59Co-NMR Knight Shift of the Superconducting NaxCoO2.yH2O

Layered Co oxide NaxCoO2.yH2O with the superconducting transition temperature Tc =4.5 K has been studied by 59Co-NMR. The Knight shift K estimated from the observed spectra for powder sample exhibits almost temperature(T)-independent behavior above Tc and decreases with decreasing T below Tc. 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 NaxCoO2.yH2O. Differences of the observed behaviors between the spectra of the non-aligned sample and the one aligned in epoxy adhesive by applying the external magnetic field are discussed.