Table of contents

The use of neon as a liquid refrigerant for high-temperature superconducting devices is briefly discussed, together with the problems posed for the liquefaction process by the small temperature difference between neon's normal liquefaction and melting points. The paper describes experiments which indicate how a Gifford - McMahon refrigerator can be made self-regulating at a temperature above the triple point of neon, so that it can be employed to condense neon used as a secondary refrigerant, without the danger of solidification.

Physical aspects of a novel read-out method for low-temperature direct detectors are analysed. The method uses relaxation oscillations in asymmetric junctions biased in the `voltage-source' regime ( are superconductors and I is an insulator). Using a model Sn-I-Pb junction, it is shown that perturbations caused in the superconducting electrode with lower critical temperature under the influence of absorbed high-energy particles may be detected by measurement of the frequency of relaxation oscillations which take place in the system. The change in the frequency is proportional to the energy of the particles.

Three types of dependences of microwave (36 GHz) surface resistance on microwave magnetic field amplitude have been found for YBaCuO samples of various qualities. For the poor-quality samples is proportional to at low fields; then it changes to linear versus and, finally, saturates at large . The intermediate-quality samples demonstrate a threshold character of the nonlinearity with a jump-like growth of at field amplitudes higher than some characteristic value. Finally, the best samples exhibit an dependence which is initially quadratic with and then grows faster with the microwave field. The results are analysed in the framework of various models for the nonlinear surface impedance of type II superconductors.

Current may flow in helical paths along the boundary between two tilted anisotropic conductors in circumstances when current flow in either conductor alone is restrained by the extreme anisotropy. We are able to explain number of features of the electrical properties of (103)/(013) oriented thin films deposited onto (110) substrates in terms of helical current paths involving the twist boundaries. These include the absence of weak link behaviour in the superconducting state for current flow along the substrate direction, the temperature dependence of the normal state resistivity for current flow in that direction, and the normal state electrical anisotropy. We predict that there are circumstances where currents will preferentially circulate around the boundaries of a tilt domain. The existence of these natural conduction loops may explain the period of recently observed quantum-interference behaviour in these films.

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 mbar. Films grown in these conditions showed surface roughness lower than 1 Å, transition temperatures above 88 K and critical current densities of about at 4.2 K in self-field.

From isothermal four-point electric resistivity measurements we have determined the chemical diffusion coefficients for the in- and out-diffusion of oxygen in epitaxial c-axis oriented films of in the temperature range 500-650 K. At constant oxygen pressure we have observed the temporal changes in resistivity, following small steps upwards or downwards in temperature. At 600 K for the in- and out-diffusion of oxygen we found about the same diffusion coefficient . The temperature dependence of could be well fitted by an Arrhenius law, yielding the same activation energy eV for the in- and out-diffusion.

The addition of platinum and the (0412) phase to melt-textured (YBCO) thick films has been shown to induce notable improvements in the flux pinning characteristics of the material. Transport critical current density values in excess of at 77 K and self-field have been achieved, with volume magnetization values in excess of at 77 K and zero applied field. Measurements of both the inter- and the intra-granular flux pinning characteristics in applied fields indicate that this behaviour is greatly improved in the Pt-doped YBCO films. The enhanced flux pinning was attributed to an increase in defect densities within the matrix material, associated with a change in the morphology of inclusions.

Magnetic properties of a series of Mg-doped granular samples were studied by means of vibrating-sample and SQUID magnetometry. The equilibrium moment measured after the sample demagnetization gave evidence that the surface barrier effects can be neglected up to 60 K. Slope of the high-field dependence was used for determination of the temperature dependence of the penetration depth and its extrapolation to T = 0, . The latter quantity was found to be significantly affected by the expression used for the data fit. The irreversible moment deduced from the magnetic hysteresis loops showed a pronounced fishtail effect. In the temperature range 50-60 K, the curve was found to scale in a similar manner as in (RE)-123 single crystals. The conventional relaxation rate dramatically changes at the fields around the remanent state. The field range of this anomaly coincides with the central peak width, and we attribute this effect to redistribution of the magnetic flux in the sample. A simple non-monotonic dependence was observed of the fishtail maximum position on the nominal Mg content. A similar but mirror dependence was found of .

Conductance measurements of junctions between a high- superconductor and a metallic oxide have been carried out along the a-b plane to examine the tunnel-junction spectra. For these measurements, in situ films have been grown on c-axis oriented thin films using the pulsed laser deposition technique. Two distinctive energy gaps have been observed along with conductance peaks around zero bias. The analysis of zero-bias conductance and energy gap data suggests the presence of midgap states located at the centre of a finite energy gap. The results obtained are also in accordance with the d-wave nature of high- superconductors.

The mechanism and development of cold-working and grain-growth texture of Bi-2212 and the corresponding inheritance of texture by Bi-2223 in Ag-sheathed tapes has been investigated. Precursor powder in the Ag tube was found to flow, rotate and fracture along the crystallites. This together with sliding among crystallites forms the basis of the deformation process in Bi-2212 texturing during the cold-working process. Bi-2212 was formed via a rapid melting process, yielding a strong epitaxial melt-textured microstructure from a subsequent heat treatment. Bi-2223 was found to form on Bi-2212, with the Bi-2223 texture being inherited from the Bi-2212 texture. This is best explained by an intercalation mechanism for Bi-2223 phase formation.

A detailed study of the phase formation mechanisms of different precursors in silver sheathed tapes is presented. The method of high temperature x-ray diffraction was used in different oxygen partial pressures and at various heating rates. Fast formation of (3321) was observed. At the same temperature where disappeared, started to form crystals. This observation leads to the assumption that the decomposition is one important factor for the growth and, hence, for the phase formation.

A cryogenic deformation process has been developed and investigated for use in the processing of Ag/Bi-Pb-Sr-Ca-Cu-O superconducting wires. A typical cryogenic deformation process used comprised rolling or pressing the wires into tapes at 77 K and then heat treating the tapes at 838- for a period of 40-60 h. It was found that the cryogenic deformation improved the density, grain alignment, Ag/oxide core interface and critical current density. Critical current densities for cryogenically rolled and pressed Ag-clad mono- and multifilamentary tapes showed a 10 to 20% increase over those observed in normal processed tapes. In comparison with hot deformation and processed tapes, the cryogenic processed tapes showed improved flux pinning capability whereas the hot deformation was found to only improve weak links. During cryogenic deformation, the hardness of the silver sheath was significantly increased, which allowed a greater pressure to be applied during the deformation process. This large pressure was found to be the principal cause of the enhanced densification, texturing and increased dislocation densities.

Textured crystals of have been grown by the platinum strip heater-floating zone technique. Texturing ratio and phase purity (Bi-2212) of the grown crystals were calculated from the x-ray diffraction data. Chemical compositions of the grown crystals were quantified from the inductively coupled plasma analysis. was found to be increased by 2 K for a lower level of substitution and a superconductor to semiconductor transition was observed for the higher order Y substitution. Oxygen stoichiometries of the Y substituted crystals were quantified from the iodometry titration method. Micro-twinning along the growth axis was revealed during etching studies for the cleaved crystals.

The influence of AgPd and Au interfaces on the formation of the (HgBi-1223) superconductor was investigated. Variations in the BaCaCuO precursor phase assemblage and the use of as the Bi source were examined in an attempt to lower the reaction temperatures suitable for the growth of HgBi-1223 in conjunction with AgPd and Au substrates. Phase pure samples of HgBi-1223 were synthesized at temperatures below . Use of as a Bi source combined with variations of the BaCaCuO precursor phase assemblage allowed the formation of some liquid phase during the reaction and resulted in dense HgBi-1223 samples with aligned colonies of grains. Magnetization measurements in fields up to 30 T indicated good intergrain coupling within the large grain colonies. Comparison of the microstructural features of bulk samples and the metal-superconductor interface showed that the interface has a positive influence on grain growth and alignment. The metallic interfaces, however, were found to have a detrimental effect on the purity of the superconducting phase due to the formation of amalgams.

Submicrometre (La-3137) superconducting powder is prepared by a polymeric polyprotic acid (PA)/ethylene glycol (EG) derived precursor method. Six different PAs, oxalic acid, succinic acid, adipic acid, malic aid, tartaric acid and citric acid, are selected. It is found that of the six PAs citric acid gives the best results in terms of the particle size, homogeneity, carbon residue and values of the resulting La-3137 superconducting powder.

Composite samples made of 123-211 DyBaCuO-based systems with 1, 4, 10 and 15% in weight additions have been synthesized. Their electrical resistivity, thermal conductivity and thermoelectric power have been measured. The electrical resistivity has also been measured in a magnetic field less than 1 T. The data show that addition has a non-trivial effect: beside mildly reducing the critical temperature of the superconductor, it increases the scattering at small concentration, and effectively reduces weak link features in the electrical property, but increases the thermal conductivity. The field influence on is shown to be .

Fluorine-doped Tl(1223) superconductors were prepared starting from mixtures of oxides (carbonates) and fluorides (in particular TlF) under moderately high gas pressure (50 bar). The thermodynamics of the phase formation were found to be modified so that the melting point was lowered and the region where a liquid phase coexists enlarged. 90% phase purity was obtained for bulk materials with the composition , prepared by a two-step synthesis including TlF among the starting materials. The critical temperature remained approximately the same as for F-free samples , but a remarkable increase of the irreversibility line was observed below 75 K. The samples showed superconducting properties even for the highest amount of fluorine introduced (about one atom per formula unit). Ag-sheathed tapes were prepared by the conventional PIT method, using ex situ reacted Tl(1223) powders or producing the reaction in situ. Transport critical current densities of about were measured. This value is slightly lower than generally observed for F-free samples, but the magnetic-field dependence was slightly improved. The tape anisotropy could be increased to a factor of 2.5 by performing cycles of mechanical pressing and heating above the formation temperature. The results (in particular neutron scattering diffraction) indicate that, with high-pressure synthesis, F remains in the sample and substitutes for O in the TlO layers in the Tl(1223) phase.