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We report systematic measurements using the ^{59}Co nuclear quadrupoleresonance(NQR) technique on the cobalt oxide superconductors Na_{x}CoO_{2}\cdot1.3H_{2}O over a wide Na content range x=0.25\sim 0.34. We find that T_cincreases with decreasing x but reaches to a plateau for x \leq0.28. In thesample with x \sim 0.26, the spin-lattice relaxation rate 1/T_1 shows a T^3variation below T_c and down to T\sim T_c/6, which unambiguously indicates thepresence of line nodes in the superconducting (SC) gap function. However, forlarger or smaller x, 1/T_1 deviates from the T^3 variation below T\sim 2 K eventhough the T_c (\sim 4.7 K) is similar, which suggests an unusual evolution ofthe SC state. In the normal state, the spin correlations at a finite wavevector become stronger upon decreasing x, and the density of states at theFermi level increases with decreasing x, which can be understood in terms of asingle-orbital picture suggested on the basis of LDA calculation.Comment: version published in J. Phys. Condens. Matter (references updated and  more added

Na content dependence of superconductivity and the spin correlations in NaxCoO2·1.3H2O

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