  Layered Co oxide NaxCoO2.yH2O with a superconducting transition temperature
Tc =4.5 K has been studied by 59Co NQR. The nuclear spin relaxation rate 1/59T1
is nearly proportional to temperature T in the normal state. In the
superconducting state, it exhibits the coherence peak and decreases with
decreasing T below ~0.8Tc. Detailed comparison of the 1/T1T values and the
magnetic susceptibilities between NaxCoO2.yH2O and NaxCoO2 implies that the
metallic state of the former system is closer to a ferromagnetic phase than
that of the latter. These experimental results impose a restriction on the
mechanism of the superconductivity.

Superconductivity;Strongly Correlated Electrons