Electronic structure and large thermoelectric power in Ca ₃Co₄O₉

Resonant photoemission spectroscopy, soft X-ray emission spectroscopy, soft X-ray absorption spectroscopy, and high-resolution (ΔE ≈ 10 meV) ultraviolet photoemission spectroscopy were performed on a layered cobalt oxide, Ca₃Co₄O₉, which has attracted interests as one of the potential thermoelectric materials because of its possession of high thermoelectric power S, relatively low electrical resistivity ρ, and small thermal conductivity κ. A narrow band of 1.5-2eV in width was observed in the photoemission spectra with its center at 1.0eV below the Fermi level (E _F). This peak turns out to be less significant when Co 2p-3d resonance takes place, indicating that it consists mainly of O 2p and of small amount of Co 3d component. Since E_F is located near the high-energy edge of this narrow band, the density of states at E_F is finite but negligibly small at room temperature. An energy-gap across E_F opens below 50K with decreasing temperature. This development of the energy-gap causes the insulating behavior; divergence both in electrical resistivity and Hall coefficient. We calculated thermoelectric power S(T) using the photoemission spectra near E_F. The calculated S(T) shows almost half of the measured value. Large thermoelectric power up to 200 μV/K observed for the Ca₃Co₄O₉ is closely related to the metallic electron transport in a less dispersive band with E_F near its band-edge.