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 Ca₃Co₄O₉, Na0.6CoO₂, and Bi₂Sr₂Co₂O₉, 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 EF in these compounds. The density of states at EF is finite but negligibly small at room temperature, because EF 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 EF determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence.