Fermi Surface ofNaxCoO2

Doping evolution of the Fermi surface topology of ${\mathrm{N}\mathrm{a}}_x{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ is studied systematically. Both local density approximation (LDA) and local spin density approximation (LSDA) predict a large Fermi surface as well as small hole pockets for doping levels $x\sim 0.5$. In contrast, the hole pockets are completely absent for all doping levels within $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$. More importantly, we find no violation of Luttinger's rule in this system. The measured Fermi surface of ${\mathrm{N}\mathrm{a}}_{0.7}{\mathrm{C}\mathrm{o}\mathrm{O}}_2$ can be explained by its half-metallic behavior and agrees with our $\mathrm{L}\mathrm{S}\mathrm{D}\mathrm{A}+U$ calculations.