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Thin films of YBa 2 Cu 3 O 7−x have been grown by chemical vapor deposition at 825 °C (5 Torr). The starting materials are tetramethylheptanedionates of Y, Ba and Cu. The superconducting properties of the films obtained on monocrystalline MgO have been studied by AC resistivity, AC susceptibility and magnetization measurements. Superconducting layers with zero resistance at 85 K and a transition width lower than 2 K are obtained. This communication will address more specifically the structural properties as well as the morphology of the layers. The films are highly orientated with their c-axis perpendicular to the substrate plane, and the pole figures show a very good in-plane orientation. The morphology of the layers has been investigated by scanning electron microscopy: we do observe a strong correlation between the roughness of the surface and the composition of the gas phase.

YBa 2 Cu 3 O 7 films, with thickness ranging between 0.08 and 0.48μm were deposited on SrTiO 3 substrates, using a newly developed aerosol MOCVD method. The critical current density of these films was measured in magnetic fields up to 4T in the temperature range from 20 to 77K using magnetization and ac screening techniques.

The texture of two series of YBCO films deposited on (100) MgO substrates by Pulsed Laser Deposition or Metal Organic Chemical Vapour Deposition was analysed using the X-ray pole figure technique. The effect of misorientations on the critical current density is studied.

Pr y Ba 2−y Cu 3 O x thin films have been synthesized by MOCVD with differents compositions on MgO (100), SrTiO 3 (100), LaAlO 3 (012) substrates. The resistivity of this oxide varies with the composition. This characteristic should allow to modify multilayer properties. Secondary neutron mass spectrometry reveals that little interdiffusion takes place at the interfaces of YBCO / PBCO stacked layers. YBa 2 Cu 3 O 7−x /PrBa 2 Cu 3 O 7−x superlattices have been grown by this technique on (100) SrTiO 3 . Satellite peaks have been observed in the X-ray diffraction spectrum.

YBa 2 Cu 3 O 7−x thin films have been deposited by Metal Organic Chemical Vapor Deposition (MOCVD) simultaneously on (001) MgO, (001) SrTiO 3 and (012) LaAlO 3 substrates. A comparison of the properties of two sets of samples has been done. The microstructure induced by the different substrates have been studied by SEM and X ray diffraction. The morphology is very sensitive to the nature of the substrate and to the overall composition of the films. The films are strongly textured, with the c axis perpendicular to the substrate but “a axis grains” are observed when the ratio Ba/Y is above 2. The highest critical temperatures and lowest resistivities are obtained on SrTiO 3 and LaAlO 3 , with T c ∼89 K and ϱ(100 K) ∼90μΩ.

Nanoscale analyses of the variation of oxygen concentration in YBa 2 Cu 3 O 7−δ samples have been carried out by Electron Energy Loss Spectroscopy on a Scanning Transmission Electron Microscope, using an electron probe of less than 10 A diameter. It appears that {110} twin planes are systematically impoverished in oxygen with respect to non-twinned parts of crystals. The variation of the average frequency of the {110} twinning in YBa 2 Cu 3 O 7−δ compounds has been also studied as a function of the T, P O2 conditions for the tetragonal-orthorhombic phase transformation. Both results are consistent with the proposed {110} twinning mechanism.

Magnetic properties of YBa 2 Cu 3 O 7 single crystals, grown either in alumina or gold crucibles, have been studied in the superconducting state with a vibrating sample magnetometer. The flux lines present in these crystals have been visualised by the high resolution Bitter pattern technique. Decoration experiments have been done in different conditions on the same crystals in field cooling mode at 4.2 K. Vortex patterns and magnetic measurements are used to correlate crystal defects and vortex structures.

EuBa 2 Cu 3 and YbBa 2 Cu 3 microcrystalltne alloys obtained by rapid solidification have been oxidized completely in flowing oxygen at a temperature higher than 900° C with subsequent slow cooling and have yield the high temperature oxides EuBa 2 Cu 3 7−δ and YbBa 2 Cu 3 O 7−δ . The onset of superconductivity occured at 92 K in the case of Eu and at 89 K in the case of Yb. The resistivity in the normal state ( ρ at 100 K) is lower than in sintered powder materials Sue to a better compaction and to a better intergrain coupling.

In order to understand the superconductive behaviour of the new high Tc superconductors, many experiments have been carried out. Among these, the measurements of a. c. complex susceptibility x = x′ + ix″ as a function of temperature are of great interestl. The in-phase component x′, corresponding to inductive variations, indicates perfect diamagnetism and flux shielding. The out-of-phase component x″, corresponding to resistive losses, can behave differently depending on which loss mechanism is involved. In the (RE) Ba2Cu3O7−δ oxides, the observed magnetic behaviour has been analyzed by several authors in terms of percolation, junctions, disorder or superconducting inclusions4,7 with the physical data apparently varying from one sample to another, depending on the synthesis methods and on the different heat treatments used. We have, therefore, performed inductive a. c. complex susceptibility as well as d. c. resistive measurements on sintered bulk and powdered samples of HoBa2Cu3O7−δ and on two types of YBa2Cu3O7−δ samples in order to correlate the resistivity, the morphology and the diamagnetic behaviour in these granular high Tc superconductors. The observed phenomena are directly dependent on the contact resistance between the grains (i. e. the ability of carrying supercurrent) and on the surface to volume ratio of the grains in a given material.