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.

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.

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.

Linear defects are important pinning sites for vortices in high-temperature superconductors. In YBa2⁢Cu3⁢O7−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 YBa2⁢Cu3⁢O7−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.

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, threading dislocations were identified as the origin of the high critical currents in YBa2Cu3O7−δ films [Nature (Lond.) 399 (1999) 439]. As these dislocations originate at the substrate–film interface, the termination of the (1 0 0) SrTiO3 substrate is expected to influence both the nucleation and the dislocation formation in YBa2Cu3O7−δ (1 2 3) 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 1 2 3 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 1 2 3 phase. Similar effects are observed in submonolayer thick films with cation stoichiometries of 1 2 2, 1 2 4 and 1 3 3. When growing several 1 2 3 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.

Using advanced magneto-optics, the field and current distributions in superconducting thin film YBa2⁢Cu3⁢O7 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.

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.

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.

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.

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.

The field dependence of the superconducting current density js(B) and the pinning energy Uc(B) in thin films of YBa2Cu3O7−δ indicates three different pinning regimes. At low fields, μ0H < B* ≈0.7ndφ0 (nd is the density of dislocations), js(B) is constant and Uc = 600K at T = 0 for all YBa2Cu3O7−δ films, independent of the density of dislocvations. This is much lower than the pinning energy of a vortex in a single dislocation, for which Uc ≈ 6000K. At intermediate fields, B* < μ0H < 0.5T, a sharp decreae of Uc is observed. For μ0H> 0.5T, Uc(B) is constant again, with Uc(B) ≈ 80K for sputtered films and Uc(B) ≈ 200K for laser ablated films.

At low temperatures dislocations are the dominant flux-pinning centers in thin films of YBa2⁢Cu3⁢O7−𝛿 deposited on (100) SrTiO3 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 YBa2⁢Cu3⁢O7−𝛿 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 𝑛disl over almost two orders of magnitude (∼1–100/μm2), the mechanism by which dislocations are induced is closely related to the YBa2⁢Cu3⁢O7−𝛿 nucleation and growth mechanism. We present evidence for preferential precipitation in the first monolayer and precipitate generated dislocations.

Kinetic roughening of flux fronts penetrating in superconducting thin films are studied by means of a high resolution magneto-optic technique. The roughening exponent (α = 0.64) and growth exponent (β = 0.65) obtained from a dynamic scaling analysis of the initial stage of flux penetration and, at small length scales, are characteristic for a static disorder dominated nonlinear diffusion such as also observed in the directed percolated depinning model. At large length scale, α = 0.46 indicates a transition towards dynamic stochastic disorder, similar to the behavior of Kardar-Parisi-Zhang systems. There is a striking similarity with the behavior of combustion fronts in burning paper.

The fluence dependence of the composition of pulsed-laser deposited YBa2Cu3O7-δ films is investigated and interpreted in terms of laser-induced target modification. Both target degradation (at fluence J<1.0 J/cm2) and diffusion-assisted preferential ablation ( 1.0<J<1.3 J/cm2) are found to be responsible for nonstoichiometric transfer. A one-dimensional, moving-boundary diffusion model is developed to describe diffusion-assisted preferential ablation. This model predicts stoichiometric transfer at large ablation rates. Indeed, for J≫ 1.3 J/cm2 stoichiometric deposition is found, resulting in precipitate-free films. However, slightly off-stoichiometric films, deposited in the diffusion-assisted preferential ablation regime, exhibit the best superconducting properties (Tc = 91.0K, ΔTc = 0.4 K) and can be produced with a remarkably high reproducibility.

We studied the superconducting current density j s (B, T) and dynamical relaxation rate Q(B, T) of laser ablated thin films of YBa 2 Cu 3 O 7−δ with various dislocation densities (n disl ∼ 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 j s (B) ∼ 5·10 11 - 1·10 12 Am −2 at T = 4.2 K and μ 0 H ≲ 0.7 B Φ 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 B Φ ≡ n disl ·Φ 0 . 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.

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.

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.

At low fluences, the [Sr]/[Ti] ratio of laser deposited SrTiO3 films appears to be a function of the laser fluence. The deviation from stoichiometry is remarkably constant in time. From an analysis of both the composition of the film and the irradiated target, we deduce a volume-diffusion-assisted preferential ablation process. At high fluences (above 1.3 J/cm2), stoichiometric SrTiO3 films are obtained. This is not due to a change in ablation mechanism, but follows from the fact that at 1.3 J/cm2 the calculated diffusion length of Sr within the irradiated target, becomes of the order of the ablation rate per shot.

Using scanning probe microscopy we observe a growth mode transition in pulsed laser deposited (PLD) YBa2Cu3O7-δ films. By increasing the background oxygen pressure the generally observed 2D-nucleation and growth mode transforms into a spiral growth mode. Keeping the oxygen pressure at increased levels and reducing the target-sub strate distance, the growth spirals disappear and 2D-nucleation and growth becomes the dominant growth mode again. We attribute the growth mode transition to a change in surface diffusivity of ∼ 2 orders of magnitude. Our experiments confirm that in PLD surface kinetics is strongly linked to plume dynamics.