  Recently, interest in Superconductor (S)-Normal (N) interfaces was renewed by
the observation of exotic proximity effects in various systems, including
S/semiconductor, S/ferromagnet, and S/topological insulator. In general, the
proximity effect is enhanced in transparent weak links where coherent Andreev
reflection is possible. Also, it is a common knowledge that the proximity
effect is, by definition, is a localized phenomenon that can only be active in
each S/N interface region. However, here we show that a three-dimensional (3D)
macroscale proximity effect is realized in few-micrometer-thick MgO/Mg2Si/MgB2
nanocomposite layers with atomically smooth and clean heterointerfaces. We
found from scanning superconducting quantum interference device (SQUID)
microscopy measurements that a normal region of more than 100x100 square
micrometers totally undergoes transition into a bulk-like superconducting state
although the normal host originally contains less than ~10 vol % of
superconducting MgB2 nanograins in a dispersed manner. In the proximity-induced
superconducting region, vortex formation and annihilation processes as well as
vortex-free Meissner regions were observed with respect to applied fields in a
similar manner as Abrikosov vortices in type-II superconductors. Furthermore,
we found that the induced superconducting layers exhibit an anisotropic
magnetization behavior, in consistent with the formation of the large-scale
superconducting coherence. This unusually extended proximity effect suggests
that disorder-induced interaction and coupling of Andreev bound states, which
are coherent superposition of time reversed electron hole pairs, is realized in
the nanocomposite. Thus, the present results not only expand the limit of the
proximity effect to bulk scales, but also provides a new route to obtain a
proximity-induced superconducting state from disordered systems.
