Influence of the upper critical field anisotropy on the transport properties of polycrystalline MgB2_{2}

The intrinsic properties of MgB$_2$ form the basis for all applications of this superconductor. We wish to emphasize that the application range of polycrystalline MgB$_2$ is limited by the upper critical field H$_{c2}$ and its anisotropy. In wires or tapes, the MgB$_2$ grains are randomly oriented or only slightly textured and the anisotropy of the upper critical field leads to different transport properties in different grains, if a magnetic field is applied and the current transport becomes percolative. The irreversibility line is caused by the disappearance of a continuous superconducting current path and not by depinning as in high temperature superconductors. Based on a percolation model, we demonstrate how changes of the upper critical field and its anisotropy and how changes of flux pinning will influence the critical currents of a wire or a tape. These predictions are compared to results of neutron irradiation experiments, where these parameters were changed systematically

Neutron irradiation of coated conductors

Various commercial coated conductors were irradiated with fast neutrons in order to introduce randomly distributed, uncorrelated defects which increase the critical current density, Jc, in a wide temperature and field range. The Jc-anisotropy is significantly reduced and the angular dependence of Jc does not obey the anisotropic scaling approach. These defects enhance the irreversibility line in not fully optimized tapes, but they do not in state-of-the-art conductors. Neutron irradiation provides a clear distinction between the low field region, where Jc is limited by the grain boundaries, and the high field region, where depinning leads to dissipation

Critical currents in weakly textured MgB2: Nonlinear transport in anisotropic heterogeneous media

A model for highly non-linear transport in heterogeneous media consisting of anisotropic particles with a preferred orientation is proposed and applied to the current transport in weakly textured magnesium diboride, MgB2. It essentially explains why, unlike in conventional superconductors, a significant macroscopic anisotropy of the critical currents can be induced by the preparation of MgB2 tapes. The field and angular dependence of the critical current is calculated for various degrees of texture and compared to experimental data

Disorder effects on the superconducting properties of BaFe1.8_{1.8}Co0.2_{0.2}As2_2 single crystals

Single crystals of superconducting BaFe$_{1.8}$Co$_{0.2}$As$_2$ were exposed to neutron irradiation in a fission reactor. The introduced defects decrease the superconducting transition temperature (by about 0.3 K) and the upper critical field anisotropy (e.g. from 2.8 to 2.5 at 22 K) and enhance the critical current densities by a factor of up to about 3. These changes are discussed in the context of similar experiments on other superconducting materials

Anisotropic critical currents in FeSe0.5Te0.5 films and the influence of neutron irradiation

We report on measurements of the superconducting properties of FeSe05Te05 thin films grown on lanthanum aluminate. The films have high transition temperatures (above 19 K) and sharp resistive transitions in fields up to 15 T. The temperature dependence of the upper critical field and the irreversibility lines are steep and anisotropic, as recently reported for single crystals. The critical current densities, assessed by magnetization measurements in a vector VSM, were found to be well above 10^9 Am-2 at low temperatures. In all samples, the critical current as a function of field orientation has a maximum, when the field is oriented parallel to the film surface. The maximum indicates the presence of correlated pinning centers. A minimum occurs in three films, when the field is applied perpendicular to the film plane. In the forth film, instead, a local maximum caused by c-axis correlated pinning centers was found at this orientation. The irradiation of two films with fast neutrons did not change the properties drastically, where a maximum enhancement of the critical current by a factor of two was found

Application prospects of MgB2_2 in view of its basic properties

Seven years after the discovery of superconductivity in magnesium diboride, the fundamental superconducting properties of this compound are well known and the peculiar current transport in polycrystalline materials is essentially understood. Based on this knowledge the ultimate performance of wires or tapes at high magnetic fields will be predicted and compared to state-of-the-art materials and to other superconductors. The key parameter for high field applications is the upper critical field, which can be strongly enhanced by impurity scattering. This fundamental property might be further optimized in bulk materials, since higher values were reported for thin films. The MgB$_2$ grains are usually very small, if prepared by the in-situ technique. The resulting high density of grain boundaries leads to strong pinning, close to the theoretical limit. On the other hand, the connectivity between the grains is still rather poor and strongly reduces the achievable critical currents, thus leaving room for further improvements