Angle-resolved photoemission study of the doping evolution of a three-dimensional Fermi surface in Na_<i>x</i>CoO₂

Abstract

We have performed angle-resolved photoemission spectroscopy of layered cobalt oxide NaxCoO2 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 a1g 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 a1g band is closely related to the various anomalies of physical properties in NaxCoO2.