^<59>Co Nuclear Quadrupole Resonance Studies of Superconducting and Nonsuperconducting Bilayer Water Intercalated Sodium Cobalt Oxides Na_xCoO_<2・y>H_2O(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We report ^<59>Co nuclear quadrupole resonance (NQR) studies of bilayer water intercalated sodium cobalt oxides Na_xCoO_2・yH_2O (BLH) with the superconducting transition temperatures, 2K < T_c ≤ 4.6K, as well as a magnetic BLH sample without superconductivity. We obtained a magnetic phase diagram of T_c and the magnetic ordering temperature T_M against the peak frequency v_3 of the ^<59>Co NQR transition I_z=±5/2←→±7/2 and found a dome-shaped superconducting phase. The ^<59>Co NQR spectrum of the nonsuperconducting BLH shows a broadening below T_M without the critical divergence of 1/T_1 or 1/T_2, suggesting an unconventional magnetic ordering. The degree of enhancement of 1/T_1T at low temperatures increases with the increase of v_3 though the optimal v_3 of approximately 12.30MHz. In the Na_xCoO_2・yH_2O system, the optimal-T_c superconductivity emerges close to the magnetic instability. T_c is suppressed near the phase boundary at v_3〜12.50MHz, which is not a conventional magnetic quantum critical point.
社団法人日本物理学会の論文
2006-06-15