Strong phase coherence and vortex matter in a fractal system with proximity-induced superconductivity

概要

We investigate vortex matter in a proximity-coupled Mg/MgO/MgB₂ nanocomposite with -30 vol. % of MgB₂ using magneto-optical imaging, scanning superconducting quantum interface device microscopy, and pinning-force analysis. This nanocomposite was prepared by spark plasma sintering (SPS) of the MgO/MgO/MgB₂ mixture powders obtained from a solid-phase reaction between Mg and B₂O₃ [Uchino et al., Phys. Rev. B 101, 035146 (2020)]. The resulting nanocomposite is characterized by scale-free (or fractal) distributions of MgB₂ components and the atomically clean MgO/MgB₂ interfaces. It is also found that high SPS temperature (1200∘C) is the key to improve the proximity-induced superconducting properties of the Mg/MgO/MgB₂ nanocomposite. This proximity-coupled system acts as a fully phase-coherent superconductor with isotropic pinning and strong superfluid phase stiffness irrespective of the low volume fraction of MgB₂. Our results demonstrate that in contrast to the case of conventional granular superconductors, the grain boundaries in the present system carry high critical currents and have high vortex-pinning efficiency, implying an excellent phase-coherent capability of the proximity-coupled fractal network.

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