59Co NQR study on superconducting NaxCoO2•yH2O

Layered Co oxide NaxCoO2.yH(2)O with a superconducting transition temperature T-c = 4.5 K has been studied by Co-59 NQR. The nuclear spin relaxation rate 1/T-59(1), 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 similar to0.8T(c). Detailed comparison of the 1/T1T values and the magnetic susceptibilities between NaxCoO2.yH(2)O 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.