We report the spin susceptibility in the superconducting state of the cobalt oxide superconductor NaxCoO2 center dot 1.3H(2)O, via the (CO)-C-59 Knight shift measurements. Along both a- and c-axis directions, the shift decreases below T-c. At T = 1.7 K, the shift along the a-axis, K-a increases with increasing the strength of magnetic field (H), following a root H variation. Together with the T-3 dependence of the spin-lattice relaxation rate, 1/T-1, these results indicate unambiguously that the Cooper pairs are in the spin-singlet form and that there exist nodes in the gap function. The data are most consistent with a d-wave symmetry. (C) 2007 Elsevier B.V. All rights reserved.
