Data taken by various kinds of methods including NMR/NQR and neutron scattering are presented for Na x CoO 2 , in particular, for Na 0.5 CoO 2 . Attentions have also been paid to the x dependence of the electronic nature. The pseudo-gap-like behavior observed in the region of x

Layered Co oxide Na x CoO 2 ÁyH 2 O with the superconducting transition temperature T c ¼ 4:5 K has been studied by 59 Co-NMR. The Knight shift K estimated from the observed spectra of a powder sample exhibits almost temperature (T)-independent behavior above T c and decreases with decreasing T below T c . This result and the existence of the coherence peak in the spin-lattice relaxation rate versus T curve reported by the present authors indicate, naively speaking, that the singlet order parameter with s-wave symmetry is realized in Na x CoO 2 ÁyH 2 O. Differences in the observed behaviors between the spectra of a nonaligned sample and those of an aligned sample embedded in epoxy adhesive by applying an external magnetic field are discussed.

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Superconducting transition temperature T c of Na z Co 1-x M x O 2 ⋅yH 2 O has been studied for M=Ir and Ga, which can substitute for Co. It has been found that T c is suppressed by the M-atom doping. The decreasing rates |dT c /dx| are of the order of 1 K/% and too small to be explained by the pair breaking mechanism for the anisotropic order parameters by non-magnetic impurities. Brief arguments are given on possible origins of the T c suppression.