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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We succeeded to image with magnetic force microscopy individual vortices in thin Nb films as well as in thin YBa2Cu3O7-δ (YBCO) films. Relying on the imaging of microfabricated gold loops, we are able to identify the exact location of the vortices within 10 nm with respect to topographic features. The center of most of the vortices is located in between the grains appearing at the film surface. For the Nb films with a larger coherence length, the pinning can be linked to the reduced film thickness in between the protruding grains. For the YBCO films with a very small coherence length, the pinning is likely to be dominated by line defects at the trenches in between the growth islands. On the other hand, Fourier analysis of the imaged vortex distribution shows that for thicker Nb films not all of the vortices are pinned in between the grains. Due to the vortex-vortex repulsion, the vortex lattice reveals short-range correlations which become more pronounced at higher fields.@en

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

Linear defects are important pinning sites for vortices in high-temperature superconductors. In YBa2Cu3O7-δ thin films, the linear defects responsible for high critical currents are threading dislocations formed near the substrate interface. Investigating the first stages of growth of pulsed-lased-deposited YBa2Cu3O7-δ on single terminated (100) SrTiO3 substrates, we study the genesis of these dislocations. We find that the formation of linear defects occurs above a certain critical layer thickness at which a coherent growth transition takes place. Coherent islands are formed, surrounded by highly strained trenches. These trenches facilitate the formation of dislocation half-loops. Such half-loops relieve the misfit strain and form misfit and threading dislocations. The number of threading dislocations thus depends on the island density. This model explains both the short-range lateral order of the threading dislocations and their decreasing density at elevated substrate temperatures.@en

Linear defects are important pinning sites for vortices in high-temperature superconductors. In (formula presented) thin films, the linear defects responsible for high critical currents are threading dislocations formed near the substrate interface. Investigating the first stages of growth of pulsed-lased-deposited (formula presented) on single terminated (100) (formula presented) substrates, we study the genesis of these dislocations. We find that the formation of linear defects occurs above a certain critical layer thickness at which a coherent growth transition takes place. Coherent islands are formed, surrounded by highly strained trenches. These trenches facilitate the formation of dislocation half-loops. Such half-loops relieve the misfit strain and form misfit and threading dislocations. The number of threading dislocations thus depends on the island density. This model explains both the short-range lateral order of the threading dislocations and their decreasing density at elevated substrate temperatures.

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Recently, threading dislocations were identified as the origin of the high critical currents in YBa₂Cu₃O_7-δ films [Nature (Lond.) 399 (1999) 439]. As these dislocations originate at the substrate-film interface, the termination of the (100) SrTiO₃ substrate is expected to influence both the nucleation and the dislocation formation in YBa₂Cu₃O_7-δ (123) films. To investigate this substrate-film interaction, we developed a new ex situ method to prepare SrO-terminated substrates in addition to the well-known TiO₂ termination. It is shown that: (i) preferential precipitation occurs when depositing submonolayer 123 films on SrO-terminated SrTiO₃ and (ii) such precipitates are absent on the TiO₂ termination. It appears that the substrate termination determines both the starting and terminating layer of the 123 phase. Similar effects are observed in submonolayer thick films with cation stoichiometries of 122, 124 and 133. When growing several 123 monolayers on SrO-terminated substrates, no precipitates are formed. Instead, to incorporate the off stoichiometry, defects are introduced. Surprisingly, the density of threading dislocations does not depend on the substrate termination.

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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

Penetration of magnetic flux in YBa2Cu3O7 superconducting thin films in an external magnetic field is visualized using a magneto-optic technique. A variety of flux patterns due to non-linear vortex diffusion is observed: (1) Roughening of the flux front with scaling exponents identical to those observed in burning paper including two distinct regimes where respectively spatial disorder and temporal disorder dominate. In the latter regime Kardar-Parisi-Zhang behavior is found. (2) Fractal penetration of flux with Hausdorff dimension depending on the critical current anisotropy. (3) Penetration as 'flux-rivers'. (4) The occurrence of commensurate and incommensurate channels in films with anti-dots as predicted in numerical simulations by Reichhardt, Olson and Nori. It is shown that most of the observed behavior is related to the non-linear diffusion of vortices by comparison with simulations of the non-linear diffusion equation appropiate for vortices.@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