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Soft X-ray photoemission study of CaB6
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 659–661
Soft X-ray photoemission study of CaB6 A. Sekiyama a,∗ , T. Sasabayashi a , A. Higashiya a , H. Fujiwara a , S. Imada a , K. Taniguchi b , H. Takagi b , T. Katsufuji c , K. Kitazawa b,d , S. Suga a a
Department of Material Physics, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan b Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8562, Japan c Department of Physics, Waseda University, Tokyo 169-8555, Japan d Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan Available online 7 March 2005
Abstract We have performed the soft X-ray photoemission and absorption studies of CaB6 . The crystal field splitting in the Ca 3d levels is estimated as 0.65 eV from the analysis of the Ca 2p X-ray absorption spectrum. It is found that the surface Ca 2p contribution is seen at the lower binding energy side of the bulk Ca 2p peak in the core-level spectra. The Ca 2p–3d resonance photoemission shows the clear enhancement of the spectral weight in the vicinity of the Fermi level. © 2005 Elsevier B.V. All rights reserved. Keywords: Photoemission; X-ray absorption; CaB6
1. Introduction
2. Experimental
After “weak ferromagnetism” in alkaline-earth hexaborides such as CaB6 doped with La has been reported [1], many studies have been performed on these materials. From an experimental viewpoint, it has been discussed whether the ferromagnetism in the doped hexaborides originates intrinsically from the bulk or not [2,3]. Recent studies have shown that the ferromagnetism is strongly suppressed by etching or acid treatment of as-grown Ca1−x Lax B6 [2,4]. So far reported low-energy angle-resolved photoemission (ARPES) studies of CaB6 show an energy gap of the order of 1 eV between the valence and conduction bands and a small electron pocket at the Fermi level (EF ) [5,6]. In order to check whether those findings reflect the intrinsic electronic structures or not, we have performed the bulk-sensitive soft X-ray photoemission (PES) [7] and absorption (XAS) study of CaB6 .
The single crystals of CaB6 etched by HNO3 were used for the experiments [4]. The XAS and PES measurements were performed at BL25SU in SPring-8. The base pressure was about 4 × 10−8 Pa. The XAS spectra were obtained by the total photoelectron yield mode. In order to obtain clean surfaces, the samples were cleaved in situ at the measuring temperature of 20 K. The surface cleanliness was confirmed by the absence of the possible Fe 2p, O 1s, N 1s and C 1s signals. The Fermi level was calibrated by measuring the gold electrically connected to the sample.
∗
Corresponding author. Tel.: +81 6 6850 6422; fax: +81 6 6845 4632. E-mail address: [email protected] (A. Sekiyama).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.100
3. Ca 2p–3d XAS spectra Fig. 1 shows the Ca 2p–3d XAS spectrum of CaB6 with the energy resolution of ∼100 meV. In addition to strong peaks at 349 and 352 eV corresponding to the 2p5 3d1 final states, weak peaks are seen at 346.8, 347.4, 348 and 351 eV. The spectral line shape is qualitatively similar to that of a 3d0 system such as CaF2 [8]. A weak shoulder structure is also observed at ∼355 eV although an origin of this is not clear at present.
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A. Sekiyama et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 659–661
found that the surface states appear in the lower binding energy side of the bulk contributions in this compound. An origin of the surface core-level shift towards the lower binding energy side is not clear at present. In the spectra, weak and broad peaks are seen at ∼354 and ∼358 eV. These are ascribed to the satellite structure originating from the charge-transfer between the valence-band and the Ca 3d states in the photoelectron final state, where the energy between the main and satellite peaks is about 6 eV.
5. Valence-band PES spectra
Fig. 1. Comparison of the experimental and calculated Ca 2p–3d X-ray absorption spectra of CaB6 .
It is known that the energy positions and intensities of the weak peaks depend on the crystal field splitting of the 3d level [8]. We have also performed the cluster model calculation for the Ca 2p XAS spectra in which the crystal field splitting is taken into account. The calculated spectrum is shown in the bottom panel of Fig. 1, well reproducing the experimental spectrum. By the fitting of the spectrum, the crystal field splitting is estimated as 0.65 ± 0.1 eV, where the eg states are lower than the t2g states.
4. Core-level PES spectra Photon energy dependence of the Ca 2p spectra is shown in Fig. 2. There are peaks at 347.8 and 351.4 eV, and shoulders at the lower binding energy side by 0.8 eV of the main peaks in the spectrum measured at hν = 730 eV, where the energy resolution was set to ∼200 meV. These shoulder structures are noticeably enhanced in the relatively surface-sensitive spectrum at hν = 400 eV with the energy resolution of ∼100 meV for which the photoelectron kinetic energy is about 50 eV. Therefore, the observed shoulders in the spectrum at hν = 730 eV originate from the surface Ca 2p contribution. Namely, it is
Fig. 2. Ca 2p core-level spectra of CaB6 measured at hν = 400 and 730 eV.
Fig. 3 demonstrates the valence-band PES spectra of CaB6 , in which peaks are recognized at ∼4, ∼6, ∼10 and ∼16 eV. According to band-structure calculations [9,10], the peaks at ∼10 and 16 eV originate from the B 2s states strongly hybridized with the 2p states. The spectral weight from EF to ∼7 eV is mainly due to the B 2p states hybridized with the B 2s and Ca 3d states. There is no clear difference between the spectra measured at hν = 340 and 730 eV. The spectral weight decreases toward EF from ∼4 eV, where the threshold of the spectra is about 1 eV. This feature suggests a clear band gap between the valence and conduction bands, which is consistent with the previous low-energy ARPES studies [5,6]. We have also measured the Ca 2p–3d resonance PES spectra as shown in Fig. 4. In the resonance spectrum at hν = 349 eV, the Ca 3d-derived spectral weight is clearly enhanced in the vicinity of EF . This result suggests that the bottom of the conduction band originates predominantly from the Ca 3d states and is located near EF . Namely, the excess electrons are induced in the bottom of the conduction band by defects as previously discussed [5]. We can notice that another large spectral weight is seen in the binding energy region of >0.5 eV. This is mainly due to an Auger contribution, which has been confirmed by measuring several resonance PES spectra with changing hν in the Ca 2p absorption edge. Such an Auger contribution implies that
Fig. 3. Valence-band photoemission spectra of CaB6 measured at hν = 344 and 730 eV. hν = 344 eV is below the Ca 2p absorption threshold. The energy resolution for these spectra was set to 200 meV.
A. Sekiyama et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 659–661
661
search (15GS0213) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The experiments were performed under the approval of the Japan Synchrotron Radiation Research Institute (2002A0428-NS1-np).
References
Fig. 4. Ca 2p–3d resonance photoemission spectra near EF with the energy resolution of ∼100 meV.
the Ca 3d states contribute partly to the valence band via the hybridization with the B 2p states.
Acknowledgements We thank H. Harima for providing the results of the bandstructure calculation. We are grateful to A. Yamasaki, T. Satonaka, K. Konoike, T. Muro and Y. Saitoh for supporting the experiments. This work was supported by a Grant-in-Aid for COE research (10CE2004) and Creative Scientific Re-
[1] D.P. Young, D. Hall, M.E. Torelli, Z. Fisk, J.L. Sarrao, J.D. Thompson, H.-R. Ott, S.B. Oseroff, R.G. Goodrich, R. Zysler, Nature 397 (1999) 412. [2] K. Matsubayashi, M. Maki, T. Tsuzuki, T. Nishioka, N.K. Sato, Nature 420 (2002) 143. [3] D.P. Young, Z. Fisk, J.D. Thompson, H.-R. Ott, S.B. Oseroff, R.G. Goodrich, Nature 420 (2002) 144. [4] K. Taniguchi, T. Katsufuji, F. Sakai, H. Ueda, K. Kitazawa, H. Takagi, Phys. Rev. B 66 (2002) 064407. [5] J.D. Denlinger, J.A. Clack, J.W. Allen, G.-H. Gweon, D.M. Poirier, C.G. Olson, J.L. Sarrao, A.D. Bianchi, Z. Fisk, Phys. Rev. Lett. 89 (2002) 157601. [6] S. Souma, H. Komatsu, T. Takahashi, R. Kaji, T. Sasaki, Y. Yokoo, J. Akimitsu, Phys. Rev. Lett. 90 (2003) 027202. [7] A. Sekiyama, K. Kadono, K. Matsuda, T. Iwasaki, S. Ueda, S. Imada, S. Suga, R. Settai, H. Azuma, Y. Onuki, Y. Saitoh, J. Phys. Soc. Jpn. 69 (2000) 2771. [8] F.M.F. de Groot, J.C. Fuggle, B.T. Thole, G.A. Sawatzky, Phys. Rev. B 41 (1990) 928. [9] H. Harima, unpublished. [10] H.J. Tromp, P. van Gelderen, P.J. Kelly, G. Brocks, P.A. Bobbert, Phys. Rev. Lett. 87 (2001) 016401.
Soft X-ray photoemission study of CaB6 A. Sekiyama a,∗ , T. Sasabayashi a , A. Higashiya a , H. Fujiwara a , S. Imada a , K. Taniguchi b , H. Takagi b , T. Katsufuji c , K. Kitazawa b,d , S. Suga a a
Department of Material Physics, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan b Department of Advanced Materials Science, University of Tokyo, Kashiwa, Chiba 277-8562, Japan c Department of Physics, Waseda University, Tokyo 169-8555, Japan d Japan Science and Technology Agency, Kawaguchi, Saitama 332-0012, Japan Available online 7 March 2005
Abstract We have performed the soft X-ray photoemission and absorption studies of CaB6 . The crystal field splitting in the Ca 3d levels is estimated as 0.65 eV from the analysis of the Ca 2p X-ray absorption spectrum. It is found that the surface Ca 2p contribution is seen at the lower binding energy side of the bulk Ca 2p peak in the core-level spectra. The Ca 2p–3d resonance photoemission shows the clear enhancement of the spectral weight in the vicinity of the Fermi level. © 2005 Elsevier B.V. All rights reserved. Keywords: Photoemission; X-ray absorption; CaB6
1. Introduction
2. Experimental
After “weak ferromagnetism” in alkaline-earth hexaborides such as CaB6 doped with La has been reported [1], many studies have been performed on these materials. From an experimental viewpoint, it has been discussed whether the ferromagnetism in the doped hexaborides originates intrinsically from the bulk or not [2,3]. Recent studies have shown that the ferromagnetism is strongly suppressed by etching or acid treatment of as-grown Ca1−x Lax B6 [2,4]. So far reported low-energy angle-resolved photoemission (ARPES) studies of CaB6 show an energy gap of the order of 1 eV between the valence and conduction bands and a small electron pocket at the Fermi level (EF ) [5,6]. In order to check whether those findings reflect the intrinsic electronic structures or not, we have performed the bulk-sensitive soft X-ray photoemission (PES) [7] and absorption (XAS) study of CaB6 .
The single crystals of CaB6 etched by HNO3 were used for the experiments [4]. The XAS and PES measurements were performed at BL25SU in SPring-8. The base pressure was about 4 × 10−8 Pa. The XAS spectra were obtained by the total photoelectron yield mode. In order to obtain clean surfaces, the samples were cleaved in situ at the measuring temperature of 20 K. The surface cleanliness was confirmed by the absence of the possible Fe 2p, O 1s, N 1s and C 1s signals. The Fermi level was calibrated by measuring the gold electrically connected to the sample.
∗
Corresponding author. Tel.: +81 6 6850 6422; fax: +81 6 6845 4632. E-mail address: [email protected] (A. Sekiyama).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.100
3. Ca 2p–3d XAS spectra Fig. 1 shows the Ca 2p–3d XAS spectrum of CaB6 with the energy resolution of ∼100 meV. In addition to strong peaks at 349 and 352 eV corresponding to the 2p5 3d1 final states, weak peaks are seen at 346.8, 347.4, 348 and 351 eV. The spectral line shape is qualitatively similar to that of a 3d0 system such as CaF2 [8]. A weak shoulder structure is also observed at ∼355 eV although an origin of this is not clear at present.
660
A. Sekiyama et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 659–661
found that the surface states appear in the lower binding energy side of the bulk contributions in this compound. An origin of the surface core-level shift towards the lower binding energy side is not clear at present. In the spectra, weak and broad peaks are seen at ∼354 and ∼358 eV. These are ascribed to the satellite structure originating from the charge-transfer between the valence-band and the Ca 3d states in the photoelectron final state, where the energy between the main and satellite peaks is about 6 eV.
5. Valence-band PES spectra
Fig. 1. Comparison of the experimental and calculated Ca 2p–3d X-ray absorption spectra of CaB6 .
It is known that the energy positions and intensities of the weak peaks depend on the crystal field splitting of the 3d level [8]. We have also performed the cluster model calculation for the Ca 2p XAS spectra in which the crystal field splitting is taken into account. The calculated spectrum is shown in the bottom panel of Fig. 1, well reproducing the experimental spectrum. By the fitting of the spectrum, the crystal field splitting is estimated as 0.65 ± 0.1 eV, where the eg states are lower than the t2g states.
4. Core-level PES spectra Photon energy dependence of the Ca 2p spectra is shown in Fig. 2. There are peaks at 347.8 and 351.4 eV, and shoulders at the lower binding energy side by 0.8 eV of the main peaks in the spectrum measured at hν = 730 eV, where the energy resolution was set to ∼200 meV. These shoulder structures are noticeably enhanced in the relatively surface-sensitive spectrum at hν = 400 eV with the energy resolution of ∼100 meV for which the photoelectron kinetic energy is about 50 eV. Therefore, the observed shoulders in the spectrum at hν = 730 eV originate from the surface Ca 2p contribution. Namely, it is
Fig. 2. Ca 2p core-level spectra of CaB6 measured at hν = 400 and 730 eV.
Fig. 3 demonstrates the valence-band PES spectra of CaB6 , in which peaks are recognized at ∼4, ∼6, ∼10 and ∼16 eV. According to band-structure calculations [9,10], the peaks at ∼10 and 16 eV originate from the B 2s states strongly hybridized with the 2p states. The spectral weight from EF to ∼7 eV is mainly due to the B 2p states hybridized with the B 2s and Ca 3d states. There is no clear difference between the spectra measured at hν = 340 and 730 eV. The spectral weight decreases toward EF from ∼4 eV, where the threshold of the spectra is about 1 eV. This feature suggests a clear band gap between the valence and conduction bands, which is consistent with the previous low-energy ARPES studies [5,6]. We have also measured the Ca 2p–3d resonance PES spectra as shown in Fig. 4. In the resonance spectrum at hν = 349 eV, the Ca 3d-derived spectral weight is clearly enhanced in the vicinity of EF . This result suggests that the bottom of the conduction band originates predominantly from the Ca 3d states and is located near EF . Namely, the excess electrons are induced in the bottom of the conduction band by defects as previously discussed [5]. We can notice that another large spectral weight is seen in the binding energy region of >0.5 eV. This is mainly due to an Auger contribution, which has been confirmed by measuring several resonance PES spectra with changing hν in the Ca 2p absorption edge. Such an Auger contribution implies that
Fig. 3. Valence-band photoemission spectra of CaB6 measured at hν = 344 and 730 eV. hν = 344 eV is below the Ca 2p absorption threshold. The energy resolution for these spectra was set to 200 meV.
A. Sekiyama et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 659–661
661
search (15GS0213) from the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The experiments were performed under the approval of the Japan Synchrotron Radiation Research Institute (2002A0428-NS1-np).
References
Fig. 4. Ca 2p–3d resonance photoemission spectra near EF with the energy resolution of ∼100 meV.
the Ca 3d states contribute partly to the valence band via the hybridization with the B 2p states.
Acknowledgements We thank H. Harima for providing the results of the bandstructure calculation. We are grateful to A. Yamasaki, T. Satonaka, K. Konoike, T. Muro and Y. Saitoh for supporting the experiments. This work was supported by a Grant-in-Aid for COE research (10CE2004) and Creative Scientific Re-
[1] D.P. Young, D. Hall, M.E. Torelli, Z. Fisk, J.L. Sarrao, J.D. Thompson, H.-R. Ott, S.B. Oseroff, R.G. Goodrich, R. Zysler, Nature 397 (1999) 412. [2] K. Matsubayashi, M. Maki, T. Tsuzuki, T. Nishioka, N.K. Sato, Nature 420 (2002) 143. [3] D.P. Young, Z. Fisk, J.D. Thompson, H.-R. Ott, S.B. Oseroff, R.G. Goodrich, Nature 420 (2002) 144. [4] K. Taniguchi, T. Katsufuji, F. Sakai, H. Ueda, K. Kitazawa, H. Takagi, Phys. Rev. B 66 (2002) 064407. [5] J.D. Denlinger, J.A. Clack, J.W. Allen, G.-H. Gweon, D.M. Poirier, C.G. Olson, J.L. Sarrao, A.D. Bianchi, Z. Fisk, Phys. Rev. Lett. 89 (2002) 157601. [6] S. Souma, H. Komatsu, T. Takahashi, R. Kaji, T. Sasaki, Y. Yokoo, J. Akimitsu, Phys. Rev. Lett. 90 (2003) 027202. [7] A. Sekiyama, K. Kadono, K. Matsuda, T. Iwasaki, S. Ueda, S. Imada, S. Suga, R. Settai, H. Azuma, Y. Onuki, Y. Saitoh, J. Phys. Soc. Jpn. 69 (2000) 2771. [8] F.M.F. de Groot, J.C. Fuggle, B.T. Thole, G.A. Sawatzky, Phys. Rev. B 41 (1990) 928. [9] H. Harima, unpublished. [10] H.J. Tromp, P. van Gelderen, P.J. Kelly, G. Brocks, P.A. Bobbert, Phys. Rev. Lett. 87 (2001) 016401.