The behavior of the superconducting current density j _s(B,T) and the dynamical relaxation rate Q(B,T) of YBa₂Cu₃O_7-δ thin films exhibits a number of features typical for strong pinning of vortices by growth induced linear defects. At low magnetic fields j_s(B) and Q(B) are constant up to a characteristic field B*, that is directly proportional to the linear defect density n_disl. The pinning energy U_c(B=0)≈600 K can be explained by half-loop excitations determining the thermal activation of vortices at low magnetic fields. Extending the Bose glass theory [D. R. Nelson and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)], we derive a different expression for the vortex pinning potential ε_r(R), which is valid for all defect sizes and describes its renormalization due to thermal fluctuations. With this expression we explain the temperature dependence of the true critical current density j_c(O,T) and of the pinning energy U_c(0,T) at low magnetic fields. At high magnetic fields μ₀H≫B* the current density experiences a power law behavior j_s(B)∼B^α with α≈-0.58 for films with low n_disl and α≈-0.8 to -1.1 for films with high n_disl. The pinning energy in this regime, U_c(high B)≈60-200 K is independent of magnetic field, but depends on the dislocation density. This implies that vortex pinning is still largely determined by the linear defects, even when the vortex density is much larger than the linear defect density. Our results show that natural linear defects in thin films form an analogous system to columnar tracks in irradiated samples. There are, however, three essential differences: (i) typical matching fields are at least one order of magnitude smaller, (ii) linear defects are smaller than columnar tracks, and (iii) the distribution of natural linear defects is nonrandom, whereas columnar tracks are randomly distributed. Nevertheless the Bose glass theory, that has successfully described many properties of pinning by columnar tracks, can be applied also to thin films. A better understanding of pinning in thin films is thus useful to put the properties of irradiated samples in a broader perspective.

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The behavior of the superconducting current density (formula presented) and the dynamical relaxation rate (formula presented) of (formula presented) thin films exhibits a number of features typical for strong pinning of vortices by growth induced linear defects. At low magnetic fields (formula presented) and (formula presented) are constant up to a characteristic field (formula presented) that is directly proportional to the linear defect density (formula presented) The pinning energy (formula presented) can be explained by half-loop excitations determining the thermal activation of vortices at low magnetic fields. Extending the Bose glass theory [D. R. Nelson and V. M. Vinokur, Phys. Rev. B 48, 13 060 (1993)], we derive a different expression for the vortex pinning potential (formula presented) which is valid for all defect sizes and describes its renormalization due to thermal fluctuations. With this expression we explain the temperature dependence of the true critical current density (formula presented) and of the pinning energy (formula presented) at low magnetic fields. At high magnetic fields (formula presented) the current density experiences a power law behavior (formula presented) with (formula presented) for films with low (formula presented) and (formula presented) to -1.1 for films with high (formula presented) The pinning energy in this regime, (formula presented) is independent of magnetic field, but depends on the dislocation density. This implies that vortex pinning is still largely determined by the linear defects, even when the vortex density is much larger than the linear defect density. Our results show that natural linear defects in thin films form an analogous system to columnar tracks in irradiated samples. There are, however, three essential differences: (i) typical matching fields are at least one order of magnitude smaller, (ii) linear defects are smaller than columnar tracks, and (iii) the distribution of natural linear defects is nonrandom, whereas columnar tracks are randomly distributed. Nevertheless the Bose glass theory, that has successfully described many properties of pinning by columnar tracks, can be applied also to thin films. A better understanding of pinning in thin films is thus useful to put the properties of irradiated samples in a broader perspective.@en

Recently [1], we have shown that dislocations are the most important flux pinning centers in Pulsed Laser Deposited YBa2Cu3O7-δ thin films. It appeared that the magnetic field upto which the critical current remains constant, is roughly equal to the matching field BΦ=ndislΦ0, with ndisl the density of dislocations. Here, we investigate the formation mechanism of these dislocations. Using wet-chemical etching in combination with Atomic Force Microscopy, we find that dislocations are induced in the first stages of film growth and persist all the way up to the film surface parallel to the c-axis. Since the substrate temperature can be used to tune the defect density ndisl, the dislocation formation mechanism is closely related to the YBa2Cu3O7-δ nucleation and growth mechanism. We propose that dislocations are induced as a result of merging of misaligned growth fronts due to the preferential formation of precipitates in the first stages of growth. Indeed, we find that we can increase the dislocation density by first depositing Y2O3 precipitates.@en

At low temperatures dislocations are the dominant flux-pinning centers in thin films of (Formula presented) deposited on (100) (Formula presented) 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 (Formula presented) 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 (Formula presented) over almost two orders of magnitude (∼1-100/μ(Formula presented)), the mechanism by which dislocations are induced is closely related to the (Formula presented) nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations.@en

Recently, we showed that natural linear defects are the origin of the high critical currents in laser ablated YBa2Cu3O7-δ films 1. Combining wet-chemical etching and Atomic Force Microscopy, we find that these dislocations are created by island coalescence during growth. Consequently, the defect density can be reproducibly varied by manipulating the density of growth islands, which in turn depends on the substrate temperature. Interestingly, the radial defect distribution function approaches zero at small distances, indicating short range order. Therefore, we are now able to study vortex, matter in films with a tailored non-random distribution of natural strong pinning sites.@en