F.C. Klaassen

The origin of high critical currents in YBa2Cu3O7-d thin films

Thin films of the high-temperature superconductor YBa2Cu3O7−δ exhibit both a large critical current (the superconducting current density generally lies between 1011 and 1012 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 YBa2Cu3O7−δ thin films offer an attractive system for investigating the properties of vortex matter in a superconductor with a tailored defect structure.

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.

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.

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.