J.M. Huijbregtse

Magnetic force microscopy of vortex pinning at grain boundaries in superconducting thin films

Abstract We succeeded to image with magnetic force microscopy individual vortices in thin Nb films as well as in thin YBa 2 Cu 3 O 7− 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.

Controlling the natural strong pinning sites in laser ablated YBa2Cu3O7-δ thin films

Abstract Recently [1], we have shown that dislocations are the most important flux pinning centers in Pulsed Laser Deposited YBa 2 Cu 3 O -δ thin films. It appeared that the magnetic field upto which the critical current remains constant, is roughly equal to the matching field B Φ = n disl Φ 0 , with n disl the density of dislocations. Here, we investigate the formation mechanim 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 n disl , the dislocation formation mechanism is closely related to the YBa 2 Cu 3 O 7−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 Y 2 O 3 precipitates.

High-quality off-stoichiometric YBa2Cu3O7−δ films produced by diffusion-assisted preferential laser ablation

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=91.0 K, ΔTc=0.4 K) and can be produced with a remarkably high reproducibility.

Vortex Pinning Regimes in thin films of YBa2Cu3O7−δ

Abstract The field dependence of the superconducting current density j s ( B ) and the pinning energy U c ( B ) in thin films of YBa 2 Cu 3 O 7−δ indicates three different pinning regimes. At low fields, μ 0 H B * ≈0.7 n d φ 0 ( n d is the density of dislocations), j s ( B ) is constant and U c = 600K at T = 0 for all YBa 2 Cu 3 O 7−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 U c ≈ 6000K. At intermediate fields, B * 0 H U c is observed. For μ 0 H > 0.5T, U c ( B ) is constant again, with U c ( B ) ≈ 80K for sputtered films and U c ( B ) ≈ 200K for laser ablated films.

Pattern formation due to non-linear vortex diffusion

Abstract Penetration of magnetic flux in YBa 2 Cu 3 O 7 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 appropriate for vortices.

YBa2Cu3O7−δ films with self-organized natural linear defects

Abstract Recently [1], we have shown that dislocations act as strong pinning sites for vortices in YBa 2 Cu 3 O 7−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.

Critical current density and dynamical relaxation rate below the matching field in thin films of YBa2Cu3O7−δ

We studied the superconducting current density js(B, T) and dynamical relaxation rate Q(B, T) of laser ablated thin films of YBa2Cu3O7−δwith 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- 1·1012Am−2at T = 4.2 K and μ0H ≲ 0.7 BΦand 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.