Th. Kluge

Absence of correlated flux pinning by columnar defects in irradiated epitaxial Bi2Sr2CaCu2O8 thin films

Abstract Using heavy-ion irradiation, we produced columnar defects of different density and orientation in epitaxial Bi 2 Sr 2 CaCu 2 O 8 thin films. Although this increases the normal state resistivity and the critical temperature is reduced proportionally to the volume fraction of damaged material, pinning-related quantities like critical current density, activation energy and depinning field are enhanced in external magnetic fields. Transport measurements in dependence of the magnetic field and its orientation consistently indicate two-dimensional pinning of pancake vortices at the columnar defects. We observe the absence of correlated flux pinning by columnar defects and compare to heavy-ion-irradiated single crystals and tapes, where line-like correlations have been observed.

Contribution of Zn Impurity Atoms to the Anisotropic Pinning Force Density of Thin Epitaxial YBa2(Cu1-xZnx)3O7-δ Films

The in-plane critical current density jcab of thin epitaxial YBa2(Cu1-xZnx)3O7-δ films (x = 0.001 ÷ 0.02) was measured in magnetic fields up to 7 T under variation of the angle between the applied magnetic field and the c-axis of the films (j|| axis of rotation, i.e. j ⊥ B). At high temperatures (i.e. T > 0.5 Tc) the jc() dependences differ significantly from those obtained for undoped films. In order to explain the resulting structures quantitatively, we propose a model based on the additional pinning mechanism arising from pointlike pinning centres due to the Zn dopant atoms.

Vortex dynamics in Bi2Sr2CaCu2O8-thin films in the presence of columnar defects

AbstractWith heavy ion irradiation we create continous amorphous columnar defects in Bi2Sr2CaCu2O8-thin films. With regard to a reliable comparison of irradiation effects three of four identical striplines on the same samples were exposed to different irradiation procedures. We performed irradiations as well parallel as under different angles with respect to the film

Scattering phase shifts in Ni and Zn doped Y-123

\vec c

Scaling behavior of U(I)-characteristics and zero-field resistive transitions in epitaxial Bi2Sr2CaCu2O8+δ thin films

-axis. Beside an enlarged normal state resistivity after irradiation the films suffer a Tc-reduction proportional to the volume of the damaged material. The activation energy ascertained from resistive transitions shows best enhancement for magnetic field values close to the matching field. Measurements of the transport critical current density, the activation energy and the irreversibility field in different field orientations show no anisotropic flux pinning due to the induced columnar defects. This can be explained by the two dimensional behaviour of the Bi2Sr2CaCu2O8-thin films.

DECOUPLING AND FLUX-LINE-DYNAMICS IN YBa2Cu3O7/PrBa2Cu3O7-SUPERLATTICES

We investigated the structural and superconducting properties ofc-axis oriented (YBa2Cu3O7)nY/(PrBa2Cu3O7)npr superlattices with thicknesses of the individual layers down to one unit cell (10≥nY≥1; 18>nPr≥ 1). By transmission electron microscopy and X-ray diffraction we find an excellent structural quality of the samples, though the quantitative analysis shows the existence of defects. In superlattices with decoupled YBa2Cu3O7 layers of two unit cell thickness we find a highTc value of 75 K. We probed the flux line structure in the superlattices by measurements of the critical current density in magnetic fields. The experiments show that the flux-line dynamics is dominated by the movement of pancake vortices.

PINNING FORCE DENSITY IN YBa2Cu3O7-FILMS AND YBa2Cu3O7/PrBa2Cu3O7-SUPERLATTICES

Cu-site substituted thin films of YBa2(Cu1-zMz)3O7-δ (M=Zn,Ni) are reinvestigated. The Tc-suppression and residual resistivities ϱo are measured as a function of the concentration z. We found that for low concentrations z < 4% the substituents occupy preferrably the in-plane Cu sites whereas for higher concentrations the chain sites are also occupied. Although the residual resistivities of Ni and Zn differ only slightly, the Tc-suppression of Zn is 2.3 times larger than that of Ni. To reconcile the measured resistivities with predictions of a two dimensional scattering model, it is necessary to assume a scattering potential of finite range and to take scattering phase shifts] δl of higher angular momentum (l > 0) into account. The different Tc-suppression rates for Zn and Ni are also discussed within this picture.

ANISOTROPIC SUPERCONDUCTING PROPERTIES OF YBa2Cu3O7 BASED THIN FILMS AND SUPERLATTICES

Abstract We investigated YBa 2 (Cu 1− z M z ) 3 O 7-δ (M Ni,Zn) thin films and determined the decrease of T c and the increase of residual resistivity due to Cu-site substitution, taking into account the CuO-chain contributions to the total conductivity. Although Zn suppresses T c stronger than Ni by a factor of 2.3 the increase of resistivity differs only slightly. Furthermore the observed resistivities are too high to be explained within scattering from point-like defects. To reconcile these contradictions, we assumed finite size scattering potentials, which lead to scattering phase shifts δ l of higher angular momebtum l > 0. T c -suppression is discussed qalitatively within this picture.