Unconventional Superconductivity Induced by Quantum Critical Fluctuations in Hydrate Cobaltate Na$_{x}$(H$_3$O)$_{z}$CoO$_{x}\cdot$ $y$H$_{2}$O -- Relationship between Magnetic Fluctuations and the Superconductivity Revealed by a Co Nuclear Quadrupole Resonance --

摘要

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.