Epitaxial Film Growth and Superconducting Behavior of Sodium−Cobalt Oxyhydrate, NaxCoO2·yH2O (x ∼ 0.3, y ∼ 1.3)

We have developed a unique multistep film growth technique, combining reactive solid-phase epitaxy (R-SPE) with an intercalation process, to fabricate epitaxial films of superconducting sodium-cobalt oxyhydrate, Na(0.3)CoO2.1.3H2O. An epitaxial film of Na(0.8)CoO2 grown on an alpha-Al2O3(0001) substrate by R-SPE was subjected to oxidation and hydration treatment, leading to the formation of a Na(0.3)CoO2.1.3H2O epitaxial film. The film exhibited metallic electrical resistivity with a superconducting transition at 4 K, similar to that of bulk single crystals. The present technique is suitable and probably the only method for the epitaxial growth of superconducting Na(0.3)CoO2.1.3H2O.