Electronic structure and thermoelectric properties of a layered cobalt oxide NaxCoO2 (0.5 ≪ x ≪ 0.8) investigated by angle-resolved photoemission spectroscopy

Angle resolved photoemission spectroscopy (ARPES) with a synchrotron radiation as an incident photon source was performed on the layered cobalt oxides Na_0.8CoO₂. The measured electronic structure was compared with that of the Na_0.7CoO₂ previously reported. The observed energy-momentum dispersion of the Na_0.8CoO₂ was essentially the same with that of the Na_0.7CoO₂ except for the small difference in the chemical potential Deltamu ~ 15 meV, which was caused by the difference in the sodium concentration. By employing the spectral conductivity reported for the Na_0.7CoO₂ and assuming the rigid band model, we calculated the thermoelectric power of the Na_0.8CoO₂. The measured thermoelectric power was quantitatively reproduced by the calculated one as well as that reported for the Na_0.7CoO₂. It is concluded that the scenario of the large thermoelectric power induced by the Boltzman-type electrical conduction in the characteristic bands previously proposed for the Na_0.7CoO₂ is applicable for the Na_0.8CoO₂.