arXiv subjects

M. Eisterer

Publications and source records attributed to M. Eisterer.

36 records · Page 2Linked to original sources

arXiv2009

Numerical extension of the power law Jc(B) to zero field in thin superconducting films

Numerical simulations of the current and field distribution in thin superconducting films are carried out for a given material law Jc(B) and as a function of the applied field H, taking the sample's self-field into account. The dependence of the critical current density on the applied field Jc(H) is computed for comparison with experiment, considering the geometry of transport measurements. We show that extrapolating the high field power law Jc \propto B^{-α} to the lowest fields results in a finite critical current at zero applied field Jc(H=0), despite the singularity of Jc(B). Moreover, particular features of the experiment, such as a low field plateau in Jc(H), are reproduced and found to be determined by the self-field.

cond-mat.supr-con↗

arXiv2009

Disorder Induced Effects on the Critical Current Density of Iron Pnictide BaFe_1.8 Co_0.2 As_2 single crystals

Investigating the role of disorder in superconductors is an essential part of characterizing the fundamental superconducting properties as well as assessing potential applications of the material. In most cases, the information available on the defect matrix is poor, making such studies difficult, but the situation can be improved by introducing defects in a controlled way, as provided by neutron irradiation. In this work, we analyze the effects of neutron irradiation on a Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ single crystal. We mainly concentrate on the magnetic properties which were determined by magnetometry. Introducing disorder by neutron irradiation leads to significant effects on both the reversible and the irreversible magnetic properties, such as the transition temperature, the upper critical field, the anisotropy, and the critical current density. The results are discussed in detail by comparing them with the properties in the unirradiated state.

cond-mat.supr-con↗

arXiv2009

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.

cond-mat.supr-con↗

arXiv2009

Consequences of the peculiar intrinsic properties of MgB2 on its macroscopic current flow

The influence of two important features of magnesium diboride on the macroscopic transport properties of polycrystalline MgB2 is discussed in the framework of a percolation model. While two band superconductivity does not have significant consequences in the field and temperature range of possible power applications, the opposite is true for the anisotropy of the upper critical field. The field dependence of the critical current densities strongly increases and the macroscopic supercurrents disappear well below the apparent upper critical field. The common scaling laws for the field dependence of the volume pinning force are altered and Kramer's plot is no longer linear, although grain boundary pinning dominates in nearly all polycrystalline MgB2 conductors. In contrast to the conventional superconductors NbTi and Nb3Sn, a significant critical current anisotropy can be induced by the preparation technique of MgB2 tapes.

cond-mat.supr-con↗

arXiv2009

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.

cond-mat.supr-con↗

arXiv2009

Disorder effects on the superconducting properties of BaFe$_{1.8}$Co$_{0.2}$As$_2$ 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.

cond-mat.supr-con↗

arXiv2009

Neutron Irradiation of Sm-1111

SmFeAsO$_{1-x}$F$_x$ was irradiated in a fission reactor to a fast (E > 0.1 MeV) neutron fluence of 4x10^${21}$ m$^{-2}$. The introduced defects increase the normal state resistivity due to a reduction in the mean free path of the charge carriers. This leads to an enhancement of the upper critical field at low temperatures. The critical current density within the grains, Jc, increases upon irradiation. The second maximum in the field dependence of Jc disappears and the critical current density becomes a monotonically decreasing function of the applied magnetic field.

cond-mat.supr-con↗

arXiv2008

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 z-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 obtained also at low fields, which is not accessible to magnetisation measurements due to the self-field of the sample. It is further shown that an evaluation of magnetisation loops, taking the self-field into account, results in a similar dependence in the field range accessible to this experiment.

cond-mat.supr-con↗

arXiv2008

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.

cond-mat.supr-con↗

arXiv2008

Application prospects of MgB$_2$ 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.

cond-mat.supr-con↗

arXiv2005

Influence of the upper critical field anisotropy on the transport properties of polycrystalline MgB$_{2}$

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.

cond-mat.supr-con↗

arXiv2004

Significance of solutions of the inverse Biot-Savart problem in thick superconductors

The evaluation of current distributions in thick superconductors from field profiles near the sample surface is investigated theoretically. A simple model of a cylindrical sample, in which only circular currents are flowing, reduces the inversion to a linear least squares problem, which is analyzed by singular value decomposition. Without additional assumptions about the current distribution (e.g. constant current over the sample thickness), the condition of the problem is very bad, leading to unrealistic results. However, any additional assumption strongly influences the solution and thus renders the solutions again questionable. These difficulties are unfortunately inherent to the inverse Biot-Savart problem in thick superconductors and cannot be avoided by any models or algorithms.

cond-mat.supr-con↗

arXiv2004

Influence of disorder on Hc2-anisotropy and flux pinning in MgB2

The upper critical field and flux pinning in MgB2 single crystals were investigated. The implications of these properties for technical applications are discussed and compared with transport properties of polycrystalline bulk samples and wires. In these untextured materials current percolation is important, especially at high magnetic fields. It is shown that the anisotropy of the upper critical field influences the "irreversibility line" and that the application range of MgB2 is limited by the smallest upper critical field (i.e., for the field direction perpendicular to the boron planes). Disorder, introduced by irradiation with neutrons, enhances the upper critical field, reduces the anisotropy and drastically changes flux pinning. While the enhanced Hc2 and the reduced anisotropy generally improve the transport properties of the polycrystalline samples, the contribution of the radiation-induced defects to flux pinning is small compared to the as-grown defect structure (grain boundary pinning).

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arXiv2003

"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.

cond-mat.supr-con↗

arXiv2003

Current percolation and anisotropy in polycrystalline MgB$_2$

The influence of anisotropy on the transport current in MgB$_2$ polycrystalline bulk samples and wires is discussed. A model for the critical current density is proposed, which is based on anisotropic London theory, grain boundary pinning and percolation theory. The calculated currents agree convincingly with experimental data and the fit parameters, especially the anisotropy, obtained from percolation theory agree with experiment or theoretical predictions.

cond-mat.supr-con↗

arXiv2002

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

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 Tc_pi ~ 13 K, and a critical temperature Tc = Tc_sigma ~ 38 K, whereas the mixed-state specific heat requires three characteristic fields, an isotropic crossover field Hc2_pi ~ 0.35 T, and an anisotropic upper critical field with extreme values Hc2_sigma_c ~ 3.5 T and Hc2_sigma_ab ~ 19 T, where the indexes πand σrefer to the 3D and 2D 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 Tc = 30 K.

cond-mat.supr-con↗

arXiv2002

Mixed state properties of superconducting MgB2 single crystals

We report on measurements of the magnetic moment in superconducting MgB2 single crystals. We find μ_0H_{c2}^c(0) = 3.2 T, μ_0H_{c2}^{ab}(0) = 14.5 T, γ= 4.6, μ_0H_c(0) = 0.28 T, and κ(T_c) = 4.7. The standard Ginzburg-Landau and London model relations lead to a consistent data set and indicate that MgB2 is a clean limit superconductor of intermediate coupling strength with very pronounced anisotropy effects.

cond-mat.supr-con↗

arXiv2001

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,a)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.

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