Disorder anisotropy of layered structure in multi-band MgB2 superconducting materials with high critical current performance
Journal of Alloys and Compounds
Layered crystal structures of various materials form through strong in-plane covalent and weaker out-of-plane bonding. The different bonding states can lead to the appearance of anisotropies not only of electronic/electrical and magnetic properties but also of structural disorder. A deeper understanding of the disorder anisotropy is essential to carry out structural modification and to enhance the material properties. However, in the case of multi-band MgB2 superconducting materials that have layered structures, including graphene-like and six-membered rings, the nature and extent of the disorder anisotropy are not well understood. Also unknown is the influence on the transport critical current performance under magnetic fields in terms of charge-carrier scattering and vortex pinning. Herein, we have investigated the disorder anisotropy to reveal the relation with the in-field superconductivity. The MgB2 phase formed by appropriate sintering conditions with carbon doping for high transport critical current performance exhibited a small anisotropy in the strain distribution and a large anisotropy in the crystallite size. The anisotropic behavior reflects small out-of-plane domains of crystallites with the strain distribution. The disordered formation may be the reason why the π band is usually dirtier than the σ band. In contrast, although the strain distribution in the in-plane structural state can be selectively tuned by carbon doping, the in-plane crystal growth is still considerably large. Such in-plane crystallization has shortcomings in terms of scattering and pinning. We therefore argue that further selective modification of the disordered structure, especially for the in-plane size properties, is a practical approach to achieve enhancement beyond the currently attainable transport performance.
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