Electronic structure and its contribution to the thermoelectric power of Ca/sub 3/Co/sub 4/O/sub 9/ and Na/sub x/CoO/sub 2/ layered cobalt oxides

摘要

Angle resolved photoemission spectroscopy (ARPES) with synchrotron radiation as an incident photon source was performed on the two different layered cobalt oxides, Ca/sub 3/Co/sub 4/O/sub 9/ and Na/sub 0.6/CoO/sub 2/. The energy-momentum dispersion was clearly observed in ARPES spectra, indicating the presence of extended and coherent Bloch states, and consequently the Boltzmann-type electrical conduction. The electronic structure near the Fermi level (E/sub F/) in Ca/sub 3/Co/sub 4/O/sub 9/ was assigned not to be those from the Ca/sub 2/CoO/sub 3/ rock-salt layers but consisting of the a/sub 1g/ and e'/sub g/ bands from the CoO/sub 2/ layers in the same manner as in Na/sub 0.6/CoO/sub 2/. The topology of the measured band was essentially the same with the calculated ones, but the energy width of the bands was greatly reduced to less than 60% of the calculated ones in both compounds most likely due to strong electron-correlation. The bilayer-splitting of the a/sub 1g/ and e'/sub g/ bands was observed for Na/sub 0.6/CoO/sub 2/ in sharp contrast with its absence in Ca/sub 3/Co/sub 4/O/sub 9/. This difference is caused by the difference in nature of the interstitial layers; thin disordered Na layer in Na/sub 0.6/CoO/sub 2/ and thick insulating Ca/sub 2/CoO/sub 3/ rock-salt layer in Ca/sub 3/Co/sub 4/O/sub 9/. Making full use of the measured electronic structure, we succeeded in qualitatively accounting for mechanism of the coexistence of a metallic electrical conduction with a large thermoelectric power.