Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in NaxCoO₂·yH₂O

Co NQR studies were performed in recently discovered superconductor NaₓCoO₂· yH_2$O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na$_x$CoO$_2$ were examined, SC bilayer-hydrate sample with $T_c ~ 4.7$ K and non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate $1/T_1$ in the SC sample, it was found that the coherence peak is absent just below $T_c$ and that $1/T_1$ is proportional to temperature far below $T_c$. These results, which are in qualitatively agreement with the previous result by Fujimoto { et al.}, suggest strongly that unconventional superconductivity is realized in this compound. In the normal state, 1/T₁T of the SC sample shows gradual increase below 100K down to Tc, whereas 1/T₁T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T₁T and χbulk in the SC sample, the increase of 1/T₁T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related with the unconventional superconductivity in this compound. The implication of this finding is discussed.