Angle-resolved photoemission spectroscopy study on the Fermi surface topology of NaxCoO2

We report a systematic study on Nax CoO2 (0.3 x 0.72) using angle-resolved photoemission spectroscopy (ARPES), with a focus on the Fermi surface topology of the metallic samples. Within this doping range, we observed only one large hexagonal Fermi surface around the -point, which comes from the a1g band. Contrary to most band calculations, our results show that the e′g bands never cross the Fermi level to form small Fermi surface pockets near the K-point. The enclosed area of the a1g Fermi surface is found to be consistent with the Luttinger theorem within the doping range in this study. We also found that, at x = 1/3, the Fermi surface coincides with the zone boundary of the 3 × √3 commensurate ordering, indicating the importance of charge fluctuations in this material. (Some figures in this article are in colour only in the electronic version) 1