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The use of superconducting (SC) materials is crucial for shielding quasi-static magnetic fields. However, the need for space-saving solutions with high shielding performance requires the development of a three-dimensional (3D) modelling procedure capable of predicting the screening properties for different orientations of the applied field. In this paper, we use a 3D numerical model based on a vector potential formulation to investigate the shielding ability of SC screens with cylindrical symmetry and a height/diameter aspect ratio close to unity, without and with the superimposition of a ferromagnetic (FM) circular shell. The chosen materials were MgB2 and soft iron. First, the outcomes of the calculations were compared with the experimental data obtained with different shielding arrangements, achieving a notable agreement in both axial field (AF) and transverse field (TF) orientations. Then, we used this validated modelling approach to investigate how the magnetic mitigation properties of a cup-shaped SC bulk can be improved by the superimposition of a coaxial FM cup. Calculations highlighted that the FM addition is very efficient in enhancing the shielding factors (SFs) in the TF orientation. Assuming a working temperature of 30 K and using a layout with the FM cup protruding over the SC one, SFs up to eight times greater than those with a single SC cup were attained at applied field up to 0.15 T, reaching values equal to or higher than 102 in the inner half of the shield. In the AF orientation, the addition of the same FM cup incurs a modest worsening at low fields, but at the same time it widens the applied field range where SF ⩾ 104 occurs near the close extremity of the shield to over 1 T.

We report on ion irradiation experiments performed on compounds belonging to the $$\hbox {BaFe}_2\hbox {As}_2$$ BaFe 2 As 2 family, each one involving the partial substitution of an atom of the parent compound (K for Ba, Co for Fe, and P for As), with an optimal composition to maximize the superconducting critical temperature $$T_c$$ T c . Employed ion beams were 3.5-MeV protons, 250-MeV Au ions, and 1.2-GeV Pb ions, but additional data from literature are also considered, thus covering a wide range of ions and energies. Microwave characterization based on the use of a coplanar waveguide resonator allowed us to investigate the irradiation-induced $$T_c$$ T c degradation, as well as the increase of normal state resistivity and London penetration depth. The damage was quantified in terms of displacements per atom (dpa). From this broad and comprehensive set of experimental data, clear scaling laws emerge, valid in the range of moderate irradiation-induced disorder (dpa up to 5 $$\times$$ × 10 $$^{-3}$$ - 3 were investigated). In these conditions, linear trends with dpa were found for all the modification rates, while a power law dependence on the ion energy was found for heavy-ion irradiation. All these scaling laws are reported and discussed throughout the paper.

We report on the anisotropy of the London penetration depth of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$, discussing how it relates to its electronic structure and how it modifies under introduction of disorder, both chemically induced (by $\mathrm{Ni}$ substitution) and irradiation induced (by 3.5-MeV protons). Indeed, ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ is particularly suitable for the study of fundamental superconducting properties due to its stoichiometric composition, exhibiting clean-limit behavior in the pristine samples and having a fairly high critical temperature, ${T}_{c}\ensuremath{\approx}35$ K. The London penetration depth ${\ensuremath{\lambda}}_{L}$ is measured with a microwave-coplanar-resonator technique that allows us to deconvolve the anisotropic contributions ${\ensuremath{\lambda}}_{L,ab}$ and ${\ensuremath{\lambda}}_{L,c}$ and obtain the anisotropy parameter ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}={\ensuremath{\lambda}}_{L,c}/{\ensuremath{\lambda}}_{L,ab}$. The ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)$ found for the undoped pristine sample is in good agreement with previous literature and is here compared to ab initio density-functional-theory and Eliashberg calculations. The dependence of ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}(T)$ on both chemical and irradiation-induced disorder is discussed to highlight which method is more suitable to decrease the direction dependence of the electromagnetic properties while maintaining a high critical temperature. Lastly, the relevance of an intrinsic anisotropy such as ${\ensuremath{\gamma}}_{\ensuremath{\lambda}}$ on application-related anisotropic parameters (critical current, pinning) is discussed in light of the recent employment of ${\mathrm{Ca}\mathrm{K}\mathrm{Fe}}_{4}{\mathrm{As}}_{4}$ in the production of wires.

IR pulsed laser radiation in air was applied to Si3N4 and Invar to obtain reliable Si3N4/Si3N4 and Si3N4/Invar adhesive bonded components. The laser pre-treatment produced a homogeneous nanostructured oxide layer on the surfaces, which effectively increased the adhesion at the adhesive/adherends interface and led to cohesive failure in the joining material. The mechanical strength of Si3N4/ Si3N4 and Si3N4/Invar joined components was measured, with and without laser nanostructuring, before and after thermal cycling from room temperature to 50 K, and it resulted that the exposure to extremely low temperatures did not affect the mechanical integrity of the joints. It was also demonstrated that this laser pre-treatment did not alter the mechanical properties of the ceramic substrate.

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

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.

We report on the combined experimental and theoretical analysis of the microwave-frequency electromagnetic response of ${\mathrm{BaFe}}_{2}{\mathrm{As}}_{2}$ single crystals with different substitutions: K in the Ba site (hole doping), Co in the Fe site (electron doping), and P in the As site (isovalent substitution). Measurements using a coplanar resonator technique lead to the experimental determination of the penetration depth and microwave conductivity as a function of temperature. The whole set of data is analyzed within a self-consistent three-band ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave Eliashberg approach, able to account for all the main observed features in the different properties. Besides the validation of the model itself, the comparison between experiment and theory allows discussing the possible role of the Fe-As planes in defining the superconducting properties of these compounds, the relevance of coherence effects, and the presence of nodes in the superconducting order parameter.

Most iron-based superconductors are characterized by the s+- symmetry of their order parameter, and are expected to go through a transition to the s++ state if enough disorder is introduced. We previously reported the observation of this transition in Ba(Fe1-xRhx)2As2 through a study of the disorder dependence of the critical temperature and low-temperature London penetration depth. In this paper we report on the analysis of the electrodynamic response of the same sample across the transition and we identify peculiarities in the behaviour of the surface resistance and normal conductivity, that can be considered as traces of the transition itself., Comment: 5 pages, 2 figures, 1 table

We report on the Eliashberg analysis of the electrodynamic response of Ba(Fe1−xRhx)2As2 single crystals across the disorder-induced s± to s++ transition. We previously experimentally identified the transition by its signature in the low temperature value of the penetration depth and observed a peculiar dependency of the critical temperature on disorder; subsequently, we proposed new hallmarks in the quasiparticle conductivity and surface impedance. Here we show that this whole set of data can be self consistently reproduced within an effective two-bands Eliashberg model with disorder treated beyond the Born approximation and with a small set of input parameters.

At the present day, iron-based superconductors (IBS) can be considered as competitors of cuprates for the fabrication of superconducting cables for high field application in the presence of particle irradiation. In particular, it has been demonstrated that 11-IBS are very robust against proton irradiation. In this paper, we analyze the effect of this kind of irradiation on the anisotropy of superconducting and pinning properties of Fe(Se,Te) thin films grown on CaF2. By electrical transport measurements, we evaluate the upper critical field as a function of the temperature and the pinning activation energy as function of the applied magnetic field, for different orientation between the material crystal structure and the field itself, in pristine and irradiated samples. The aim is to recognize if and how proton irradiation modifies the anisotropy of the material properties.

In this work, an experimental approach toward reaching new insights into the mechanism of superconductivity in the iron-based compounds is presented. The temperature dependence of the penetration depth and of the microwave conductivity of high-quality Ba(Fe1-xCox)2As2 and (Ba1-xKx)Fe2As2 single crystals have been obtained by a 7.97-GHz superconducting coplanar resonator technique, in a perturbative approach. The proposed technique is suitable to obtain the absolute value of the penetration depth and shows high sensitivity to small samples. For the Co-doped BaFe2As2, the comparison of crystals with different shape factors allows distinguishing the in-plane and out-of-plane components of the penetration depth. Peaks in the normalized quasiparticle conductivity of both Co-doped and K-doped crystals are shown and discussed.

Irradiation effects are often used to improve the effective pinning in superconductors, but sometimes they can reveal detrimental for superconducting properties. Fe(Se,Te) has been proven to be a very robust material against irradiation, in particular proton irradiation, thus configuring as an ideal material to work in harsh environments such as particle accelerators or fusion reactors. Anyway, the study of the pinning activation energy in Fe(Se,Te) thin film irradiated by 3.5 MeV protons suggests that this treatment can modify the anisotropy of the films pinning. Thus here we present the result of further investigation analyzing the effect of proton irradiation on the critical current and the pinning force both for the magnetic field applied parallel and perpendicular to the sample surface. We find that, although a slight effect on the critical current anisotropy is observed, the pinning landscape is not affected by the irradiation process. This confirms that Fe(Se,Te) can be considered for devices working in harsh environments.

Microwave measurements of the London penetration depth and critical temperature ${T}_{c}$ were used to show evidence of a disordered-driven transition from ${s}_{\ifmmode\pm\else\textpm\fi{}}$ to ${s}_{++}$ order parameter symmetry in optimally doped $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}{)}_{2}{\mathrm{As}}_{2}$ single crystals, where disorder was induced by means of 3.5 MeV proton irradiation. Signatures of such a transition, as theoretically predicted [V. D. Efremov et al., Phys. Rev. B 84, 180512(R) (2011)], are found as a drop in the low-temperature values of the London penetration depth and a virtually disorder-independent superconducting ${T}_{c}$. We show how these experimental observations can be described by multiband Eliashberg calculations in which the effect of disorder is accounted for in a suitable way. To this aim, an effective two-band approach is adopted, allowing us to treat disorder in a range between the Born approximation and the unitary limit.

In this paper we explore the effects of 3.5 MeV proton irradiation on Fe(Se,Te) thin films grown on CaF2. In particular, we carry out a systematic experimental investigation with different irradiation fluences up to 7.30x10^16 cm^-2 and different proton implantation depths, in order to clarify whether and to what extent the critical current is enhanced or suppressed, what are the effects of irradiation on the critical temperature, the resistivity and the critical magnetic fields, and finally what is the role played by the substrate in this context. We find that the effect of irradiation on superconducting properties is generally small as compared to the case of other iron-based superconductors. Such effect is more evident on the critical current density Jc, while it is minor on the transition temperature Tc, on the normal state resistivity and on the upper critical field Hc2 up to the highest fluences explored in this work. In addition, our analysis shows that when protons implant in the substrate far from the superconducting film, the critical current can be enhanced up to 50% of the pristine value at 7 T and 12 K, while there is no appreciable effect on critical temperature and critical fields together with a slight decrease in resistivity. On the contrary, when the implantation layer is closer to the film-substrate interface, both critical current and temperature show a decrease accompanied by an enhancement of the resistivity and the lattice strain. This result evidences that possible modifications induced by irradiation in the substrate may affect the superconducting properties of the film via lattice strain. The robustness of the Fe(Se,Te) system to irradiation induced damage makes it a promising compound for the fabrication of magnets in high-energy accelerators., Comment: 14 pages, 11 figures, submitted to Superconductor Science and Technology

One of the open issues concerning iron-based superconductors is whether the s± wave model is able to account for the overall effects of impurity scattering, including the low rate of decrease of the critical temperature with the impurity concentration. Here we investigate Ba1−x K x Fe2As2 crystals where disorder is introduced by Au-ion irradiation. Critical temperature, T c , and London penetration depth, λ L , were measured by a microwave resonator technique, for different values of the irradiation fluence. We compared experimental data with calculations made on the basis of the three-band Eliashberg equations, suitably accounting for the impurity scattering. We show that this approach is able to explain in a consistent way the effects of disorder both on T c and on λ L (T), within the s± wave model. In particular, a change of curvature in the low-temperature λ L (T) curves for the most irradiated crystals is fairly well reproduced.

We present a $^{75}\mathrm{As}$ nuclear magnetic resonance (NMR) and resistivity study of the effect of 5.5 MeV proton irradiation on the optimal electron doped $(x=0.068)$ and overdoped $(x=0.107)$ $\mathrm{Ba}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x})}_{2}{\mathrm{As}}_{2}$ iron based superconductors. While the proton induced defects only mildly suppress the critical temperature and increase residual resistivity in both compositions, sizable broadening of the NMR spectra was observed in all the irradiated samples at low temperature. The effect is significantly stronger in the optimally doped sample where the Curie Weiss temperature dependence of the line width suggests the onset of ferromagnetic correlations coexisting with superconductivity at the nanoscale. $1/{T}_{2}$ measurements revealed that the energy barrier characterizing the low energy spin fluctuations of these compounds is enhanced upon proton irradiation, suggesting that the defects are likely slowing down the fluctuations between $(0,\ensuremath{\pi})$ and $(\ensuremath{\pi}$,0) nematic ground states.

We investigate the penetration depth of high-quality ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals by a planar waveguide resonator technique, in a cavity perturbation approach. The experimental ${\ensuremath{\lambda}}_{L}$ is compared to calculations based on the three-band Eliashberg equations within the $s\ifmmode\pm\else\textpm\fi{}$ wave model. To this end, the anisotropy of the penetration depth is taken into account. In fact, the agreement between theory and experiment is remarkable. The low-temperature value of the in-plane penetration depth, ${\ensuremath{\lambda}}_{L,ab}(5\phantom{\rule{4.pt}{0ex}}\text{K})=220\phantom{\rule{4pt}{0ex}}\mathrm{nm}$, and the total plasma frequency, ${\ensuremath{\omega}}_{p}=1.0\phantom{\rule{4pt}{0ex}}\mathrm{eV}$, are also consistent with earlier results. This overall consistency validates the model itself, thus allowing us to estimate parameters that are missing in literature, such as the plasma frequency for each band: it turns out that ${\ensuremath{\omega}}_{p,1}={\ensuremath{\omega}}_{p,3}=0.32\phantom{\rule{4pt}{0ex}}\mathrm{eV}$ and ${\ensuremath{\omega}}_{p,2}=0.89\phantom{\rule{4pt}{0ex}}\mathrm{eV}$, with the subscripts 1 and 2 denoting the two hole bands and 3 the equivalent electron band.

This paper compares the shielding properties of superconducting (SC) and superimposed superconducting/ferromagnetic (SC/FM) systems, consisting of cylindrical cups with an aspect ratio of height/radius close to unity. Both bilayer structures, with the SC cup placed inside the FM one, and multilayer structures, made up of two SC and two FM alternating cups, have been considered. Induction magnetic field values have been calculated by means of a finite element model based on the vector potential formulation, simultaneously taking into account the non-linear properties of both the SC and FM materials. The analysis highlights that at low applied fields, the presence of a height difference between the edges of the SC/FM cups, as well as a suitable choice of the lateral gap between the cups, is a key factor in obtaining hybrid structures with a shielding potential comparable to, or even higher than, that of the single SC cup. In contrast, at high applied fields, all the hybrid arrangements investigated always provide much greater shielding factors than the SC cup alone. The computation results show that at both low and high applied fields, the multilayer solutions are the hybrid shields with the highest efficiency.

Magneto-optical imaging with an indicator film is used for investigating the magnetic pattern in ferromagnetic material specimens whose optical properties do not allow the direct visualization of their magnetic pattern. The magneto-optical indicator is a ferromagnetic film with low coercitivity that detects the magnetic field from a magnetic source on which it is placed. The measurement of the indicator film is performed with a polarization microscope. In this way, the magnetic pattern generated by ferromagnets, superconductors, or current carrying conductors has been successfully visualized. By means of epitaxial iron garnet films, we obtained the quantitative imaging of the stray field induced by the magnetic microstructure in ferromagnetic films and crystals. The quantitative measurement of the perpendicular component of the magnetic field, obtained by a suitable nonlinear calibration, is demonstrated for single domain walls in ferromagnetic materials (in this paper for manganite and garnet films).

International audience; MgB2 bulk superconductors were shaped by Spark Plasma Sintering (SPS) process and the superconducting properties were measured. The obtained materials were 98% dense. Trapped field higher than 1.4 T was measured at 20 K. Otherwise, the possibility to use MgB2 for magnetic levitation has been investigated. The levitation force was investigated below 35 K. The results show that the SPS process allows to produce dense samples with enhanced superconductor characteristics as a result of improved grains connection and better densification of the bulk MgB2 superconductors, which could be a viable variant for magnetic levitation applications and/or as cryo-magnets. © 2016.

CaO–Al2O3 (CA) and SiO2–Al2O3–Y2O3 (SAY) glass-ceramics are promising candidates for SiC/SiC indirect joints. In view of their use in locations where high radiation level is expected (i.e. fusion plants) it is important to investigate how radiation-induced damage can modify the material microstructure. To this aim, pellets of both types were irradiated with 5.5 MeV 4He+ ions at an average temperature of 75 °C up to a fluence of almost 2.3·1018 cm−2. This produces a displacement defect density that increases with depth and reaches a value of about 40 displacements per atom in the ion implantation region, where the He-gas reaches a concentration of several thousands of atomic parts per million. X-ray diffractometry and scanning electron microscopy showed no change in the microstructure and in the morphology of the pellet surface. Moreover, a transmission electron microscopy investigation on cross-section lamellas revealed the occurrence of structural defects and agglomerates of He-bubbles in the implantation region for the CA sample and a more homogeneous He-bubble distribution in the SAY pellet, even outside the implantation layer. In addition, no amorphization was found in both samples, even in correspondence to the He implantation zone. The radiation damage induced only occasional micro-cracks, mainly located at grain boundaries (CA) or within the grains (SAY).

In this work, we present an approach towards achieving a detailed knowledge on the effects of irradiation in iron-based superconductors with high-energy heavy ions. Ba1−xKxFe2As2 single crystals (x = 0.42) were irradiated with 250 MeV Au-ions. The crystals were shaped to be thin plates with thickness lower than the ion implantation depth and bases perpendicular to the c-axis. By means of the quantitative magneto-optical imaging technique we attained the local current density in pristine and irradiated sample regions without any model assumption. After irradiation a strong critical current density (Jc) enhancement was measured, overcoming a factor 3 at T = 4 K with an irradiation fluence φ = 9.7 × 109 cm-2. Moreover, correlating point by point Jc and induction magnetic field maps, a contribution of the Au-ion induced defects to the anisotropic out of plane pinning (i.e. pinning in the direction parallel to the ion path) turned up. This contribution shows a maximum at applied fields quite lower than the nominal dose equivalent field. Furthermore, the temperature dependence of the penetration depth was evaluated before and after high-fluence irradiations by investigating the high-frequency behavior of these crystals using a microwave coplanar resonator technique. Fitting the experimental curves with a power law, we found a decrease of the power exponent, more pronounced at the higher fluences, that can supports the s± wave model

We report about the functional exploiting of the High-Energy Heavy-Ion (HEHI) lithography aimed at modulating in confined regions both structural and electrical properties of high temperature superconducting YBa 2 Cu 3 O 7− x (YBCO) films. We discuss the physical behaviors related to the HEHI modification of such films by means of 3D columnar defects, exhibiting nanometric cross section. A major part of this paper is devoted to the viability of well-defined (B,T,J) phase-diagram zones, where electrical decoupling between as-grown and modified parts occurs and so guarantees external-signal localization. In order to stress out complementary behaviors into different phase-diagram regions of locally modified YBCO films, we present low-temperature magnetic imaging, accounting for the case where a spatially continuous Meissner state holds on throughout the whole sample. In this case, the continuity of the Meissner state hampers localization of dissipative signals in the nanostructured region. On the contrary, inside the (B,T,J) phase-diagram zones belonging to the temperature range where the Meissner state is spatially broken, the temperature range of dissipation confinement under external stimuli can be functionally exploited. Then the main target of a road map pointing towards position-sensitive infrared superconducting sensors is enlightened by means of crucial experimental results. With this respect, a device layout, consisting into HEHI modified YBCO film grown either on YSZ or MgO substrates, is chosen to test the functional behavior of infrared detectors working above liquid nitrogen temperature and, in the case of MgO substrate, displaying sub-millisecond response-time.

This paper investigates the possibility of using superconductive elements as passive magnetic shields, in order to mitigate the magnetic field in limited regions around magnetic resonance scanners where medical staff could operate. The analysis is performed through the Finite Element Method by assuming a relative permeability close to zero to simulate the behavior of superconductive materials. Some shielding solutions employing superconductive sheets are proposed, also evaluating the disturbance on the field homogeneity in the center of the superconducting MRI. Possible synergies between the superconductive shields and the ferromagnetic elements, employed as hybrid shields or field concentrators, are finally discussed.

Broadband electromagnetic characterization of hot plasmas and electron beams requires real-time, direct detection of high flux of electromagnetic radiation. We report about a project aimed at creating robust THz detectors based on YBa2Cu3O7-x (YBCO) films, working around liquid nitrogen temperature and in a regime of low dissipation. The sensor layout mainly consists in two contiguous meanders that are integrated on the same substrate. One of them is nanostructured by local high-energy heavy-ion irradiation; the other one (as-grown YBCO) is used for thermo-electric signal reference. By measuring the voltage response to THz excitation of YBCO film, at a given bias current and temperature, on selected substrates (Yttria Stabilized Zirconia (YSZ) and MgO), it turns out that only the nanostructured part is brought in the dissipative state when exposed to THz radiation. We present estimated noise equivalent power and relaxation times for YBCO grown on YSZ and on MgO substrates. On the latter, we experimentally checked out the device response to time-varying Far-Infrared (FIR) signals up to 800 Hz. The proposed device turns out to fit the necessary requirements for measurement of electron cyclotron emission of hot plasmas or of electron beams in the FIR spectrum.

Magnetic shielding properties of MgB2 bulk samples synthesized by the SPS (Spark–Plasma–Sintering) technique were characterized in low applied magnetic fields at temperatures ranging from 20 to 37 K. The used growth technique allows one to produce this compound in different shapes and sizes required for shielding applications. In this framework, shielding magnetic-induction field profiles generated by MgB2-based shield components, shaped as planar thick disks, were measured by means of a suitable Hall probe in-plane array. The magnetic field distribution at different vertical distances above the sample was also obtained by a micrometric motion of the probe ensemble. Magnetic field profiles were then analyzed in the framework of the critical state model and the critical current density, Jc, was evaluated. The Jc magnitude indicates that the material under test is a good candidate for passive magnetic shield manufacturing up to temperatures close to the transition one.

Research and application of THz technology, pursued all over the last century, are currently focused either on high-performance sources and detectors or on low-cost and rugged devices that are suitable for widespread use. Besides semiconductors, high temperature superconductors are good candidates for next generation superconducting bolometers. We employ the high energy heavy ion (HEHI) lithography for modulating the local structural and electrical properties of high temperature superconducting YBa2Cu3O7 (YBCO) films in a functional way. In this report, measurements of the photoresponse of a prototype device to electromagnetic radiation in the far infrared band (THz region) are presented. It is demonstrated that HEHI lithography is an effective tool to enhance the absorption band of the YBCO film in the infrared spectrum, enabling the THz detection of YBCO sensors, in the superconducting state and above the liquid nitrogen temperature.

The electrodynamic response of Ba1−x K x Fe2As2 single crystals at the microwave frequencies has been investigated by means of a coplanar resonator technique, at different values of non-magnetic disorder introduced into the samples by heavy-ion irradiation. The surface impedance Z s = R s + iX s conforms to the classical skin effect above the critical temperature. Below T c, R s monotonically decreases while X s shows a peak, which evolves as a function of the irradiation fluence. The disorder-dependent Z s (T) curves are analyzed within a two-fluid model, suitably modified to account for a finite quasiparticle fraction at T = 0. The analysis gives, for the unirradiated crystal, quasiparticle relaxation times τ that are in good agreement with previous literature. Smaller τ values are deduced for the disordered crystals, both in the normal and in the superconducting states. The limits of application of the model are discussed.

Heavy Ion Lithography was used for local tuning the superconducting properties (critical temperature and critical current) of high temperature superconducting films through the introduction of ion induced nanostructures confined in micrometric areas. These modified areas were patterned on a device layout that was designed for the wide-band bolometric detection of electromagnetic radiation. The device was biased with constant current at a specific temperature in order to obtain the localization of the electromagnetic excitation only into the modified areas. The device was tested with coherent light stimuli in the visible spectrum and the photoresponse was measured with high frequency real-time sampling. The localization of the photoexcitation was demonstrated by scanning the laser beam across the modified and as-grown areas of the device.

The aim of the present work is the optimization of the Si /buffer-layer/YBCO multilayer deposition process so as to grow superconducting films of quality suitable for device applications. The structural properties of the Si/CeO 2 system, obtained by RF magnetron sputtering of CeO 2 targets in Ar atmosphere, have been studied. More than 50 films have been deposited and some of them submitted to post-deposition annealing treatments both in N 2 and O 2 atmospheres. The presence of an unwanted amorphous SiO 2 layer at the Si/CeO 2 interface compromises the YBCO c-axis orientation, and therefore the sharpness of the R versus T transition. A newly designed deposition system has been realized: it has been specially conceived for obtaining bi- and tri-layers, adopting two targets in YSZ and CeO 2, respectively. Results on YSZ/Si and CeO 2/ YSZ/Si systems obtained with the new machine are presented and discussed: (100) oriented YSZ films with nominal thickness of 40 nm have been obtained. The CeO 2 film subsequently deposited has the desired (100) orientation. The YBCO film, in the final YBCO/YSZ/CeO 2/ Si configuration, is c-axis oriented.

This paper reports on an ac susceptometric analysis to evaluate the ac losses as a function of frequency and temperature at different external dc applied fields in MgB/sub 2/ twin bulk samples. The two investigated samples come from the same preparation batch. In agreement with partial results in literature, it turns out that the ac losses exhibit, as a function of frequency, a nonmonotonic behavior that is strongly temperature-dependent. In the lower range of analyzed temperatures the results are significantly affected by high-energy heavy ion irradiation. These last findings put forward puzzling questions about the creep dynamics in MgB/sub 2/. Finally the issue of a comparison with the ac losses in Ag sheathed/Bi2223 tapes is addressed. The comparison is made by means of measurements once performed with the same apparatus. It turns out that, taking into account the expected differences due to different ac applied fields, the absolute values of the dissipated energy density in MgB/sub 2/ and Ag/Bi2223 monofilamentary tape are of the same magnitude order and exhibit a similar behavior.

The paper reports on results concerning the Si/YSZ/CeO/sub 2//YBCO multilayer prepared by means of magnetron sputtering. Such multilayer is considered for the possibility to compound the integration of YBCO films with silicon-based devices, with the unique, in perspective, properties of YBCO concerning photon sensors. We characterized YBCO films in such multilayer configuration by means of structural and dc transport measurements. It turns out that some granularity affects the transport properties of the YBCO film and lowers the critical currents. However, the lower temperature E-J characteristics are sharp enough to consider exploiting the transition between under critical and over critical (flux flow) state for future silicon-integrated broad-band photon sensors.

This paper reports on nonuniform heavy ion irradiations of YBCO coplanar resonators aimed at controlling vortex-induced microwave dissipation and nonlinearity. The adopted moving-target irradiation technique allows producing tailored profiles of columnar defect density to get an optimal compromise between defect-induced enhancement of vortex pinning and addition of dissipative scattering. The same samples were characterized before and after irradiation in order to evaluate the surface impedance, by means of data analysis suitable to account also for the substrate properties. In the case of the modulated dose profile, matching the fundamental-mode field distribution along the resonator, the onset of nonlinearity is shifted toward higher circulating power, without increasing dissipation. Finally, the possibility to locally modify by irradiation the properties of the material to obtain the confinement of external perturbations, while maintaining the current continuity throughout the whole device, has been preliminarily tested.

A very significant figure of merit of a superconductor, for what concerns perspective applications, is related to its radiation hardness. In this paper we focus on the effect of high energy ions (4.2 GeV Au-ions) and low energy proton (3.5 MeV) irradiation on the superconducting properties of MgB2 tapes and films, respectively. Ions and energies are chosen as representative of the main radiation exposure in hard ambient. For sake of comparison, the behaviour of conventional high-Tc Superconductor with weak pinning, as Ag-sheated/Bi2Sr2Ca2Cu3Ox tapes, irradiated with 0.25 GeV Au-ions, is also reported. The main analysis methods consists in ac/dc magnetic and magneto-optical analysis. The obtained results indicate an high radiation hardness of MgB2 as a valuable figure of merit with respect to the application of this material in hard ambient, as well as, a radiation-induced significant enhancement of the critical current density and a large shift of the irreversibility line towards high temperatures in Ag-sheated/Bi-2223 tapes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)