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The emergence of global phase coherence due to proximity effect in heterogeneous and disordered superconductor systems has been an issue of long-standing interest. Recently, we have reported that a highly disordered fractal MgO/MgB2 nanocomposite exhibits bulk-like superconducting properties with isotropic pinning, showing an excellent phase-coherent capability irrespective of the low volume fraction (~30 vol. %) of MgB2 [Uchino et al., Phys. Rev. B 101, 035146 (2020); Teramachi et al,, Phys. Rev. B 108, 155146 (2023)]. In this work, we show from 3D focused ion beam scanning electron microscopy data that in the nanocomposite, a complex MgO/MgB2 microstructure spreads isotropically throughout the sample with a constant fractal dimension of ~1.67. Atomic-resolution scanning transmission electron microscopy has revealed that the interfaces are atomically clean and free from amorphous grain boundaries. Detailed ac susceptibility measurements have demonstrated a smooth crossover from an intragranular to an intergranular superconducting regime. Also, spatially-resolved cathodoluminescence measurements have demonstrated that oxygen vacancies in the MgO-rich phase tend to aggregate near the MgO/MgB2 boundary regions, forming long channels of oxygen vacancies through the nanocomposite. These channels of oxygen vacancies are likely to be responsible for the long-range carrier transfer and the related proximity effect via coherent tunneling of charge carriers among the oxygen vacancy sites. Our results imply that the fractal-like MgO/MgB2 microstructure with atomically clean interfaces will induce the phase coherent transport of charge carries in the MgO-rich regions, leading to the observed long-range proximity effect and the resulting bulk-like superconductivity in this highly disordered system.

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• Creative Commons BY Attribution 4.0 International
  

  Copyright 2025 Author(s). This article is distributed under a Creative Commons Attribution (CC BY) License.

Keyword: nanocomposite, MgO/MgB2, Defects

Date published: 2025-10-28

Publisher: AIP Publishing

Journal:

• Journal of Applied Physics (ISSN: 00218979) vol. 138 issue. 16 163902

Funding:

• Ministry of Education, Culture, Sports, Science and Technology JPMXP1223NM0163, JPMXP1223MS1024 and JPMXP1224MS1017 (ARIM)

Manuscript type: Author's version (Accepted manuscript)

MDR DOI: https://doi.org/10.48505/nims.6042

First published URL: https://doi.org/10.1063/5.0293448

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Updated at: 2025-12-19 12:01:04 +0900

Published on MDR: 2025-12-19 20:30:49 +0900