Thin films of the high-temperature superconductor YBa 2 Cu 3 O 7 ؊ ␦ exhibit both a large critical current (the superconducting current density generally lies between 10 11 and 10 12 A m −2 at 4.2 K in zero magnetic field) and a decrease in such currents with magnetic field that point to the importance of strong vortex pinning along extended defects 1,2 . But it has hitherto been unclear which types of defect-dislocations, grain boundaries, surface corrugations and anti-phase boundaries-are responsible. Here we make use of a sequential etching technique to address this question. We find that both edge and screw dislocations, which can be mapped
Kinetic roughening of flux fronts penetrating in superconducting thin films are studied by means of a high resolution magneto-optic technique. The roughening exponent ͑a 0.64͒ and growth exponent ͑b 0.65͒ obtained from a dynamic scaling analysis of the initial stage of flux penetration and, at small length scales, are characteristic for a static disorder dominated nonlinear diffusion such as also observed in the directed percolated depinning model. At large length scale, a 0.46 indicates a transition towards dynamic stochastic disorder, similar to the behavior of Kardar-Parisi-Zhang systems. There is a striking similarity with the behavior of combustion fronts in burning paper.
At low temperatures dislocations are the dominant flux-pinning centers in thin films of YBa 2 Cu 3 O 7Ϫ␦ deposited on ͑100͒ SrTiO 3 substrates ͓B. Dam et al., Nature ͑London͒ 399, 439 ͑1999͔͒. Using a wet-chemical etching technique in combination with atomic force microscopy, both the length and the lateral dislocation distribution are determined in laser ablated YBa 2 Cu 3 O 7Ϫ␦ films. We find that ͑i͒ dislocations are induced in the first stages of film growth, i.e., close to the substrate-film interface, and persist all the way up to the film surface parallel to the c axis, resulting in a uniform length distribution, and ͑ii͒ the radial dislocation distribution function exhibits a universal behavior: it approaches zero at small distances, indicating short-range ordering of the defects. This self-organization of the growth-induced correlated disorder makes epitaxial films completely different from single crystals with randomly distributed columnar defects created by means of heavy-ion irradiation. Since the substrate temperature can be used to tune the dislocation density n disl over almost two orders of magnitude ͑ϳ1-100/m 2 ͒, the mechanism by which dislocations are induced is closely related to the YBa 2 Cu 3 O 7Ϫ␦ nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations.
At low fluences, the [Sr]/[Ti] ratio of laser deposited SrTiO3 films appears to be a function of the laser fluence. The deviation from stoichiometry is remarkably constant in time. From an analysis of both the composition of the film and the irradiated target, we deduce a volume-diffusion-assisted preferential ablation process. At high fluences (above 1.3 J/cm2), stoichiometric SrTiO3 films are obtained. This is not due to a change in ablation mechanism, but follows from the fact that at 1.3 J/cm2 the calculated diffusion length of Sr within the irradiated target, becomes of the order of the ablation rate per shot.
Linear defects are important pinning sites for vortices in high-temperature superconductors. In YBa 2 Cu 3 O 7Ϫ␦ thin films, the linear defects responsible for high critical currents are threading dislocations formed near the substrate interface. Investigating the first stages of growth of pulsed-lased-deposited YBa 2 Cu 3 O 7Ϫ␦ on single terminated ͑100͒ SrTiO 3 substrates, we study the genesis of these dislocations. We find that the formation of linear defects occurs above a certain critical layer thickness at which a coherent growth transition takes place. Coherent islands are formed, surrounded by highly strained trenches. These trenches facilitate the formation of dislocation half-loops. Such half-loops relieve the misfit strain and form misfit and threading dislocations. The number of threading dislocations thus depends on the island density. This model explains both the short-range lateral order of the threading dislocations and their decreasing density at elevated substrate temperatures.
Although vortex pinning in laser-ablated YBa 2 Cu 3 O 7−δ films on (100) SrTiO 3 is dominated by threading dislocations (Dam B et al (1999) Nature 399 439), many other natural pinning sites are present. To identify the contribution from twin planes, surface corrugations and point defects, we manipulate the relative densities of all defects by post-annealing films with various as-grown dislocation densities, n disl. While a universal magnetic field B dependence of the transport current density j s (B, T) is observed (independently of n disl , temperature T and the annealing treatment), the defect structure changes considerably. Correlating the microstructure to j s (B, T), it becomes clear that surface roughness, twins and point defects are not important at low magnetic fields compared to linear defect pinning. Transmission electron microscopy indicates that threading dislocations are not part of grain boundaries nor are they related to the twin domain structure. We conclude that j s (B, T) is essentially determined by pinning along threading dislocations, naturally induced during the growth process. Even in high magnetic fields, where the vortex density outnumbers n disl , it appears that linear defects stabilize the vortex lattice by means of the vortex-vortex interaction.
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