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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 -- | | Y.Ihara
; H.Takeya
; K.Ishida
; H.Ikeda
; C.Michioka
; K.Yoshimura
; K.Takada
; T.Sasaki
; H.Sakurai
; E.Takaayama-Muromachi
; | | Date: |
8 Jun 2006 | | Subject: | Superconductivity; Strongly Correlated Electrons | | Abstract: | A Co nuclear-quadrupole-resonance (NQR) measurement was performed on various bilayered hydrate Na_x(H_3O)_zCoO_xcdot yH_2O with different values of the superconducting and magnetic-ordering temperatures, T_c and T_M. From measurements of the temperature and sample dependence of the NQR frequency, it was revealed that the NQR frequency is changed due to the change of the electric field gradient (EFG) along the c-axis
u_zz rather than the asymmetry of EFG within the ab-plane. In addition, it is considered that the change of
u_zz is determined mainly by the trigonal distortion of the CoO_2 block layers along the c axis from the relationships between
u_zz and the various physical parameters. We found a tendency that the samples with
u_zz larger than 4.2 MHz show magnetic ordering, whereas the samples with lower
u_zz show the superconductivity. We measured nuclear spin-lattice relaxation rate 1/T_1 in these samples, and found that the magnetic fluctuations depend on sample. The higher-T_c sample has a stronger magnetic fluctuations at T_c. From the relationship between
u_zz and T_c or T_M, we suggest that the NQR frequency is regarded as a tuning parameter to determine the ground state of the system, and develop the phase diagram using the
u_zz. The phase diagram we developed shows that the highest-T_c sample is located at the point where T_M is considered to be zero, and suggests that the superconductivity is induced by the quantum critical fluctuations. We strongly suggest that the hydrate cobaltate superconductor is an example of magnetic-fluctuation-mediated superconductivity argued in the heavy-fermion compounds. The coexistence of the superconductivity and magnetism observed in the sample with the highest
u_zz is also discussed on the basis of our experiments. | | Source: | arXiv, cond-mat/0606212 | | Services: | Forum | Review | PDF | Favorites | |
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