Structural origin of long-range proximity effect in highly disordered fractal MgO/MgB2 nanocomposites: Roles of interface, geometry, and defect

The emergence of global phase coherence due to the 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 the 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 carriers in the MgO-rich regions, leading to the observed long-range proximity effect and the resulting bulk-like superconductivity in this highly disordered system.