Relationship between magnetic fluctuations and superconductivity in Nax(H3O)zCoO2·yH2O revealed by a Co nuclear quadrupole resonance

Nuclear quadrupole frequency ( ν Q ) and spin-lattice relaxation rate 1/ T 1 of Co were measured on various bilayered hydrate (BLH) Na x CoO 2 · y H 2 O ( y ∼ 1.3) with different values of superconducting (SC) and magnetic-transition temperatures, T c and T M , together with non-SC monolayered hydrate (MLH) Na x CoO 2 · y H 2 O ( y ∼ 0.7). In the temperature range above 70 K, 1/ T 1 T in all samples follows the similar temperature dependence as each other. In the BLH compounds, 1/ T 1 T is enhanced with decreasing temperature below 70 K, and the values of 1/ T 1 T at T c are larger in the higher- ν 3 sample, where ν 3 is the peak frequency of the NQR spectrum arising from ±5/2 ↔ ±7/2 transitions. The magnetic ordering is indicated from the prominent divergence of 1/ T 1 T at T M and the presence of the internal field at the Co nuclear site in the samples with ν 3 higher than 12.5 MHz. We analyze the temperature dependence of 1/ T 1 T in all samples on the basis of identical formula, and discuss the relationship between magnetic fluctuations and superconductivity in the BLH cobaltate.