  Due to their graphene-like properties after oxygen reduction, incorporation
of graphene oxide (GO) sheets into correlated-electron materials offers a new
pathway for tailoring their properties. Fabricating GO nanocomposites with
polycrystalline MgB2 superconductors leads to an order of magnitude enhancement
of the supercurrent at 5 K/8 T and 20 K/4 T. Herein, we introduce a novel
experimental approach to overcome the formidable challenge of performing
quantitative microscopy and microanalysis of such composites, so as to unveil
how GO doping influences the structure and hence the material properties. Atom
probe microscopy and electron microscopy were used to directly image the GO
within the MgB2, and we combined these data with computational simulations to
derive the property-enhancing mechanisms. Our results reveal synergetic effects
of GO, namely, via localized atomic (carbon and oxygen) doping as well as
texturing of the crystals, which provide both inter and intra granular flux
pinning. This study opens up new insights into how low-dimensional
nanostructures can be integrated into composites to modify the overall
properties, using a methodology amenable to a wide range of applications.
