23Na NMR Studies of Non-superconducting and Superconducting Na0.35CoO2-1.3H2O (Tc < 1.8 and 4.5 K)

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We report 23Na NMR studies of the polycrystalline samples of bilayer hydrated cobalt oxides Na0.35CoO2-1.3H2O with the superconducting transition temperatures Tc < 1.8 and 4.5 K. The magnitude of the 23Na nuclear spin-lattice relaxation rate 23(1/T1) significantly decreased from non-hydrated Na0.7CoO2 to the bilayer hydrates. We found that the temperature dependence of 23(1/T1) of the non-superconducting bilayer hydrate is similar to that of the non-hydrated Na0.7CoO2, the spin dynamics of which is understood by A-type (intra-layer ferromagnetic and inter-layer antiferromagnetic) spin fluctuations. The superconducting phase is located close to the A-type magnetic instability.