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Co nuclear-quadrupole-resonance (NQR) measurements were performed on variousbilayered hydrate cobaltate Na_x(H_3O)_zCoO_2\cdot yH_2O with different valuesof the superconducting and magnetic-ordering temperatures, T_c and T_M,respectively. From measurements of the temperature and sample dependence of theNQR frequency, it was revealed that the NQR frequency is changed by the changeof the electric field gradient (EFG) along the c axis \nu_{zz} rather than theasymmetry of EFG within the ab-plane. In addition, it is considered that thechange of \nu_{zz} is gaverned mainly by the trigonal distortion of the CoO_2block layers along the c axis, from the relationships between \nu_{zz} and thevarious physical parameters. We found the tendency that samples with \nu_{zz}larger than 4.2 MHz show magnetic ordering, whereas samples with lower \nu_{zz}show superconductivity. We measured the nuclear spin-lattice relaxation rate1/T_1 in these samples, and found that magnetic fluctuations depend on samples.The higher-\nu_{zz} sample has stronger magnetic fluctuations at T_c. From therelationship between \nu_{zz} and T_c or T_M, we suggest that the NQR frequencycan be regarded as a tuning parameter to determine the ground state of thesystem, and develop the phase diagram using \nu_{zz}. This phase diagram showsthat the highest-T_c sample is located at the point where T_M is considered tobe zero, which suggests that the superconductivity is induced by quantumcritical fluctuations. We strongly advocate that the hydrate cobaltatesuperconductor presents an example of the magnetic-fluctuation-mediatedsuperconductivity argued in the heavy-fermion compounds. The coexistence ofsuperconductivity and magnetism observed in the sample with the highest\nu_{zz} is also discussed on the basis of the results of our experiments.Comment: 11 pages, 14 figure

journal of the physical society of japan

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–