Unconventional superconductivity near quantum critical point revealed by Co-NQR measurements on Nax(H3O)zCoO2·yH2ONax(H3O)zCoO2·yH2O

The nuclear quadrupole resonance (NQR) frequency of Co nuclei and the nuclear spin–lattice relaxation rate 1/T11/T1 were measured on several bilayered hydrate (BLH) Nax(H3O)zCoO2·yH2ONax(H3O)zCoO2·yH2O (y∼1.3y∼1.3) with variety of superconducting (SC) and magnetic transition temperatures, TcTc and TMTM, together with non-SC mono-layered hydrate (MLH) Nax(H3O)zCoO2·yH2ONax(H3O)zCoO2·yH2O (y∼0.7y∼0.7). In the high temperature region above 70 K, 1/T1T1/T1T in all the samples follows the same temperature dependence which is interpreted as the pseudogap behavior. In the BLH compounds, 1/T1T1/T1T increases with decreasing temperature below 70 K, and the values of 1/T1T1/T1T at TcTc are large in high-TcTc samples. The magnetic ordering is ascertained from the observation of the prominent divergence of 1/T1T1/T1T at TMTM in the samples whose NQR frequency is higher than 12.5 MHz. The temperature dependence of 1/T1T1/T1T is found to be consistently expressed by a unique function with two fitting parameters. We analyze the temperature dependence of 1/T1T1/T1T on the basis of this function, and investigate the relationship between the magnetic fluctuations and superconductivity in the BLH compounds.