Angle-resolved photoemission study of the doping evolution of a three-dimensional Fermi surface in Na_xCoO₂

We have performed angle-resolved photoemission spectroscopy of layered cobalt oxide Na_xCoO₂ for a wide doping range (0.35 ≤ x ≤ 0.85) with synchrotron radiation, and determined the doping dependence of the Fermi surface and band structure. The two-dimensional (2D) cylindrical a_1g Fermi surface at x = 0.35 gradually increases the dimensionality upon Na doping and eventually transforms into a 3D-like Fermi surface at x = 0.77, indicating the strong inter-layer coupling between adjacent CoO2 layers in the highly doped region. We found that the characteristic shape of the top of the a_1g band is closely related to the various anomalies of physical properties in Na_xCoO₂.