Probing defect ordering in the Curie-Weiss metallic phase of NaxCoO2*

Single crystals of Na$_{2/3}$CoO$_{2}$, Na$_{2/3}$CoO$_{1.98}$, and Na$_{0.71}$CoO$_{2}$, which are metallic Curie-Weiss paramagnets, have been investigated by nuclear magnetic resonance (NMR) techniques, in order to clarify the Na atomic ordering among these samples. By analyzing the $^{23}$Na and $^{59}$Co NMR spectra, we confirm that the Na vacancies arrange orderly in Na$_{2/3}$CoO$_{1.98}$ and Na$_{0.71}$CoO$_{2}$, so that a superlattice structure is formed due to such Na ordering. In addition, the oxygen vacancies in Na$_{2/3}$CoO$_{1.98}$ can be located by the NMR spectra. As for the Na$_{2/3}$CoO$_{2}$ single crystal, a long-range Na order is not observed.