$^{17}$O and $^{59}$Co NMR Studies of Strongly Correlated Electrons in Na$_{x}$CoO$_{2}$

The anomalous electronic properties of triangular-lattice system Na$_{x}$CoO$_{2}$ has been attracting strong interest over the last several years since the discovery of superconductivity in hydrated Na$_{1/3}$CoO$_{2}\cdot _{4/3}$[H$_{2}$O]. The electronic phase diagram of these materials is quite rich, as the physical properties depend very strongly on Na concentration. Here we report our $^{17}$O and $^{59}$Co NMR studies of the local electronic properties and low-frequency spin dynamics in these materials for a variety of Na concentrations [1,2]. \newline \newline [1] F.L. Ning, T. Imai, B.W. Statt, and F.C. Chou, PRL \underline {93} (2004) 237201.\newline [2] F.L. Ning and T. Imai, PRL \underline {94} (2005) 227004.