- Email: [email protected]
Photoemission spectroscopy of Ce-filled skutterudites
ARTICLE IN PRESS
Physica B 378–380 (2006) 177–178 www.elsevier.com/locate/physb
Photoemission spectroscopy of Ce-filled skutterudites M. Matsunamia,, K. Horibaa, M. Taguchia, K. Yamamotoa, A. Chainania, Y. Takataa, Y. Senbab, H. Ohashib, H. Sugawarac, H. Satod, H. Harimae, S. Shina,f a
RIKEN/SPring-8, Hyogo 679 5148, Japan JASRI/SPring-8, Hyogo 679 5148, Japan c Faculty of Integrated Arts and Sciences, The University of Tokushima, Tokushima 770 8502, Japan d Department of Physics, Tokyo Metropolitan University, Tokyo 192 0397, Japan e Department of Physics, Kobe University, Kobe 657 8501, Japan f Institute of Solid State Physics, University of Tokyo, Kashiwa, Chiba 277 85817, Japan b
Abstract We have investigated the electronic structure of filled skutterudites CeFe4 P12 and CeOs4 Sb12 by means of Ce 3d–4f resonant photoemission spectroscopy. The difference in the spectral property of on-resonant photoemission spectra can be explained by the degree of hybridization between conduction band states and Ce 4f states. In addition, the spectral intensity at the Fermi level for CeFe4 P12 is significantly lower than that for CeOs4 Sb12 . The results indicate that a Kondo resonance exists in CeOs4 Sb12 while Kondo resonance is suppressed in CeFe4 P12 . r 2006 Elsevier B.V. All rights reserved. PACS: 71.10.Hf; 71.27.þa; 75.30.Mb Keywords: Filled skutterudite; CeFe4 P12 ; CeOs4 Sb12 ; Photoemission spectroscopy
The filled skutterudite compounds with a general formula RT4 X12 (R ¼ rare earth; T ¼ Fe, Ru and Os; X ¼ P, As and Sb) have attracted much attention since they exhibit a variety of strongly correlated electron phenomena. Among these compounds, CeFe4 P12 is a semiconductor with an energy gap of 130 meV [1] and CeOs4 Sb12 shows anomalous temperature-dependent transport behavior [2]. These properties suggest a Kondo semiconducting character derived from the hybridization between conduction (c) band states and Ce 4f states. Since the compounds have different lattice parameters in the same crystal structure, the degree of c–f hybridization may vary significantly. Hence, it is interesting to make a comparative study of their electronic structures. In this study, in order to clarify the relation between their physical properties and c–f hybridization, we have performed X-ray absorption spectroscopy (XAS) and Ce 3d–4f resonant Corresponding author. Tel.: +81 791 58 2933; fax: +81 791 58 2934.
E-mail address: [email protected] (M. Matsunami). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.068
photoemission spectroscopy (PES) for CeFe4 P12 and CeOs4 Sb12 . Single crystals of CeFe4 P12 and CeOs4 Sb12 used in this work were grown by the Sn-flux [1] and Sb-self-flux method [4], respectively. XAS and resonant PES experiments were performed at undulator beamline BL17SU in SPring-8. XAS spectra were recorded using total electron yield method. PES spectra were measured using a hemispherical electron analyzer, SCIENTA SES-2002. The overall energy resolution was set to 200 meV for the spectra of wide energy region, and 100 meV for the spectra near Fermi level (E F ). Clean surfaces were obtained by fracturing in situ at the measurement temperature of 22 K. Fig. 1(a) shows Ce 3d–4f XAS spectra of CeFe4 P12 and CeOs4 Sb12 at 22 K. For CeOs4 Sb12 , the main peak structure at hn882 eV is consistent with the multiplet structure of 3d9 4f 2 final state. The multiplet features are less clear in CeFe4 P12 . This behavior is related to the observation of a weak satellite structure at 888 eV marked ‘‘S’’ in Fig. 1(a).
ARTICLE IN PRESS M. Matsunami et al. / Physica B 378–380 (2006) 177–178
178
On-Res. (hv ∼ 882 eV) Off-Res. (hv ∼ 876 eV)
Ce 3d5/2-4f XAS
T = 22 K
CeOs4Sb12
CeFe4P12
Intensity (arb. units)
S
875 880 885 890 hν (eV)
10 (b)
8
6 4 2 0 Binding Energy (eV)
Fig. 1. (a) Ce 3d–4f X-ray absorption spectra of CeFe4 P12 and CeOs4 Sb12 at 22 K. (b) On- and off-resonant photoemission spectra for CeFe4 P12 and CeOs4 Sb12 at 22 K.
The satellite feature is absent for CeOs4 Sb12 . Since the weak satellite is attributed to 3d9 4f 1 final state originating from the 4f 0 state in the ground state [3], this result is an evidence for the valence fluctuation in CeFe4 P12 and a nearly localized nature of f state in CeOs4 Sb12 . It reflects the degree of c–f hybridization in the two compounds. The Fig. 1(b) shows Ce 3d–4f resonant PES spectra for CeFe4 P12 and CeOs4 Sb12 at 22 K. On- and off-resonant PES spectra are measured at 882 and 876 eV, respectively, as indicated by arrows in XAS spectra shown in Fig. 1(a). The significant enhancement of spectral intensity for both compounds represents the Ce 4f component in on-resonant spectra. In typical Ce compounds, on-resonant (Ce 4f) spectrum can be described within the framework of the single impurity Anderson model. In this model, since the ground state is represented by a linear combination of the f 0 and f 1 states, the Ce 4f spectrum is composed of the poorly screened f 0 final state at 2–3 eV below E F and wellscreened f 1 final state corresponding to so-called Kondo resonance peak just above E F . Fig. 1(b) shows a clear observation of f 0 peak at 3:5 eV for CeOs4 Sb12 , while the corresponding structure at 2–3 eV for CeFe4 P12 has very weak intensity. These results also indicate that the degree of c–f hybridization for CeFe4 P12 is much stronger than that for CeOs4 Sb12 . The fact that a remarkable deviation from trivalent lanthanide contraction law has been observed for CeFe4 P12 , but not for CeOs4 Sb12 [5], can also be explained in terms of the degree of c–f hybridization. Fig. 2 shows the on-resonant PES spectra in the vicinity of E F for CeFe4 P12 and CeOs4 Sb12 . The positions of the prominent peak at 0:6 eV for CeFe4 P12 and at 0:1 eV for CeOs4 Sb12 are qualitatively consistent with the peak
CeFe4P12
CeOs4Sb12
CeOs4Sb12
(a)
Au
T = 22 K
Intensity (arb. units)
Intensity (arb. units)
CeFe4P12
On-Resonance
2.0
1.5
1.0 0.5 Binding Energy (eV)
0.0
-0.5
Fig. 2. On-resonant photoemission spectra near Fermi level for CeFe4 P12 and CeOs4 Sb12 . The Fermi edge of gold was used to calibrate the energy scale.
position of Ce 4f partial density of states (DOS) reported by band calculations in the local density approximation [6,7]. For CeFe4 P12 , the spectral intensity at E F is significantly lower than that for CeOs4 Sb12 , indicating the absence of a Kondo resonance peak in CeFe4 P12 . The result is consistent with Kondo semiconducting property [1]. Taking into account the strong c–f hybridization in CeFe4 P12 as described above, the semiconducting property may originate from the c–f hybridization gap. Note that for CeOs4 Sb12 , large spectral intensity at E F is clearly observed in the on-resonant PES spectrum, but not for off-resonant PES spectrum as shown in Fig. 1(b). This result may indicate that the pseudogap is formed in non-f-derived DOS, but not in f-derived DOS. Higher resolution PES measurements are essential to study the low-energy behavior in the vicinity of E F . This work was supported by a Grant-in-Aid for Scientific Research Priority Area ‘‘Skutterudite’’ (nos. 15072203 and 15072206) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References [1] [2] [3] [4] [5] [6] [7]
H. Sato, et al., Phys. Rev. B 62 (2000) 15125. E.D. Bauer, et al., J. Phys.: Condens. Matter 13 (2001) 4495. J.C. Fuggle, et al., Phys. Rev. B 27 (1983) 4637. H. Sugawara, et al., Phys. Rev. B 71 (2005) 125127. F. Grandjean, et al., J. Phys. Chem. Solids 45 (1984) 877. L. Nordstro¨m, D.J. Singh, Phys. Rev. B 53 (1996) 1103. H. Harima, K. Takegahara, J. Phys.: Condens. Matter 15 (2001) S2081.
Physica B 378–380 (2006) 177–178 www.elsevier.com/locate/physb
Photoemission spectroscopy of Ce-filled skutterudites M. Matsunamia,, K. Horibaa, M. Taguchia, K. Yamamotoa, A. Chainania, Y. Takataa, Y. Senbab, H. Ohashib, H. Sugawarac, H. Satod, H. Harimae, S. Shina,f a
RIKEN/SPring-8, Hyogo 679 5148, Japan JASRI/SPring-8, Hyogo 679 5148, Japan c Faculty of Integrated Arts and Sciences, The University of Tokushima, Tokushima 770 8502, Japan d Department of Physics, Tokyo Metropolitan University, Tokyo 192 0397, Japan e Department of Physics, Kobe University, Kobe 657 8501, Japan f Institute of Solid State Physics, University of Tokyo, Kashiwa, Chiba 277 85817, Japan b
Abstract We have investigated the electronic structure of filled skutterudites CeFe4 P12 and CeOs4 Sb12 by means of Ce 3d–4f resonant photoemission spectroscopy. The difference in the spectral property of on-resonant photoemission spectra can be explained by the degree of hybridization between conduction band states and Ce 4f states. In addition, the spectral intensity at the Fermi level for CeFe4 P12 is significantly lower than that for CeOs4 Sb12 . The results indicate that a Kondo resonance exists in CeOs4 Sb12 while Kondo resonance is suppressed in CeFe4 P12 . r 2006 Elsevier B.V. All rights reserved. PACS: 71.10.Hf; 71.27.þa; 75.30.Mb Keywords: Filled skutterudite; CeFe4 P12 ; CeOs4 Sb12 ; Photoemission spectroscopy
The filled skutterudite compounds with a general formula RT4 X12 (R ¼ rare earth; T ¼ Fe, Ru and Os; X ¼ P, As and Sb) have attracted much attention since they exhibit a variety of strongly correlated electron phenomena. Among these compounds, CeFe4 P12 is a semiconductor with an energy gap of 130 meV [1] and CeOs4 Sb12 shows anomalous temperature-dependent transport behavior [2]. These properties suggest a Kondo semiconducting character derived from the hybridization between conduction (c) band states and Ce 4f states. Since the compounds have different lattice parameters in the same crystal structure, the degree of c–f hybridization may vary significantly. Hence, it is interesting to make a comparative study of their electronic structures. In this study, in order to clarify the relation between their physical properties and c–f hybridization, we have performed X-ray absorption spectroscopy (XAS) and Ce 3d–4f resonant Corresponding author. Tel.: +81 791 58 2933; fax: +81 791 58 2934.
E-mail address: [email protected] (M. Matsunami). 0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.01.068
photoemission spectroscopy (PES) for CeFe4 P12 and CeOs4 Sb12 . Single crystals of CeFe4 P12 and CeOs4 Sb12 used in this work were grown by the Sn-flux [1] and Sb-self-flux method [4], respectively. XAS and resonant PES experiments were performed at undulator beamline BL17SU in SPring-8. XAS spectra were recorded using total electron yield method. PES spectra were measured using a hemispherical electron analyzer, SCIENTA SES-2002. The overall energy resolution was set to 200 meV for the spectra of wide energy region, and 100 meV for the spectra near Fermi level (E F ). Clean surfaces were obtained by fracturing in situ at the measurement temperature of 22 K. Fig. 1(a) shows Ce 3d–4f XAS spectra of CeFe4 P12 and CeOs4 Sb12 at 22 K. For CeOs4 Sb12 , the main peak structure at hn882 eV is consistent with the multiplet structure of 3d9 4f 2 final state. The multiplet features are less clear in CeFe4 P12 . This behavior is related to the observation of a weak satellite structure at 888 eV marked ‘‘S’’ in Fig. 1(a).
ARTICLE IN PRESS M. Matsunami et al. / Physica B 378–380 (2006) 177–178
178
On-Res. (hv ∼ 882 eV) Off-Res. (hv ∼ 876 eV)
Ce 3d5/2-4f XAS
T = 22 K
CeOs4Sb12
CeFe4P12
Intensity (arb. units)
S
875 880 885 890 hν (eV)
10 (b)
8
6 4 2 0 Binding Energy (eV)
Fig. 1. (a) Ce 3d–4f X-ray absorption spectra of CeFe4 P12 and CeOs4 Sb12 at 22 K. (b) On- and off-resonant photoemission spectra for CeFe4 P12 and CeOs4 Sb12 at 22 K.
The satellite feature is absent for CeOs4 Sb12 . Since the weak satellite is attributed to 3d9 4f 1 final state originating from the 4f 0 state in the ground state [3], this result is an evidence for the valence fluctuation in CeFe4 P12 and a nearly localized nature of f state in CeOs4 Sb12 . It reflects the degree of c–f hybridization in the two compounds. The Fig. 1(b) shows Ce 3d–4f resonant PES spectra for CeFe4 P12 and CeOs4 Sb12 at 22 K. On- and off-resonant PES spectra are measured at 882 and 876 eV, respectively, as indicated by arrows in XAS spectra shown in Fig. 1(a). The significant enhancement of spectral intensity for both compounds represents the Ce 4f component in on-resonant spectra. In typical Ce compounds, on-resonant (Ce 4f) spectrum can be described within the framework of the single impurity Anderson model. In this model, since the ground state is represented by a linear combination of the f 0 and f 1 states, the Ce 4f spectrum is composed of the poorly screened f 0 final state at 2–3 eV below E F and wellscreened f 1 final state corresponding to so-called Kondo resonance peak just above E F . Fig. 1(b) shows a clear observation of f 0 peak at 3:5 eV for CeOs4 Sb12 , while the corresponding structure at 2–3 eV for CeFe4 P12 has very weak intensity. These results also indicate that the degree of c–f hybridization for CeFe4 P12 is much stronger than that for CeOs4 Sb12 . The fact that a remarkable deviation from trivalent lanthanide contraction law has been observed for CeFe4 P12 , but not for CeOs4 Sb12 [5], can also be explained in terms of the degree of c–f hybridization. Fig. 2 shows the on-resonant PES spectra in the vicinity of E F for CeFe4 P12 and CeOs4 Sb12 . The positions of the prominent peak at 0:6 eV for CeFe4 P12 and at 0:1 eV for CeOs4 Sb12 are qualitatively consistent with the peak
CeFe4P12
CeOs4Sb12
CeOs4Sb12
(a)
Au
T = 22 K
Intensity (arb. units)
Intensity (arb. units)
CeFe4P12
On-Resonance
2.0
1.5
1.0 0.5 Binding Energy (eV)
0.0
-0.5
Fig. 2. On-resonant photoemission spectra near Fermi level for CeFe4 P12 and CeOs4 Sb12 . The Fermi edge of gold was used to calibrate the energy scale.
position of Ce 4f partial density of states (DOS) reported by band calculations in the local density approximation [6,7]. For CeFe4 P12 , the spectral intensity at E F is significantly lower than that for CeOs4 Sb12 , indicating the absence of a Kondo resonance peak in CeFe4 P12 . The result is consistent with Kondo semiconducting property [1]. Taking into account the strong c–f hybridization in CeFe4 P12 as described above, the semiconducting property may originate from the c–f hybridization gap. Note that for CeOs4 Sb12 , large spectral intensity at E F is clearly observed in the on-resonant PES spectrum, but not for off-resonant PES spectrum as shown in Fig. 1(b). This result may indicate that the pseudogap is formed in non-f-derived DOS, but not in f-derived DOS. Higher resolution PES measurements are essential to study the low-energy behavior in the vicinity of E F . This work was supported by a Grant-in-Aid for Scientific Research Priority Area ‘‘Skutterudite’’ (nos. 15072203 and 15072206) of the Ministry of Education, Culture, Sports, Science and Technology, Japan.
References [1] [2] [3] [4] [5] [6] [7]
H. Sato, et al., Phys. Rev. B 62 (2000) 15125. E.D. Bauer, et al., J. Phys.: Condens. Matter 13 (2001) 4495. J.C. Fuggle, et al., Phys. Rev. B 27 (1983) 4637. H. Sugawara, et al., Phys. Rev. B 71 (2005) 125127. F. Grandjean, et al., J. Phys. Chem. Solids 45 (1984) 877. L. Nordstro¨m, D.J. Singh, Phys. Rev. B 53 (1996) 1103. H. Harima, K. Takegahara, J. Phys.: Condens. Matter 15 (2001) S2081.