Evidence for transition of Fermi-surface topology in highly doped Na _xCoO₂

We have performed angle-resolved photoemission spectroscopy of Na _xCoO₂ (x=0.65 and 0.77) to clarify the origin of antiferromagnetic (AF) transition. We determined the electronic band structure along the out-of-plane direction by varying photon energy, and revealed that a three-dimensional electron pocket emerges at Γ point in the AF sample (x=0.77) while it is absent in the non-AF counterpart (x=0.65), showing the transition of Fermi-surface topology with the band filling. The present result suggests that the emergence of the electron pocket is closely related to the various anomalies of the physical properties in Na-rich cobaltates.