$^{23}$Na NMR Study of Layered Superconductor $Na_xCoO_2- yH_2O$
  We measured $^{23}$Na-NMR spectra and nuclear spin-lattice relaxation rates $^{23}(1/T_1)$ of superconducting and non-superconducting bilayer hydrate $Na_xCoO_2-yH_2O$ (x ~ 0.3, y ~ 1.3). The central resonance frequency shows a small but various shift due to the difference in the shielding effect by intercalated $H_2O$ molecules. The different shielding effect also gives a large difference in the magnitude of $^{23}(1/T_1)$. 
H. Ohta, C. Michioka, Y. Itoh, K. Yoshimura, H. Sakurai, E.   Takayama-Muromachi, K. Takada and T. Sasaki