Papers by Keyword: Superconducting Oxide

Abstract: Since the discovery of high-temperature superconductivity of cuprate oxides, it has been clear that it is strongly affected by the oxygen content, which is also a crucial factor to determine other physical properties of high Tc superconductors. Non-stoichiometric (interstitial) oxygen strongly influences the physical properties of various superconducting oxides, in particular by creating conducting holes. It is now ascertained that the amount of holes injected depends not only on the content of interstitial oxygen, but also on its ordering. Rearrangement of the oxygen ordering may occur even below room temperature due to the unusual high mobility of these atoms. This way, mechanical spectroscopy is one of the most adequate techniques for the study of the mobility (diffusion) of oxygen atoms. This technique allows the determination of the jump frequency of an atomic species precisely, regardless of the model or the different possible types of jumps. In order to evaluate the mobility and the effect of oxygen content on these oxides, ceramic samples we prepared and submitted to several oxygen removal cycles alternately with mechanical relaxation measurements. As for SBCO, it was assumed that the peak was due to O(1)-O(5) jumps of oxygen atoms at the chain terminals or in chain fragments in the orthorhombic phase. In the case of BSCCO, the results showed complex anelastic relaxation structures, which were attributed to interstitial oxygen atom jumps between two adjacent CuO planes.

Abstract: Magnetic field texturing of superconducting oxides has shown the possible existence of intrinsic solid nuclei surviving above the melting temperature Tm and governing the solidification. Tiny crystals could survive above Tm and act as growth nuclei with undercooling ratios θ= (T-Tm)/Tm larger than the theoretical value −2/3 if a negative supplementary volume energy −ε v is added in the Gibbs free energy change associated to the formation of a critical cluster. A double layer of opposite charges could create the solid-liquid interface electrostatic -εv. The observed maximum values θ1 and the dimensionless surface energies α1ls calculated for 38 elements assuming that their melts homogeneous,
 used to determine εv(θ). The εv values at T=Tm were equal to 21.7% of the fusion heat per volume unit. The quantity α2ls 3× Sm was nearly the same for all elements, α2ls being the dimensionless surface energy and Sm the fusion entropy. After melting these tiny crystals around Tm2=1.20Tm, all the undercooling ratios could tend to -2/3. The bidimensional texture of Bi2212, Bi2223 tapes can be induced by these nuclei during crystal growth when the prereacted compounds in the sheath are melted and annealed at a weak overheating temperature smaller than a critical value.

Abstract: The composite SmBa2Cu3O7-δ (Sm-123), obtained by the substitution of the ion Y for Sm in the very well known and studied YBa2Cu3O7-δ (Y-123), is potentially attractive for better understanding superconductivity mechanisms and for its applications as electronic devices. Sm-123 samples show higher critical temperatures than Y-123 ones do and a larger solubility of Sm in Ba-Cu-O solvent, which makes their growth process faster. When oxygen is present interstitially, it strongly affects the physical properties of the material. The dynamics of oxygen can be investigated by anelastic spectroscopy measurements, a powerful technique for the precise determination of the oscillation frequency and the internal friction when atomic jumps are possible. Anelastic spectroscopy allows determining the elasticity modulus (related to the oscillation frequency) and the elastic energy loss (related to the internal friction) as a function of the temperature. The sample was also investigated by X-ray diffraction (XRD), scanning electronic microscopy (SEM), and electric resistivity. The results obtained show a thermally activated relaxation structure composed by at least 3 relaxation processes. These processes may be attributed to the jumps of oxygen atoms present of the Cu-O plane in the orthorhombic phase.