Observation of Bulk Superconductivity in NaxCoO2.yH2O and NaxCoO2.yD2O Powder and Single Crystals

Poly- and single-crystalline Na${}_x$CoO${}_2$ has been successfully intercalated with H${}_2$O and D${}_2$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 Na${}_x$CoO${}_2\cdot$yH${}_2$O single crystals shows: (a) almost zero resistivity below T${}_c$, and (b) an abrupt upturn at $T^{\ast }\sim$ 52 K in both the $ab$ plane and the $c$ direction. The implications of our results on the possible superconducting mechanism will be discussed.