JOSEPHSON JUNCTION NETWORK AS A TOOL TO SIMULATE INTERGRAIN SUPERCONDUCTING CHANNELS IN YBCO FILMS

Recent considerations on the physics of YBa 2 Cu 3 O 7-δ films made possible explaining their transport properties as flow of supercurrents through links between the granular structure of the film. The present work deals with the analysis of the Josephson junction network as a discrete set of parallel junctions (1D array) in quasi-static conditions and is aimed to compare the results of the simulations with the experimental findings, in particular with the plateau-like features in the critical current dependence on the magnetic field. Different regimes and vortex phases have been individuated and discussed

Tuning the microwave electromagnetic properties of biochar-based composites by annealing

Abstract We report on the effects of thermal treatment of biochar embedded in epoxy-based composites on their microwave electrical properties, linking such properties to the material structure investigated by Raman, X-ray photoelectron spectroscopy, and X-ray diffraction. Annealing temperatures in the range 900–1500 °C and biochar concentrations in the epoxy matrix in the range from 5 to 25 wt.% were investigated. The microwave analysis, in the range from 250 MHz to 6 GHz, allowed us to determine the complex permittivity of composites and, through a proper deconvolution technique, to determine the contribution of biochar inclusions alone. High values of real permittivity (up to 220) and conductivity (up to 17 S/m) were evaluated for the biochar particles at 5 GHz, after the 1500 °C thermal treatment. A clear correlation between electrical properties and the biochar microstructure emerged from the dataset, with real permittivity and conductivity increasing as carbon inclusions transform from amorphous to nanocrystalline graphite. Conversely, the percentage of aromatic carbon has a weaker influence on the microwave properties. This study opens to the possibility of tailoring the high-frequency properties of biochar and biochar composites through proper thermal treatments

Spectroscopic studies of the superconducting gap in the 12442 family of iron-based compounds

The iron-based compounds of the so-called 12442 family are very peculiar in various respects. They originate from the intergrowth of 122 and 1111 building blocks, display a large in-plane vs. out-of-plane anisotropy, possess double layers of FeAs separated by insulating layers, and are generally very similar to double-layer cuprates. Moreover, they are stoichiometric superconductors because of an intrinsic hole doping. Establishing their superconducting properties, and in particular the symmetry of the order parameter, is thus particularly relevant in order to understand to what extent these compounds can be considered as the iron-based counterpart of cuprates. In this work we review the results of various techniques from the current literature and compare them with ours, obtained in Rb-12442 by combining point-contact Andreev-reflection spectroscopy and coplanar waveguide resonator measurements of the superfluid density. It turns out that the compound possesses at least two gaps, one of which is certainly nodal. The compatibility of this result with the theoretically allowed gap structures, as well as with the other results in literature, is discussed in detail.Comment: 16 pages, 12 figure

Confinement of fluxons by surface columnar defects in Bi1.8Pb0.33Sr1.87Ca2Cu3Oy tapes

We study the effects of surface columnar defects created along about 5% of the sample thickness by means of 0.25 GeV Au ions at different doses on Bi1.8Pb0.33Sr1.87Ca2Cu3Oy high-quality tapes on the vortex dynamics in the sample bulk. Strong phenomena of vortex localization inside the bulk are revealed by shifts of the irreversibility lines (IL's) as well as by their after-irradiation shape. The enhanced IL's exhibit specific characteristics, such as a Bose-glass-like behavior up to quite high fields, with a dose-dependent onset point. Moreover, the irreversible regime expands with decreasing defect density. Such results are consistent with the setting up of confined vortices morphologies. Experimental data concerning the IL with the field orthogonal to the columnar defect direction as well as critical current density enhancements are also reported and discussed

UV-Led curable coatings containing porcupine-like carbon structures: thermal, dynamic-mechanical and electrical properties

Multi-functional coatings based on a UV-LED curable epoxy-acrylate resin and different loadings (up to 10 wt.%) of carbonaceous structures having a peculiar porcupine (PuP)-like morphology were formulated. More specifically, biochar-based particles derived from cellulose nanocrystals were modified through the growth of carbon nanofibers onto their outer surface, aiming at combining the effects of globular and high aspect ratio structures in a single filler. The introduction of increasing amounts of PuPs induced a progressive increase of the material storage modulus and thermal conductivity and a remarkable enhancement of the electrical conductivity, thus pointing out the effectiveness of the proposed approach of surface modifying biochar particles in obtaining composite films with superior properties

Mechanical, electrical, thermal and tribological behavior of epoxy resin composites reinforced with waste hemp-derived carbon fibers

Short hemp fibers, an agricultural waste, were used for producing biochar by pyrolysis at 1000°C. The so-obtained hemp-derived carbon fibers (HFB) were used as filler for improving the properties of an epoxy resin using a simple casting and curing process. The addition of HFB in the epoxy matrix increases the storage modulus while damping factor is lowered. Also, the incorporation of HFB induces a remarkable increment of electrical conductivity reaching up to 6 mS/m with 10 wt% of loading. A similar trend is also observed during high frequency measurements. Furthermore, for the first time wear of these composites has been studied. The use of HFB is an efficient method for reducing the wear rate resistance and the friction coefficient (COF) of the epoxy resin. Excellent results are obtained for the composite containing 2.5 wt% of HFB, for which COF and wear rate decrease by 21% and 80%, respectively, as compared with those of the unfilled epoxy resin. The overall results prove how a common waste carbon source can significantly wide epoxy resin applications by a proper modulation of its electrical and wear properties

High Frequency Electromagnetic Shielding by Biochar-Based Composites

We report on the microwave shielding efficiency of non-structural composites, where inclusions of biochar—a cost effective and eco-friendly material—are dispersed in matrices of interest for building construction. We directly measured the complex permittivity of raw materials and composites, in the frequency range 100 MHz–8 GHz. A proper permittivity mixing formula allows obtaining other combinations, to enlarge the case studies. From complex permittivity, finally, we calculated the shielding efficiency, showing that tailoring the content of biochar allows obtaining a desired value of electromagnetic shielding, potentially useful for different applications. This approach represents a quick preliminary evaluation tool to design composites with desired shielding properties starting from physical parameters

Microwave analysis of the interplay between magnetism and superconductivity in EuFe 2 (As 1 -x P x ) 2 single crystals

This paper presents a microwave analysis of the interplay between magnetism and superconductivity in an iron-based ferromagnetic superconductor. By comparing the complex rf susceptibility with magnetic force images, the authors discuss the nature of the observed phase transitions and the possible presence of a quantum critical point

Study of the thermal distribution for YBCO based Transition Edge Bolometers working above 77 K

ransition Edge Bolometers (TEB) are among the simplest radiation detectors. The straightforward operation mode provides good results only if it is combined with a careful thermal optimization.In a TEB, the strong dependence of the electrical resistivity on the temperature in its transition zone enables the detection of a variation of the local temperature which can reach tens of µK. For this reason, it is essential to study the thermal profile of the superconducting active part of the detector, hence its substrate, to make it as homogeneous as possible.Irradiated YBa 2 Cu 3 O 7-x (YBCO) films can be used for position sensitive detection of infrared radiation. A TEB with a double meander pattern, one of which with a reduced critical temperature due to irradiation with high-energy heavy ions, was designed to work in a portable cryostat at a temperature above the liquid nitrogen (LN 2 ) point.In this work, we present a series of Finite Element Method simulations (using COMSOL Multiphysics ® ) aimed at the optimization of the thermal distribution above the YBCO film. Once the optimal working point for the device is found, various materials for the bolometer hosting are tested to identify the combination that provides the most homogeneous temperature distribution. The optimal configurations are then analyzed in response to a sudden change in the PID current to determine the one which presents the best behavior in a transient situation