Unconventional superconductivity and nearly ferromagnetic spin fluctuations in NaxCoO2•γH2O

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor NaxCoO2-yH(2)O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same NaxCoO2 were examined, a SC bilayer-hydrate sample with T-c similar to 4.7 K and a 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 qualitative 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 a gradual increase below 100 K down to T-c, whereas 1/T1T of the non-SC sample shows the Korringa behavior in this temperature range. From the comparison between 1/T1T and chi(bulk) in the SC sample, the increase in 1/T1T is attributed to nearly ferromagnetic fluctuations. These remarkable findings suggest that the SC sample possesses nearly ferromagnetic fluctuations, which are possibly related to the unconventional superconductivity in this compound. The implication of this finding is discussed.