H. Kuhn

Current capability of filaments depending on their position in (Bi,Pb)2Sr2Ca2Cu3O10+δ ‐multifilament tapes

The microstructures of single filaments of silver‐sheathed (Bi,Pb)2Sr2Ca2Cu3O10+δ‐multifilament tapes are investigated by polarized light and scanning electron microscopy. Large differences in the microstructure of filaments depending on their position in the cross section of the tape are found. The influence of this microstructure on the critical current density is investigated by magneto‐optics. A drastic reduction of the critical current density by a factor of about 10 of the outer, compared to the filaments in the center of the tape is observed.

Critical current limiting structural defects in melt‐textured YBa2Cu3O7−δ

Structural analyses and investigations of flux penetration of typically melt‐textured YBa2Cu3O7−δ samples were performed. This allows regions of suppressed critical current density to be directly related to microstructural defects. At small‐angle grain boundaries the critical current density was found to be reduced by a factor of up to 20.

Flux-lines of inversed sign in YBa2Cu3O7−δ thin films

Abstract Using the high-resolution Faraday technique (HRF), we observed directly the nucleation of flux-lines of opposite sign in the remanent state of YBaCuO thin films. These negative flux-lines are nucleated by the stray fields caused by the pinned vortices. Due to the large demagnetizing factor of the thin film, this flux is able to penetrate the sample. Such a coupled behaviour can be observed directly. Experimental evidence for the existence of vortices of opposite sign is given. The influence of various polarizer/ analyzer settings on these observations is discussed. Furthermore, we show effects of flux-creep in thin films and the role of the negative flux-lines in this behavior is analyzed.

Magneto-optical study of flux penetration in heavy-ion irradiated high-Tc single crystals

Abstract Single crystals of Bi2Sr2CaCu2O8+δ (Bi-2212) and of DyBa2Cu3O7−δ have been exposed to high-energy heavy-ion irradiation (230 MeV 58Ni-ions, 0.5 GeV 127I-ions and 1.0 GeV 208Pb-ions) with various fluences perpendicular to the sample surface and hence parallel to the c-axis of the crystals. The irradiation with Ni-ions produces spherical regions of amorphized material, whereas the iodine and lead ions create columnar defects along the paths of the projectiles in the superconductors. Using the high-resolution Faraday effect (HRF) technique, domain patterns of the Shubnikov phase are investigated. From the measured flux-density profiles, the acting local pinning forces and critical current densities are determined. To allow direct comparison of the irradiation effects under the same experimental conditions, one half of each crystal was covered by an absorber during the irradiation. Fluence-dependent measurements of the pinning forces are performed and the influence of irradiation on flux-creep effects is also investigated. All heavy-ion irradiated samples show an enhancement of the measured critical current densities up to a factor of 33 as compared to the unirradiated region of the same crystal. The effective activation energies for flux creep remain unchanged after each kind of irradiation in both types of single crystals.

Magnetic phase transitions and structural deficiencies in superconducting YNiBC

Abstract Samples of the composition YNi 2 B x C with 1 ≤ x ≤ 2 are prepared on a powder-metallurgical route at 1300°C. This procedure yields specimens with magnetic transitions between 11 K and 24 K and boron deficiencies as compared to the ideal stoichiometry YNi 2 B 2 C. Measurements of the magnetization in combination with chemical analysis and electron microprobe analyzer measurements indicate a correlation between the onset transition temperatures T c and the boron deficiencies in the phase YNi 2 B x C. Two transitions at 12K ≤ T ≤ 15.6K and 16K ≤ T ≤ 24K are clearly distinguished.

Flux penetration into YBa2Cu3Ox thin films covering substrate step edges

Using the high‐resolution Faraday effect, the flux penetration into YBa2Cu3Ox thin films grown over one or two substrate step edges is directly observed at a temperature of T=5 K. The regions at the steps are easily penetrated by the flux already present at low applied external magnetic fields due to the locally enhanced stray fields. The step edges are found to separate the sample magnetically into independent parts. It is also shown that the local observation of flux penetration allows detection of the influence of defects in the thin‐film samples on the domain structures in a direct way.

HIGH-PRECISION LASER CUTTING OF HIGH-TEMPERATURE SUPERCONDUCTORS

A laser‐cutting technique was used to pattern high‐temperature superconducting single crystals with high precision into various shapes. The low mean power of 135 mW of the laser enables one to produce cuts of arbitrary shape without affecting the superconducting parameters. This was proven by local observations of the magnetic flux distribution using the magnetooptical Faraday effect.

Geometry dependence of intergranular currents in sintered YBa2Cu3O7−δ

Abstract Using standard four-point terminal measurements, the intergranular critical current densities j J c ( T, B ) of sintered YBa 2 Cu 3 O 7−δ samples are determined. The dependence of the critical current density on the sample geometry (height h and width b of parallel- epiped samples) is measured systematically for various temperatures (5K ≤ T ≤ 91 K) and low applied external magnetic fields (0 mT ≤ μ 0 H ext ≤ 10 mT). It is shown that the intergranular critical current density increases on reducing the sample height or width, if no external magnetic field is applied to the sample. The results are discussed using a model assuming the currents flowing in a surface layer of the sample. A reduction of the height or width of the sample changes the circumferences and not the current itself, thus implying an enhancement of the critical current density, j J c ( T, B ). It is found that the increase of j J c ( T, B ) on reducing the sample dimensions is limited by the Josephson penetration depth, λ J ( T ). This penetration depth, λ J ( T ), turns out to be a very important parameter to reach high intergranular critical current densities in sintered high- T c superconductors.