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Para- to antiferro-magnetic phase transition of CeSb studied by ultrahigh-resolution angle-resolved photoemission spectroscopy
ARTICLE IN PRESS
Physica B 351 (2004) 268–270
Para- to antiferro-magnetic phase transition of CeSb studied by ultrahigh-resolution angle-resolved photoemission spectroscopy T. Itoa,*, S. Kimuraa, H. Kitazawab b
a UVSOR Facility, Institute for Molecular Science, Okazaki, 444-8585, Japan Nano-Materials Laboratory, National Institute for Materials Science (NIMS), Tsukuba, 305-0047, Japan
Abstract Temperature-dependent angle-resolved photoemission spectroscopy has been performed on CeSb to study the origin of its complicated magnetic phase transition. In paramagnetic phase (T ¼ 30 K), we have found that the electronic structure near the Fermi level (EF) consists of the hole-like Sb 5p band at the G point and the electron-like Ce 5d bands at the X point. With decreasing temperature across TN (TN ¼ 10 K), both the energy shift and the energy splitting of the bands appear along the GX high-symmetry line. While the energy shift of the bands is consistent with the pf mixing model, the energy splitting has not been expected in the model so far. On the other hand, by comparing with the recent calculation based on the pf+dp mixing model, we found a qualitative agreement between the experiment and the calculation. This result suggests the importance of the dp mixing effect to interpret the mechanism of the magnetic phase transition of CeSb. r 2004 Elsevier B.V. All rights reserved. PACS: 79.60.i; 71.27.+a; 71.20.Eh Keywords: Angle-resolved photoemission spectroscopy; Band structure; Magnetic phase transition
CeSb is a typical low-carrier strongly correlated electron system, where the anomalously complicated magnetic phase transition has been observed as a function of temperature and/or magnetic field [1]. It has been believed that the magnetic phase diagram was qualitatively understood by the Ce 4f–Sb 5p mixing model (pf mixing model) suggested theoretically by Kasuya [2], since the model explains well, the heavy Fermi surface observed in *Corresponding author. Fax: +81-564-54-7079. E-mail address: [email protected] (T. Ito).
the ferro-magnetic phase by the de Haas–van Alphen experiments [3] and the change of the band structure from the para to the antiferro-para magnetic phases detected by an angle-resolved photoemission (ARPES) experiment [4]. On the other hand, the recent optical and magneto-optical experiments suggest the importance of the Ce 5d–Sb 5p mixing effect from the comparison with the band structure calculation based on the pf+pd mixing model [5,6]. Thus, the appropriate starting point to explain the complicated magnetic phase transition has not been achieved yet. In this paper,
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.06.022
ARTICLE IN PRESS T. Ito et al. / Physica B 351 (2004) 268–270
we report high-resolution ARPES on CeSb at the para- and type-IA antiferro magnetic (AF) phase to elucidate the temperature dependence of the electronic structure. Single crystals of CeSb were grown by the Bridgman method in tungsten furnaces. The ARPES measurements were carried out using a MBS Toyama A-1 electron analyzer with a GAMMADATA discharge lamp and a toroidal grating monochromator. The energy and angular resolution was set at 15 meV and 70.1 , respectively. We used He Ia resonance line (21.218 eV) to excite photoelectrons. The single crystals were cleaved on the (0 0 1) plane in situ under vacuum of 2 108 Pa to obtain a clean mirror-like surface. The Fermi level of the sample was referred to a gold film evaporated onto the sample substrate. Fig. 1(a) and (b) shows the ‘‘band structure’’ near the Fermi level (EF) of CeSb around the G(X) point along the GX-XWX emission plane measured at the para- (T ¼ 30 K) and the antiferro(T ¼ 5 K) magnetic phase, respectively. The experimental band structures have been derived by taking the second derivative of the moderately smoothed spectra and plotting the intensity as a function of the wave vector and the binding energy. The bright areas correspond to ‘‘bands’’. It should be noted here that we have combined the second derivative of the energy distribution curves and momentum distribution curves to show the dispersive features clear [7]. In the reported ARPES study [4], it has been suggested that
269
the ARPES measurement of CeSb with He Ia photons belongs to the so-called ‘‘band-gap’’ case, in which high-symmetry lines (e.g., both the GX and XWX lines) appear as prominent structures owing to the lack of appropriate final states in the photoexcitation process [8]. Thus, the bands with their top at the G point are assigned as the valence Sb 5p bands (A1(0 ) and A2(0 ) in Fig. 1), while ones with their bottom at the X point as the conduction Ce 5d band (A3(0 ) in Fig. 1). When we compare the band structure of the paramagnetic CeSb with the reported one [4], the former agrees well with the previous ARPES, while the latter has not been observed possibly due to the insufficient energy and/or momentum resolutions. From the para- to AF magnetic phase, we have observed two types of changes in the band structure near EF. (1) The energy positions of the Sb 5p (A1(0 ) and A2(0 )) and the Ce 5d bands (A3(0 )) around the G(X) point shift to the lower and the higher binding energies, respectively (see Fig. 1(c)). (2) The additional band B with its top around 200 meV appears at the antiferromagnetic phase. It has been believed that the magnetic phase transition in CeSb can be understood under the framework of the pf mixing model [2,4] where the Sb 5p bands at the G point expect to be pushed up to EF following the simultaneous downward shift of the Ce 5d bands. Thus, the temperature dependence of the former seems to be consistent with the pf mixing model. On the other hand, the latter is not explained by the model, since the band B is
Fig. 1. Experimental band structure of paramagnetic: (a) and antiferromagnetic, (b) CeSb derived by ARPES. Dashed lines with symbols (A1(0 )-A2(0 ), B) are guide for eyes. Circles and triangles correspond to the ARPES peak positions assigned in (c). (c) Temperature dependence of ARPES spectra at the G(X) point.
ARTICLE IN PRESS 270
T. Ito et al. / Physica B 351 (2004) 268–270
suggestive of the band energy-splitting which has not been expected by the pf mixing model. To understand the anomalous feature in Fig. 1(b), we have compared the experimental band structure at AF phase with the recent theoretical calculation based on the pf+pd mixing model [6]. As a result, we have found a qualitative agreement between the experiment and the calculation. In the calculation, the band energy-splitting originating in the pf+pd mixing effect along the GX-axis has been expected, while the pf mixing model has no energy splitting for the corresponding band. This result suggests the importance of the pd mixing effect to interpret the exact mechanism of the magnetic phase transition in CeSb other than the pf mixing effect. The details of the comparison between the experiment and the calculation will be published elsewhere.
Acknowledgements We thank Dr. F. Ishiyama for valuable discussions. This work was supported by grants from the Ministry of Education, Culture and Science of Japan. References [1] J. Rossat-Mignod, et al., J. Magn. Magn. Mater. 52 (1985) 111. [2] T. Kasuya, Physica B 215 (1995) 88. [3] R. Settai, et al., J. Phys. Soc. Japan 63 (1994) 3026. [4] H. Kumigashira, et al., Phys. Rev. B 56 (1997) 13654. [5] S. Kimura, et al., J. Phys. Soc. Japan 71 (2002) 2200. [6] F. Ishiyama, O. Sakai, J. Phys. Soc. Japan 72 (2003) 2071. [7] S.-I. Fujimori, et al., Phys. Rev. B 67 (2003) 144507. [8] T. Grandke, L. Ley, M. Cardona, Phys. Rev. B 18 (1978) 3847.
Physica B 351 (2004) 268–270
Para- to antiferro-magnetic phase transition of CeSb studied by ultrahigh-resolution angle-resolved photoemission spectroscopy T. Itoa,*, S. Kimuraa, H. Kitazawab b
a UVSOR Facility, Institute for Molecular Science, Okazaki, 444-8585, Japan Nano-Materials Laboratory, National Institute for Materials Science (NIMS), Tsukuba, 305-0047, Japan
Abstract Temperature-dependent angle-resolved photoemission spectroscopy has been performed on CeSb to study the origin of its complicated magnetic phase transition. In paramagnetic phase (T ¼ 30 K), we have found that the electronic structure near the Fermi level (EF) consists of the hole-like Sb 5p band at the G point and the electron-like Ce 5d bands at the X point. With decreasing temperature across TN (TN ¼ 10 K), both the energy shift and the energy splitting of the bands appear along the GX high-symmetry line. While the energy shift of the bands is consistent with the pf mixing model, the energy splitting has not been expected in the model so far. On the other hand, by comparing with the recent calculation based on the pf+dp mixing model, we found a qualitative agreement between the experiment and the calculation. This result suggests the importance of the dp mixing effect to interpret the mechanism of the magnetic phase transition of CeSb. r 2004 Elsevier B.V. All rights reserved. PACS: 79.60.i; 71.27.+a; 71.20.Eh Keywords: Angle-resolved photoemission spectroscopy; Band structure; Magnetic phase transition
CeSb is a typical low-carrier strongly correlated electron system, where the anomalously complicated magnetic phase transition has been observed as a function of temperature and/or magnetic field [1]. It has been believed that the magnetic phase diagram was qualitatively understood by the Ce 4f–Sb 5p mixing model (pf mixing model) suggested theoretically by Kasuya [2], since the model explains well, the heavy Fermi surface observed in *Corresponding author. Fax: +81-564-54-7079. E-mail address: [email protected] (T. Ito).
the ferro-magnetic phase by the de Haas–van Alphen experiments [3] and the change of the band structure from the para to the antiferro-para magnetic phases detected by an angle-resolved photoemission (ARPES) experiment [4]. On the other hand, the recent optical and magneto-optical experiments suggest the importance of the Ce 5d–Sb 5p mixing effect from the comparison with the band structure calculation based on the pf+pd mixing model [5,6]. Thus, the appropriate starting point to explain the complicated magnetic phase transition has not been achieved yet. In this paper,
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.06.022
ARTICLE IN PRESS T. Ito et al. / Physica B 351 (2004) 268–270
we report high-resolution ARPES on CeSb at the para- and type-IA antiferro magnetic (AF) phase to elucidate the temperature dependence of the electronic structure. Single crystals of CeSb were grown by the Bridgman method in tungsten furnaces. The ARPES measurements were carried out using a MBS Toyama A-1 electron analyzer with a GAMMADATA discharge lamp and a toroidal grating monochromator. The energy and angular resolution was set at 15 meV and 70.1 , respectively. We used He Ia resonance line (21.218 eV) to excite photoelectrons. The single crystals were cleaved on the (0 0 1) plane in situ under vacuum of 2 108 Pa to obtain a clean mirror-like surface. The Fermi level of the sample was referred to a gold film evaporated onto the sample substrate. Fig. 1(a) and (b) shows the ‘‘band structure’’ near the Fermi level (EF) of CeSb around the G(X) point along the GX-XWX emission plane measured at the para- (T ¼ 30 K) and the antiferro(T ¼ 5 K) magnetic phase, respectively. The experimental band structures have been derived by taking the second derivative of the moderately smoothed spectra and plotting the intensity as a function of the wave vector and the binding energy. The bright areas correspond to ‘‘bands’’. It should be noted here that we have combined the second derivative of the energy distribution curves and momentum distribution curves to show the dispersive features clear [7]. In the reported ARPES study [4], it has been suggested that
269
the ARPES measurement of CeSb with He Ia photons belongs to the so-called ‘‘band-gap’’ case, in which high-symmetry lines (e.g., both the GX and XWX lines) appear as prominent structures owing to the lack of appropriate final states in the photoexcitation process [8]. Thus, the bands with their top at the G point are assigned as the valence Sb 5p bands (A1(0 ) and A2(0 ) in Fig. 1), while ones with their bottom at the X point as the conduction Ce 5d band (A3(0 ) in Fig. 1). When we compare the band structure of the paramagnetic CeSb with the reported one [4], the former agrees well with the previous ARPES, while the latter has not been observed possibly due to the insufficient energy and/or momentum resolutions. From the para- to AF magnetic phase, we have observed two types of changes in the band structure near EF. (1) The energy positions of the Sb 5p (A1(0 ) and A2(0 )) and the Ce 5d bands (A3(0 )) around the G(X) point shift to the lower and the higher binding energies, respectively (see Fig. 1(c)). (2) The additional band B with its top around 200 meV appears at the antiferromagnetic phase. It has been believed that the magnetic phase transition in CeSb can be understood under the framework of the pf mixing model [2,4] where the Sb 5p bands at the G point expect to be pushed up to EF following the simultaneous downward shift of the Ce 5d bands. Thus, the temperature dependence of the former seems to be consistent with the pf mixing model. On the other hand, the latter is not explained by the model, since the band B is
Fig. 1. Experimental band structure of paramagnetic: (a) and antiferromagnetic, (b) CeSb derived by ARPES. Dashed lines with symbols (A1(0 )-A2(0 ), B) are guide for eyes. Circles and triangles correspond to the ARPES peak positions assigned in (c). (c) Temperature dependence of ARPES spectra at the G(X) point.
ARTICLE IN PRESS 270
T. Ito et al. / Physica B 351 (2004) 268–270
suggestive of the band energy-splitting which has not been expected by the pf mixing model. To understand the anomalous feature in Fig. 1(b), we have compared the experimental band structure at AF phase with the recent theoretical calculation based on the pf+pd mixing model [6]. As a result, we have found a qualitative agreement between the experiment and the calculation. In the calculation, the band energy-splitting originating in the pf+pd mixing effect along the GX-axis has been expected, while the pf mixing model has no energy splitting for the corresponding band. This result suggests the importance of the pd mixing effect to interpret the exact mechanism of the magnetic phase transition in CeSb other than the pf mixing effect. The details of the comparison between the experiment and the calculation will be published elsewhere.
Acknowledgements We thank Dr. F. Ishiyama for valuable discussions. This work was supported by grants from the Ministry of Education, Culture and Science of Japan. References [1] J. Rossat-Mignod, et al., J. Magn. Magn. Mater. 52 (1985) 111. [2] T. Kasuya, Physica B 215 (1995) 88. [3] R. Settai, et al., J. Phys. Soc. Japan 63 (1994) 3026. [4] H. Kumigashira, et al., Phys. Rev. B 56 (1997) 13654. [5] S. Kimura, et al., J. Phys. Soc. Japan 71 (2002) 2200. [6] F. Ishiyama, O. Sakai, J. Phys. Soc. Japan 72 (2003) 2071. [7] S.-I. Fujimori, et al., Phys. Rev. B 67 (2003) 144507. [8] T. Grandke, L. Ley, M. Cardona, Phys. Rev. B 18 (1978) 3847.