Unconventional Superconductivity and Nearly Ferromagnetic Spin Fluctuations in Na_xCoO₂·yH₂O

Abstract

Co nuclear-quadrupole-resonance (NQR) studies were performed in the recently discovered superconductor Na_xCoO₂·yH₂O to investigate physical properties in the superconducting (SC) and normal states. Two samples from the same Na_xCoO₂ were examined, a SC bilayer-hydrate sample with T_c∼4.7 K and a non-SC monolayer-hydrate sample. From the measurement of nuclear-spin lattice relaxation rate 1⁄T₁ in the SC sample, it was found that the coherence peak is absent just below T_c and that 1⁄T₁ 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 χ_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.