The single exponent feature, generally noticed on thin films grown on SrTiO3 substrate and associated to a strong single vortex regime, has been observed in thin films grown on LaAlO3, only in the parallel configuration. At the end, this overall comparison can be useful to develop a technological material able to compete with high temperature superconductors.

We have shown that the superconducting properties of FeSe 0.5 Te 0.5 thin films are strongly dependent on the growth conditions and, in particular, the in-plane lattice constant of the substrate influences the crystallographic lattice parameters of the films, affecting the strain and is responsible for strong enhancements of the critical temperature T c and for the introduction of different pinning mechanisms. Here we compare the structural, electrical and transport properties of superconducting Fe(Se 0.5 , Te 0.5 ) epitaxial films deposited through pulsed laser ablation on three different substrates namely lanthanum aluminate (LaAlO 3 ), strontium titanate (SrTiO 3 ), and calcium fluoride (CaF 2 ). We analyze in particular the critical current density J c as a function of the temperature and magnetic field, and its anisotropy, which is related to the different pinning mechanisms in play. The film grown on SrTiO 3 exhibits a higher critical current when the field is perpendicular to the film surface, opposite to what happens in the sample grown on LaAlO 3 due to the presence of extrinsic pinning along the c -axis, while we observe almost no anisotropy on the thin film grown on CaF 2 .

Superconducting electron-doped Nd2−xCexCuO4±δ films have been successfully prepared on (001) SrTiO3 substrates by an automated dc sputtering system. The composition of the samples has been measured by wavelength dispersive spectroscopy, and the crystalline structure has been investigated by high resolution X-ray diffraction technique. Measurements by backscattered electrons has been also used in order to deeply analyze the microstructure of the samples. We have found that the quality of the films is highly sensitive to the deposition conditions and that the superconductivity is reached only after suitable heat treatments at temperature above to 850°C in an oxygen-reducing atmosphere. The films deposited with the optimized conditions show a high degree of epitaxial growth, moreover an increase of c lattice parameter is found in all the superconducting samples. X-ray reflectivity measurements on thin films by using synchrotron radiation proved that smooth surfaces (roughness less than 2nm) and neat interfaces characterize the films through the whole thickness.

The transport properties of manganite thin films characterized by a weak-localization transition have been studied. Detailed voltage noise measurements show a specific 1/f noise spectrum below the transition temperature. A theoretical interpretation in terms of universal conductance fluctuations explains the nature of the unconventional electric noise, suggesting a direct connection between the weak-localization phenomenon and universal conductance fluctuations. The universal nature of the mechanism allows its detection in different systems under the weak-localization regime.

Trilayers made of perovskite-type ferromagnetic (F), antiferromagnetic (AF) and superconducting (S) films were fabricated and their microstructural properties studied. Epitaxially strained La2/3Ca1/3MnO3/La1/3Ca2/3MnO3/YBa2Cu3O7−δ (F/AF/S) trilayers were grown in situ by dc-sputtering technique onto (001)-oriented SrTiO3 (STO) substrates. Whereas the thickness of the bottom (F) and top (S) layer was fixed to 74 nm and 100 nm, respectively, that of the intermediate AF layer was varied between ∼4 nm and ∼9 nm. The crystalline quality of the samples was checked by X-Ray diffraction (XRD) analysis. The θ–2θ scans and reciprocal space maps at (002) and (013) Bragg reflections provided clear evidence for the heteroepitaxial growth of the trilayers as well as for the absence of secondary phases. Due to high epitaxiality of the trilayers, their magnetic response was already shown to develop an excellent magnetic anisotropy to clearly visualize the superconducting diamagnetism and ferromagnetic behavior at a same temperature. The superconducting diamagnetic response at 5 K has now been used to calculate the superconducting current density of the S layer as a function of an applied magnetic field. The results confirmed the uncoupling role of the AF layer. The presence of well defined AF barriers (La1/3Ca2/3MnO3) in the trilayers was also evidenced by recording the I–V characteristic in cross configuration, which showed an anisotropic behavior as well. The results achieved suggest that such oxide heterostructures could play an important role in the search for novel devices based on magnetic junctions.