The excess 1/f noise investigations of YBCO films on SrTiO₃ substrates are reported.The epitaxial films produced by the laser ablation are characterized by the high perfectstructure, sharp superconducting transition, strong pinning and very low excess 1/f noise.It is observed that the level at normal state depends on the dislocation density. The bestnoise properties are observed for films with the dislocation density of (8-12) disl/Μm².These films have noise Hooge-parameters of around (1.6-3)×10^-6, which is 3 order lessthan published values. The calculation of the bolometer based on investigated films,shows the possibility to reach the noise equivalent power limited by only the phononnoise in the modulating frequency range beginning above 0.1 Hz.

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Although vortex pinning in laser-ablated YBa2Cu3O7-δ films on (100) SrTiO3 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, ndisl. While a universal magnetic field B dependence of the transport current density js (B, T) is observed (independently of ndisl, temperature T and the annealing treatment), the defect structure changes considerably. Correlating the microstructure to js (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 js (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 ndisl, it appears that linear defects stabilize the vortex lattice by means of the vortex-vortex interaction.@en

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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Using advanced magneto-optics, the field and current distributions in superconducting thin film (formula presented) rings in an externally applied magnetic field are studied experimentally. The observations are in close agreement with numerical calculations. During the initial flux penetration and field reversal a highly nonuniform current distribution is observed. In particular, concentric counterrotating current loops occur during field reversal. We explore implications of these results for the determination of critical currents and penetration fields from bulk magnetization measurements.

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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

The noise characteristics of YBa2Cu3O7-x films prepared by the laser ablation technique on SrTiO3 substrates were studied. The epitaxial YBCO films possess a perfect structure and are characterized by a narrow superconducting transition, strong pinning, and a very small flicker noise. In the normal state, the 1/f noise increases with the dislocation density. The minimum noise level with a Hooge parameter of (2-3) × 10-6 was observed at a disc location density of 8-12 μm-2.@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

The field dependence of the superconducting current density j_s(B)and the pinning energy U_C(B) in thin films of YBa₂Cu₃O_7-δindicates three different pinning regimes. At low fields, μH < B* ≈ 0.7n_d,Φ₀s the density of dislocations), j_sB) is constant and U_c = 600 K at T 0 for all YBa₂Cu₃O_7-δfilms, independent of the density of dislocations. This is much lower than the pinning energy of a vortex in a single dislocation, for which U_c ≈ 6000 K. At intermediate fields, B* < μ₀H < 0.5 T, a sharp decrease of U_c is observed. For μ₀H > 0.5 T, U_c(B) is constant again, with U_C(B) ≈ 80 K for sputtered films and U_C(B) ≈ 200 K for laser ablated films.

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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 [1], we have shown that dislocations act as strong pinning sites for vortices in YBa2Cu3O7-δ films. Using a wet-chemical etching technique in combination with Atomic Force Microscopy, we investigate the distribution of these natural linear defects in laser ablated films. We find that: (i) dislocations are induced in the early stages of film growth at the substrate-film interface and persist up to the film surface, resulting in a uniform length distribution and (ii) the in-plane defect distribution exhibits short-range ordering. This self-organization makes films completely different from e.g. single crystals with artificial columnar defects.@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

We studied the superconducting current density js (B, T) and dynamical relaxation rate Q(B, T) of laser ablated thin films of YBa2Cu3O7-δ with various dislocation densities (ndisl ∼ 7 - 70 μm-2). In these films dislocations act as strong pinning centers like columnar defects in heavy ion-irradiated crystals. The matching field clearly manifests itself in a constant js(B) ∼ 5.1011 - 1.1012 Am-2 at T = 4.2 K and μ0H ≲ 0.7·BΦ and a constant Q(B). In contrast to irradiated crystals, in films we do not observe a peak in the relaxation rate Q versus T below the matching field BΦ = ndisl · Φ0. Instead, in thin films Q(T) increases monotonously with increasing temperature. We conclude that the influence of vortex-vortex interactions in thin films is greatly reduced due to the non-random distribution of pinning sites and the low matching field.@en

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.

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