Electronic structure and its contribution to the thermoelectric power of Ca3Co4O9 and NaxCoO2 layered cobalt oxides

Angle resolved photoemission spectroscopy (ARPES) with synchrotron radiation as an incident photon source was performed on the two different layered cobalt oxides, Ca₃Co₄O₉ and Na_0.6CoO₂. The energy-momentum dispersion was clearly observed in ARPES spectra, indicating the presence of extended and coherent Bloch states, and consequently the Boltzmann-type electrical conduction. The electronic structure near the Fermi level (E_F) in Ca₃Co₄O₉ was assigned not to be those from the Ca₂CoO₃ rock-salt layers but consisting of the a_1g and e´_g bands from the CoO₂ layers in the same manner as in Na_0.6CoO₂. The topology of the measured band was essentially the same with the calculated ones, but the energy width of the bands was greatly reduced to less than 60% of the calculated ones in both compounds most likely due to strong electron-correlation. The bilayer-splitting of the a_1g and e´_g bands was observed for Na_0.6CoO₂ in sharp contrast with its absence in Ca₃Co₄O₉. This difference is caused by the difference in nature of the interstitial layers; thin disordered Na layer in Na_0.6CoO₂ and thick insulating Ca₂CoO₃ rock-salt layer in Ca₃Co₄O₉. Making full use of the measured electronic structure, we succeeded in qualitatively accounting for mechanism of the coexistence of a metallic electrical conduction with a large thermoelectric power.