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

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 degrees 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.