Contribution of electronic structure to the large thermoelectric power in layered cobalt oxides

Abstract

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 Ca 3 Co 4 O 9 , Na 0.6 CoO 2 , and Bi 2 Sr 2 Co 2 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 F in these compounds. The density of states at E F is finite but negligibly small at room temperature, because E 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 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.

Role of Temperature Dependent Chemical Potential on Thermoelectric Power

“…The validity of this argument is further confirmed from the consistency between the calculated S %" F ðTÞ and the measured one for the various materials in our previous papers. [4][5][6][7][8][9][10][11] Those materials indeed show the magnitude of thermoelectric power less than 100 mV/K in the whole temperature range of measurement.…”

“…The former assumption is often used in theoretical calculations, [18][19][20] and the latter assumption was experimentally confirmed to be acceptable in some materials in the vicinity of " F . [4][5][6][7][8][9][10][11] The employed models of electronic density of states Nð"Þ and the spectral conductivity ð"; TÞ are shown in Figs. 1(a)-(f).…”

“…[9][10][11] Finally, the thermoelectric power was calculated with the Mott formula, and the resulting data is labeled as S Mott ðTÞ. The temperature dependence of the chemical potential is generally ignored when the Mott formula is employed, because the variation of chemical potential with temperature is believed to be very small in metallic materials.…”

“…We investigated the origin of these unusual behaviors observed in some thermoelectric materials, and found that the fine electronic structure near the chemical potential that is generally ignored in the standard theories plays an essential role to realize the unusual behaviors. [4][5][6][7][8][9][10][11] Although the role of the fine electronic structure near the chemical potential was clearly revealed, the effects of temperature dependent chemical potential on thermoelectric power have not been well investigated yet. This problem is clearly understood by knowing the fact that the temperature dependence of thermoelectric power of many materials was quantitatively accounted for with ignoring the temperature dependence of chemical potential, [4][5][6][7][8][9][10][11] while the extremely large thermoelectric power observable in some semiconductors 12,13) should be attributed to the temperature dependence of chemical potential.…”

Thermoelectric properties of the misfit cobaltate Ca3Co4O9

“…Huang et al [18] have investigated the structural, electrical, and thermal transport properties of CCO ceramics sintered in a high magnetic field, arguing that this technique is efficient for enhancing the thermopower. High-resolution photoemission spectroscopy has been used by Takeuchi et al [6,7] to investigate the electronic structure. These authors argue that the high thermopower is due to the metallic conduction in the hexagonal CoO 2 layers.…”