The behavior of the superconducting current density 𝑗𝑠⁡(𝐵,𝑇) and the dynamical relaxation rate 𝑄⁡(𝐵,𝑇) of YBa2⁢Cu3⁢O7−𝛿 thin films exhibits a number of features typical for strong pinning of vortices by growth induced linear defects. At low magnetic fields 𝑗𝑠⁡(𝐵) and 𝑄⁡(𝐵) are constant up to a characteristic field 𝐵*, that is directly proportional to the linear defect density 𝑛disl. The pinning energy 𝑈𝑐⁡(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 ɛ𝑟⁡(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 𝑗𝑐⁡(0,𝑇) and of the pinning energy 𝑈𝑐⁡(0,𝑇) at low magnetic fields. At high magnetic fields 𝜇0⁢𝐻≫𝐵* the current density experiences a power law behavior 𝑗𝑠⁡(𝐵)∼𝐵𝛼, with 𝛼≈−0.58 for films with low 𝑛disl and 𝛼≈−0.8 to -1.1 for films with high 𝑛disl. The pinning energy in this regime, 𝑈𝑐⁡(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.

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.

Recently, we showed that natural linear defects are the origin of the high critical currents in laser ablated YBa 2 Cu 3 O 7-δ films. 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.

Thin films of the high-temperature superconductor YBa₂Cu₃O(7-δ) exhibit both a large critical current (the superconducting current density generally lies between 10 and 10 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. 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 quantitatively by this technique, are the linear defects that provide the strong pinning centres responsible for the high critical currents observed in these thin films. Moreover, we find that the superconducting current density is essentially independent of the density of linear defects at low magnetic fields. These natural linear defects, in contrast to artificially generated columnar defects, exhibit self-organized short-range order, suggesting that YBa₂Cu₃O(7-δ) thin films offer an attractive system for investigating the properties of vortex matter in a superconductor with a tailored defect structure.