Spin-singlet superconductivity and antiferromagnetic correlations for the field-aligned powder of the triangular lattice NaxCoO2 · yH2O

Nuclear magnetic resonance and relaxation measurements of the 59Co nuclei for the well-defined field-aligned powder of the triangular lattice superconductor NaxCoO2 · yH2O with a nearly optimal composition for the transition temperature Tc 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 Tc. A small enhancement of two-dimensional antiferromagnetic spin correlations at some nonzero wave vector may emerge near Tc.