59Co-NMR Knight Shift of Superconducting NaxCoO2·yH2O

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Abstract
Layered Co oxide Na x CoO 2 · y H 2 O with the superconducting transition temperature T c =4.5 K has been studied by 59 Co-NMR. The Knight shift K estimated from the observed spectra of a powder sample exhibits almost temperature ( T )-independent behavior above T c and decreases with decreasing T below T c . This result and the existence of the coherence peak in the spin–lattice relaxation rate versus T curve reported by the present authors indicate, naively speaking, that the singlet order parameter with s -wave symmetry is realized in Na x CoO 2 · y H 2 O. Differences in the observed behaviors between the spectra of a nonaligned sample and those of an aligned sample embedded in epoxy adhesive by applying an external magnetic field are discussed.

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