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

Nuclear quadrupole frequency (nu(Q)) and spin-lattice relaxation rate 1/T-1 of Co were measured on various bilayered hydrate (BLH) NaxCoO2 center dot yH(2)O (y similar to 1.3) with different values of superconducting (SC) and magnetic-transition temperatures, T-c and T-M, together with non-SC monolayered hydrate (MLH) NaxCoO(2) center dot yH(2)O (y similar to 0.7). In the temperature range above 70 K, 1/T1T in all samples follows the similar temperature dependence as each other. In the BLH compounds, 1/T1T is enhanced with decreasing temperature below 70 K, and the values of 1/T1T at T-c are larger in the higher-nu(3) sample, where nu(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/T1T at T-M and the presence of the internal field at the Co nuclear site in the samples with nu(3) higher than 12.5 MHz. We analyze the temperature dependence of 1/T1T in all samples on the basis of identical formula, and discuss the relationship between magnetic fluctuations and superconductivity in the BLH cobaltate. (C) 2007 Elsevier B.V. All rights reserved.