Pronounced increase of the thermopower in $\rm Na_xCoO_2$ in the large-$x$ regime ($x\ge$0.75)

We report for the first time the systematic transport study on high doped Na$_x$CoO$_2$ over series of samples which lie ($ 0.75 \leq x \leq 1$). We discovered two distinctive regions of phase diagram exist in high Na doping level : one is characterized by largely enhanced thermopower (200 -- 300 $\mu$V/K at $\sim$ 130K) yet highly metallic resistivity in $ 0.75 \alt x \alt 0.88$ (region I). The other (region II) is emerged in $0.89 \alt x \alt 0.97$ , in which the electronic phase is mixed with the region I phase and $x=1$ of non-conducting end compound and the mixture seems to occur mainly along the layer. We will discuss the newly revised phase diagram of the high Na part and speculate on a relation to the characteristics of Na layer.