ARPES studies on NaxCoO2

We report systematic angle-resolved photoemission studies on Na$_{x}$CoO$_{2}$ single crystals for a wide range of Na concentrations. We observe a large Fermi surface centered at the $\Gamma $ point, which satisfies Luttinger theorem. However, the small Fermi surface pockets predicted by band theory near the K points are not observed. Instead, ``sinking islands'' with the binding energy of 100 -- 200 meV are observed. In addition, we observe a strong ``kink'' in the band dispersion at $\sim $ 70 meV binding energy, with weak dependence on momentum, doping, and temperature, suggesting a strong coupling between electrons and a collective mode, possibly an oxygen phonon mode.