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Abstract: Nuclear magnetic resonance and relaxation measurements of the 59Co nuclei for the well-defined field-aligned powder of the triangular lattice superconductor Na x CoO2 · yH2O with a nearly optimal composition for the transition temperature T c have been performed. Detailed analyses indicate that the Knight shifts for the directions parallel and perpendicular to the CoO2 plane and the spin-lattice relaxation rates, taken by making an inevitable deterioration of specimen minimal, are significantly smaller than most of the data reported to date, and that this compound is classified into one of the unconventional spin-singlet superconductors as suggested by recent works for single crystals with a lower T c. A small enhancement of two-dimensional antiferromagnetic spin correlations at some nonzero wave vector may emerge near T c. [Copyright &y& Elsevier]

The Li de-intercalation effects for the trimerized state of the geometrically frustrated triangular lattice LiV O(2) are investigated through measurements of x-ray diffraction, magnetic susceptibility and magic-angle spinning-nuclear magnetic resonance (MAS-NMR). Li(1-x)V O(2) with 0 ≤ x ≤ 0.14 obtained with a soft-chemistry synthesis is a single phase. All of the data are understood by considering that the partial substitution for V(3+) (spin-1) with V(4+) (spin-½) leads to the coexistence of the spin-singlet and spin-½ trimers.