[en] Epitaxial titanium diboride thin films have been deposited on sapphire substrates by pulsed laser ablation technique. Structural properties of the films have been studied during the growth by reflection high energy electron diffraction (RHEED) and ex-situ by means of X-ray diffraction techniques; both kinds of measurements indicate a good crystallographic orientation of the TiB₂ film both in plane and along the c axis. A flat surface has been observed by atomic force microscopy imaging. Electrical resistivity at room temperature resulted to be five times higher than the value reported for single crystals. The films also resulted to be very stable at high temperature, which is very promising for using this material as a buffer layer in the growth of magnesium diboride thin films

[en] We present thermal conductivity and specific heat measurements on MgB₂ and Mg-AlB₂ samples. Thermal properties have been analyzed by using a two-gap model in order to estimate the gap amplitudes, Δ(0)_π and Δ(0)_σ and the intra-band scattering rates, Γ_σσ and Γ_ππ. Δ(0)_σ rapidly decreases as a function of Al doping and disorder, while Δ(0)_π is rather constant. Γ_σσ and Γ_ππ are increased by disorder, being Γ_ππ more affected than Γ_σσ

[en] A novel magnetic procedure has been introduced for the determination of the degree of texturing of the superconducting grains in Ag-sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O_x tapes. Our new method is based on the observation that, in the BSCCO family of superconductors, a relevant anisotropy is present between the in-plane and out-of-plane critical current densities of individual crystallites. By neglecting the magnetic moment of the single grain due to out-of-plane circulating currents, and by comparing magnetic hysteresis loops measured within different field orientations with respect to the tape direction, it has been possible to determine the average degree of texture of an assembly of BSCCO superconducting grains. As a result of our simple approach, we have measured the texture evolution of the precursor grains during the cold deformation process of unreacted multifilamentary (Bi,Pb)₂Sr₂Ca₂Cu₃O_x tapes; moreover, we have been also able to distinguish between longitudinal and transverse texture of the grains with respect to the tape direction. Finally, the usefulness of our method has been verified on tapes prepared with various precursor powders and cold deformation techniques. (author)

[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] The employment of superconducting thin films as electronic devices seems to be the most probable future application of these materials. For this application, thin films with extremely smooth surface and very good superconducting as well as structural properties are required. We deposited YBCO epitaxial thin films by pulsed laser ablation using a pulsed molecular oxygen flow synchronous with the arrival of the ablated species on the substrate. We studied the dependence of their surface morphology on the deposition pressure and we found an evident improvement in the surface quality by lowering the deposition pressure below 5x10^-3 mbar. Films grown in these conditions showed surface roughness lower than 1 A, transition temperatures above 88 K and critical current densities of about 10⁷ A cm^-2 at 4.2 K in self-field. (author)

[en] Magnetic measurements on type-II superconducting thin films subjected to a transverse external field can be explained by a model that takes demagnetizing effects into account. They carried out measurements of magnetic flux distribution on the film surface by means of a self made apparatus, based on two Hall probes and a differential transformer and they tested the validity of a recent model presented in the literature

[en] In this work we investigate the effect of each different heat treatment stage in the fabrication of Bi₂Sr₂CaCu₂O_8+δ superconducting wires on intra-grain and inter-grain superconducting properties. We measure magnetic critical temperature T _c values and transport critical current density J _c at temperatures from 4 K to 40 K and in fields up to 7 T. From an analysis of the temperature dependence of the self-field critical current density J _c( T ) that takes into account weak link behavior and the proximity effect, we study grain boundary (GB) transparency to supercurrents; we also establish a relationship between GB oxygenation in the different steps of the fabrication process and GB transparency to supercurrents. We find that GB oxygenation starts in the first crystallization stage, but it becomes complete in the plateau at 836 °C and in slow cooling stages and is further enhanced in the prolonged post-annealing step. Such oxygenation makes GBs more conductive, thus improving the inter-grain J _c value and temperature dependence. On the other hand, from inspection of the T _c values in the framework of the phase diagram dome, we find that grains are already oxygenated in the crystallization step up to the optimal doping, while successive slow cooling and post-annealing treatments further enhance the degree of overdoping, especially if carried out in oxygen atmosphere rather than in air. (paper)

[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] Twisted and not twisted multifilamentary BSCCO (2223) - Ag tapes, prepared by the PIT technique, have been characterized by SQUID magnetometry with the magnetic field applied both parallel and perpendicular to the c-axis. From these measurements and estimation of the critical current densities J_ab with Hext parallel to the c-axis J_ab planes and J_c is obtained. These values are in fairly good agreement with those obtained by transport measurements. The tapes turned out to be very homogeneous and practically no difference was found in the hysteresis loops of twisted and not twisted tapes, which suggests that the mean grain alignment is the same in the two types of samples and the current path probably follows the 'brick wall' model

[en] Femtosecond spectroscopy has been used to investigate the quasiparticle relaxation times in a nearly optimally doped chalcogenide superconducting FeSe_0.5Te_0.5 thin film. It is characterized by an enhancement of the critical temperature T_c = 19.1 K by strain. Experimental results indicate the existence of a temperature dependence of the quasiparticle recombination time τ_R in the superconducting state where an energy gap Δ(T) is present. A fitting procedure based on a BCS-like assumption for τ_R gives Δ₀ = 4.1 ± 0.4 meV for the superconducting energy gap extrapolated at T = 0, which corresponds to 2Δ₀/k_BT_c = 4.9 ± 0.5. Accordingly, an estimation of the characteristic phonon energy and the electron–phonon coupling constant λ is reported and discussed. The obtained value λ ≈ 0.6 is in good agreement with other similar determinations obtained by different techniques and numerical approaches. The small value of λ confirms that a phonon-mediated coupling cannot be the only mechanism leading to the formation of the superconducting state in FeSeTe films. (paper)