E. Stangl

Particulates on pulsed-laser deposited YBaCuO films

Abstract Different types of particulates observed on high temperature superconducting YBaCuO films prepared by pulsed-laser deposition on MgO substrates are investigated as a function of deposition parameters and classified according to their size, shape, and chemical composition. Among the parameters studied in detail are the oxygen pressure, the substrate temperature, the laser energy, fluence, spot size, and the number of pulses. The interrelations between the different particulates, the ablation and deposition rates, the shape of the visible plasma plume, and the film properties have been investigated.

STM observation of dislocation chains suitable for flux pinning in YBa2Cu3O7−x films

Abstract Systematic variations of the film-growth conditions during pulsed-laser deposition (PLD) with regard to substrate temperature, oxygen partial pressure and substrate quality, allow us to establish correlations between the as-grown defect structure and the current transport properties of c -axis oriented YBa 2 Cu 3 O 7− x films. By investigating the film surface with scanning electron microscopy (SEM) and scanning tunneling microscopy (STM) the nature and the density of various defects as well as the general film-growth mode were determined. SEM pictures show the overall surface roughness and the presence of other phases (outgrowths), while higher-resolution STM pictures reveal small-scale defects down to the nm range. Transport current measurements on patterned parts of the same films show differences in the magnitude of J c as well as in its field and angle dependence. Influences of the defect structure on intrinsic pinning, weak-link properties and special features in the angular dependence of J c are discussed. Dislocation chains extending over several layers along the c -axis within the terraces of growth spirals are of special interest with regard to flux pinning. The existence of these dislocation chains can be directly implied from the present data. A higher substrate roughness and a faster growth speed of the terraces along the ab plane seem to favor the appearance of the dislocation chains.

The morphology of YBa2Cu3O7−α thin films investigated by STM

Abstract Scanning tunnelling microscopy (STM) was employed to study the surface of c -axis oriented YBCO thin films. The films with a thickness of typically about 300 nm were grown onto single crystal (100) MgO by pulsed laser deposition. In addition to the well known growth spirals, “frozen in” clusters were observed. The latter result in a high density of holes with sizes ranging from a few to about 50 nm. These holes may serve as pinning sites. Sub-unit cell steps were found in regions between neighboring growth spirals. In samples exposed to air for several weeks, a change in surface morphology was found. Samples that were patterned by wet-chemical etching did not show the characteristic growth steps. Irradiation with fast neutrons slightly increased the density of nm sized holes.

Critical currents in Tl-2212 and Tl-2223 thin films

Abstract The critical currents of Tl-2212 and Tl-2223 thin films grown by sputtering and post annealing on LaAlO3 have been measured as a function of the angle between the magnetic field and the c-axis at various temperatures and magnetic fields. The results show higher critical current densities in Tl-2223 than in Tl-2212 at low temperatures. Moreover, Jc is strongly field dependent in both films, suggesting a weak pinning structure. Scaling of Jcab(θ) with the c-axis component of the applied field is observed at all temperatures in both films, which confirms the 2D-nature of the flux lines. Furthermore, the thickness dependence of Jc was investigated for Tl-2212. The effect of fast neutron and Au ion irradiation has been investigated on both films. Large enhancements of flux pinning and Jc are observed, in particular at high temperatures, without significant changes of Tc. STM studies of the surface morphology and of the growth mechanism are also presented. Steps of unexpected height are observed on the surface of both films. The surface steps are shown to develop as a consequence of different stacking sequences. A striking feature in Tl-2223 is the occurrence of blocks with non-oriented terraces. These blocks are attributed to the growth of other phases in the films.

Pulsed-laser deposited GdBaSrCu3O7−δ and GdBa2Cu3O7−δ films

Abstract Laser-deposited superconducting thin films of GdBa 2 Cu 3 O 7−δ (Gd-123) and GdBaSrCu 3 O 7–δ (GBSCO) are compared with respect to their electrical properties, surface morphology, and structure. The maximum zero-resistance temperatures of Gd-123 and GBSCO films on MgO substrates are T C0 = 93.5 K and 78 K, respectively. The critical current density of Gd-123 films is crucially influenced by their granular morphology, whereas the superconducting properties of GBSCO films are mainly determined by the concentration and distribution of oxygen.

Properties of laser-deposited TmBaSrCu3O7−δ thin films

Abstract Single-phase thin films of superconducting TmBaSrCu 3 O 7−δ have been deposited on (100) MgO substrates by pulsed-laser deposition from multi-phase targets. The morphology of these films is similar to that of TmBa 2 Cu 3 O 7−δ films. The differences in the electrical properties are discussed in terms of intrinsic differences between the two compounds. With TmBaSrCu 3 O 7−δ films critical temperatures of T c0 = 72 K and critical current densities of j c (57 K) = 10 6 A/cm 2 have been achieved.

Classification of particulates on pulsed-laser deposited Y—Ba—Cu—O films

Abstract Particulates observed on high temperature superconducting Y—Ba—Cu—O films prepared by pulsed-laser deposition on MgO substrates, are classified by their size, shape and chemical composition. Their occurrence is related to the deposition parameters and to the shape of the visible plasma plume.

Anisotropic resistivity in pulsed-laser deposited Bi2Sr2CaCu2O8+δ films

Abstract An orientational dependence of the resistivity and critical current density was observed inc-axis oriented Bi 2 Sr 2 CaCu 2 O 8+δ films grown on MgO substrates by pulsed-laser deposition. As derived from X-ray diffraction and scanning electron microscopy, the anisotropy is caused by the small tilt angle of thec-axis with respect to the substrate surface.

Fabrication of TmBa2Cu3O7−δ films by pulsed-laser deposition

Abstract Thin films of superconducting TmBa2Cu3O7−δ have been deposited on (100) MgO substrates by pulsed-laser deposition. The zero-resistance temperatures achieved were, typically, Tc0=88 K and the critical current densities, jc (77 K), in excess of 106 A/cm2.

Flux pinning in oxygen deficient YBa2Cu3O7−δ thin films

Abstract The critical current density of laser ablated YBa 2 Cu 3 O 7−δ thin films was studied as a function of systematically changing oxygen stoichiometry. By measuring the field and temperature dependence of J c in different films, we investigate the effect of oxygen deficiency on the dimensionality and on flux pinning.