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 ${\mathrm{Ca}}_{3}{\mathrm{Co}}_{4}{\mathrm{O}}_{9},$ ${\mathrm{Na}}_{0.6}{\mathrm{CoO}}_{2},$ and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Co}}_{2}{\mathrm{O}}_{9},$ 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 ${E}_{\mathrm{F}}$ in these compounds. The density of states at ${E}_{\mathrm{F}}$ is finite but negligibly small at room temperature, because ${E}_{\mathrm{F}}$ 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 ${E}_{\mathrm{F}}$ determined by the photoemission measurement. The calculated S shows fairly good consistency with the measured value both in its magnitude and the temperature dependence.