M. Zehetmayer

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.

Assessment of the local supercurrent densities in long superconducting coated conductors

The authors report on measurements of the local supercurrent density in long Y1Ba2Cu3O7−δ based coated conductors by the magnetoscan technique. Significant inhomogeneities were found, which are well resolved by the resulting magnetic field map. A single central line scan along the length of the conductor reflects the inhomogeneities over the entire width of the sample, thus offering the possibility of very fast characterization. Modifying the applied field leads to different results highlighting either the overall critical current or details of the defect structure. In addition, numerical simulations of the current dynamics were carried out for a qualitative and quantitative interpretation of the results.

Assessing the spatial and field dependence of the critical current density in YBCO bulk superconductors by scanning Hall probes

Although the flux density map of a bulk superconductor provides in principle sufficient information for calculating the magnitude and the direction of the supercurrent flow, the inversion of the Biot?Savart law is ill conditioned for thick samples, thus rendering this method unsuitable for state of the art bulk superconductors. If a thin (<1?mm) slab is cut from the bulk, the inversion is reasonably well conditioned and the variation of the critical current density in the sample can be calculated with adequate spatial resolution. Therefore a novel procedure is employed, which exploits the symmetry of the problem and solves the equations non-iteratively, assuming a planar thickness-independent current density. The calculated current density at a certain position is found to depend on the magnetic induction. In this way the average field dependence of the critical current density Jc(B) is also obtained at low fields, which is not accessible to magnetization measurements due to the self-field of the sample. It is further shown that an evaluation of magnetization loops, taking the self-field into account, results in a similar dependence in the field range accessible to this experiment.

Limitations for the trapped field in large grain YBCO superconductors

The actual limitations for the trapped field in YBa2Cu3O7?? (YBCO) monoliths are discussed. The influence of the sample geometry and of the critical current density on the trapped field is investigated by numerical calculations. The field dependence of the critical current density strongly influences the trapped field. A nonlinear relationship between the sample size, the critical current density and the resulting trapped field is derived. The maximum achievable trapped field in YBCO at 77?K is found to be around 2.5?T. This limit is obtained for reasonable geometries and high but realistic critical current densities. Such high fields have not been reached experimentally so far, due to non-optimized flux pinning and material inhomogeneities. These inhomogeneities can be directly assessed by the magnetoscan technique, and their influence is discussed. Significant differences between the a-?and the c-growth sectors were found. Limitations due to cracks and non-superconducting inclusions (e.g.?211 particles) are estimated and found to be candidates for variations of Jc on a millimetre length scale, as observed in experiments.

Connectivity and critical currents in polycrystalline MgB2

Current transport in polycrystalline magnesium diboride is highly non-uniform (percolative) due to the presence of secondary phases and also due to the intrinsic anisotropy of the material. The influence of secondary phases on the transport properties of MgB2 was investigated. Bulk samples were prepared from a mixture of MgB2 and MgO powders by the ex situ technique in order to vary the MgO content systematically. The samples were characterized by resistive and magnetization measurements. The reduced MgB2 fraction is modeled by a reduced effective cross section (connectivity), which was assessed directly by the experiments. The presence of MgO also increases the percolation threshold, which reduces the zero resistivity (or irreversibility) field.

Changes in the transition temperature after irradiation and annealing in single crystalline YBa2Cu3O7−δ

Abstract We present a model for the defect dynamics in YBa 2 Cu 3 O 7− δ crystals elucidating the changes in the transition temperature after neutron irradiation and subsequent annealing. These changes are found to be closely related to the calculated concentration of vacancies at the Cu–O chain sites.

Interaction of vortices in anisotropic superconductors with isotropic defects

To assess a superconductors suitability for high-current applications, it is imperative to know how its critical current density () depends on magnetic field and its orientation. We present a comprehensive study of the anisotropy of the in-plane in iron-based superconducting single crystals carried out by an advanced magnetometry technique. As is governed by material defects capable of vortex pinning, we investigated three samples with three essentially different defect landscapes representing weak, strong and intermediate pinning. In the latter case, a second maximum in the field dependence of the critical current (fishtail effect) occured, allowing us to investigate the influence of this effect on the angular dependence of in an intermediate pinning regime for the first time in a quantitative way. For weak and strong pinning, the influence of the field orientation on the in-plane can be described by the anisotropic scaling theory. For weak pinning, we find the predicted scaling for the field () and for the in-plane current (unity). For strong pinning, we find the same scaling for the field, but show for the first time that an additional scaling factor () emerges for the in-plane critical current density. We attribute this new scaling rule (double scaling) to the presence of defects which are larger than the vortex core.

Simulation of the current dynamics in a superconductor induced by a small permanent magnet: application to the magnetoscan technique

The magnetoscan is a rather new technique for investigating local inhomogeneities of large bulk high-Tc superconductors. Local (shielding) currents, which are induced by a small permanent magnet slightly above the sample surface, generate a magnetic field that is recorded by a Hall probe. We report on a theoretical analysis of such a three-dimensional system using numerical simulations to calculate the current dynamics during a magnetoscan. The magnetoscan signal is analysed for different experimental setups and different critical current densities of the superconductor. We show that the shape of small defects (~1 mm in size) is almost exactly displayed, even if the radius of the permanent magnet is much larger than the defect size. The critical current density of larger inhomogeneities can be directly determined at the sample surface, if all details of the experimental setup are known. Different aspects of how to improve the experimental resolution are discussed.

Critical current anisotropy in Nd-1111 single crystals and the influence of neutron irradiation

We report on angle-resolved magnetization measurements on NdFeAsO0.65F0.35 ( Nd-1111) single crystals. The field dependence of the critical current density, Jc, is non-monotonic in these crystals at all orientations and temperatures due to the fishtail effect, which strongly influences the angular dependence of Jc. The currents decrease as the field is tilted from the crystallographic c-axis at low fields, but increase at high fields. A peak occurs in the angular dependence of Jc at intermediate fields. The critical currents are significantly enhanced after irradiation with fast neutrons and the fishtail disappears. The different current anisotropies at low and high fields, however, persist. We discuss the data in the framework of the anisotropic scaling approach and propose a transition from dominant pinning by large defects of low density at low fields to pinning by small defects of high density at high fields in the pristine crystal. Strong pinning dominates at all fields after the irradiation, and the angular dependence of Jc can be described via anisotropic scaling only after an appropriate extension to this pinning regime.

How the macroscopic current correlates with the microscopic flux-line distribution in a type-II superconductor: an experimental study

We present a study of the real-space flux-line lattice (FLL) of pristine and neutron irradiated conventional type-II superconductors using scanning tunnelling microscopy. Our work is focused on the magnetic field range, where the critical current density shows a second peak as a result of neutron irradiation. Scanning tunnelling microscopy images, including more than 2000 flux lines, are used to evaluate various microscopic parameters describing the disorder of the FLL, such as the defect density, the nearest neighbour distances and correlation functions. These parameters are compared with the macroscopic critical current density of the samples. The results show a direct correlation of the micro- and macroscopic properties. We observe a clear transition from an ordered to a disordered lattice at the onset of the second peak. Moreover, we discuss the defects of the FLL and their accumulation to large clusters in the second peak region.