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

TL;DR

This paper investigates how the anisotropy of the upper critical field in polycrystalline MgB$_2$ affects its transport properties, using a percolation model and neutron irradiation experiments to understand limitations in applications.

Contribution

It introduces a percolation-based model linking upper critical field anisotropy and flux pinning to transport properties in MgB$_2$, supported by experimental data.

Findings

Upper critical field anisotropy impacts critical current in MgB$_2$.

Neutron irradiation modifies flux pinning and anisotropy.

Transport properties are governed by percolation thresholds.

Abstract

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.