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Electronic structure near the Fermi level in the Ca3 Co4O9 layered cobalt oxide
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 849–852
Electronic structure near the Fermi level in the Ca3Co4 O9 layered cobalt oxide Tsunehiro Takeuchia,∗ , Takeshi Kondob , Takio Kitaob , Kazuo Sodac , Masahiro Shikanod , Ryoji Funahashid , Masashi Mikamid , Uichiro Mizutanib a EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan c Department of Quantum Engineering, Nagoya University, Nagoya 464-8603, Japan National Institute of Advanced Industrial Science and Technology Kansai, Ikeda 563-8577, Japan b
d
Available online 11 March 2005
Abstract Synchrotron radiation angle resolved photoemission spectroscopy measurements were performed on Ca3 Co4 O9 , which is one of promising candidates for the thermoelectrical material because of its possession of the large thermoelectric power, metallic electrical conduction, and low thermal conductivity. Three dispersing bands were observed in the energy range of E = 0–0.9 eV below the Fermi level, and one of these bands constructs a hole-like Fermi surface (FS) centered at point. These bands were successfully assigned as those from two-dimensionally spanned CoO2 layers by observing their periodicity in the reciprocal space. It was also found that the FS possesses a hexagonal shape with By using tight-binding fit on the band that crosses EF , its edge center aligned on the direction of the primitive reciprocal lattice vector K. we successfully reproduced the characteristic features in the density of states that is responsible for the large thermoelectric power and the metallic electrical conduction. © 2005 Elsevier B.V. All rights reserved. PACS: 72.15.jf; 71.20.Be Keywords: ARPES; Cobalt oxide; Triangle lattice; Tight-binding fit; Thermoelectric power
1. Introduction Recently, layered cobalt oxides characterized by twodimensionally spanned CoO2 triangular lattice consisting of edge-shared CoO6 octahedrons, such as Ca3 Co4 O9 , Nax CoO2 , Bi2 Sr2 Co2 O9 , and Tl0.4 (Sr0.9 O)1.12 CoO2 , were reported to possess a large thermoelectric power exceeding 100 V/K at room temperature with maintaining metallic electrical resistivity less than 10 m cm [1–7]. Some of these cobalt oxides show low thermal conductivity due most likely to the misfit structure between the CoO2 layers and other layers. These layered cobalt oxides have been regarded as one of the most promising candidates for the practical ther∗
Corresponding author. Tel.: +81 52 789 5620; fax: +81 52 789 3724. E-mail address: [email protected] (T. Takeuchi).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.235
moelectric material, because large thermoelectric power and high electrical conductivity together with a low thermal conductivity are of necessities for the development of a practical thermoelectric material. Their high stability at high temperatures further promoted us to utilize the cobalt oxides as a thermoelectric device at high temperatures. In this study, angle resolved photoemission spectroscopy (ARPES) measurements were carried out for one of the typical layered cobalt oxides, Ca3 Co4 O9 , to discuss its thermoelectrical properties in terms of the momentum resolved band structure. By using the measured energy–momentum (ε–k) dispersion, we discuss the topological features of the band that crosses the Fermi level (EF ), because it should play a dominant role in determining the electron transport properties. We demonstrate that information about the band structure near EF obtained in this study lends a great support to
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T. Takeuchi et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 849–852
the scenario we have previously proposed on the basis of angle integrated photoemission data; the large thermoelectric power and metallic electrical conduction is simply brought about by the electron transport in a narrow band with EF near its band edge [8,9].
2. Experimental procedure High-quality single crystals were grown by the flux method. The details of the sample preparation were reported elsewhere [7]. Angle resolved photoemission spectroscopy (ARPES) measurements with an incident energy of hν = 22 eV were carried out at the port 011 Undulator4mNIM in the Synchrotron Radiation Center, Wisconsin, USA. We used Gammadata-Scienta SES2002 hemispherical analyzer for the ARPES measurement. Energy resolution of the measurement was about 20 meV which was estimated by the energy width of the intensity reduction from 90 to 10% at the Fermi level of the reference gold. The angle resolution was 0.45◦ . Although the best angle resolution of the SES2002 is much better than 0.45◦ , we did use this rather poor one partly because more intense signal is obtained with poorer angler resolutions and partly because 0.45◦ is small enough to correspond only√to 0.6% of the primitive reciprocal lattice = 2π/( 3a/2) of the two-dimensionally spanned vector |K| CoO2 triangular lattice, where a represents the distance between the nearest cobalt atoms in the CoO2 planes. The orientation of the single crystal were determined by taking the Laue pictures, and the crystals are accurately mounted on the ARPES sample holder within the accuracy of ±0.5◦ . The clean surfaces were prepared by cleaving samples under the ultrahigh vacuum condition with a base pressure better than 1×10−10 Torr. We measured ARPES spectra at 40 K along two different momentum lines; -M and -K. The atomic arrangements in a CoO2 triangular lattice and the corresponding hexagonal Brillouin zone are schematically drawn in Fig. 1(b) and (c), respectively.
3. Results and discussions Fig. 2(a) shows the second energy-derivative of the 2 along the -K direction. Ob ARPES intensity ∂2 I(ε, k)/∂ε viously, the bands are dispersing with momentum. This experimental fact indicates that the Ca3 Co4 O9 single crystal we employed in out present analysis had good quality over the macroscopic area of a few hundred micron in radius that is the size of the incident photon-beam. In our previous work with the angle integrated photoemission spectroscopy, five peaks were observed in the valence band at −1.0, −2.9, −3.5, −5.0 and −6.2 eV [8]. By taking a careful look on the present ARPES data, we found that the valence band indeed possesses five bands as the those observed in the angle integrated spectra. Since electron transport properties are dominantly determined by the bands crossing EF , ARPES spectra near EF should be carefully investigated in detail. The second energyderivative of the ARPES intensity in the energy range of 0–1.6 eV below EF are shown in Fig. 2(b) and (c) for -K and -M direction, respectively. The bands observed at this energy range was previously assigned as the triply degenerated anti-bonding t2g bands from the CoO2 layers [9]. One may easily realize that the peak is indeed made up with three bands and that one of the bands disperse across EF to construct the Fermi surface. We found that the high-symmetry points in the bands agree with the point and the M point in the hexagonal Brillouin of the triangular lattice of the CoO2 layers. We can, thus, strongly argue that the bands observed at EF ∼ 1.0 eV is not provided by the rock-salt layers of the square lattice but by the CoO2 layers of the triangular lattice. were obtained by Momentum distribution curves A(k)’s at an energy ε as shown cutting the ARPES intensity I(ε, k) in Fig. 3(a). The momentum providing the peak in each A(k) was precisely determined by the Lorentzian-fitting. By plotting the obtained peak-momentum as a function of energy, we successfully constructed the ε–k dispersion near EF . The resulting data are shown in Fig. 3(b). It is easily noticed from
Fig. 1. Schematic drawing of (a) the structure of the Ca3 Co4 O9 in the a–c plane, (b) two-dimensionally spanned CoO2 triangular lattice in the a–b plane, and (c) the corresponding hexagonal first Brillouin zone in the two-dimensional reciprocal space. CoO2 layers are stacked between rock-salt layers (Ca2 CoO3 ). are rotated from that of direct lattice by 60◦ . The direction of primitive reciprocal lattice vectors |K|
T. Takeuchi et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 849–852
851
be translated into the simple relation of mM ≤ mK , which is schematically drawn in the inset of Fig. 3(b). Since both curves should be collapsed in the same eigenvalue at point, the smaller effective mass in one direction would provide larger Fermi velocity vF and smaller Fermi wave vector |k F | than those with heavier effective mass. To gain deep insight into the nature of the band crossing EF , we employed here the tight-binding approximation on the experimentally determined ε–k dispersion shown in Fig. 4. The function we employed is 2kx kx ε = ε0 − 2t cos √ + 2 cos √ cos(ky ) 3 3 4kx 2kx − 2t cos √ + 2 cos √ cos(2ky ) . (1) 3 3
Fig. 2. The second energy-derivative of the ARPES intensity: (a) valence band along the -K direction and (b) and (c) bands in the vicinity of EF along the -K and -M directions, respectively. Five different bands are observable in the valence band. The highest energy band in (a) consists of three bands, which can be seen in (b) and (c). Fermi wave vectors are also observable in (b) and (c).
Fig. 3(b) that the Ca3 Co4 O9 has an anisotropy in the Fermi surface and Fermi velocity; larger |kF | and smaller vF were observed along the -K direction. If we assume a constant effective mass along the -M direction (mM ) together with a constant mass along the -K (mK ) with different magnitude, the difference in the |kF | and vF described above can
Here t and t represent the transfer integral between atomic orbitals connected with a and those with 2a, where a is one of in the two-dimensional trianthe primitive vectors (a and b) gular lattice. We ignored the transfer integrals t
associated with a + b because dxy orbitals make a one-dimensional bond-array along the a. t
is, therefore, expected to be much smaller than t . Moreover, it is important to note that the tightbinding fit with t
gave us a wrong result that is not consistent with the band-dispersion we observed in our present ARPES measurement. As a consequence of the tight-binding fit with t and t , hexagonal FS and the ∼250 meV bandwidth observed in the present ARPES measurement were successfully reproduced. The resulting parameters are ε0 = −72.6 meV, t = −25.4 meV, and t = 7.2 meV. It is also found that a flat portion exists above EF around the point that was predicted by the LDA band calculation for Na0.5 CoO2 [10]. Density of states near the Fermi level N(ε) was calculated from the ε–k dispersion obtained by the tight-binding fit. The resulting N(ε) have two singularities, one of which is observed above EF and the other at ∼120 meV below E F . The singularity above EF and the other at −100 meV are brought about by the flat portions around the point and K
Fig. 3. (a) Momentum distribution curves obtained along the -M direction. The MDC curve with thick line corresponds to MDC at EF . (b) ε–k dispersions along the -M and -K directions determined by the peak-momentum of each MDC. Larger |kF | and smaller vF are observed along the -K direction than those along the -M direction. This experimental fact simply indicates the larger effective mass along the -M direction than that along -K as schematically drawn in inset.
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T. Takeuchi et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 849–852
Fig. 4. (a) ε–k dispersion of the band crossing EF obtained by the tight-binding fit. (b) Contour plot of the band. The Fermi surface is drawn with thick lines. (c) Density of states deduced from the ε–k dispersion. Hole-like FS is observed with its center at the point. Very flat portion extends at point that causes a peak in the density of states above EF .
point, respectively. Note here that the singularity above EF had been observed in the photoemission spectrum measured at high temperature [8,9]. The presence of the other singularity at −120 meV, unfortunately, had not confirmed yet, most likely because other bands are overlapped on it. On the basis of angle integrated photoemission data, we previously reported that (a) presence of the band edge just above EF and (b) an increasing N(ε) with decreasing energy cause the large thermoelectric power of the Ca3 Co4 O9 [8,9]. Obviously the former condition (a) is realized in the present data; the high-energy-edge of the band which crosses EF exists a few tenths of meV above the Fermi level. The latter condition (b) might be related to the presence of the singularities at 120 meV. We consider, however, that this single-band picture is too simple to fully account for the large thermoelectric power because the group velocity of the conduction electrons in the vicinity of such a singularity at −120 meV would be very small and that their contribution to the energy flow would be reduced even if the N(ε) is very large. It is, therefore, argued that presence of the other bands below −100 meV, that was indeed observed in the present ARPES measurement, would additionally contribute in increasing the thermoelectric power of the Ca3 Co4 O9 .
4. Conclusion In this study, we performed ARPES measurement on one of the typical layered cobalt oxides, Ca3 Co4 O9 , of high
thermoelectric power. We have successfully determined the band dispersions in the Ca3 Co4 O9 and found that the highest partially occupied band comes from the two-dimensionally spanned CoO2 layers and have very narrow energy width only of ∼250 meV. The Fermi level was located at a few tenths of meV below the band edge. We conclude, therefore, that the presence of this very narrow band with the Fermi level near its band edge is responsible for the large thermoelectric power observed for the Ca3 Co4 O9 .
References [1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685– R12687. [2] K. Fujita, T. Mochida, K. Nakamura, Jpn. J. Appl. Phys. 40 (2001) 4644–4647. [3] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166–175. [4] R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, S. Sodeoka, Jpn. J. Appl. Phys. 39 (2000) L1127–L1129. [5] R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, Mater. Trans. 42 (2001) 956–960. [6] S. H´ebert, S. Lambert, D. Pelloquin, A. Maignan, Phys. Rev. B 64 (2001) 171201-1–171201-4. [7] M. Shikano, R. Funahashi, Appl. Phys. Lett. 82 (2003) 1851–1853. [8] T. Takeuchi, T. Kondo, K. Soda, U. Mizutani, R. Funahashi, M. Shikano, S. Tsuda, T. Yokoya, S. Shin, T. Muro, J. Electron Spectrosc. Relat. Phenom. 137–140 (2004) 595–599. [9] T. Takeuchi, T. Kondo, T. Takami, H. Takahashi, H. Ikuta, U. Mizutani, K. Soda, R. Funahashi, M. Shikano, M. Mikami, S. Tsuda, T. Yokoya, S. Shin, T. Muro, Phys. Rev. B 69 (2004) 125410. [10] D.J. Singh, Phys. Rev. B 61 (2000) 13397–13402.
Electronic structure near the Fermi level in the Ca3Co4 O9 layered cobalt oxide Tsunehiro Takeuchia,∗ , Takeshi Kondob , Takio Kitaob , Kazuo Sodac , Masahiro Shikanod , Ryoji Funahashid , Masashi Mikamid , Uichiro Mizutanib a EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan Department of Crystalline Materials Science, Nagoya University, Nagoya 464-8603, Japan c Department of Quantum Engineering, Nagoya University, Nagoya 464-8603, Japan National Institute of Advanced Industrial Science and Technology Kansai, Ikeda 563-8577, Japan b
d
Available online 11 March 2005
Abstract Synchrotron radiation angle resolved photoemission spectroscopy measurements were performed on Ca3 Co4 O9 , which is one of promising candidates for the thermoelectrical material because of its possession of the large thermoelectric power, metallic electrical conduction, and low thermal conductivity. Three dispersing bands were observed in the energy range of E = 0–0.9 eV below the Fermi level, and one of these bands constructs a hole-like Fermi surface (FS) centered at point. These bands were successfully assigned as those from two-dimensionally spanned CoO2 layers by observing their periodicity in the reciprocal space. It was also found that the FS possesses a hexagonal shape with By using tight-binding fit on the band that crosses EF , its edge center aligned on the direction of the primitive reciprocal lattice vector K. we successfully reproduced the characteristic features in the density of states that is responsible for the large thermoelectric power and the metallic electrical conduction. © 2005 Elsevier B.V. All rights reserved. PACS: 72.15.jf; 71.20.Be Keywords: ARPES; Cobalt oxide; Triangle lattice; Tight-binding fit; Thermoelectric power
1. Introduction Recently, layered cobalt oxides characterized by twodimensionally spanned CoO2 triangular lattice consisting of edge-shared CoO6 octahedrons, such as Ca3 Co4 O9 , Nax CoO2 , Bi2 Sr2 Co2 O9 , and Tl0.4 (Sr0.9 O)1.12 CoO2 , were reported to possess a large thermoelectric power exceeding 100 V/K at room temperature with maintaining metallic electrical resistivity less than 10 m cm [1–7]. Some of these cobalt oxides show low thermal conductivity due most likely to the misfit structure between the CoO2 layers and other layers. These layered cobalt oxides have been regarded as one of the most promising candidates for the practical ther∗
Corresponding author. Tel.: +81 52 789 5620; fax: +81 52 789 3724. E-mail address: [email protected] (T. Takeuchi).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.235
moelectric material, because large thermoelectric power and high electrical conductivity together with a low thermal conductivity are of necessities for the development of a practical thermoelectric material. Their high stability at high temperatures further promoted us to utilize the cobalt oxides as a thermoelectric device at high temperatures. In this study, angle resolved photoemission spectroscopy (ARPES) measurements were carried out for one of the typical layered cobalt oxides, Ca3 Co4 O9 , to discuss its thermoelectrical properties in terms of the momentum resolved band structure. By using the measured energy–momentum (ε–k) dispersion, we discuss the topological features of the band that crosses the Fermi level (EF ), because it should play a dominant role in determining the electron transport properties. We demonstrate that information about the band structure near EF obtained in this study lends a great support to
850
T. Takeuchi et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 849–852
the scenario we have previously proposed on the basis of angle integrated photoemission data; the large thermoelectric power and metallic electrical conduction is simply brought about by the electron transport in a narrow band with EF near its band edge [8,9].
2. Experimental procedure High-quality single crystals were grown by the flux method. The details of the sample preparation were reported elsewhere [7]. Angle resolved photoemission spectroscopy (ARPES) measurements with an incident energy of hν = 22 eV were carried out at the port 011 Undulator4mNIM in the Synchrotron Radiation Center, Wisconsin, USA. We used Gammadata-Scienta SES2002 hemispherical analyzer for the ARPES measurement. Energy resolution of the measurement was about 20 meV which was estimated by the energy width of the intensity reduction from 90 to 10% at the Fermi level of the reference gold. The angle resolution was 0.45◦ . Although the best angle resolution of the SES2002 is much better than 0.45◦ , we did use this rather poor one partly because more intense signal is obtained with poorer angler resolutions and partly because 0.45◦ is small enough to correspond only√to 0.6% of the primitive reciprocal lattice = 2π/( 3a/2) of the two-dimensionally spanned vector |K| CoO2 triangular lattice, where a represents the distance between the nearest cobalt atoms in the CoO2 planes. The orientation of the single crystal were determined by taking the Laue pictures, and the crystals are accurately mounted on the ARPES sample holder within the accuracy of ±0.5◦ . The clean surfaces were prepared by cleaving samples under the ultrahigh vacuum condition with a base pressure better than 1×10−10 Torr. We measured ARPES spectra at 40 K along two different momentum lines; -M and -K. The atomic arrangements in a CoO2 triangular lattice and the corresponding hexagonal Brillouin zone are schematically drawn in Fig. 1(b) and (c), respectively.
3. Results and discussions Fig. 2(a) shows the second energy-derivative of the 2 along the -K direction. Ob ARPES intensity ∂2 I(ε, k)/∂ε viously, the bands are dispersing with momentum. This experimental fact indicates that the Ca3 Co4 O9 single crystal we employed in out present analysis had good quality over the macroscopic area of a few hundred micron in radius that is the size of the incident photon-beam. In our previous work with the angle integrated photoemission spectroscopy, five peaks were observed in the valence band at −1.0, −2.9, −3.5, −5.0 and −6.2 eV [8]. By taking a careful look on the present ARPES data, we found that the valence band indeed possesses five bands as the those observed in the angle integrated spectra. Since electron transport properties are dominantly determined by the bands crossing EF , ARPES spectra near EF should be carefully investigated in detail. The second energyderivative of the ARPES intensity in the energy range of 0–1.6 eV below EF are shown in Fig. 2(b) and (c) for -K and -M direction, respectively. The bands observed at this energy range was previously assigned as the triply degenerated anti-bonding t2g bands from the CoO2 layers [9]. One may easily realize that the peak is indeed made up with three bands and that one of the bands disperse across EF to construct the Fermi surface. We found that the high-symmetry points in the bands agree with the point and the M point in the hexagonal Brillouin of the triangular lattice of the CoO2 layers. We can, thus, strongly argue that the bands observed at EF ∼ 1.0 eV is not provided by the rock-salt layers of the square lattice but by the CoO2 layers of the triangular lattice. were obtained by Momentum distribution curves A(k)’s at an energy ε as shown cutting the ARPES intensity I(ε, k) in Fig. 3(a). The momentum providing the peak in each A(k) was precisely determined by the Lorentzian-fitting. By plotting the obtained peak-momentum as a function of energy, we successfully constructed the ε–k dispersion near EF . The resulting data are shown in Fig. 3(b). It is easily noticed from
Fig. 1. Schematic drawing of (a) the structure of the Ca3 Co4 O9 in the a–c plane, (b) two-dimensionally spanned CoO2 triangular lattice in the a–b plane, and (c) the corresponding hexagonal first Brillouin zone in the two-dimensional reciprocal space. CoO2 layers are stacked between rock-salt layers (Ca2 CoO3 ). are rotated from that of direct lattice by 60◦ . The direction of primitive reciprocal lattice vectors |K|
T. Takeuchi et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 849–852
851
be translated into the simple relation of mM ≤ mK , which is schematically drawn in the inset of Fig. 3(b). Since both curves should be collapsed in the same eigenvalue at point, the smaller effective mass in one direction would provide larger Fermi velocity vF and smaller Fermi wave vector |k F | than those with heavier effective mass. To gain deep insight into the nature of the band crossing EF , we employed here the tight-binding approximation on the experimentally determined ε–k dispersion shown in Fig. 4. The function we employed is 2kx kx ε = ε0 − 2t cos √ + 2 cos √ cos(ky ) 3 3 4kx 2kx − 2t cos √ + 2 cos √ cos(2ky ) . (1) 3 3
Fig. 2. The second energy-derivative of the ARPES intensity: (a) valence band along the -K direction and (b) and (c) bands in the vicinity of EF along the -K and -M directions, respectively. Five different bands are observable in the valence band. The highest energy band in (a) consists of three bands, which can be seen in (b) and (c). Fermi wave vectors are also observable in (b) and (c).
Fig. 3(b) that the Ca3 Co4 O9 has an anisotropy in the Fermi surface and Fermi velocity; larger |kF | and smaller vF were observed along the -K direction. If we assume a constant effective mass along the -M direction (mM ) together with a constant mass along the -K (mK ) with different magnitude, the difference in the |kF | and vF described above can
Here t and t represent the transfer integral between atomic orbitals connected with a and those with 2a, where a is one of in the two-dimensional trianthe primitive vectors (a and b) gular lattice. We ignored the transfer integrals t
associated with a + b because dxy orbitals make a one-dimensional bond-array along the a. t
is, therefore, expected to be much smaller than t . Moreover, it is important to note that the tightbinding fit with t
gave us a wrong result that is not consistent with the band-dispersion we observed in our present ARPES measurement. As a consequence of the tight-binding fit with t and t , hexagonal FS and the ∼250 meV bandwidth observed in the present ARPES measurement were successfully reproduced. The resulting parameters are ε0 = −72.6 meV, t = −25.4 meV, and t = 7.2 meV. It is also found that a flat portion exists above EF around the point that was predicted by the LDA band calculation for Na0.5 CoO2 [10]. Density of states near the Fermi level N(ε) was calculated from the ε–k dispersion obtained by the tight-binding fit. The resulting N(ε) have two singularities, one of which is observed above EF and the other at ∼120 meV below E F . The singularity above EF and the other at −100 meV are brought about by the flat portions around the point and K
Fig. 3. (a) Momentum distribution curves obtained along the -M direction. The MDC curve with thick line corresponds to MDC at EF . (b) ε–k dispersions along the -M and -K directions determined by the peak-momentum of each MDC. Larger |kF | and smaller vF are observed along the -K direction than those along the -M direction. This experimental fact simply indicates the larger effective mass along the -M direction than that along -K as schematically drawn in inset.
852
T. Takeuchi et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 849–852
Fig. 4. (a) ε–k dispersion of the band crossing EF obtained by the tight-binding fit. (b) Contour plot of the band. The Fermi surface is drawn with thick lines. (c) Density of states deduced from the ε–k dispersion. Hole-like FS is observed with its center at the point. Very flat portion extends at point that causes a peak in the density of states above EF .
point, respectively. Note here that the singularity above EF had been observed in the photoemission spectrum measured at high temperature [8,9]. The presence of the other singularity at −120 meV, unfortunately, had not confirmed yet, most likely because other bands are overlapped on it. On the basis of angle integrated photoemission data, we previously reported that (a) presence of the band edge just above EF and (b) an increasing N(ε) with decreasing energy cause the large thermoelectric power of the Ca3 Co4 O9 [8,9]. Obviously the former condition (a) is realized in the present data; the high-energy-edge of the band which crosses EF exists a few tenths of meV above the Fermi level. The latter condition (b) might be related to the presence of the singularities at 120 meV. We consider, however, that this single-band picture is too simple to fully account for the large thermoelectric power because the group velocity of the conduction electrons in the vicinity of such a singularity at −120 meV would be very small and that their contribution to the energy flow would be reduced even if the N(ε) is very large. It is, therefore, argued that presence of the other bands below −100 meV, that was indeed observed in the present ARPES measurement, would additionally contribute in increasing the thermoelectric power of the Ca3 Co4 O9 .
4. Conclusion In this study, we performed ARPES measurement on one of the typical layered cobalt oxides, Ca3 Co4 O9 , of high
thermoelectric power. We have successfully determined the band dispersions in the Ca3 Co4 O9 and found that the highest partially occupied band comes from the two-dimensionally spanned CoO2 layers and have very narrow energy width only of ∼250 meV. The Fermi level was located at a few tenths of meV below the band edge. We conclude, therefore, that the presence of this very narrow band with the Fermi level near its band edge is responsible for the large thermoelectric power observed for the Ca3 Co4 O9 .
References [1] I. Terasaki, Y. Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685– R12687. [2] K. Fujita, T. Mochida, K. Nakamura, Jpn. J. Appl. Phys. 40 (2001) 4644–4647. [3] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166–175. [4] R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, S. Sodeoka, Jpn. J. Appl. Phys. 39 (2000) L1127–L1129. [5] R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani, Mater. Trans. 42 (2001) 956–960. [6] S. H´ebert, S. Lambert, D. Pelloquin, A. Maignan, Phys. Rev. B 64 (2001) 171201-1–171201-4. [7] M. Shikano, R. Funahashi, Appl. Phys. Lett. 82 (2003) 1851–1853. [8] T. Takeuchi, T. Kondo, K. Soda, U. Mizutani, R. Funahashi, M. Shikano, S. Tsuda, T. Yokoya, S. Shin, T. Muro, J. Electron Spectrosc. Relat. Phenom. 137–140 (2004) 595–599. [9] T. Takeuchi, T. Kondo, T. Takami, H. Takahashi, H. Ikuta, U. Mizutani, K. Soda, R. Funahashi, M. Shikano, M. Mikami, S. Tsuda, T. Yokoya, S. Shin, T. Muro, Phys. Rev. B 69 (2004) 125410. [10] D.J. Singh, Phys. Rev. B 61 (2000) 13397–13402.