Contribution of electronic structure to the large thermoelectric power in layered cobalt oxides

With a strong help of high-resolution photoemission spectroscopy we demonstrate in this paper that the large thermoelectric power observed in the layered cobalt oxides, such as Ca_3Co_4O_9, Na_0.6CoO_2, and Bi_2Sr_2Co_2O_9, can be well accounted for with the Boltzmann-type metallic electrical conduction. An intense peak with 1.5–2 eV in width was observed in the photoemission spectra with its center at 1.0 eV below the Fermi level E_F in these compounds. The density of states at E_F is finite but negligibly small at room temperature, because E_F is located near the high-energy edge of this narrow band. We calculated thermoelectric power S using the Boltzmann transport equation with the electronic structure near E_F determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence.