F. Hillmer

Preparation and structural characterization of thin epitaxial Bi2Sr2CaCu2O8+δ films with Tc in the 90 K range

Abstract Thin films of Bi 2 Sr 2 CaCu 2 O 8+δ with T c up to 92 K were prepared by an in situ sputtering method on SrTiO 3 (100) and LaAlO 3 (100) substrates. A strong c -axis orientation of the film growth with low mosaic spread and full epitaxy within the ab -plane were confirmed by X-ray diffraction in Bragg-Brentano and four-circle geometry. Rutherford backscattering and channeling confirm the correct film composition and highly textured growth with a minimum yield of 23%. The surface morphology of the films was examined by STM and SEM studies, revealing a low density of precipitates and the absence of screw dislocations. The depth profile of the composition was investigated by secondary neutrals mass spectroscopy (SNMS).

Thermally activated dissipation in epitaxial Bi2Sr2CaCu2O8 + δ films evidence for the occurence of vortex strings with three-dimensional fluctuations in low magnetic fields

Abstract The thermally activated dissipation in highly anisotropic Bi 2 Sr 2 CaCu 2 O 8 + δ superconducting thin films was investigated by conventional resistive measurements, with the magnetic field B = μ 0 H applied perpendicular to the ( a, b ) plane and ranging between 5 mT and 3 T. We observed a significant change in the form of the “irreversibility line” (determined with a constant-resistivity criterion) and of the magnetic-field dependence of the activation energy at an applied field value B cr ≈ 30 mT. This change is discussed in connection to the predicted “dimensional crossover” in the vortex system, in the case of highly anisotropic superconductors. For the investigated specimen, we found that the model of thermally activated plastic flux motion with double vortex kink formation describes the irreversibility line at intermediate field values. This indicates that the “pancake” vortices in the (CuO 2 ) 2 blocks are not completely decoupled, at least up to 3 T. In the low field region, due to the predicted increase of the tilt modulus in the vortex system, the above model becomes inappropriate, and the form of the irreversibility line can be explained in terms of thermally assisted flux flow in a three-dimensional vortex ensemble, with a coherence length along the flux line of the order of 50 nm.

Heavy-ion-induced effects on the transport critical current density in epitaxial 2212-BSCCO thin films

Abstract We report on heavy-ion induced effects on the transport critical current density J ct in expitaxial thin films of Bi-2212. The films were irradiated with different ions at a temperature of 70 K. In-situ measurements showed an increase of J ct with the applied ion dose. We found enhancement factors of up to 150 in magnetic fields below 2 T for irradiation with 197 Anu- and 238 U-ions. Applying 40 Ca- and 197 Au-ions to the same sample in two consecutive experiments we could show that the columnar defects produced by the heavy ions are more effective pinning sites than scattered displacement cascades.

Characterization of epitaxial Bi2Sr2CaCu2O8+δ thin films

We report a dc sputtering method for the fullin situ preparation of Bi2Sr2CaCu2O8+δ thin films on SrTiO3 and LaAlO3.Tc values of more than 90 K can be achieved by oxidizing annealing below the melting point, followed by a reducing anneal at 500°C. The structural properties of the films are revealed by X-ray diffraction in Bragg-Brentano geometry (strongc-axis orientation with FWHM (0 0 10)=0.3) and also byΦ scans (epitaxy within the substrate plane). Rutherford backscattering and channeling confirmed the correct composition of the cations while the minimum yield,χmin, is 23%. Depth profiles by SNMS show a very homogeneous distribution of the cations with no detectable loss of bismuth near the surface. The surface morphology of the films was studied by SEM and by STM. Patterning of the films in lateral geometry can be performed by photolithographic techniques without degradation ofTc.

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

Abstract Up to now it is a controversially discussed question whether the Kosterlitz-Thouless (KT) theory can be applied to the high temperature superconductors. Despite of experimental evidences there are theoretical reservations mainly based on the finite anisotropy and resulting linear contributions to the logarithmic vortex-antivortex interaction. A considerable number of experimental results however can be consistently explained by the less-restrictive 2D-Coulomb-gas approach. Highly anisotropic, epitaxial Bi-2212 thin films with a mass-anisotropy parameter exceeding 150 offer a model system for the study of these aspects of vortex dynamics in quasi 2D-superconductors.

In situ-preparation, characterization and transport-properties of high quality Bi2Sr2CaCu2Oy-thin films

Abstract We report the preparation of Bi 2 Sr 2 CaCu 2 O y -thin films with inductively measured T c -values of up to 92 K by dc-sputtering from a single stoichiometric target in pure oxygen atmosphere. The excellent structural properties of these films are revealed by X-ray diffraction in Bragg-Brentano-geometry, Φ-Scans and RBS. In detail we present measurements of the resistive transitions in magnetic fields up to 10 T, the depinning-line and the field-dependence of the activation energy. Measurements of the critical current density ( j c (70 K )=10 6 A / cm 2 ) exhibit a comparatively slow decline of j c with increasing magnetic field. The angular dependences of j c and of the magnetoresistance in external fields confirm the two dimensional behavior of the 2212-compound, in the latter case even up to 4 K above T c . Furthermore we found a clear correlation between the critical temperature of the films and their carrier density and mobility.