Correlation between Superconducting Transition TemperatureTcand Increase of Nuclear Spin–Lattice Relaxation Rate Devided by Temperature 1/T₁TatTcin the Hydrate Cobaltate NaxCoO₂·yH₂O

We have performed Co-nuclear quadrupole resonance (NQR) studies on NaₓCoO₂· yH₂O compounds with different Na (x) and hydrate (y) contents. Two samples with different Na contents but nearly the same Tc values (x = 0.348, Tc = 4.7 K ; x = 0.339, Tc = 4.6 K) were investigated. The spin-lattice relaxation rate 1/T₁ in the superconducting (SC) and normal states is almost the same for the two samples except just above Tc. NQR measurements were also performed on different-hydrate-content samples with different Tc values, which were prepared from the same Na-content (x = 0.348) sample. From measurements of 1/T₁ using the different-hydrate-content samples, it was found that a low-Tc sample with Tc = 3.9 K has a larger residual density of states (DOS) in the SC state and a smaller increase of 1/T₁T just above Tc than a high-Tc sample with Tc = 4.7 K. The former behavior is consistent with that observed in unconventional superconductors, and the latter suggests the relationship between Tc and the increase in DOS just above Tc. This increase, which is seemingly associated with the two-dimensionality of the CoO₂ plane, is considered to be one of the most important factors for the occurrence of superconductivity.