NMR studies of Na$_{x}$CoO$_{2}$

Sodium cobaltate Na$_{x}$CoO$_{2}$ is the parent phase of triangular-lattice superconductor Na$_{1/3}$CoO$_{2}$-[H$_{2}$O]$_{4/3}$. Co ions take a mixed-valence state of +4-x in this system. Since Co$^{4+}$ and Co$^{3+}$ possess spin S=1/2 and S=0, respectively, one may view the CoO$_{2}$ layers as charge doped S=1/2 triangular-lattice. CoO$_{2}$ layers exhibit a rich variety of strongly correlated electron behavior as a function of sodium concentration x, ranging from itinerant antiferromagnet (x$\sim $0.84 and 0.5), ``Curie-Weiss metal'' (x$\sim $0.71) to Fermi liquid (x$\sim $1/3). In this talk, I will discuss our NMR studies of Na$_{x}$CoO$_{2}$ for various Na concentration x, with particular emphasis on $^{59}$Co evidence for charge ordering in the insulating ground state of Na$_{0.5}$CoO$_{2}$ [1-4]. \newline \newline [1] F.L. Ning et al., arXiv : 0711.4023 \newline [2] F.C. Chou et al., arXiv : 0709.0085 \newline [3] F.L. Ning et al., PRL 94, 227004 (2005) \newline [4] F.L. Ning et al., PRL 93, 237201 (2004)