Na Nmr Study of Non-superconducting Double-layer Hydrate Na X Coo 2 · Yh 2 O

We report Na NMR studies of the polycrystalline samples of double-layer hydrated cobalt oxides NaxCoO2 · yH2O (x ∼035 and y ∼ 1.3) with the superconducting transition temperatures Tc <1.8 K and ∼4.5 K, and the dehydrated NaxCoO2 (x ∼0.35). The hyperfine field and the electric field gradient at the Na sites in the non-hydrated Na0.7CoO2 and the dehydrated Na0.35CoO2 are found to be significantly reduced by the hydration, which indicates a strong shielding effect of the intercalated water molecules on the Na sites. The temperature dependence of Na nuclear spin-lattice relaxation rate 1/23T1 of the non-superconducting double-layer hydrate NaxCoO2 ·yH2O is found to be similar to that of the non-hydrated Na0.7CoO2, whose spin dynamics is understood by A-type (intra-layer ferromagnetic and inter-layer antiferromagnetic) spin fluctuations. The superconducting phase is located close to the quantum critical point with the A-type magnetic instability.