⁵⁹Co, ²³Na, and ¹H NMR Studies of Double-Layer Hydrated Superconductors Na_xCoO₂ · yH₂O

We present our NMR studies of double-layer hydrated cobalt oxides NaxCoO2 center dot yH(2)O (x approximate to 0.35, y approximate to 1.3) with various T-c = 0-4.8 K and magnetic transition temperatures. High-resolution H-1 NMR spectrum served as an evidence for the existence of H3O+ oxonium ions. Na-23 nuclear spin-lattice relaxation rates served to detect local field fluctuations sensitive to T-c. Co-59 nuclear quadrupole resonance (NQR) spectra served to classify the various T-c samples. From the classification by Co-59 NQR frequency, the double-layer hydrated compounds were found to have two Superconducting phases closely located to a magnetic phase. In the normal state and at a magnetic field in the ab-plane, two Co-59 NMR signals with different Knight shifts and different Co-59 nuclear spin-lattice relaxation times T-59(1) were observed. The two Co-59 NMR signals suggest magnetic disproportionation of two Co sites or XY-anisotropy of a single Co site. Non Korringa behavior and power law behavior in zero-field NQR 1/T-59(1) above and below T-c suggest non-Fermi liquid and unconventional superconductivity.