H.W. Weber

On the calculation of Jc from magnetization measurements on superconductors

Abstract A simple but reliable procedure to evaluate J c ( B ) from magnetization measurements has been developed. It applies to the fully penetrated state and to situations where the flux density gradient has reversed completely. In an approach consistent with the Bean model, a uniform current density is assumed to flow throughout the entire sample being correlated to the corresponding average magnetic induction in the sample. In combination with the anisotropic Bean model, we present evaluations of J c ( B ) for a typical single crystalline high-temperature superconductor.

Neutron irradiation of MgB2 bulk superconductors

Sintered samples of MgB2 were irradiated in a fission reactor. Defects in the bulk microstructure are produced during this process mainly by the 10B(n,α)7Li reaction while collisions of fast neutrons with the lattice atoms induce much less damage. Self-shielding effects turn out to be very important and lead to a highly inhomogeneous defect distribution in the irradiated samples. The resulting disorder enhances the normal state resistivity and the upper critical field. The irreversibility line shifts to higher fields at low temperatures and the measured critical current densities increase following irradiation.

Defect cascades produced by neutron irradiation in YBa2Cu3O7−δ

Abstract Defect cascades produced by fast neutron irradiation of YBa 2 Cu 3 O 7−δ single crystals were studied by transmission electron microscopy. The visible defects were found to have sizes between 1 and 5 nm. Defect densities were obtained as a function of neutron fluence between 2x10 21 and 8x10 21 m −2 ( E >0.1 MeV) and compared to damage calculations. The measured defect density scales linearly with fluence and amounts to 1x10 22 m −3 at a neutron fluence of 2x10 21 m −2 . The defect stability was studied at room temperature and through annealing to 400°C.

Anisotropy Effects in Superconductors

1. The Upper Critical Field.- R-1 On the Theory of Macroscopic Anisotropy Phenomena in Type-II Superconductors.- R-2 Experimental Investigations on the Anisotropy of the Upper Critical Field in Type-II Superconductors.- 2. Hc2 and Related Properties.- C-1 The Influence of Fermi Surface Anisotropy on Hc2(T).- C-2 Impurity Dependence of Hc2 Anisotropy in Niobium.- C-3 Microscopic Anisotropy Parameters of Niobium.- C-4 Anisotropy of Hc2 in PbTl-Alloys.- C-5 Comments on Hc2 at Low Temperatures.- C-6 The Influence of Magnetic Anisotropy on the Properties of Niobium in the Mixed State.- C-7 Anisotropy of the Stress Dependence of Critical Parameters in Uniaxial Superconductors.- 3. Flux Lines.- R-3 Magnetization and Flux Line Lattice in Anisotropic Superconductors.- R-4 Morphology of Flux Line Lattices in Single-Crystalline Type-II Superconductors.- 4. Flux Lines, Domains and Magnetization.- C-8 Correlations between Flux Line Lattice and Crystal Lattice.- C-9 Experiments on the Correlation between the Flux Line Lattice and the Crystal Lattice in Superconducting Lead Films.- C-10 Anisotropy in the Intermediate State of Superconducting Mercury.- C-11 Mixed State Anisotropy of Superconducting Vanadium.- C-12 Measurement of Torque due to Anisotropy of the Magnetization Vector in Superconducting Niobium.- 5. Energy Gap.- R-5 Microscopic Calculations of Energy Gap Anisotropy.- R-6 An Evaluation of the Validity of Superconducting Evidence for Anisotropy and Multiple Energy Gaps.- 6. Energy Gap and Related Properties.- C-13 Tunneling Junction Phenomena: An Answer to Unanswered Questions.- C-14 An Analysis of Evidence for Superconducting Energy Gap and Pairing Interaction Anisotropy for Two Types of Experiments.- C-15 A Nearly Free Electron Model of the BCS Gap Equation: Energy Gap Anisotropy in Gallium.- C-16 Specific Heat of Superconducting Zinc.- 7. Other Topics.- C-17 Unusual Resistance Effect Shown in a Periodic S-N-S System (Pb-Sn Lamellar Eutectic).- C-18 The Upper Critical Field of Superconducting Polysulfur Nitride, (SN)x - (Abstract only).- List of Participants.- Author Index.

Unusual effects of anisotropy on the specific heat of ceramic and single crystal MgB2

Abstract The two-gap structure in the superconducting state of MgB 2 gives rise to unusual thermodynamic properties which depart markedly from the isotropic single-band BCS model, both in their temperature- and field dependence. We report and discuss measurements of the specific heat up to 16 T on ceramic, and up to 14 T on single crystal samples, which demonstrate these effects in the bulk. The behavior in zero field is described in terms of two characteristic temperatures, a crossover temperature T c,π ≅13 K, and a critical temperature T c = T c,σ ≅38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field μ 0 H c2,π ≅0.35 T, and an anisotropic upper critical field with extreme values μ 0 H c2,σ, c ≅3.5 T and μ 0 H c2,σ, ab ≅19 T, where the indexes π and σ refer to the three-dimensional and two-dimensional sheets of the Fermi surface. Irradiation-induced interband scattering tends to move the gaps toward a common value, and increases the upper critical field up to ∼28 T when T c ≅30 K.

Anisotropic current flow and demagnetization corrections in the Bean model

Abstract In the present paper corrections to the standard Bean model are considered which turn out to be essential for the analysis of magnetization measurements on single crystals of high temperature superconductors. Because of the sensitivity of the Bean model to the actual geometrical shape of the superconductor in the plane perpendicular to the field direction, appropriate equations have to be employed. This becomes even more important if anisotropic current flow, e.g. in the ac plane of YBCO single crystals, is considered. In addition, demagnetization corrections of the measured magnetization curves are introduced. An analysis of experimental results based on these extensions of the standard Bean model shows that demagnetization corrections affect the j c values in the crystal by +21% to −6% in the worst case, whereas the inclusion of anisotropic current flow leads to an enhancement of current anisotropies by at least 100%.

Magnetic properties of fullerene superconductors

A review of the superconducting magnetic properties of doped fullerenes is presented. Experimental results on the main superconducting properties, such as the critical fields and the characteristic lengths, are critically discussed. Different methods to evaluate the lower critical field are discussed. Finally, experimental data on properties connected to flux pinning, such as the critical current density, the irreversibility line and the pinning force, are summarized and discussed.

Enhancement of transport critical current densities in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes at 77 K following fast neutron irradiation

We report on significant enhancements of the transport critical current densities Jc in (Bi,Pb)2Sr2Ca2Cu3O10/Ag tapes at 77 K and higher magnetic fields after neutron irradiation. This enhancement is attributed to an improvement in the flux pinning capability of this material by the neutron‐induced defects. The angular dependence of Jc is still consistent with two‐dimensionality, i.e., flux pinning of pancake and/or Josephson vortices is directly confirmed by this transport measurement. Fast neutron irradiation also affects the weak links in a way which agrees with previous results on unaligned ceramics. However, there are some peculiarities which need further investigation.

'Magnetoscan': a modified Hall probe scanning technique for the detection of inhomogeneities in bulk high temperature superconductors

We present a novel technique for the investigation of local variations of the critical current density in large bulk superconductors. In contrast to the usual Hall probe scanning technique, the sample is not magnetized as a whole before the scan, but locally by a small permanent magnet, which is fixed near the Hall probe, during the scanning process. The resulting signal can be interpreted as a qualitative measure of the local shielding currents flowing at the surface.

Mechanical behavior of the ITER TF model coil ground insulation system after reactor irradiation

Abstract The mechanical properties of glass fiber reinforced plastics (GFRPs) suggested for the turn and ground insulation of the ITER toroidal field (TF) coils are subject to extensive investigations with respect to their design requirements at present. The insulation system used for the ITER TF model coil, manufactured by European industry, consists of a boron-free R-glass fiber reinforced tape, vacuum-pressure impregnated in a DGEBA epoxy system and partly interleaved with polyimide-foils (e.g. Kapton-H-foils). In order to assess the material performance under the actual operating conditions of ITER-FEAT, the system was irradiated in the TRIGA reactor (Vienna, Austria) to neutron fluences of 5×1021 and 1×1022 m−2 (E>0.1 MeV). The composite was screened at 77 K using static tensile, short-beam-shear (SBS) as well as double-lap-shear tests prior to and after irradiation. Furthermore, tension–tension fatigue measurements were done in order to simulate the pulsed ITER-FEAT operation. We observe that the mechanical strength and the fracture behavior of these GFRPs after irradiation are strongly influenced by the three factors: the winding direction of the tape, the quality of fabrication and the delamination process.