Hydration-induced anisotropic spin fluctuations in Na_xCoO ₂ 1.3H₂O superconductor

We report ⁵⁹Co NMR studies in single crystals of the cobalt oxide superconductor Na_0.42CoO₂1.3H₂O (T _c=4.25 K) and its parent compound Na_0.42CoO₂. We find that both the magnitude and the temperature (T) dependence of the Knight shifts are identical in the two compounds above T_c. The spin-lattice relaxation rate (1/T₁) is also identical above T₀∼60 K for both compounds. Below T₀, the unhydrated sample is found to be a non-correlated metal that well conforms to the Fermi liquid theory, while spin fluctuations develop in the superconductor. These results indicate that water intercalation does not change the density of states at the Fermi level or the carrier density but its primary role is to bring about spin fluctuations. Our result shows that, in the hydrated superconducting compound, the electron correlation is anisotropic. Namely, the spin fluctuation around the finite wave vector is much stronger along the a-axis direction than that along the c-axis direction.