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High resolution soft X-ray photoemission of Kondo insulator YbB12
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673
High resolution soft X-ray photoemission of Kondo insulator YbB12 A. Shigemotoa,∗ , S. Imadaa , A. Sekiyamaa , A. Yamasakia , A. Irizawaa , T. Murob , Y. Saitohc , F. Igad , T. Takabataked , S. Sugaa a
d
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan b Japan Synchrotron Research Institute, Mikazuki, Sayo, Hyogo 679-5198, Japan c Japan Atomic Energy Research Institute, SPring-8, Mikazuki, Sayo, Hyogo 679-5148, Japan Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan Available online 25 February 2005
Abstract We have measured photoemission spectra of a single crystal YbB12 , which is known to be a Kondo insulator, with high energy resolution at 700 eV in the temperature range between 200 and 20 K to study the Kondo resonance behavior in the bulk. The sharp bulk Yb2+ 4f13 peaks observed on fractured YbB12 , which is only weakly influenced by the surface electronic structures, facilitate high accuracy evaluation of the temperature dependences of the Kondo peak energy and the bulk valence. © 2004 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 79.60.−i Keywords: Kondo insulator; Photoemission
YbB12 is known to be a Kondo insulator with a temperature dependent energy gap induced by strong electron correlation. Below the Kondo temperature (TK ), the electric resistivity increases in accordance with the increase of the hybridization strength between the local 4f electron and the conduction electrons. Meanwhile, the magnitude of the hybridization is characterized by the Kondo temperature TK . The TK of YbB12 is around 220 K (∼ =19 meV), which is estimated as 3Tmax , where Tmax = 75 K corresponds to the maximum of the magnetic susceptibility [1,2]. In YbB12 , the Kondo peak has been observed by the low excitation energy photoemission spectroscopy [3,4]. Besides the contributions of the spectral weights of the boron 2p and ytterbium 5d states, the spectral weights of the surface Yb2+ 4f components overlap strongly with those of the bulk Yb2+ 4f components near the Fermi level (EF ). This makes it difficult to accurately estimate the valence in the bulk by low excitation energy photoemission spectroscopy. Therefore we have performed high-energy photoemission spectroscopy of ∗
Corresponding author. Tel.: +81 6 6850 6422; fax: +81 6 6845 4632. E-mail address: [email protected] (A. Shigemoto). 0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.144
YbB12 by using synchrotron radiation at the excitation energy hν of 700 eV with high energy resolution. At this energy, the ratio of the photoionization cross sections Yb 4f/B 2p is 104 times larger than at hν = 21.2 eV and the meanfree path of the photoelectrons reaches several Yb layers beneath the sample surface. Therefore the results at hν = 700 eV are much more sensitive to the bulk Yb 4f electronic states. We measured photoemission spectra of a single crystal YbB12 with use of the synchrotron radiation at BL25SU of SPring-8. Emitted photoelectrons were analyzed by the Scienta SES-200 analyzer. The base pressure in the analyzer chamber was ∼4.0 × 10−8 Pa. We fractured samples at 200 K and took spectra in the sequence of T = 200, 150, 75, 40 and 20 K. The spectrum of Pd, which was electrically connected to the sample, was measured at each temperature to accurately determine the EF position. In addition, Au is used for energy calibration. The energy resolution estimated by Au Fermi edge was 60 meV. Fig. 1(a) shows the valence-band spectrum at 20 K on the fractured surface. The spectral features in the range from 5 to 12 eV are assigned to the 4f12 final state multiplet structures. The atomic multiplet calculation can well reproduce the 4f12
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A. Shigemoto et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673
Fig. 1. Photoemission spectra of YbB12 at hν = 700 eV. (a) Experimental spectrum obtained at 20 K. (b) The total fitting result including Yb2+ bulk, subsurface, and surface components. For fitting of Yb3+ multiplets, the atomic calculation by Gerken is employed. (c) Yb2+ bulk peaks with the aymmetric parameter α = 0.15. (d) Yb2+ subsurface and surface peaks. The asymmetric parameter α for the subsurface peaks is taken to be the same as for the Yb2+ bulk peaks.
multiplet structures [5]. The 4f13 final state multiplets are observed between EF and 3 eV, where the spectrum is governed by the 4f13 spin-orbit doublet and the binding energy of the 4f7/2 peak is estimated to be 34 meV at 20 K. Although the Yb2+ surface peaks (4f5/2 , 4f7/2 ) have been drastically suppressed, they are still recognizable as broad humps near 2 and 1 eV (Fig. 1a). Then, we have performed numerical fitting to the experimental spectra. The reproduced spectrum is shown in Fig. 1(b). The Yb2+ bulk components are calculated with the Ma-
han’s asymmetric parameter α = 0.15 [6]. We have also assumed the third spin-orbit doublet representing the subsurface components. The mean valence of the Yb ion is then estimated to be 2.94 at 20 K and 2.96 ± 0.01 at 200 K. These values were not so accurately estimated by the low energy excitation photoemission spectroscopy [3]. Fig. 2 shows the detailed temperature dependence of the Yb2+ (4f7/2 ) and Yb3+ ( 3 H6 ) 4f peaks. The spectrum is normalized by the intensity of the Yb3+ ( 3 H6 ) 4f peak. The Yb2+ 4f13 peaks shift slightly toward lower binding energies with decreasing the temperature. The amounts of the energy shifts of both 4f5/2 and 4f7/2 peaks are estimated as 6 meV at most. On the other hand, the spectrum shows a noticeable peak shift of the Yb3+ multiplet structures. The energy shift toward higher binding energies is about 30 meV with decreasing the temperature. The energy separation between the center of gravity of the Yb2+ and Yb3+ 4f peaks at 0 K is approximated in the single impurity Anderson model by εf + Uff in the lowest order [7], where εf and Uff represent the energy level of an f electron and on-site Coulomb repulsive energy between the Yb 4f electrons. However, the spectra at finite temperatures should be calculated by non crossing approximation (NCA) calculation in the single impurity model. The peak shifts of both 4f13 and 4f12 peaks with temperature are qualitative consistent with the prediction of NCA with vertex correction. In summary, we have measured photoemission spectra of YbB12 at hν = 700 eV and obtained the bulk Yb 4f electronic structures. We have observed the temperature dependent energy shifts of the Yb2+ 4f components are at most −6 meV, whereas the Yb3+ multiplet peaks shift up to 30 meV toward higher binding energies as the temperature decreases. This result is qualitatively explicable by an NCA calculation. The mean valence of Yb ions is accurately estimated as 2.94 at 20 K and 2.96 at 200 K.
Fig. 2. The temperature dependence of the Yb2+ 4f7/2 peak and the Yb3+ 3 H6 component. The spectral intensities have been normalized by the intensity of 3 H peak. 6
A. Shigemoto et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673
This work was supported by a Grant-in-Aid for Creative Scientific Reasearch (15GS0213) from the Ministry of Education, Science, Sports and Culture, Japan. References [1] F. Iga, N. Shimizu, T. Takabatake, J. Magn. Magn. Mater. 177–181 (1998) 337. [2] N.E. Bickers, D.L. Cox, J.W. Wilkins, Phys. Rev. Lett. 54 (1985) 230.
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[3] T. Susaki, A. Sekiyama, K. Kobayashi, T. Mizokawa, A. Fujimori, M. Tsunekawa, T. Muro, T. Matsushita, S. Suga, H. Ishii, T. Hanyu, A. Kimura, H. Namatame, M. Taniguchi, T. Miyahara, F. Iga, M. Kasaya, H. Harima, Phys. Rev. Lett. 77 (1996) 4269. [4] T. Susaki, A. Fujimori, Y. Takeda, M. Taniguchi, M. Arita, K. Shimada, H. Namatame, S. Hiura, F. Iga, T. Takabatake, J. Phys. Soc. Jpn. 70 (2001) 756. [5] F. Gerken, J. Phys. F13 (1983) 703. [6] G.D. Mahan, Phys. Rev. 163 (1967) 612. [7] H. Sato, K. Yoshikawa, K. Hiraoka, M. Arita, K. Fujimoto, K. Kojima, T. Muro, Y. Saitoh, A. Sekiyama, S. Suga, M. Taniguchi, Phys. Rev. B 69 (2004) 165101.
High resolution soft X-ray photoemission of Kondo insulator YbB12 A. Shigemotoa,∗ , S. Imadaa , A. Sekiyamaa , A. Yamasakia , A. Irizawaa , T. Murob , Y. Saitohc , F. Igad , T. Takabataked , S. Sugaa a
d
Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan b Japan Synchrotron Research Institute, Mikazuki, Sayo, Hyogo 679-5198, Japan c Japan Atomic Energy Research Institute, SPring-8, Mikazuki, Sayo, Hyogo 679-5148, Japan Graduate School of Advanced Sciences of Matter, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan Available online 25 February 2005
Abstract We have measured photoemission spectra of a single crystal YbB12 , which is known to be a Kondo insulator, with high energy resolution at 700 eV in the temperature range between 200 and 20 K to study the Kondo resonance behavior in the bulk. The sharp bulk Yb2+ 4f13 peaks observed on fractured YbB12 , which is only weakly influenced by the surface electronic structures, facilitate high accuracy evaluation of the temperature dependences of the Kondo peak energy and the bulk valence. © 2004 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 79.60.−i Keywords: Kondo insulator; Photoemission
YbB12 is known to be a Kondo insulator with a temperature dependent energy gap induced by strong electron correlation. Below the Kondo temperature (TK ), the electric resistivity increases in accordance with the increase of the hybridization strength between the local 4f electron and the conduction electrons. Meanwhile, the magnitude of the hybridization is characterized by the Kondo temperature TK . The TK of YbB12 is around 220 K (∼ =19 meV), which is estimated as 3Tmax , where Tmax = 75 K corresponds to the maximum of the magnetic susceptibility [1,2]. In YbB12 , the Kondo peak has been observed by the low excitation energy photoemission spectroscopy [3,4]. Besides the contributions of the spectral weights of the boron 2p and ytterbium 5d states, the spectral weights of the surface Yb2+ 4f components overlap strongly with those of the bulk Yb2+ 4f components near the Fermi level (EF ). This makes it difficult to accurately estimate the valence in the bulk by low excitation energy photoemission spectroscopy. Therefore we have performed high-energy photoemission spectroscopy of ∗
Corresponding author. Tel.: +81 6 6850 6422; fax: +81 6 6845 4632. E-mail address: [email protected] (A. Shigemoto). 0368-2048/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.144
YbB12 by using synchrotron radiation at the excitation energy hν of 700 eV with high energy resolution. At this energy, the ratio of the photoionization cross sections Yb 4f/B 2p is 104 times larger than at hν = 21.2 eV and the meanfree path of the photoelectrons reaches several Yb layers beneath the sample surface. Therefore the results at hν = 700 eV are much more sensitive to the bulk Yb 4f electronic states. We measured photoemission spectra of a single crystal YbB12 with use of the synchrotron radiation at BL25SU of SPring-8. Emitted photoelectrons were analyzed by the Scienta SES-200 analyzer. The base pressure in the analyzer chamber was ∼4.0 × 10−8 Pa. We fractured samples at 200 K and took spectra in the sequence of T = 200, 150, 75, 40 and 20 K. The spectrum of Pd, which was electrically connected to the sample, was measured at each temperature to accurately determine the EF position. In addition, Au is used for energy calibration. The energy resolution estimated by Au Fermi edge was 60 meV. Fig. 1(a) shows the valence-band spectrum at 20 K on the fractured surface. The spectral features in the range from 5 to 12 eV are assigned to the 4f12 final state multiplet structures. The atomic multiplet calculation can well reproduce the 4f12
672
A. Shigemoto et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673
Fig. 1. Photoemission spectra of YbB12 at hν = 700 eV. (a) Experimental spectrum obtained at 20 K. (b) The total fitting result including Yb2+ bulk, subsurface, and surface components. For fitting of Yb3+ multiplets, the atomic calculation by Gerken is employed. (c) Yb2+ bulk peaks with the aymmetric parameter α = 0.15. (d) Yb2+ subsurface and surface peaks. The asymmetric parameter α for the subsurface peaks is taken to be the same as for the Yb2+ bulk peaks.
multiplet structures [5]. The 4f13 final state multiplets are observed between EF and 3 eV, where the spectrum is governed by the 4f13 spin-orbit doublet and the binding energy of the 4f7/2 peak is estimated to be 34 meV at 20 K. Although the Yb2+ surface peaks (4f5/2 , 4f7/2 ) have been drastically suppressed, they are still recognizable as broad humps near 2 and 1 eV (Fig. 1a). Then, we have performed numerical fitting to the experimental spectra. The reproduced spectrum is shown in Fig. 1(b). The Yb2+ bulk components are calculated with the Ma-
han’s asymmetric parameter α = 0.15 [6]. We have also assumed the third spin-orbit doublet representing the subsurface components. The mean valence of the Yb ion is then estimated to be 2.94 at 20 K and 2.96 ± 0.01 at 200 K. These values were not so accurately estimated by the low energy excitation photoemission spectroscopy [3]. Fig. 2 shows the detailed temperature dependence of the Yb2+ (4f7/2 ) and Yb3+ ( 3 H6 ) 4f peaks. The spectrum is normalized by the intensity of the Yb3+ ( 3 H6 ) 4f peak. The Yb2+ 4f13 peaks shift slightly toward lower binding energies with decreasing the temperature. The amounts of the energy shifts of both 4f5/2 and 4f7/2 peaks are estimated as 6 meV at most. On the other hand, the spectrum shows a noticeable peak shift of the Yb3+ multiplet structures. The energy shift toward higher binding energies is about 30 meV with decreasing the temperature. The energy separation between the center of gravity of the Yb2+ and Yb3+ 4f peaks at 0 K is approximated in the single impurity Anderson model by εf + Uff in the lowest order [7], where εf and Uff represent the energy level of an f electron and on-site Coulomb repulsive energy between the Yb 4f electrons. However, the spectra at finite temperatures should be calculated by non crossing approximation (NCA) calculation in the single impurity model. The peak shifts of both 4f13 and 4f12 peaks with temperature are qualitative consistent with the prediction of NCA with vertex correction. In summary, we have measured photoemission spectra of YbB12 at hν = 700 eV and obtained the bulk Yb 4f electronic structures. We have observed the temperature dependent energy shifts of the Yb2+ 4f components are at most −6 meV, whereas the Yb3+ multiplet peaks shift up to 30 meV toward higher binding energies as the temperature decreases. This result is qualitatively explicable by an NCA calculation. The mean valence of Yb ions is accurately estimated as 2.94 at 20 K and 2.96 at 200 K.
Fig. 2. The temperature dependence of the Yb2+ 4f7/2 peak and the Yb3+ 3 H6 component. The spectral intensities have been normalized by the intensity of 3 H peak. 6
A. Shigemoto et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 671–673
This work was supported by a Grant-in-Aid for Creative Scientific Reasearch (15GS0213) from the Ministry of Education, Science, Sports and Culture, Japan. References [1] F. Iga, N. Shimizu, T. Takabatake, J. Magn. Magn. Mater. 177–181 (1998) 337. [2] N.E. Bickers, D.L. Cox, J.W. Wilkins, Phys. Rev. Lett. 54 (1985) 230.
673
[3] T. Susaki, A. Sekiyama, K. Kobayashi, T. Mizokawa, A. Fujimori, M. Tsunekawa, T. Muro, T. Matsushita, S. Suga, H. Ishii, T. Hanyu, A. Kimura, H. Namatame, M. Taniguchi, T. Miyahara, F. Iga, M. Kasaya, H. Harima, Phys. Rev. Lett. 77 (1996) 4269. [4] T. Susaki, A. Fujimori, Y. Takeda, M. Taniguchi, M. Arita, K. Shimada, H. Namatame, S. Hiura, F. Iga, T. Takabatake, J. Phys. Soc. Jpn. 70 (2001) 756. [5] F. Gerken, J. Phys. F13 (1983) 703. [6] G.D. Mahan, Phys. Rev. 163 (1967) 612. [7] H. Sato, K. Yoshikawa, K. Hiraoka, M. Arita, K. Fujimoto, K. Kojima, T. Muro, Y. Saitoh, A. Sekiyama, S. Suga, M. Taniguchi, Phys. Rev. B 69 (2004) 165101.