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.

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The hysteresis exhibited by ceramic superconductors can be modeled using a phenomenological critical current density of the form Jc=α/(B0+B) with suitable extensions. At low temperatures (near 4 K), satisfactory results can be obtained using this form and neglecting the equilibrium magnetization. However, we show that at higher temperatures the incorporation of the equilibrium magnetization becomes more important. At 77 K, the commonly observed shape of the curves is dominated by the equilibrium magnetization. An approximate model including the equilibrium magnetization is described and compared to measurements made at 77 K.