Electronic structure and large thermoelectric power in Ca 3Co 4O 9

Abstract

Resonant photoemission spectroscopy, soft X-ray emission spectroscopy, soft X-ray absorption spectroscopy, and high-resolution (Δ E≈10 meV) ultraviolet photoemission spectroscopy were performed on a layered cobalt oxide, Ca 3Co 4O 9, 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 1.5–2 eV in width was observed in the photoemission spectra with its center at 1.0 eV 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 50 K 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 Ca 3Co 4O 9 is closely related to the metallic electron transport in a less dispersive band with E F near its band-edge.