Various questions concerning pinning and flux creep in high- temperature superconductors are addressed. First the pinning force by point defects in CuO2-layers in Bi:2212 and Y:123 single crystals is computed. Then some possibilities for the pin mechanism of the recently discovered screw dislocations in sputtered Y:123 films are discussed. Finally, we turn to the issue of flux creep in these materials and the relation between activation barrier and current density. The relevance of different creep models is further elucidated by the numerical solution of the non-linear creep equation for infinite slab and cylinder geometries and various initial field and current distributions.

A second-order vortex-solid melting transition in twinned Y-Ba-Cu-O single crystals is manifested by two independent types of electrical transport measurements: the electric field (E) versus current density (J) isotherms, and the ac resistivity (ρ) vs current frequency (ω) isotherms. Universal static and dynamic exponents (ν ≈ 2/3 and Ζ ≈ 3, respectively) are found for magnetic fields ranging from 1 to 90 kOe, frequencies ranging from 0 to 2 MHz, magnetic directions parallel and perpendicular to the crystal c-axis, as well as samples with and without proton irradiations. At microwave frequencies, we find that the vortex dissipation in Nd-Ce-Cu-O epitaxial films is consistent with the viscous motion of individual vortices, due to the break down of the critical scaling theory in the high frequency limit.

A detailed examination of the Y2BaCuO5 (211)/ YBa2Cu3O7−x (123) interface in several melt-processed 123 samples prepared using different methods was undertaken using analytical electron microscopy. It is found that there exists a significant increase in the a-b planar stacking fault density in 123, near the 211/123 interface. When viewed along [001], these faults appear as disks with diameter from a few to 30 nm and are bounded by dislocation loops. Most stacking faults are confined to the (001) basal plane. The size and density of defects around the 211 particles suggest that these defects could act as effective flux-pinning sites and may explain the observations of increased Jc with increasing volume fraction of 211 and a maximum in Jc when the applied field parallel to the c-axis.

X-ray diffraction shows a preferred orientation of the 1:2:3 crystallites with the (ab) planes parallel to the longitudinal axis of the bar-shaped specimen, and a random orientation of the c axes. The dc magnetic hysteresis and time-dependent response were measured parallel to the (ab) planes or parallel to the random axis. The orientation dependence of the shape of the dc magnetic hysteresis proved the anisotropie nature of the flux-line structure and flux pinning. The experimental points were fitted to theoretical equations, allowing quantitative characterization of the orientation and temperature-dependent pinning. Time-dependent response was found to be logarithmic below a critical temperature in the range of 0.5 Tc. Activation energies determined in this region by assuming thermally assisted flux flow were found to be significantly higher than those in polycrystalline YBCO. Several activation processes or steps could be clearly distinguished. The field-cooled and zero-field-cooled specimens did not show significant differences, neither did the transport and magnetic critical current. This proves the presence of clean grain boundaries in this investigated high-quality sample.

We report a study of magnetic flux penetration into a single crystal superconductor, using the technique of 1-dimensional neutron depolarisation. In these experiments we have applied magnetic fields of up to 1.7 kOe to the crystal, perpendicular to the polarisation direction of the neutron beam. By analysing the polarisation of the transmitted neutron beam, after removal of the applied field, we are able to determine the remanent magnetisation in the crystal as well as its decay over periods from 10 to 105 seconds. This technique allows us to study both field and time dependent behaviour.

We are developing a model to explain these observations, using the Bean critical state model to calculate the bulk trapped flux with the assumption of simple spin precession in a uniaxial magnetic environment. We have been able to relate the field dependence of the remanent magnetisation to the lower critical field Hcl and the field at which magnetisation saturation is reached 2H*-Hcl. We also relate the time dependence to the flux pinning energy U which determines the rate of flux relaxation. Measurements of this type could be used to plot the magnetic phase diagram of high temperature superconductors, to study anisotropy and flux creep in such crystals.

We have carried out a systematic study of the detailed I-V characteristic in magnetic fields of the YBa2Cu3O7 (YBCO) single crystals and c-axis oriented thin films. For all the samples studied, there exists a temperature at which the I-V satisfies simple power law. The I-V characteristic also follows the scaling behavior predicted by the phase transition model based on the random disorder. The derived critical exponents suggest the occurrence of 3D to 2D crossover of the flux motion. The critical field at which the crossover occurs depends on the sample thickness.

YBa2Cu3O6+x (Y-123) wires with Er2BaCuO5 (Er-211) and Y2BaCuO5 (Y-211) additions were vertically zone melted using a single element resistive furnace. The microstructure and magnetic Jc of wire segments taken from different sections of well textured wires were analyzed and compared. In both Er-211 and Y-211 doped wires the 123 plate width and the magnetic Jc decreased with increasing 211 additions. Microstructural analysis on the Er-211 doped wires confirms that Er-211 dissolves in the melt and results in the formation of a YzEr(1-z)Ba2Cu3O6+x superconducting phase upon solidification which shows improved magnetic properties over Y-211 doped wires.

High-energy heavy-ion irradiation has been shown to be effective in introducing flux-pinning defects in Y123 single crystals. Contrary to electron, low energy proton, and fast neutron irradiation, high-energy heavy-ion irradiation produces defects beneficial to flux pinning through the electronic (rather than nuclear) stopping mechanism. This type of stopping results in tubular defects many microns long that can increase the range of irreversible magnetic behavior to higher fields. In contrast, dense arrays of small defects (size < 100 Ang.) produce enhanced critical currents at low fields but do not extend the range of irreversible behavior significantly. We have used two ion-energy combinations in which the electronic stopping powers vary by nearly two orders of magnitude. The difference in stopping powers results in very different defect types, ranging from well oriented and dense (Xe) to much less well oriented and shorter (O) tubular defects. A novel analysis of the ac μ data indicates that the strong pinning introduced by the Xe irradiation breaks the vortex glass picture in favor of an individual pinning model, while O irradiation does not.

The (RE1Ba2Cu3O7)m (RE2Ba2Cu3O7)n superlattices (SL) with the superporiod d along [100]. [010] and [001] have been considered as a single crystals with primitive cells formally multiplied duo to ordering of rare earth atoms in an initial hypothetical mixed system (RE1pRE21-p)Ba2Cu3O7 By means of symmetry methods it is found that the suporporiodicity in those SL, unlike Ge/Si or GaAs/AlAs. keeps the initial space group Pmmm at any m and n. On the contrary, the obtained Table of the SL atoms distribution over Wyckoff positions indicates that the type, multiplicity and the number of the positions essentially depend on d and Cm.nD. Those data are used for group thoorotical classification of the fundamental vibrations of SL's. The analysis of spectrum changes under transformation from the initial compound to SL is performed. In particular, it is found out that already at the lowest m and n the oven vibrations of RE, Cul and Ol, occupying only one-fold positions in the initial structure, occur. Also for each d the typo of spectrum is shown to depend on both m+n and the fact if the pair (m.n) is even-oven, odd-odd or oven-odd.

This paper details the experimental results for the influence of transport currents on the ac magnetisation of high-Tc superconductors. It has been found that the variation of the third harmonic amplitude V3 with an ac field can still be described by square and cubic laws for low and high ac fields, respectively, the same relationships as observed in the absence of transport currents. The only significant difference is that the magnitude of V3 in the low field region increases linearly with transport current. Consequently, the crossover point (from square law to cubic law) moves to higher ac fields and the variation of V3 with transport current is less pronounced in the high field region. The results are explained in terms of an effective increase in Hg, which is a parameter in Kim critical state model, with increase of transport current.

The growth and stability of the (Bi1-xPbx)2Sr2Ca2Cu3Oy (Bi-2223) phase contained in silver-sheathed wires has been investigated by a combination of x-ray diffraction, scanning electron microscopy, energy dispersive x-ray analysis, and transmission electron microscopy. Silver tubes loaded with Bi-2223 precursor powders were processed into filaments using established metallurgical techniques. The filaments were then heat-treated at selected temperatures (800 to 845°C) for a range of times (10 to 6000 min) in a 7.5% oxygen atmosphere. From these studies it has been possible to investigate the time-temperature-oxygen pressure domains wherein Bi2Sr2CaCu2O8 (Bi-2212) + second phases transform to Bi-2223. Fractional conversion (Bi-2212 --> Bi-2223) versus time data show good conformance to the kinetic model for a diffusion-controlled reaction at the interface between thin sheets and a fine powder or a fluid. Quenching experiments also reveal that the Bi-2223 phase is stable in a limited temperature interval between 810 and 830°C.

We report one amorphous and four new crystalline phases in the Bi-Sr-0 system. The structure of three of the crystalline phases was identified. The Br10Bi6O24-y phase, which is cubic with space group Fm3m and cell parameter a = 0.8471(1) nm. The SreBi10O27-y phase, which is tetragonal with the space group I/mmm, and cell parameters a = 0.6007(1), c = 0.8376(1) nm. The SreBi10O27-y phase, which is tetragonal with space group I/mmm and cell parameters a = 1.3221(1), c = 0.4249(1) nm. We show that in the crystalline structures Sr and Bi occupy equivalent sites.

Large single crystals of Tl2Ba2CaCu2O8 superconductors were grown by a two step self-flux process. Crystals are plate-like. The in-plane crystal sizes are as large as 5mmX5mm and the thickness range from 0.01mm to 0.8mm. The critical currents were extracted from I-V curves measured at different temperatures. jc was about 2.4×104 A/cm2 at liquid nitrogen temperature.

This study is a part of an on-going research programme in Oxford to produce Bi-based ceramic superconductor wires. It aims at optimising processing parameters in order to achieve desired properties of the material. It has been shown that the 2212 BSCCO material can be successfully produced using the melt processing technique. High critical current density has been obtained in the form of wire and thin film/1,2/. Although several studies have been recently reported regarding the crystallisation of the 2212 phase/3/ and the effect of the oxygen stoichiometry on the properties of the material/4,5/, there is limited knowledge in the literature about the details of the processing of the 2212 material enclosed in silver. The present study is undertaken to elaborate the microstructural evolution mechanism of the 2212 phase in silver when it is cooled from the melted state, and to establish the effect of the cooling rate from the annealing temperature on the electric properties.

Bulk YBCO samples have been prepared for the first time by using Metal-Organic Deposition (MOD) technique.

Solutions of Y, Ba and Cu neodecanoates have been used in order to obtain directly YBCO powders (not exactly, but approximately stoichiometric) after a back-up and a pyrolysis step at 500°C. The powders have been successively pressed and subjected to two annealing steps in order to built up the YBCO structure, to increase grain dimensions and to adjust oxygen stoichiometry. The samples, which are disk-shaped, 1 mm thick and 1 cm wide, display a sharp superconducting transition at 93 K and possess a relatively large critical current.

Diffraction patterns indicate a strong degree of orientation of (a,b) planes along the surface. The superconducting gap has been measured by SIS' tunneling with Nb and by investigation on SS' Andreev reflection.

In this work the possibility of enhancing the critical current density in proton implanted surface layers on YBCO bulk is discussed. A complete transport characterization shows that, for what concerns two samples from different irradiation runs, the critical current density has been enhanced. Increases of other meaningful parameters, such as average pinning energy and vortex-antivortex coupling strength, have also been found, although the relative effect of the implanted layer on the whole specimen cannot be evaluated in a fully quantitative way.

High quality YBCO films have been grown on GaAs-based substrates via depositing MgO epitaxial buffer layers prior to YBCO growth. The critical temperature of the best YBCO film, Tc(0) was 87K and Jc>6.7×104 A/cm2 at 77K. The MgO buffer layers are usually [100] oriented along the normal of GaAs (100) substrates with full-width-half-maximum (FWHM) of rocking curve varying from 1–3 degree. We found that the uniformity and quality of MgO buffer layers are very sensitive to the pre-deposition preparation of GaAs-based substrates. Nonuniform MgO buffer layers are often obtained on those substrates prepared by wet chemical processing. Reproducible, controlled formation of the MgO buffer layer was achieved using an antimony passivation scheme, after molecular beam epitaxial (MBE) growth of the desired structure of the substrate.

Various models have been proposed to interpret ac susceptibility curves measured on sintered high-To superconductors (HTSC) and to deduce parameters like critical current densities and critical fields. To get more clearness, systematic studies of position and height of the intergranular maximum in χ'' have been performed on several ceramic HTSC samples in dependence of both ac field amplitude and sample size. In the present paper, the data are used to discuss the various models proposed in the literature. It is shown that modifications of the critical state model using incorrect jc(Hi) functions as well as other interpretations of the χ'' peak yield not actually intrinsic but sample size and geometry dependent parameters. The dependence of both position and height of the intergranular maximum in χ '' on ac field amplitude and sample size can be explained in the framework of a modified critical state model using a power law for the jc(Hi) dependence.

Various compositions Bi-Sr-Ca-Cu-O thin films having 2212–1 ike structure have been synthesized on MgO substrates using Molecular Beam Epitaxy. Four Knudsen metal sources were utilized to prepare alloy films which were subsequently post annealed in air atmosphere. Quenching of the films from the annealing temperature, 800°C-820°C, increased the superconducting transition temperature Tc. The highest Tcs, 90 K - 95 K, were attained for compositions with slightly lower Ca and Sr content if compared to the nominal 2212 and 2223 ones. Plasma frequency of free carriers, ωp, determined from optical reflectivity measurements have been found to increase with the average density of CuO2 units. However, no substantial shift ofωp was displayed by the highest Tc films.

Hot-pressed YBa2Cu3O6+δ superconductor with plate-like grains annealed at the partial melting temperature ( ∼ 1,000 C) followed by slow cooling in oxygen atmosphere has resulted in enhanced magnetization hysteresis loops which show weak dependence on magnetic field. The X-ray diffraction pattern exhibits the significant features associated with the reflection (003) and (006), and a higher compaction density in hot-pressed samples compared with cold-pressed samples was observed from the SEM micrographs. Morphology study and microstructural analysis by SEM and TEM, respectively, indicate that grains were well aligned during hot-pressing mostly with their c-axes parallel to the pressing axis, and many intersecting twins were actually formed during post-oxygenation. Local oxygen-deficient regions likely existing in these hot-pressed and oxygenated samples are believed to act as effective flux pinning centers at high temperatures (∼ 50 K and above). The measurement of magnetic relaxation was employed to interpret the observed pinning behavior. The critical magnetization current density (Jc) calculated using the Bean model and the average grain dimension is in the order of 104 Amp/cm2 at 75 K up to 1 T. Because of a shorter period of oxygenation and a weak field dependence of Jc values, the process of hot-pressing combined with partial melting post-oxygenation is technologically practical in forming various shapes as desired from oxide superconductors.