NMR and neutron scattering studies on NaxCoO2·yH2O

NMR/NQR, neutron scattering, and various other kinds of measurements have been carried out for hydrated Co oxides Na x CoO 2 . yH 2 O(or Na x CoO 2 . yD 2 O), and the mother system Na x CoO 2 by using both polycrystalline and single-crystal samples, to clarify the symmetry of the order parameter or the microscopic origin of the superconductivity (SC). The NMR Knight shifts are suppressed by the occurrence of the SC for both magnetic field directions, parallel and perpendicular to the CoO 2 planes, indicating that the Cooper pairs are in the singlet state. Inelastic neutron scattering data of Na 0.44 CoO 2 indicate the existence of weak spin fluctuations at Q points in the reciprocal space corresponding to both intra-plane and inter-plane magnetic correlations, while no indication of the ferromagnetic fluctuations has been detected in the inelastic scattering data taken for large single crystals of the superconducting Na x CoO 2 . yD 2 O. The magnetic structure determined in the magnetically ordered phase of Na 0.5 CoO 2 indicates that the magnetic correlation is basically antiferromagnetic within a CoO 2 plane. The rate of the T c -suppression by non-magnetic impurities is too small to be explained by considering isotropic order parameters. All these results indicate that the singlet SC is realized in Na x CoO 2 . yH 2 O. Only the result that the curve of the nuclear relaxation rate 1/ T 1 plotted against T does not have a definite coherence peak, cannot simply be explained by the isotropic order parameter.