Unconventional Superconductivity Induced by Quantum Critical Fluctuations in Hydrate Cobaltate Nax(H3O)zCoO2·yH2O –Relationship between Magnetic Fluctuations and Superconductivity Revealed by Co Nuclear Quadrupole Resonance–

Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Na x (H 3 O) z CoO 2 · y H 2 O with different values of the superconducting and magnetic-ordering temperatures, T c and T M , respectively. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed by the change of the electric field gradient (EFG) along the c axis ν z z rather than the asymmetry of EFG within the a b -plane. In addition, it is considered that the change of ν z z is gaverned mainly by the trigonal distortion of the CoO 2 block layers along the c axis, from the relationships between ν z z and the various physical parameters. We found the tendency that samples with ν z z larger than 4.2 MHz show magnetic ordering, whereas samples with lower ν z z show superconductivity. We measured the nuclear spin–lattice relaxation rate 1/ T 1 in these samples, and found that magnetic fluctuations depend on samples. The higher-ν z...