Natural strong pinning sites in laser-ablatedYBa2Cu3O7−δthin films

At low temperatures dislocations are the dominant flux-pinning centers in thin films of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ deposited on (100) ${\mathrm{SrTiO}}_{3}$ substrates [B. Dam et al., Nature (London) 399, 439 (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 ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ films. We find that (i) 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 (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 ${n}_{\mathrm{disl}}$ over almost two orders of magnitude (\ensuremath{\sim}1--100/\ensuremath{\mu}${\mathrm{m}}^{2}$), the mechanism by which dislocations are induced is closely related to the ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}\ensuremath{\delta}}$ nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations.