We report a systematic angle-resolved photoemission study on ${\mathrm{Na}}_x{\mathrm{CoO}}_2$ for a wide range of Na concentrations $(0.3\le x\le 0.72)$. In all the metallic samples at different $x$, we observed (i) only a single holelike Fermi surface centered around $\Gamma$ and (ii) its area changes with $x$ according to the Luttinger theorem. We also observed a surface state that exhibits a larger Fermi surface area. The $e_g^{\prime }$ band and the associated small Fermi surface pockets near the $K$ points predicted by band calculations are found to sink below the Fermi energy in a manner almost independent of the doping and temperature.

• Received 17 January 2005

DOI:https://doi.org/10.1103/PhysRevLett.95.146401