Co-NMR Knight Shift of Na_xCoO_2・yH_2O Studied in Both Superconducting Regions of the T_c-ν_<Q3> Phase Diagram Divided by the Nonsuperconducting Phase(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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In the temperature (T)-ν_<Q3> phase diagram of Na_xCoO_2・yH_2O, there exist two superconducting regions of ν_<Q3> separated by the nonsuperconducting region, where ν_<Q3> is usually estimated from the peak position of the ^<59>Co-NQR spectra of the ±5/2 ↔ ±7/2 transition and well-approximated here as ν_<Q3>≅3ν_Q, ν_Q being the interaction energy between the nuclear quadrupole moment and the electric field gradient. We have carried out measurements of the ^<59>Co-NMR Knight shift (K) for a single crystal in the higher-ν_<Q3> superconducting phase and found that K begins to decrease with decreasing T at T_c for both magnetic field directions parallel and perpendicular to CoO_2-planes. The result indicates together with the previous ones that the superconducting pairs are in the spin-singlet state in both phases, excluding the possibility of the spin-triplet superconductivity in this phase diagram. The superconductivity of this system spreads over the wide ν_<Q3> regions, but is suppressed in the narrow region located at the middle point of the region possibly due to charge instability.
社団法人日本物理学会の論文
2008-06-15