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

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  • Ihara Y.
    Department of Physics, Graduate School of Science, Kyoto University
  • Takeya H.
    Department of Physics, Graduate School of Science, Kyoto University
  • Ishida K.
    Department of Physics, Graduate School of Science, Kyoto University International Innovation Center, Kyoto University
  • Ikeda H.
    Department of Physics, Graduate School of Science, Kyoto University
  • Michioka C.
    Department of Chemistry, Graduate School of Science, Kyoto University
  • Yoshimura K.
    Department of Chemistry, Graduate School of Science, Kyoto University
  • Takada K.
    Nanoscale Materials Center, National Institute for Materials Science
  • Sasaki T.
    Nanoscale Materials Center, National Institute for Materials Science
  • Sakurai H.
    Advanced Nano Materials Laboratory, National Institute for Materials Science
  • Takayama-Muromachi E.
    Advanced Nano Materials Laboratory, National Institute for Materials Science

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  • 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—
  • Unconventional Superconductivity Induced by Quantum Critical Fluctuations in Hydrate Cobaltate Na<sub><i>x</i></sub>(H<sub>3</sub>O)<sub><i>z</i></sub>CoO<sub>2</sub>·<i>y</i>H<sub>2</sub>O –Relationship between Magnetic Fluctuations and Superconductivity Revealed by Co Nuclear Quadrupole Resonance–

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Co nuclear-quadrupole-resonance (NQR) measurements were performed on various bilayered hydrate cobaltate Nax(H3O)zCoO2·yH2O with different values of the superconducting and magnetic-ordering temperatures, Tc and TM, 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 νzz rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of νzz is gaverned mainly by the trigonal distortion of the CoO2 block layers along the c axis, from the relationships between νzz and the various physical parameters. We found the tendency that samples with νzz larger than 4.2 MHz show magnetic ordering, whereas samples with lower νzz show superconductivity. We measured the nuclear spin–lattice relaxation rate 1⁄T1 in these samples, and found that magnetic fluctuations depend on samples. The higher-νzz sample has stronger magnetic fluctuations at Tc. From the relationship between νzz and Tc or TM, 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 νzz. This phase diagram shows that the highest-Tc sample is located at the point where TM 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 νzz is also discussed on the basis of the results of our experiments.

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