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 Tc 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 Tc and down to T∼Tc/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 Tc (∼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.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics