[en] A good control of the Tl content in the Tl(1223) phase was achieved by developing a high pressure synthesis (typically 50 bar He/1 bar O₂) which allows the production of 95 % pure ceramics with a critical temperature of about 120 K. Mono- and multifilamentary Ag-sheathed tapes were prepared by the powder-in-tube technique with pre-reacted powder. Transport critical current densities of 15000 A / cm² for short-length (3 cm) and 10000 A / cm² for long length (2 m) were reached for monofilamentary tapes (7 K and 0 T). For multifilamentary tapes the critical current density (6000 A / cm²) appears to be independent of the superconducting filament thickness down to about 10 μm. Studies on the dependence of the transport properties on temperature and magnetic field showed that the transport current is dominated by weak-link phenomenon. The existence of a critical current at very high magnetic fields indicates the presence of well connected current paths through the sample. Studies on the distribution of the current showed that the current flows uniformly inside the tapes

[en] The capability of manufacturing long superconducting MgB₂ wires with already remarkable critical currents makes this material a very promising candidate for future applications. Tapes are prepared by the powder-in-tube technique. After the cold working procedure typically carried out by wire drawing and cold rolling, it has been found that a final sintering step carried out in argon atmosphere is a key process for further improving the superconducting properties of the conductors. To study the effect of the deformation and heat treatment processes, we performed neutron scattering experiment. Due to the high penetration depth of neutron inside matter, it was possible to analyse the MgB₂ phase still wrapped in the Ni sheath. Our studies were carried out by a full spectra refinement by the Rietveld method. In the starting superconducting powder a large Mg deficiency was observed. In the tapes we found that the large forces applied during the cold working induced a large MgB₂ lattice deformation, and that it is partly relaxed during the final sintering process. An important correlation of the residual stress with the critical temperature and the pinning properties was pointed out. We also observed the appearance of detrimental secondary phases during the sintering process. In particular, the MgB₂ phase reacted with the nickel sheath and MgB₂Ni_2.5 was formed at temperatures higher than 850 deg C. These results are of basic importance for a further optimization of the transport properties at moderate fields where applications of MgB₂ tapes are already envisageable

[en] The use of cryogenics micro calorimeters with Transition Edge Sensors for high-resolution spectroscopy for space applications puts several constraints on the detector's performances. Among several requirements, the long term stability of TES from a chemical and physical point of view is one of the most important. Iridium is a very interesting material for TES fabrication due to its excellent chemical stability and low transition temperature. Starting from the promising results we had with a cryogenic microcalorimeter with TES made of an Ir single crystal, we are developing a procedure to produce patterned Ir thin films by means of the Pulsed Laser Deposition (PLD). PLD with an infrared laser, in UHV, allows very clean evaporation process, good efficiency (about 4 nm/shot) and low dissipated power in the vacuum chamber (about 1 W). We have applied a lift-off mask technique that allows to heat the substrate during the deposition, which improves the adhesion and the thermal conductivity and circumvents the needs of very aggressive etchant solutions

[en] We study the growth conditions of Cu₂O thin films deposited on MgO (0 0 1) and SrTiO₃ (0 0 1) substrates by pulsed laser ablation, in order to explore the compatibility between semiconducting p-type Cu₂O and other perovskite oxides in view of the fabrication of oxide electronics heterostructures. We find that in both cases perfect epitaxy, high crystalline quality and good out-of-plane orientation are achieved. In this context, epitaxy plays a major role in driving the phase formation. On the other hand, in films deposited at temperatures higher than 700 deg. C transport is inhibited by poor grain connectivity, which is an inevitable consequence of the necessity for the crystal to release the lattice strain. Instead, better connectivity and bulk-like values of resistivity, as well as good crystallinity and orientation, are obtained for films deposited at 650 deg. C. This should be kept in mind for the fabrication of stacked layer oxide heterostructures, where deep grooves between adjacent grains would be a serious drawback both for vertical and planar transport

[en] Among perovskite oxides strontium titanate (STO) SrTiO₃ undergoes a metal-insulator transition at very low carrier concentration and exhibits high mobility values at low temperature. We exploited such electrical properties and the structural compatibility of perovskite oxide materials in realizing ferroelectric field effect epitaxial heterostructures. By pulsed laser deposition, we grew patterned field effect devices, consisting of lanthanum doped STO and Pb(Zr,Ti)O₃. Such devices showed a resistance modulation up to 20%, consistent with geometrical parameters and carrier concentration of the semiconducting channel

[en] Fluorine substitution in the Bi(2223), Bi(2212) and Tl(1223) superconducting phases was studied. The authors obtained superconducting structures, never observed before, of the Bi-based superconductors by a low temperature (200--400 C) fluorination process. Fluorine substitutes completely the oxygen sites in the Bi layers and additional F atoms are inserted in the structure. As a consequence, changes in the arrangements of cation and anions were induced, especially in the Bi and partially in the Sr layer. F-doped Tl(1223) has been prepared in the same way as Bi(2223) and Bi(2212) (low temperature fluorination), but also starting from precursor containing fluorides of different elements. No significant differences in the crystal structure have been observed between the Tl-based samples with F inclusions and without. The critical temperature (116 K) remains unchanged but a significant increase of the irreversibility field at low temperature was found

[en] Fluorine-doped Tl(1223) superconductors were prepared starting from mixtures of oxides (carbonates) and fluorides (in particular TlF) under moderately high gas pressure (50 bar). The thermodynamics of the phase formation were found to be modified so that the melting point was lowered and the region where a liquid phase coexists enlarged. 90% phase purity was obtained for bulk materials with the composition Tl_0.6Pb_0.5Sr_1.8Ba_0.2Ca_1.9Cu₃O_8.5-δF_0.5, prepared by a two-step synthesis including TlF among the starting materials. The critical temperature remained approximately the same as for F-free samples (∼115 K), but a remarkable increase of the irreversibility line was observed below 75 K. The samples showed superconducting properties even for the highest amount of fluorine introduced (about one atom per formula unit). Ag-sheathed tapes were prepared by the conventional PIT method, using ex situ reacted Tl(1223) powders or producing the reaction in situ. Transport critical current densities of about 10 000 A cm^-2 were measured. This value is slightly lower than generally observed for F-free samples, but the magnetic-field dependence was slightly improved. The tape anisotropy could be increased to a factor of 2.5 by performing cycles of mechanical pressing and heating above the formation temperature. The results (in particular neutron scattering diffraction) indicate that, with high-pressure synthesis, F remains in the sample and substitutes for O in the TlO layers in the Tl(1223) phase. (author)

[en] A novel technique based on a low-temperature single-step solid state reaction of stoichiometric quantities of HgO and Ba₂CuO₃ to produce superconducting HgBa₂CuO_4+x samples has been developed. The 'as-prepared' polycrystalline Hg-1201 samples had a T_c of 95 K, as obtained by the diamagnetic onset and resistivity measurements. The crystal symmetry was tetragonal with lattice parameters a = 3.876(1) AA, and c = 9.509(4) AA in agreement with literature data. Sample morphology consists of grains (average dimension 30 μ m) larger than in previously published works and of a needle-like structure embedding the whole sample. These results were highly reproducible over several batches. The resistive transition in magnetic field up to 8 T and a set of magnetic measurements is presented. From these measurements we obtain information about the irreversibility line, the critical current field dependence and the presence of weak links. (author)

[en] As-grown superconducting MgB₂ thin films were deposited by pulsed laser deposition on magnesium oxide and sapphire substrates. Starting from a non-stoichiometric, Mg and B mixed-powder target, we were able to grow the superconducting phase during the film deposition, without any further annealing process. So far, samples grown in the temperature range of 400-450 deg. C, and at an argon buffer pressure of the order of 10^-2 mbar turned out to be superconducting with an onset temperature of the resistive transition at about 25 K. Even if the deposition process still needs to be fully optimized, we have demonstrated that this method allows us to achieve in situ deposition of as-grown superconducting thin films. This procedure could therefore be promising for the deposition of high-quality epitaxial MgB₂ thin films. (author)

[en] CERN has recently started a design study for a possible next-generation high-energy hadron–hadron collider (Future Circular Collider—FCC-hh). The FCC-hh study calls for an unprecedented center-of-mass collision energy of 100 TeV, achievable by colliding counter-rotating proton beams with an energy of 50 TeV steered in a 100 km circumference tunnel by superconducting magnets which produce a dipole field of 16 T. The beams emit synchrotron radiation at high power levels, which, to optimize cryogenic efficiency, is absorbed by a beam-facing screen, coated with copper, and held at 50 K in the current design. The surface impedance of this screen has a strong impact on beam stability, and copper at 50 K allows for a limited beam stability margin only. This motivates the exploration of whether high-temperature superconductors (HTS), the only known materials possibly having a surface impedance lower than copper under the required operating conditions, would represent a viable alternative. This paper summarizes the FCC-hh requirements and focuses on identifying the best possible HTS material for this purpose. It reviews in particular the properties of Tl-based HTS, and discusses the consequent motivation for developing a deposition process for such compounds, which should be scalable to the size of the FCC components. (paper)