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Magnetic phase transition of CeSb studied by high-resolution angle-resolved photoemission
Iourr~al o[ Magnetism and Magnetic Materials 1"/7-181 (1998) 1027-t028
,i~
j~ ELSEVIER
Journal of
magnetism and magnetic materials
Magnetic phase transition of CeSb studied by high-resolution angle-resolved photoemission T. Takahashi*, H. Kumigashira, H.-D. Kim, A. Ashihara, A. Chainani, T. Yokoya, A. Uesawa, T. Suzuki Department q[ Pkysics, Tohoku University, Sendai 980- 77, Japan
Abstract We observed a change of Fermi-surface topology of CeSb across the magnetic phase transition using temperaturedependent high-resolution angle-resolved photoemission spectroscopy. The experimental results show that the complicated magnetic structure of CeSb strongly relates to its unique electronic structure as predicted by the anisotropic p-f mixing model. ~ 1998 Elsevier Science B.V. All rights reserved. K
Semimetallic CeSb shows a complicated magnetic phase transition with temperature and/or external magnetic field [I]. It shows the first-order phase transition from the paramagnetic (P-) to the antiferroparamagnetic (AFP-) phase at T~ = 16 K, being followed by at least six different AFP phases at lower temperature, then transforming into the antiferromagnetic (AF) phase at 6 K. It has been proposed that the anisotropic p-f mixing plays an essential role in the phase transition [2]. In contrast to de Haas-van Alphen measurement, angle-resolved photoentission spectroscopy (ARPES) has a wider temperature range of measurement as well as a capability to directly identify the character of Fermi surface (electronor hole-like) and its exact location in the Brillouin zone. ARPES measurement was carried out with a highresolution (HR) ARPES spectrometer constructed at Tohoku University. Angular and energy resolution were set at about 1 and 50 meV, respectively. Single crystals were cleaved in situ along the ( 1 0 0) plane at 30 K in the spectrometer (base pressure 2 x 10- t t Tort). Fig. 1 shows HR-ARPES spectra of the P-phase at 30 K and the fifth AFP-phase (AFP5) at 10 K, measured along F-X direction in the Brillouin zone. HR-ARPES spectra of both phases exhibit remarkable and systematic
*Corresponding author. Fax: +81222176419: e-mail: t.takahashi~ msp.phys.tohoku.ac.jp.
changes as a function of polar angle ((9), indicating the complicated band structure of CeSb. We find that the overall band dispersion looks similar in both phases as well as to that of CeBi [3]. In the near-EF region, we have identified two types of Fermi-level (EF) crossings of dispersive bands; (1) two hole pockets at F point as marked by 'h~' and 'h2' in Fig. 1, and (2) an electron pocket at X(M) point marked by "e'. Comparing HR-ARPES spectra near Er of two phases, we find clear difference in the intensity and the position of peaks, which should reflect the change of the band structure and the Fermi surface topology across the phase transition. To see the change more clearly, we show the ~band dispersion' for both phases in Fig. 2, which were derived front the HRARPES spectra in Fig. 1 by taking the second derivative after smoothing and plotting tile intensity in the squareroot scale by gradual shading as a function of the wave vector parallel to the 1--X(M) direction [4]; dark parts correspond to bands. Fig. 2 shows the existence of two hole pockets at Y point and an electron pocket at X(M) point in good agreement with the prediction of the band calculation [5]. According to the band calculation based on the p-f mixing model, the hole pockets are ascribed to the Sb 5P3/2 states while the electron pocket has a dominant Ce 5dt2g character. Ce ions in CeSb are thought to be trivalent, so that the Ce 4fs~2 state splits into the Ks quartet and the 1-7 doublet due to the cubic crystal field, with the Fs state being located at a higher energy.
0304-8853/98/S19.00 0 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 4 4 0 - X
1028
L Takahashi et al. / Journal of Magnetism and Magnetic Materials 177-181 (1998) 102 7-1028
.......F-X !a) 1 ..... - ~ i CeSb T=30K ~!
~"
b
LLA/Z °:
EF
0.5
0
ea)
.=~ 1.0 1.5
~=
51 =
~5o
'10
~,~ £
' 10
!15 °
~
i15'
F--A--X X--W--X Wave Vector
(a/
EF ' 20 °
20
O
0.5
1.0
25
h~ t.5 30 4
3 2 1 EF Binding Energy (eV)
4
3 2 1 Ev Binding Energy (eV)
(b)
F Z ~ S
Z - - M - - A
Wave Vector
Fig. 1. High-resolution angle-resolved photoemission spectra of (a) paramagnetic (30 K) and (b) antiferroparamagnetic (10 K) CeSb, measured with He I photons (21.2 eV) along F-X(M) direction in the Brillouin zone.
Fig. 2. Experimental band structure of (a) paramagnetic and (b) antiferroparamagnetic CeSb determined by the present HRARPES measurements. Dark parts correspond to the energy bands and dashed white lines are guide to eyes.
The key mechanism of p-f mixing model is the anisotropic hybridization between the Sb 5p312 state and the Ce 4fFs, since both have the same Fs symmetry. As found in Fig. 2, the volume of the hole pockets at F point and the electron pocket at X(M) point increase simultaneously when the temperature is decreased from 30 K (P-phase) to 10 K (AFP5-phase). This change is clearly viewed by upward shift o f ' h i ' and 'h2' bands near F point beyond Ev and simultaneous downward shift of 'e' band near the X(M) point. The p-f mixing model predicts that the anisotropic p-f mixing becomes stronger below Tw as a consequence of formation of a ferromagnetic layer with the fully polarized 4ITs state, because the wave function of Ce 4fF s state expands toward the Sb 5p3~2 orbital within the layer. This bonding-and-ant/bonding effect through the p - f mixing causes an upward shift of the Sb 5p3!2 band across the Fermi level. In order to compensate the increased hole number, the Ce 5dt2g electron band at X point shifts downward across Ev. Thus, the volume of both hole and electron pockets increase below TN, in good agreement with the HR-ARPES results. In summary, we observed a change of Fermi-surface topology of CeSb across the magnetic phase transition from the P- to the AFP-phase using high-resolution angle-resolved photoemission spectroscopy (HR-
ARPES). We found that the complicated magnetic structure of CeSb has a close connection with the unique band structure as predicted by the p - f mixing model. The present study has demonstrated that HR-ARPES is a useful experimental technique to study the change of band structure across the temperature dependent magnetic phase transition of heavy fermion materials. We are very grateful to O. Sakai and R. Pittini for useful discussion. H.K. and T.Y. thank the Japan Society for the Promotion of Science for financial support. This work was supported by grants from the N E D O and the Ministry of Education, Science and Culture of Japan. References
[1] J. Rossat-Mignod, J.M. Effantin, P. Buffet, T. Chattopadhyay, L.P. Regnault, H. Bartholin, C. Vettier, O. Vogt, D. Ravot, J.C. Achart, J. Magn. Magn. Mater. 52 (1985) 1ll. [2] T. Kasuya, Physica B 215 (1995) 88. [3] H. Kumigashira, S.-H. Yang, T. Yokoya, A. Chainani, T. Takahashi, A. Uesawa, T. Suzuki, Y. Kaneta, Phys. Rev. B 54 (1996) 9341. [4] F.J. Himpsel, Adv. Phys. 32 (I983) 1. [5] O. Sakai, Y. Kaneta, T. Kasuya, Jpn. J. Appl. Phys. 26 (1987) 447.
,i~
j~ ELSEVIER
Journal of
magnetism and magnetic materials
Magnetic phase transition of CeSb studied by high-resolution angle-resolved photoemission T. Takahashi*, H. Kumigashira, H.-D. Kim, A. Ashihara, A. Chainani, T. Yokoya, A. Uesawa, T. Suzuki Department q[ Pkysics, Tohoku University, Sendai 980- 77, Japan
Abstract We observed a change of Fermi-surface topology of CeSb across the magnetic phase transition using temperaturedependent high-resolution angle-resolved photoemission spectroscopy. The experimental results show that the complicated magnetic structure of CeSb strongly relates to its unique electronic structure as predicted by the anisotropic p-f mixing model. ~ 1998 Elsevier Science B.V. All rights reserved. K
Semimetallic CeSb shows a complicated magnetic phase transition with temperature and/or external magnetic field [I]. It shows the first-order phase transition from the paramagnetic (P-) to the antiferroparamagnetic (AFP-) phase at T~ = 16 K, being followed by at least six different AFP phases at lower temperature, then transforming into the antiferromagnetic (AF) phase at 6 K. It has been proposed that the anisotropic p-f mixing plays an essential role in the phase transition [2]. In contrast to de Haas-van Alphen measurement, angle-resolved photoentission spectroscopy (ARPES) has a wider temperature range of measurement as well as a capability to directly identify the character of Fermi surface (electronor hole-like) and its exact location in the Brillouin zone. ARPES measurement was carried out with a highresolution (HR) ARPES spectrometer constructed at Tohoku University. Angular and energy resolution were set at about 1 and 50 meV, respectively. Single crystals were cleaved in situ along the ( 1 0 0) plane at 30 K in the spectrometer (base pressure 2 x 10- t t Tort). Fig. 1 shows HR-ARPES spectra of the P-phase at 30 K and the fifth AFP-phase (AFP5) at 10 K, measured along F-X direction in the Brillouin zone. HR-ARPES spectra of both phases exhibit remarkable and systematic
*Corresponding author. Fax: +81222176419: e-mail: t.takahashi~ msp.phys.tohoku.ac.jp.
changes as a function of polar angle ((9), indicating the complicated band structure of CeSb. We find that the overall band dispersion looks similar in both phases as well as to that of CeBi [3]. In the near-EF region, we have identified two types of Fermi-level (EF) crossings of dispersive bands; (1) two hole pockets at F point as marked by 'h~' and 'h2' in Fig. 1, and (2) an electron pocket at X(M) point marked by "e'. Comparing HR-ARPES spectra near Er of two phases, we find clear difference in the intensity and the position of peaks, which should reflect the change of the band structure and the Fermi surface topology across the phase transition. To see the change more clearly, we show the ~band dispersion' for both phases in Fig. 2, which were derived front the HRARPES spectra in Fig. 1 by taking the second derivative after smoothing and plotting tile intensity in the squareroot scale by gradual shading as a function of the wave vector parallel to the 1--X(M) direction [4]; dark parts correspond to bands. Fig. 2 shows the existence of two hole pockets at Y point and an electron pocket at X(M) point in good agreement with the prediction of the band calculation [5]. According to the band calculation based on the p-f mixing model, the hole pockets are ascribed to the Sb 5P3/2 states while the electron pocket has a dominant Ce 5dt2g character. Ce ions in CeSb are thought to be trivalent, so that the Ce 4fs~2 state splits into the Ks quartet and the 1-7 doublet due to the cubic crystal field, with the Fs state being located at a higher energy.
0304-8853/98/S19.00 0 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 4 4 0 - X
1028
L Takahashi et al. / Journal of Magnetism and Magnetic Materials 177-181 (1998) 102 7-1028
.......F-X !a) 1 ..... - ~ i CeSb T=30K ~!
~"
b
LLA/Z °:
EF
0.5
0
ea)
.=~ 1.0 1.5
~=
51 =
~5o
'10
~,~ £
' 10
!15 °
~
i15'
F--A--X X--W--X Wave Vector
(a/
EF ' 20 °
20
O
0.5
1.0
25
h~ t.5 30 4
3 2 1 EF Binding Energy (eV)
4
3 2 1 Ev Binding Energy (eV)
(b)
F Z ~ S
Z - - M - - A
Wave Vector
Fig. 1. High-resolution angle-resolved photoemission spectra of (a) paramagnetic (30 K) and (b) antiferroparamagnetic (10 K) CeSb, measured with He I photons (21.2 eV) along F-X(M) direction in the Brillouin zone.
Fig. 2. Experimental band structure of (a) paramagnetic and (b) antiferroparamagnetic CeSb determined by the present HRARPES measurements. Dark parts correspond to the energy bands and dashed white lines are guide to eyes.
The key mechanism of p-f mixing model is the anisotropic hybridization between the Sb 5p312 state and the Ce 4fFs, since both have the same Fs symmetry. As found in Fig. 2, the volume of the hole pockets at F point and the electron pocket at X(M) point increase simultaneously when the temperature is decreased from 30 K (P-phase) to 10 K (AFP5-phase). This change is clearly viewed by upward shift o f ' h i ' and 'h2' bands near F point beyond Ev and simultaneous downward shift of 'e' band near the X(M) point. The p-f mixing model predicts that the anisotropic p-f mixing becomes stronger below Tw as a consequence of formation of a ferromagnetic layer with the fully polarized 4ITs state, because the wave function of Ce 4fF s state expands toward the Sb 5p3~2 orbital within the layer. This bonding-and-ant/bonding effect through the p - f mixing causes an upward shift of the Sb 5p3!2 band across the Fermi level. In order to compensate the increased hole number, the Ce 5dt2g electron band at X point shifts downward across Ev. Thus, the volume of both hole and electron pockets increase below TN, in good agreement with the HR-ARPES results. In summary, we observed a change of Fermi-surface topology of CeSb across the magnetic phase transition from the P- to the AFP-phase using high-resolution angle-resolved photoemission spectroscopy (HR-
ARPES). We found that the complicated magnetic structure of CeSb has a close connection with the unique band structure as predicted by the p - f mixing model. The present study has demonstrated that HR-ARPES is a useful experimental technique to study the change of band structure across the temperature dependent magnetic phase transition of heavy fermion materials. We are very grateful to O. Sakai and R. Pittini for useful discussion. H.K. and T.Y. thank the Japan Society for the Promotion of Science for financial support. This work was supported by grants from the N E D O and the Ministry of Education, Science and Culture of Japan. References
[1] J. Rossat-Mignod, J.M. Effantin, P. Buffet, T. Chattopadhyay, L.P. Regnault, H. Bartholin, C. Vettier, O. Vogt, D. Ravot, J.C. Achart, J. Magn. Magn. Mater. 52 (1985) 1ll. [2] T. Kasuya, Physica B 215 (1995) 88. [3] H. Kumigashira, S.-H. Yang, T. Yokoya, A. Chainani, T. Takahashi, A. Uesawa, T. Suzuki, Y. Kaneta, Phys. Rev. B 54 (1996) 9341. [4] F.J. Himpsel, Adv. Phys. 32 (I983) 1. [5] O. Sakai, Y. Kaneta, T. Kasuya, Jpn. J. Appl. Phys. 26 (1987) 447.