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(H3O)(z)CoO2 center dot yH(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 upsilon(zz) rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of upsilon(zz) is gaverned mainly by the trigonal distortion of the CoO2 block layers along the c axis, from the relationships between v,., and the various physical parameters. We found the tendency that samples with upsilon(zz) larger than 4.2 MHz show magnetic ordering, whereas samples with lower vzz show superconductivity. We measured the nuclear spin-lattice relaxation rate l/T-1 in these samples, and found that magnetic fluctuations depend on samples. The higher-v, sample has stronger magnetic fluctuations at T-c. From the relationship between upsilon(zz) and T-c or T-M, we suggest that the NQR frequency can be regarded as a tuning parameter to determine the ground state of the system, and develop the phase diagram using upsilon(zz). This phase diagram shows that the highest-T-c sample is located at the point where T-M is considered to be zero, which suggests that the superconductivity is induced by quantum critical fluctuations. We strongly advocate that the hydrate cobaltate superconductor presents an example of the magnetic-fluctuation-mediated superconductivity argued in the heavy-fermion compounds. The coexistence of superconductivity and magnetism observed in the sample with the highest upsilon(zz) is also discussed on the basis of the results of our experiments.