Na content dependence of superconductivity and the spin correlations in Na_xCoO₂·1.3H₂O

We report systematic measurements using the ⁵⁹Co nuclear quadrupole resonance (NQR) technique on the cobalt oxide superconductors Na _xCoO₂·1.3H₂O over a wide Na content range x ≤ 0.25-0.34. We find that T_c increases with decreasing x but reaches a plateau for x≤0.28. In the sample with x∼0.26, the spin-lattice relaxation rate 1/T₁ shows a T³ variation below T_c and down to T∼T_c/6, which unambiguously indicates the presence of line nodes in the superconducting (SC) gap function. However, for larger or smaller x,1/T₁ deviates from the T³ variation below T∼2 K even though the T_c (∼4.7 K) is similar, which suggests an unusual evolution of the SC state. In the normal state, the spin correlations at a finite wavevector become stronger upon decreasing x, and the density of states at the Fermi level increases with decreasing x, which can be understood in terms of a single-orbital picture suggested on the basis of LDA calculation.