Large thermoelectric power generated by the van Hove singularity in Na x CoO 2
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1044-U02-07
Large thermoelectric power generated by the van Hove singularity in NaxCoO2 Tsunehiro Takeuchi1,2, and Syuhei Kuno3 1 EcoTopia Science Institute, Nagoya University, Nagoya, 464-8603, Japan 2 Department of Crystalline Materials Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603, Japan 3 Department of Crystalline Materials Science, Nagoya University, Nagoya, 464-8603, Japan ABSTRACT Mechanism leading to the large thermoelectric power and metallic electrical conduction observed in NaxCoO2 was investigated by means of angle resolved photoemission spectroscopy and the Bloch-Boltzmann theory. As a result of thermoelectric power calculation using the experimentally determined electronic structure under the assumptions of rigid band and constant mean free path, we found that the electrical conduction of NaxCoO2 were accounted for with the Boltzmann transport equation over a wide carrier-concentration range. Analysis using the simplest tight-binding bands revealed that the two-dimensional hexagonal lattices including the crystalline structure of the present NaxCoO2 produce a characteristic spectral conductivity leading to the large thermoelectric power and metallic electrical conduction. INTRODUCTION Layered cobalt oxides, such as NaxCoO2, Bi2Sr2Co2O9, and Ca3Co4O9, were found to possess large thermoelectric power and metallic electrical conduction. [1-3] Those properties are two of the three necessities of thermoelectric materials. The layered cobalt oxides, therefore, were widely considered as promising candidates for the practical thermoelectric materials. Surprisingly, the carrier concentration of these materials were distributed over 1021~1022 cm-3 in the same manner as that in typical metallic phases possessing very small thermoelectric power less than 10 µV / K . Thus a large number of attentions have been focused on the mechanism leading to the thermoelectric power exceeding 100 µV / K with the large carrier concentration. Two possible mechanisms for the large thermoelectric power were frequently discussed; the band structure oriented mechanism [4,5] and entropy induced one [6]. Positive temperature coefficient with rather small magnitude of the electrical resistivity [1-3] observed in these cobalt oxides seems to lend a support to the former mechanism. The former scenario is easily confirmed if the electronic structure of the layered cobalt oxides is precisely determined. Therefore we employed, in our previous work, the high-resolution angle resolved photoemission spectroscopy measurements on the Na0.7CoO2 [7] and investigated its energy-momentum ( ε − k ) dispersion near the Fermi level ( ε F ) in detail. By using the experimentally determined electronic structure and the Bloch-Boltzmann theory, we found that the large thermoelectric power with the metallic electrical conduction is brought about by the well defined Bloch states with a unique spectral conductivity σ (ε ) characterized by the large peak just below ε F . In order to further confirm the scenario of “electronic structure ind
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