A structural study of the YBa2Cu3O7−x superconductor system

X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD) and analytical scanning electron microscopy have been employed to investigate the structure and microstructure together with the chemical nature of the YBa₂Cu₃O_7–x superconductor system at different stages of preparation. XRD studies showed that the original mixture assumed a structure quite similar to that of the final superconductor phase after being sintered at about 900° C in air. While XPS results showed no evidence for copper in any valency other than 2, X-ray-induced Auger features confirmed the existence of monovalent copper at low temperatures. A small shoulder on the O1s spectrum in the final compound proved to be variant with the temperature. The intensity of this shoulder increased relative to the main O1s peak by lowering the temperature. This is believed to be due to the formation of monovalent copper on the top of the sample. Microstructural studies together with point analysis and X-ray mapping showed the existence of different phases in the final compound.     

Elastic constants of the high-temperature ceramic superconductor YBa2Cu3O7−x

Journal of Materials Science Letters -     

Charge-transfer in YBa2Cu3O x

The charge-transfer hypothesis is shown to be inconsistent with data for YBa₂Cu₃O_x: (i) The two-step behavior ofT _c(x) (with jumps from zero to ≈60 K and then to ≈90 K) is not reflected as a similar, statistically significant two-step behavior in the bond-valence-sum charge of cuprate-plane Cu ions (as once believed), (ii) as a consequence of the law of conservation of charge, the derivatives of the layer charges with respect to oxygen contentx for both the Ba-O layers and the charge-reservoir Cu-O chains have the opposite signs to those predicted, and (iii) the charge-transfer observed for superconducting YBa₂Cu₃O_x is not sufficient to produce superconductivity, as demonstrated by insulating PrBa₂Cu₃O_x, which has virtually the same layer charges.     

High Tc superconductor polymer composites based on YBa2Cu3O7−x

This work reports on the study of the microstructure and electrical properties (conduction and superconduction) of YBa₂Cu₃O_7–x (Y) and polypropylene (PP) composites. For the purpose of enhancing and improving their conductive properties, different amounts of carbon black (N) and copper (Cu) were incorporated into these composites, and the effects of N, Cu and Y were determined, both on the microstructure of PP and on the electrical properties of the resulting composites. The complex systems on the basis of PP/Cu/Y and PP/N/Y were sintered in order to study their mechanical characteristics, their morphology and the superconductor properties of the end products.     

Fabrication of YBa2Cu3O7−x bulk superconductor from the powder without reoxygenation

The fabrication of YBa₂Cu₃O_7–x sintered ceramic and YBa₂Cu₃0_7–x melt product from the powder without additional reoxygenation is reported. The YBa₂Cu3O_7–x is fabricated inside a commercial silicon carbide tube. The SiC tube effectively does not react with the YBa₂Cu3O_7–x and acts to prevent oxygen from diffusing out of the YBa₂Cu3O_7–x . The fabrication procedure can, in principle, be used with any tube enclosure which neither reacts with YBa₂Cu3O_7–x nor allows diffusion of oxygen to the outside atmosphere.     

A study of textured YBa2Cu3O7-x superconductor

The effect of the starting powders, prepared by different heat treatment processes on the grain-oriented YBCO bulk materials, were studied by X-ray diffraction, scanning electron microscopy and a quantum design SQUID magnetometer. Results suggest that the specimens, which were calcined at 750° C for 10 h, heated at 950° C for 24 h, and then sintered at 930° C for 20 h, were preferentiallyc-axis oriented. The grain-orientation was found to be due to the partial melting of the YBCO compound.     

Photoemission spectroscopy of YBa2Cu3O6+x

High-resolution angle-integrated photoemission measurements were made on a sample of the compound Y₁Ba₂Cu₃O_6+x, for which it was shown that the onset of superconductivity was as high as 93 K. The density of states associated with the Cu 3d levels appears in an intense band about 4 eV wide beginning about 1.5 eV below the Fermi level. A small wing of this band extends up to the Fermi level, where the density of states is relatively low. These results are in good agreement with the projected density of states of Cu as obtained in recent calculations.     

Compressibility and thermal expansion of YBa2Cu3O x

The linear compressibilities and the thermal expansion of an untwinned, nearly optimal doped YBa₂Cu₃O_6.94single crystal were measured along the three crystallographic axes with a high-resolution dilatometer mounted in a high-pressure cell. The measurements were performed in a temperature range from 50 K to 320K under hydrostatic gas pressure of maximal 0.65 GPa. At a temperature of 300 K the measured bulk modulus is reduced and shows a very strong enhancement under increasing pressure due to pressure-induced oxygen ordering. This ordering process can also be related to a glass transition seen in the thermal expansion above 225 K. At lower temperatures, when oxygen ordering is frozen, the bulk modulus is increased and shows a significantly reduced enhancement under pressure. This pressure dependence, however, is still anomalous high, probably caused by bonds of extremely different compressibilities within the unit cell. The widely spread literature data of the bulk modulus can be explained by this high pressure dependence.     

Investigation of the many-particle spectrum of superconductor YBa2Cu3O7

We investigate the electronic structure of superconductor YBa₂Cu₃O₇ through a molecular cluster approach. The calculations are performed self-consistently through a semi-empirical LCAO technique, and correlation effects are taken into account by a configuration interaction procedure (INDO-CI). Our results for the larger clusters yield a density of states (in the valence band) that is in good agreement with the experimental data. We obtain for the ground-state a strongp-d covalency, resulting in a width of around 8 eV for the valence band. The interactions of Cu(1) and (2) with O(1),(2),(3) and (4) are analysed, showing how hybridization occurs. In all cases we calculate the Mott-Hubbard energy U taking into account many-body effects.     

Microwave surface impedance of YBa2Cu3O7−x :S-wave versusD-wave pairing

The low temperature behavior of the microwave surface resistanceR _s and the penetration provides information on the pairing state. Fully oxygenated films with low de-resistivity in the normal state exhibit an exponential temperature dependence corresponding to values of the energy gap up to 10 meV. For oxygen deficient films a linearR _s(T) was observed, similar to single crystals. This can be explained qualitatively by induced superconductivity in the copper-oxygen chains.