Observation of Bulk Superconductivity in ${\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{o}\mathrm{O}}_2·y{\mathrm{H}}_{\mathrm{2}}\mathrm{O}$ and ${\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{o}\mathrm{O}}_2·y{\mathrm{D}}_2\mathrm{O}$ Powder and Single Crystals

Poly- and single-crystalline ${\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ has been successfully intercalated with ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{D}}_2\mathrm{O}$ as confirmed by x-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with $T_c$ close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on $T_c$ of less than 0.2 K. Investigation of the resistivity anisotropy of ${\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{o}\mathrm{O}}_2·y{\mathrm{H}}_2\mathrm{O}$ single crystals shows (a) almost zero resistivity below $T_c$, and (b) an abrupt upturn at $T^{*}\sim 52\mathrm{K}$ in both the $ab$ plane and the $c$ direction. The implications of our results on the possible superconducting mechanism will be discussed.