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Hard X-ray core level photoemission of vanadium oxides
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 841–843
Hard X-ray core level photoemission of vanadium oxides N. Kamakuraa,∗ , M. Taguchia , K. Yamamotoa,c , K. Horibaa , A. Chainania , Y. Takataa , E. Ikenagad , H. Namatamee , M. Taniguchie , M. Awajid , A. Takeuchid , K. Tamasakub , Y. Nishinob , D. Miwab , T. Ishikawab , Y. Uedaf , K. Kobayashie , S. Shina,f a b
Soft X-ray Spectroscopy Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan Coherent X-ray Optics Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan c Department of Mathematical Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan d JASRI/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan e HiSOR, Hiroshima University, Higashi-Hiroshima, Hirosima 739-8526, Japan f Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Available online 4 March 2005
Abstract We have studied vanadium oxides corresponding to formally 3d0 (V2 O5 ), 3d1 (VO2 ), and 3d2 (V2 O3 ) configurations and compare them with mixed valent Na0.33 V2 O5 , using hard X-ray core level photoemission spectroscopy (PES), which enables to probe bulk electronic states due to large escape depth of high kinetic energy photoelectrons. The quasi-one-dimensional mixed-valence compound Na0.33 V2 O5 and threedimensional V2 O3 undergo metal–insulator (MI) transition at TMI = 135 and 160 K, respectively. The observed V 2p core level spectra of V2 O5 and VO2 show a single peak with chemical shifts corresponding to the V5+ and V4+ , while two components (V5+ and V4+ ) are observed in Na0.33 V2 O5 . Metallic V2 O3 shows a bulk character additional peak at low binding energy in the V 2p core levels. We also study the V 1s core level spectra of these compounds, which are possible only using hard X-rays and confirm the systematics in valence states of vanadium oxides. The temperature dependence of the low binding energy peak in V 1s spectra across the MI transition in V2 O3 suggests an additional screening channel available in the Mott–Hubbard correlated metal phase. © 2005 Elsevier B.V. All rights reserved. Keywords: Hard X-ray photoemission; Vanadium oxides
1. Introduction Vanadium oxides show a variety of electronic and magnetic properties [1]. V2 O3 is known as a typical three-dimensional Mott–Hubbard system, in which a metal–insulator (MI) transition occurs at TMI = 160 K from a paramagnetic metal (PM) to antiferromagnetic insulator (AFI). V2 O5 has a layered crystal structure, and is known to be a band insulator with the band gap of 2.0 eV. VO2 is thought to be a monoclinic structure with dimerization along the c-axis coupled to a semiconductor–metal transition at 340 K. The quasi-one- dimensional system Na0.33 V2 O5 also shows a metal–insulator transition at 135 K accompanied by ∗
Corresponding author. E-mail address: [email protected] (N. Kamakura).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.203
a charge ordering of V4+ and V5+ with the formal ratio of 1:5 [2,3]. The formal oxidation states of V ions are V3+ (3d2 ) for V2 O3 , V4+ (3d1 ) for VO2 , and V5+ (3d0 ) for V2 O5 , whereas the mixing between the V 3d and O 2p bands enhances the occupation numbers relative to the formal occupations. PES give important informations for the electronic structure of these V oxides and have been extensively studied using ultraviolet ray and soft X-ray [4–7]. Those studies have shown the importance of V 3d–3d Coulomb interaction and the hybridization between V 3d and O 2p bands, etc. Hard X-ray photoemission spectroscopy (HX-PES) has been recently developed as a probe of the bulk electronic structure [8–12]. Since HX-PES studies of the 2p core levels for Cr-doped V2 O3 [11] and La1−x Srx MnO3 [12] show the clear low binding energy feature, we have studied the V core levels of the V2 O3 , VO2 , Na0.33 V2 O5 , and V2 O5 with various valence
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configurations and structures using HX-PES. The electron mean free path (MFP) of photoelectrons with 5425 eV kinetic energy, which corresponds to the kinetic energy of photoelectrons from V 2p excited using a photon energy 5940 eV, is es˚ timated to be about ∼80 A[13]. This MFP is four times larger ˚ of photoelectrons by Al K␣ [13]. V than the MFP (∼ 20 A) 1s core level is located at about 5465 eV binding energy, and can also be investigated using the present HX-PES technique.
2. Experimental HX-PES was performed at the beam line, BL47XU, of SPring-8 [8,9]. The V2 O3 , V2 O5 , and Na0.33 V2 O5 singlecrystal samples were fractured in a vacuum of better than 2 × 10−7 Pa. Since the electron emission angle was set to ˚ The total en70◦ , the probing depth is estimated to be 70 A. ergy resolution was set to 400 meV. The experiment has been performed at 250 K for V2 O5 , 300 K for VO2 (semiconductor phase), 200 K for Na0.33 V2 O5 (metallic phase). The observed spectra of these V oxides have been compared with the spectra of V2 O3 at 250 K (PM phase). The temperature dependence of V 1s spectrum is also studied for the V2 O3 at 250 K (PM phase) and 90 K (AFI phase).
3. Results and discussion Fig. 1 shows the V 2p spectra of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 . The V 2p spectra of V2 O5 and VO2 reveal a single peak and the peak position of these compounds shows the difference due to the number of 3d electrons, that is, 3d0 (V5+ ) for V2 O5 and 3d1 (V4+ ) for VO2 . In Na0.33 V2 O5 , on the other hand, V 2p spectrum shows a double peak structure, which match the peak energies observed in V2 O5 and VO2 ,
Fig. 1. V 2p photoemission spectra of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 measured with hν = 5940 eV. The V 2p spectrum of V2 O3 in PM phase shows additional peaks at low binding energy to the main peak, which are labelled A.
i.e. V5+ and V4+ . NMR study for Na0.33 V2 O5 also show a similar mixed valent configuration with V4+ and V5+ [2]. The V 2p spectrum of V2 O3 shows a broad and complicated line-shape, with additional peaks at low binding energies to the 2p3/2 and 2p1/2 main peaks (512.5 and 520.5 eV, labelled A). The broad line-shape originates from the multiplets of 2p5 3d2 and 2p5 3d3 L configurations, while the low binding energy feature (A) is attributed to screening by coherent states at the Fermi level (EF ) [11]. In the model described in Ref. [11], the changes in the valence band states as reported in ref. [14] are modeled using discrete states called coherent states to represent the quasiparticle density of states at EF . The density of states at EF is known to show temperature dependence corresponding to the gap opening in the AFI phase [15]. The low binding energy feature in V 2p core levels also show temperature dependence, that is, the low binding energy well-screened feature disappears in the AFI phase [11]. The feature (A) was actually observed as a weak feature in earlier V 2p spectra obtained using soft X-ray PES [7], but it was not very clear. The present study shows feature (A) is clear in V 2p spectra, indicating that screening by coherent states at EF is more effective in the bulk sensitive measurement. Fig. 2 shows the V 1s photoemission spectra for the V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 . The V 1s photoemission spectra of these V oxides are qualitatively consistent with the observed V 2p spectra. The main peak of V 1s spectra in V2 O5 and VO2 consist of a single peak, corresponding to the valence configuration of V5+ and V4+ , respectively. In Na0.33 V2 O5 , a double peak structure is observed in the main peak of V 1s, confirming the V4+ and V5+ configurations. In the V 1s spectrum of V2 O3 , the low binding energy feature (labelled B) is observed as in the case of V 2p spectrum (labelled A in Fig. 1). The appearance of the low binding
Fig. 2. V 1s photoemission spectra of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 measured with hν = 5940 eV. The V 1s spectrum of V2 O3 in PM phase shows additional peaks at low binding energy to the main peak, which are labelled B.
N. Kamakura et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 841–843
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consisting of V5+ and V4+ components. The V 2p and 1s core levels of V2 O3 in the PM phase show an additional low binding energy feature. A cluster model calculation shows that this low binding energy feature originates from screening by coherent states at EF , consistent with its absence in the AFI phase. This low binding energy feature is not observed in the core levels of V2 O5 , VO2 , and Na0.33 V2 O5 . For VO2 , the photoemission experiment is performed at 300 K, that is, in the semiconductor phase, and it would be important to study it in the high temperature metallic phase. Acknowledgments We thank Drs. T. Takeuchi, M. Arita, and J.J. Kim for help with experiments and Profs. K. Katsumata, S.W. Lovesey, and T. Yokoya for valuable discussions. Fig. 3. V 1s photoemission spectra of V2 O3 in the PM and AFI phase. The calculated spectrum for the PM phase is shown in the bottom, where the screening by coherent states is also included.
energy feature (B) in V 1s spectrum in spite of the lower probing depth is consistent with the result of the V 2p soft X-ray PES results which showed a weak low binding energy feature [7]. In Fig. 3, the temperature dependence of V 1s spectra ((a) AFI and (b) PM phase) is illustrated. The observed V 1s spectrum of V2 O3 shows clear temperature dependence across the MI transition, where the low binding energy feature disappears in the AFI phase. For the V 1s spectrum of the PM phase, we show the calculated spectrum obtained using a cluster model. The calculation includes the charge transfer from coherent states at EF [11], which well reproduces the observed V 1s spectrum. The final state 1s1 3d2 mostly contributes to the main line, while the 1s1 3d3 C state, due to screening by coherent states, forms the low binding energy feature. The observed temperature dependence also indicates that the low binding energy feature is surely related to the density of states at EF , since the absence of the low binding energy feature in the AFI phase is consistent with the gap opening in the AFI phase. Another interesting observation in the V 1s spectra of Fig. 3, is the existence of a satellite structure at 5478 eV binding energy. The calculation also reproduces the satellite structure as a charge transfer (CT) satellite formed by 1s1 3d3 L state. The V 1s spectra of V2 O5 , VO2 , and Na0.33 V2 O5 also show the CT satellite (Fig. 2), which have been observed in 2s core level photoemission spectra using soft X-ray [5].
4. Conclusion We have studied the V 2p and 1s core levels of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 using HX-PES. The V 2p and 1s core level spectra of V2 O5 and VO2 show a single peak corresponding to the V5+ and V4+ valence configurations, while that of Na0.33 V2 O5 shows the double peak structure
References [1] M. Imada, A. Fujimori, Y. Tokura, Rev. Mod. Phys. 70 (1998) 1039. [2] M. Itoh, N. Akimoto, H. Yamada, M. Isobe, Y. Ueda, Phys. Soc. Jpn. 69 (Suppl. B) (2000) 69. [3] K. Okazaki, A. Fujimori, T. Yamauchi, Y. Ueda, Phys. Rev. B 69 (2004) 140506R. [4] S. Shin, S. Suga, M. Taniguchi, M. Fujisawa, H. Kanzaki, A. Fujimori, H. Daimon, Y. Ueda, K. Kosuge, S. Kachi, Phys. Rev. B 41 (1990) 4993. [5] R. Zimmermann, R. Claessen, F. Reinert, P. Steiner, S. H¨ufner, J. Phys.: Condens. Matter 10 (1998) 5697. [6] A.E. Bocquet, T. Mizokawa, K. Morikawa, A. Fujimori, S.R. Barman, K. Maiti, D.D. Sarma, Y. Tokura, M. Onoda, Phys. Rev. B 53 (1996) 1161. [7] R.L. Kurtz, V.E. Henrich, Phys. Rev. B 28 (1983) 6699; K.E. Smith, V.E. Henrich, Phys. Rev. B 50 (1994) 1382. [8] K. Kobayashi, M. Yabashi, Y. Takata, T. Tokushima, S. Shin, K. Tamasaku, D. Miwa, T. Ishikawa, H. Nohira, T. Hattori, Y. Sugita, O. Nakatsuka, A. Sakai, S. Zaima, Appl. Phys. Lett. 83 (2003) 1005. [9] Y. Takata, K. Tamasaku, T. Tokushima, D. Miwa, S. Shin, T. Ishikawa, M. Yabashi, K. Kobayashi, J.J. Kim, T. Yao, T. Yamamoto, M. Arita, H. Namatame, M. Taniguchi, Appl. Phys. Lett. 84 (2004) 4310. [10] A. Chainani, T. Yokoya, Y. Takata, K. Tamasaku, M. Taguchi, T. Shimojima, N. Kamakura, K. Horiba, S. Tsuda, S. Shin, D. Miwa, Y. Nishino, T. Ishikawa, M. Yabashi, K. Kobayashi, H. Namatame, M. Taniguchi, K. Takada, T. Sasaki, H. Sakurai, E. Takayama-Muromachi, Phys. Rev. B 69 (2004) 180508. [11] M. Taguchi, A. Chainani, N. Kamakura, K. Horiba, Y. Takata, E. Ikenaga, T. Yokoya, S. Shin, K. Kobayashi, K. Tamasaku, Y. Nishino, D. Miwa, M. Yabashi, T. Ishikawa, T. Mochiku, K. Hirata, K. Motoya, preprint cond-mat/0404200. [12] K. Horiba, M. Taguchi, A. Chainani, Y. Takata, E. Ikenaga, H. Namatame, M. Taniguchi, M. Awaji, A. Takeuchi, D. Miwa, Y. Nishino, K. Tamasaku, T. Ishikawa, H. Kumigashira, M. Oshima, M. Lippmaa, M. Kawasaki, H. Koinuma, K. Kobayashi, S. Shin, preprint condmat/0405442. [13] NIST Electron Inelastic-Mean-Free-Path Database: Ver. 1.1 (2000). [14] S.-K. Mo, J.D. Denlinger, H.-D. Kim, J.-H. Park, J.W. Allen, A. Sekiyama, A. Yamasaki, K. Kadono, S. Suga, Y. Saitoh, T. Muro, P. Metcalf, G. Keller, K. Held, V. Eyert, V.I. Anisimov, D. Vollhardt, Phys. Rev. Lett. 90 (2003) 186403. [15] S. Shin, Y. Tezuka, T. Kinoshita, T. Ishii, T. Kashiwakura, M. Takahasi, Y. Suda, J. Phys. Soc. Jpn. 64 (1995) 1230.
Hard X-ray core level photoemission of vanadium oxides N. Kamakuraa,∗ , M. Taguchia , K. Yamamotoa,c , K. Horibaa , A. Chainania , Y. Takataa , E. Ikenagad , H. Namatamee , M. Taniguchie , M. Awajid , A. Takeuchid , K. Tamasakub , Y. Nishinob , D. Miwab , T. Ishikawab , Y. Uedaf , K. Kobayashie , S. Shina,f a b
Soft X-ray Spectroscopy Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan Coherent X-ray Optics Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5148, Japan c Department of Mathematical Sciences, Osaka Prefecture University, Sakai, Osaka 599-8531, Japan d JASRI/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan e HiSOR, Hiroshima University, Higashi-Hiroshima, Hirosima 739-8526, Japan f Institute for Solid State Physics, University of Tokyo, Kashiwanoha, Kashiwa, Chiba 277-8581, Japan Available online 4 March 2005
Abstract We have studied vanadium oxides corresponding to formally 3d0 (V2 O5 ), 3d1 (VO2 ), and 3d2 (V2 O3 ) configurations and compare them with mixed valent Na0.33 V2 O5 , using hard X-ray core level photoemission spectroscopy (PES), which enables to probe bulk electronic states due to large escape depth of high kinetic energy photoelectrons. The quasi-one-dimensional mixed-valence compound Na0.33 V2 O5 and threedimensional V2 O3 undergo metal–insulator (MI) transition at TMI = 135 and 160 K, respectively. The observed V 2p core level spectra of V2 O5 and VO2 show a single peak with chemical shifts corresponding to the V5+ and V4+ , while two components (V5+ and V4+ ) are observed in Na0.33 V2 O5 . Metallic V2 O3 shows a bulk character additional peak at low binding energy in the V 2p core levels. We also study the V 1s core level spectra of these compounds, which are possible only using hard X-rays and confirm the systematics in valence states of vanadium oxides. The temperature dependence of the low binding energy peak in V 1s spectra across the MI transition in V2 O3 suggests an additional screening channel available in the Mott–Hubbard correlated metal phase. © 2005 Elsevier B.V. All rights reserved. Keywords: Hard X-ray photoemission; Vanadium oxides
1. Introduction Vanadium oxides show a variety of electronic and magnetic properties [1]. V2 O3 is known as a typical three-dimensional Mott–Hubbard system, in which a metal–insulator (MI) transition occurs at TMI = 160 K from a paramagnetic metal (PM) to antiferromagnetic insulator (AFI). V2 O5 has a layered crystal structure, and is known to be a band insulator with the band gap of 2.0 eV. VO2 is thought to be a monoclinic structure with dimerization along the c-axis coupled to a semiconductor–metal transition at 340 K. The quasi-one- dimensional system Na0.33 V2 O5 also shows a metal–insulator transition at 135 K accompanied by ∗
Corresponding author. E-mail address: [email protected] (N. Kamakura).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.203
a charge ordering of V4+ and V5+ with the formal ratio of 1:5 [2,3]. The formal oxidation states of V ions are V3+ (3d2 ) for V2 O3 , V4+ (3d1 ) for VO2 , and V5+ (3d0 ) for V2 O5 , whereas the mixing between the V 3d and O 2p bands enhances the occupation numbers relative to the formal occupations. PES give important informations for the electronic structure of these V oxides and have been extensively studied using ultraviolet ray and soft X-ray [4–7]. Those studies have shown the importance of V 3d–3d Coulomb interaction and the hybridization between V 3d and O 2p bands, etc. Hard X-ray photoemission spectroscopy (HX-PES) has been recently developed as a probe of the bulk electronic structure [8–12]. Since HX-PES studies of the 2p core levels for Cr-doped V2 O3 [11] and La1−x Srx MnO3 [12] show the clear low binding energy feature, we have studied the V core levels of the V2 O3 , VO2 , Na0.33 V2 O5 , and V2 O5 with various valence
842
N. Kamakura et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 841–843
configurations and structures using HX-PES. The electron mean free path (MFP) of photoelectrons with 5425 eV kinetic energy, which corresponds to the kinetic energy of photoelectrons from V 2p excited using a photon energy 5940 eV, is es˚ timated to be about ∼80 A[13]. This MFP is four times larger ˚ of photoelectrons by Al K␣ [13]. V than the MFP (∼ 20 A) 1s core level is located at about 5465 eV binding energy, and can also be investigated using the present HX-PES technique.
2. Experimental HX-PES was performed at the beam line, BL47XU, of SPring-8 [8,9]. The V2 O3 , V2 O5 , and Na0.33 V2 O5 singlecrystal samples were fractured in a vacuum of better than 2 × 10−7 Pa. Since the electron emission angle was set to ˚ The total en70◦ , the probing depth is estimated to be 70 A. ergy resolution was set to 400 meV. The experiment has been performed at 250 K for V2 O5 , 300 K for VO2 (semiconductor phase), 200 K for Na0.33 V2 O5 (metallic phase). The observed spectra of these V oxides have been compared with the spectra of V2 O3 at 250 K (PM phase). The temperature dependence of V 1s spectrum is also studied for the V2 O3 at 250 K (PM phase) and 90 K (AFI phase).
3. Results and discussion Fig. 1 shows the V 2p spectra of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 . The V 2p spectra of V2 O5 and VO2 reveal a single peak and the peak position of these compounds shows the difference due to the number of 3d electrons, that is, 3d0 (V5+ ) for V2 O5 and 3d1 (V4+ ) for VO2 . In Na0.33 V2 O5 , on the other hand, V 2p spectrum shows a double peak structure, which match the peak energies observed in V2 O5 and VO2 ,
Fig. 1. V 2p photoemission spectra of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 measured with hν = 5940 eV. The V 2p spectrum of V2 O3 in PM phase shows additional peaks at low binding energy to the main peak, which are labelled A.
i.e. V5+ and V4+ . NMR study for Na0.33 V2 O5 also show a similar mixed valent configuration with V4+ and V5+ [2]. The V 2p spectrum of V2 O3 shows a broad and complicated line-shape, with additional peaks at low binding energies to the 2p3/2 and 2p1/2 main peaks (512.5 and 520.5 eV, labelled A). The broad line-shape originates from the multiplets of 2p5 3d2 and 2p5 3d3 L configurations, while the low binding energy feature (A) is attributed to screening by coherent states at the Fermi level (EF ) [11]. In the model described in Ref. [11], the changes in the valence band states as reported in ref. [14] are modeled using discrete states called coherent states to represent the quasiparticle density of states at EF . The density of states at EF is known to show temperature dependence corresponding to the gap opening in the AFI phase [15]. The low binding energy feature in V 2p core levels also show temperature dependence, that is, the low binding energy well-screened feature disappears in the AFI phase [11]. The feature (A) was actually observed as a weak feature in earlier V 2p spectra obtained using soft X-ray PES [7], but it was not very clear. The present study shows feature (A) is clear in V 2p spectra, indicating that screening by coherent states at EF is more effective in the bulk sensitive measurement. Fig. 2 shows the V 1s photoemission spectra for the V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 . The V 1s photoemission spectra of these V oxides are qualitatively consistent with the observed V 2p spectra. The main peak of V 1s spectra in V2 O5 and VO2 consist of a single peak, corresponding to the valence configuration of V5+ and V4+ , respectively. In Na0.33 V2 O5 , a double peak structure is observed in the main peak of V 1s, confirming the V4+ and V5+ configurations. In the V 1s spectrum of V2 O3 , the low binding energy feature (labelled B) is observed as in the case of V 2p spectrum (labelled A in Fig. 1). The appearance of the low binding
Fig. 2. V 1s photoemission spectra of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 measured with hν = 5940 eV. The V 1s spectrum of V2 O3 in PM phase shows additional peaks at low binding energy to the main peak, which are labelled B.
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consisting of V5+ and V4+ components. The V 2p and 1s core levels of V2 O3 in the PM phase show an additional low binding energy feature. A cluster model calculation shows that this low binding energy feature originates from screening by coherent states at EF , consistent with its absence in the AFI phase. This low binding energy feature is not observed in the core levels of V2 O5 , VO2 , and Na0.33 V2 O5 . For VO2 , the photoemission experiment is performed at 300 K, that is, in the semiconductor phase, and it would be important to study it in the high temperature metallic phase. Acknowledgments We thank Drs. T. Takeuchi, M. Arita, and J.J. Kim for help with experiments and Profs. K. Katsumata, S.W. Lovesey, and T. Yokoya for valuable discussions. Fig. 3. V 1s photoemission spectra of V2 O3 in the PM and AFI phase. The calculated spectrum for the PM phase is shown in the bottom, where the screening by coherent states is also included.
energy feature (B) in V 1s spectrum in spite of the lower probing depth is consistent with the result of the V 2p soft X-ray PES results which showed a weak low binding energy feature [7]. In Fig. 3, the temperature dependence of V 1s spectra ((a) AFI and (b) PM phase) is illustrated. The observed V 1s spectrum of V2 O3 shows clear temperature dependence across the MI transition, where the low binding energy feature disappears in the AFI phase. For the V 1s spectrum of the PM phase, we show the calculated spectrum obtained using a cluster model. The calculation includes the charge transfer from coherent states at EF [11], which well reproduces the observed V 1s spectrum. The final state 1s1 3d2 mostly contributes to the main line, while the 1s1 3d3 C state, due to screening by coherent states, forms the low binding energy feature. The observed temperature dependence also indicates that the low binding energy feature is surely related to the density of states at EF , since the absence of the low binding energy feature in the AFI phase is consistent with the gap opening in the AFI phase. Another interesting observation in the V 1s spectra of Fig. 3, is the existence of a satellite structure at 5478 eV binding energy. The calculation also reproduces the satellite structure as a charge transfer (CT) satellite formed by 1s1 3d3 L state. The V 1s spectra of V2 O5 , VO2 , and Na0.33 V2 O5 also show the CT satellite (Fig. 2), which have been observed in 2s core level photoemission spectra using soft X-ray [5].
4. Conclusion We have studied the V 2p and 1s core levels of V2 O5 , VO2 , Na0.33 V2 O5 , and V2 O3 using HX-PES. The V 2p and 1s core level spectra of V2 O5 and VO2 show a single peak corresponding to the V5+ and V4+ valence configurations, while that of Na0.33 V2 O5 shows the double peak structure
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