G.M. Stollman

Analysis of the critical current density in high-Tc superconducting films

Abstract Current models used to analyse the temperature dependence of the critical current density of high- T c superconductors are reviewed. The relation between the critical current density and the preparation conditions of thin films in the Y-Ba-Cu-O and the Bi(Pb)-Ca-Sr-Cu-O systems is studied. It is found that in thin films made by high-vacuum sputtering, laser ablation or thermal decomposition of metal-organic compounds the critical current can be described by models for granular superconductors. Films made by ultra-high vacuum evaporation are very homogeneous and have a much higher critical current density of 1.8 × 10 11 A/m 2 at 20 K. In this case flux motion determines the critical current density.

Frequency and field dependence of the irreversibility line in a YBa2Cu3O7−δ film

Abstract The irreversibility line of an epitaxial YBa 2 Cu 3 O 7−δ film was determined using AC susceptibility experiments. It is shown that the irreversibility line depends on both the amplitude of the AC field and the measuring frequency. We argue that, therefore this line represents a crossover from pinning to flow regimes. The power law behaviour which is predicted by flux creep is only found in a limited field range and is dependent on h AC .

Preparation, patterning, and properties of thin YBa2Cu3O7−δ films

High TC superconducting thin films were prepared on (100) SrTiO3 substrates by dc triode sputtering and subsequent annealing. In these films Hall‐bar structures having a width down to 5 μm were patterned using a reactive ion etching technique. Superconductivity above 77 K was observed. When compared with the original film there is only a small reduction in TC. The critical current density determined by electrical measurements is substantially reduced. On the other hand, the critical current density in the bulk of the grains as measured by the torque on a film is not reduced by the patterning process. It is suggested that superconductor‐normal metal‐superconductor junctions between the grains account for this difference.

Oxygen content, lattice constants and Tc of Bi2Sr2-xLaxCuO6+δ with 0.0 ⩽ x ⩽ 1.0

Abstract Compounds in the solid solution Bi2Sr2-xLaxCuO6+δ with 0.0 ⩽ x ⩽ 1.0 were investigated by X-ray powder diffraction, magnetic and electrical measurements. The samples are prepared starting from spray-dried powders. The oxygen contents were obtained from determinations of the oxidizing powder. The Tc was found to increase with increasing lanthanum content up to 25 K at x = 0.4. Samples with x > 0.6 are not superconducting. The oxygen content increases with lanthanum content while the formal copper valence remains approximately constant.

Critical currents and magnetic relaxation of epitaxial YBa2Cu3O7 − δ films☆

Abstract Critical currents of YBa 2 Cu 3 O 7 − δ epitaxial films, obtained by co-evaporation of yttrium, copper and BaF 2 in an ultrahigh vacuum system, were measured using resistometric and magnetic torque methods. Typical values of j c

Resistive states in thin films of Y2Ba4Cu8O16−δ

2 × 10 11 A m −2 at 4.2 K and 4 × 10 10 A m −2 at 77 K were found. In spite of their good quality these films exhibited large magnetic relaxation even at 4.2 K. Using the results of Monte-Carlo simulations of thermally activated flux motion for the analysis of the relaxation data typical values of 25 ± 3 meV were found for the activation energy U of flux motion at 4.2 K. It is shown that the occurrence of high j c values and small activation energies is not inconsistent in high T c superconductors because of their short coherence lengths.

Preparation and transport properties of superconducting layers in the Ca-Sr-Bi-Cu-O system

Abstract The resistance of a preferentially oriented thin film of Y2Ba4Cu8O16−δ has been measured as a function of temperature and fields up to 20 T oriented parallel to the c-axis. The normal state resistance could be well fitted by the Bloch-Gruneisen theory. Below Tc (≈ 80 K) the resistance transitions both in field and temperature could be separated in three regimes. A regime where the data can be analysed in terms of a model combining thermally activated flux flow and flux pinning due to dislocation lines. The upper critical field Bc2, obtained from this analysis, exhibits an upward curvature at low temperatures, possibly indicating a mixed s-d wave pairing. In the second regime just below Bc2 viscous forces are increasingly important for flux-line motion and the resistance is determined by the flux-flow resistance ϱf. At fields higher than Bc2 we find a large contribution of superconducting fluctuations to the conduction.

Degradation of oriented superconducting thin films

Abstract Superconducting layers in the CaSrBiCuO system are prepared by thermal decomposition of metal carboxylates using a spin-coating and a dip-coating method onto ceramic MgO substrates. The samples consist of a tetragonal calcium-strontium-bismuth-cuprate and two bismuth-free calcium-strontium-cuprates. A step in the resistance versus temperature curve is observed which, together with the influence of magnetic fields, is interpreted as typical for a granular superconductor. The analysis shows that the critical current density is determined by domains of the order of some unit cells. The strong dependence of the superconducting transition on the orientation of an applied magnetic field is probably caused by the anisotropic layer structure. The coherence length perpendicular to the c-axis of the material is estimated to be ξ ab (0) = 4.0 nm and parallel to the c-axis ξ c (0) = 0.6 nm.

Critical current as a function of temperature in thin YBa2Cu3O7−δ films

Abstract Thin highly oriented superconducting YBa 2 Cu 3 O 7−δ (123) films are produced by laser ablation deposition on SrTiO 3 (100). The quality of the best layer produced enables us to study the degradation behaviour of 123 films. The X-ray diffraction pattern only shows ( h 00) and (00 l ) peaks of the 123 material. Resistance versus temperature, R ( T ), measurements indicate that two superconducting transitions are taking place with onsets at T = 93.0 K and 90.1 K. The 10–90% width of both transitions is 0.6 K. I - V curves show a linear dependence of the critical current I c on T near T c . The degradation of this film in air results in a change in both the R ( T ) and the I c ( T ) behaviour.

Thermally activated flux motion in high-Tc superconductors

Abstract High —Tc superconducting thin films were prepared on (100) SrTiO3 substrates by DC triode sputtering. In these films Hall bar structures with a width down to 5 μm were patterned using a reactive ion etching technique. Superconductivity above 77 K was observed. When compared with the original film there is only a small reduction inTc, however, the critical current density is substantially reduced. It is suggested that a combination of superconductor-insulator-superconductor and superconductor-normal metal-superconductor junctions account for the temperature dependence of the critical current density.