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We have performed Co-nuclear quadrupole resonance (NQR) studies onNa$_{x}$CoO$_{2}\cdot y$H$_{2}$O compounds with different Na ($x$) and hydrate($y$) contents. Two samples with different Na contents but nearly the same$T_c$ values ($x$ = 0.348, $T_c$ = 4.7 K ; $x$ = 0.339, $T_c$ = 4.6 K) wereinvestigated. The spin-lattice relaxation rate $1/T_1$ in the superconducting(SC) and normal states is almost the same for the two samples except just above$T_c$. NQR measurements were also performed on different-hydrate-contentsamples with different $T_c$ values, which were prepared from the sameNa-content ($x$ = 0.348) sample. From measurements of $1/T_1$ using thedifferent-hydrate-content samples, it was found that a low-$T_c$ sample with$T_c = 3.9$ K has a larger residual density of states (DOS) in the SC state anda smaller increase of $1/T_1T$ just above $T_c$ than a high-$T_c$ sample with$T_c$ = 4.7 K. The former behavior is consistent with that observed inunconventional superconductors, and the latter suggests the relationshipbetween $T_c$ and the increase in DOS just above $T_c$. This increase, which isseemingly associated with the two-dimensionality of the CoO$_2$ plane, isconsidered to be one of the most important factors for the occurrence ofsuperconductivity.Comment: 5 pages, 5 figures, To be published in J. Phys. Soc. Jp

journal of the physical society of japan

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