  In the temperature (T)-nuQ3 phase diagram of NaxCoO2 \dot yH2O, there exist
two superconducting regions of nuQ3 separated by the nonsuperconducting region,
where nuQ3 is usually estimated from the peak position of the 59Co-NQR spectra
of the 5/2-7/2 transition and well-approximated here as nuQ3~3nuQ,nuQ being the
interaction energy between the nuclear quadrupole moment and the electric field
gradient. We have carried out measurements of the 59Co-NMR Knight shift (K) for
a single crystal in the higher-nuQ3 superconducting phase and found that K
begins to decrease with decreasing T at Tc for both magnetic field directions
parallel and perpendicular to CoO2-planes. The result indicates together with
the previous ones that the superconducting pairs are in the spin-singlet state
in both phases, excluding the possibility of the spin-triplet superconductivity
in this phase diagram. The superconductivity of this system spreads over the
wide nuQ3 regions, but is suppressed in the narrow region located at the middle
point of the region possibly due to charge instability.
