Natural strong pinning sites in laser-ablated YBa2Cu3O7-δ thin films

At low temperatures dislocations are the dominant flux-pinning centers in thin films of YBa2Cu3O7-delta deposited on (100) SrTiO3 substrates [B. Dam et nl., Nature (London) 399, 139 (1999)]. 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 YBa2Cu3O7-delta A films. We find that (i) dislocations are induced in die 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 (ii) 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 (similar to 1-100/mu m(2)), the mechanism by which dislocations are induced is closely related to the YBa2CU3O7-delta nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations.