Vortex Lattice Melting in YBa2Cu4O8 with H∥ab

We report c-axis resistivity measurements in the mixed state of YBa₂Cu₄O₈ with the magnetic field applied parallel to the a-, b-, and c-axes. For all orientations of the magnetic field, a kink is observed in the resistive transition, associated with the first-order melting of an anisotropic three-dimensional vortex lattice. Whereas the melting lines for Hb and Hc obey the expected relation H _m = H ₀(1 – T/T _c ) ⁿ with n = 1.5, H _m (T) for Ha follows a different temperature dependence with a lower exponent. This result is consistent with a suppression of the melting line due to a reduction in the dimensionality of YBa₂Cu₄O₈ in this field geometry, as observed in normal-state magnetoresistance measurements.     

Disordered Vortex Phases in YBa2Cu3O7?δ

The disordered vortex phases induced by line and point pinning in YBa₂Cu₃O_7– are explored. At high defect densities, only a single disordered solid separated from the liquid phase by a melting line is observed. At low defect densities the topology of the phase diagram changes dramatically, with a vortex lattice phase adjoining disordered phases at high or low field. Critical points at the termination of the first-order melting line separate the lattice and the disordered phases. The line defect disordered phases follow the expected Bose glass behavior, while the point defect disordered phases do not exhibit the expected vortex glass behavior.     

Enhancement of Vortex Pinning in YBa2Cu3O7?δ -BaHfO3 Superconductor-Insulator System

A systematic study on phase evolution, bulk density, resistivity, critical transition temperature (T _C), critical current density (J _C) and in-field transport critical current density (J _C?CB) of pure and nano particles of BaHfO₃ added YBa₂Cu₃O_7??? (YBCO) samples of different weight percentages has been carried out. The result shows a significant improvement in the electrical transport properties of the superconductor, namely the J _C, J _C?CB characteristics and flux pinning force (F _p) of the BaHfO₃ added samples. The transition temperature also was found to increase marginally. A maximum transport J _C of 742?A/cm² at 77?K (for x=2?wt%) was observed for BaHfO₃ added samples against 147?A/cm² for pure YBCO sample. The enhancement of the critical current density in the YBCO?CBaHfO₃ sample is attributed to the formation of an insulating and non-reacting YBa₂HfO_5.5 phase, acting as artificial pinning centers in the matrix. The introduction of nano particles of BaHfO₃ in bulk YBCO increases the pinning force density from 30×10³?N/m³ to 424×10³?N/m³ at 77 K. The improvement in sintering density due to an optimum value 2?wt% BaHfO₃ addition in bulk YBCO indicates its potential use in electrical devices and technology. The enhancement in the superconducting properties, particularly in the J _C?CB characteristics due to BaHfO₃ addition, seems to have great technological significance.     

Thermally Assisted Flux Creep of a Driven Vortex Lattice in Untwinned YBa2Cu3O7−δ Single Crystals

We have studied the driven vortex lattice in untwinned, clean YBa ₂ Cu ₃ O _7– single crystals, showing the first order (melting) transition T _m . At high enough driving currents (j 10 ³ A/cm ² , j j _c ) and temperatures T < T _m , a clear distinction is found between two different behaviours of the moving vortex lattice. The onset of dissipation is characterized by a noisy flux creep with a temperature independent activation energy U(j). At higher temperatures, the creep regime crosses over into a flux flow regime with linear resistivity. Apart from the dip in resistivity at T _m , associated with the peak effect and usually assigned to a softening of the shear modulus, the resistivity of the flowing flux lattice continuously extends into the vortex-liquid.     

Formation temperature of bosons in high-T c YBa2Cu3O7−y systems

At different equilibrium temperatures, a precise measurement of the electrical resistivity (R) is performed for high T _c polycrystalline YBa₂Cu₃O_7–y systems. T ² law is applied in the part of the R-T relationship far above the superconducting transition temperature. Since Landau and Pomeranchuk predicted the T ² law arising from electron-electron scattering, a higher critical temperature of T ² law is defined as a start point (T _es) of electron-electron scattering. On the other hand, we defined a lower critical temperature of T ² law as the formation temperature (T _ee) of an electron-electron pair, i.e. the formation temperature of boson. T _ee relates to the offset temperature of the superconducting transition .     

Anisotropy of in-plane thermal conductivity in YBa2Cu3Oδδδ

The thermal conductivity of two untwinned single crystals of YBa₂Cu₃O_7-.     

Oxygen Exchange in YBa2Cu3O7 – δ

The oxygen exchange in YBa₂Cu₃O_{7 –} materials of different densities was studied by thermogravimetry over wide ranges of temperatures and oxygen pressures. The results were correlated with x-ray diffraction (b – a), T _c (inductance method), and T _c data. According to polarized-light microscopy and coulometric titration results, the annealing conditions have a strong effect on the uniformity of oxygen distribution in the basal plane.     

Vortex Dynamics in YBa2Cu2O7?�� Films and Superlattices: A?Magnetization?Relaxation Study

The relaxation of the irreversible magnetization of optimally doped YBa₂Cu₃O_7??? (YBCO) films and YBa₂Cu₃O_7??? /PrBa₂Cu₃O_7??? (YBCO/PrBCO) superlattices in a magnetic field H oriented along the c axis was investigated over a large temperature T interval. The observed behavior is dominated by the presence of a crossover elastic vortex creep at low T?Cplastic creep at high T generated by the T-dependent current density J of the macroscopic currents induced in the specimen during experiments. Magnetization relaxation in the flux-creep annealing regime indicates the reduction of the creep crossover temperature with decreasing J, which suggests that in static conditions the disordered vortex phase at high H behaves like a plastically pinned vortex assembly, rather than an elastic vortex glass. The often reported scaling of the electric field E?Ctransport current density J characteristics in agreement with an elastic vortex glass?Cvortex fluid transition seems to originate from the creep crossover generated by an increasing J.     

Dip-Coating of YBa2Cu3O7?δ and YBa2Cu3O7?δ-Ag Films on Polycrystalline Rare-Earth Barium Niobate Substrates

Dip-coating and partial melting technique have been used to fabricate high quality superconducting YBa₂Cu₃O_7– and YBa₂Cu₃O_7–-Ag thick films with T _c(0)=92 K on polycrystalline REBa₂NbO₆ (RE=Pr, Nd, Sm, and Eu) substrates. The superconducting films showed excellent adhesion to the REBa₂NbO₆ substrate. The effect of Ag addition in YBa₂Cu₃O_7– on the current density, microstructure, and crystal orientation of the superconducting films developed on the above substrates have been discussed in detail. Dip-coating technique was found to be one of the easiest method for obtaining good quality superconducting YBa₂Cu₃O_7– thick films with thickness as low as 3 m even on polycrystalline substrates.     

Enhancement of Pinning Role by Nonstoichiometry in YBa2Cu3O7?y Superconductors

Considerably improved flux pinning and critical current density J c values have been achieved in Y-deficient Y-123 superconductors by directional solidification in air. In comparison with the regular Y-123 composition, Y-deficient one also has an orthorhombic structure and Y-123 main crystal phase remains in it. Whereas with the shortage of Y, Y_1–x Ba₂Cu₃O_7–y can be regarded as (YBa₂Cu₃)_1–x O_7–t(Ba₂Cu₃) _x O _t–y or (YBa₂Cu₃O_7–z )( _x ^YO _z–y ), so there may develop several kinds of microstructure defects as pinning sites in the system, such as highly dense, fine-scale, and faultlike defects, as well as localized superstructure, which are able to induce the increasing in flux pinning and J c values in higher external magnetic fields. This kind of simple nonstoichiometric route could lead to a commercial technique for flux-pinning enhancement in Y-123 bulk materials.     

Fabrication of the New Y3Ba5Cu8O y Superconductor Using Melt–Powder–Melt–Growth Method and Comparison with YBa2Cu3O7?x

YBaCuO superconducting sample with nominal composition in the stoichiometric ratio of 3:5:8 was fabricated by using the Melt?CPowder?CMelt?CGrowth (MPMG) process. Microstructural property of the sample was examined by X-rays diffraction (XRD) pattern and also observed at micrographs taken by both a polarized optical microscope and a scanning electron microscope (SEM) with energy dispersion analysis of X-rays (EDAX). Identification of chemical composition of the sample was investigated by the SEM-EDAX. Electric and magnetic properties were performed with a standard four-probe dc technique and magnetic levitation force measurements, respectively. In addition, all of these properties were compared with a standard MPMG YBa₂Cu₃O_7?x ?(Y123) superconductor. According to the obtained results, it was determined that empirical resulting formula of the sample was Y₃Ba_4.89Cu_8.17O _y ?(Y358) and also the magnetic levitation force of the Y358 was 169.8 mN under zero-field-cooled (ZFC) regime at 77 K while that of the Y123 was 99.3 mN.     

Preparation and characterization of high-T c superconducting YBa2 Cu3 O7−x filaments

High-T _c superconducting filaments of the Y-Ba-Cu-O system were prepared using a low-cost suspension spinning method, where the solvent was removed by a phase inversion technique. The Y-Ba-Cu oxide precursor, containing polysulphone (PSF), was spun as a filament into a precipitating medium, removing the solvent by phase inversion and using water as a non-solvent. The green product filament was washed, dried and subjected to a heat treatment to remove the binding material and generate the oxide in, the appropriate superconducting phase. Stoichiometry, porosity and grain size were investigated by scanning electron microscopy, energy-dispersive X-ray analysis and electron probe micro analysis, while crystal structure was checked by X-ray diffraction analysis. Resistivity measurements as a function of temperature were performed by the four-point method and typicalT _c values of 88 K were observed, whileJ _c in the range of 125 A cm^–2 at 77 K and zero field.     

Analysis of deformed YBa2Cu3O7−δ

In this study, polycrystalline YBa₂Cu₃O_7– (123) was deformed under controlled conditions with a confining pressure of 1.0GPa, temperatures of 25, 500 and 800° C, and a strain rate of 10^–4 sec^–1 in order to ascertain the micromechanisms of deformation that give rise to the macroscopic plastic behaviour. The deformed material was analysed using optical microscopy, transmission electron microscopy (TEM), and a SQUID magnetometer to study the effects of deformation on the microstructure of YBa₂Cu₃O_7– and how changes in the microstructure affected the superconducting properties. The results of these preliminary experiments suggest that the 123 material will be very difficult to deform plastically as slip occurs only on the (001) plane. The lack of multiple slip systems implies that this material will show some brittle behaviour up to a very high homologous temperature. Even when plastic behaviour can be sustained for high strains it may require high annealing temperatures to remove lattice imperfections which impede the superconducting currents. Densification by high pressure deformation may make reoxygenation difficult due to the reduced diffusion rates between the grains. These factors combined suggest that traditional fabrication techniques are not applicable to the 123 material. More work needs to be carried out to determine how annealing affects the microstructures of deformed materials and how these changes in microstructure affect the superconducting properties of these materials.     

Novel approaches in the melt-texturing of YBa2Cu3O7−y

Electrochemically pre-textured samples have been subjected to melt-processing in order to produce dense and highly textured bulk samples of YBa₂Cu₃O_7–y (YBCO). Full oxygenation of these samples has been achieved at high processing temperatures of 720 °C by electrochemical titration, in order to increase the superconducting transition temperature to > 89 K. These samples show a large magnetic hysteresis, and the value of J _c calculated using the Bean model is in the range 4000–6000 A cm^–2 at 77 K and 1 tesla magnetic field, and is independent of the applied field in that range. In another variation of the melt-processing technique — referred to as isothermal melt-textured growth — highly textured samples have been produced by the movement of the solidification front at a constant temperature in an oxygen activity gradient.     

Sintering of YBa2Cu3O7−x compacts

Heating of YBa₂Cu₃O_7−x compacts above about 930°C is shown to induce liquid formation. Presence of the liquid phase results in excellent densification, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities (≈ 500 A/cm²) have been obtained in these solid-state sintered samples.