- Email: [email protected]
Hard X-ray valence-band and core-level photoelectron spectroscopy of strongly correlated electron systems
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175 www.elsevier.com/locate/nima
Hard X-ray valence-band and core-level photoelectron spectroscopy of strongly correlated electron systems Kenya Shimada Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 2-313, Higashi-Hiroshima, Hiroshima 739-0046, Japan Available online 6 June 2005
Abstract Valence-band and core-level photoelectron spectroscopy (PES) has been successfully used to study strongly correlated electron systems such as CeRhAs, YbInCu4, and YbB12 at photon energies of hn6 keV with high-energy resolution, DE1602270 meV. Due to the long penetration depth of electrons with high kinetic energy, the photoelectron spectra obtained reflect the bulk electronic properties. The merits of utilizing both hard X-ray PES and VUV–SX PES to investigate the electronic states of strongly correlated electron systems in the bulk are presented. r 2005 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.27.+a; 75.30.Mb; 79.60.i Keywords: Hard X-ray photoelectron spectroscopy; Kondo semiconductor; Valence-transition; Core-level; Valence-band
Since the pioneering work of Lindau et al. involving hard X-ray photoelectron spectroscopy (HXPES) of gold using a photon energy of hn8 keV [1], there have been many improvements in the design of light sources, high-resolution monochromators, and electron-energy analyzers. Electrons with kinetic energies, E K ¼ 202200 eV ( have an inelastic mean free path (IMFP), l5 A (Fig. 1(a)) [2]. An examination of the electronic states using this kinetic energy range reveals that electronic states derived from both the surface and the bulk are reflected in the photoelectron spectra. Tel.: +81 82 424 6293; fax: +81 82 424 6294.
E-mail address: [email protected].
If the thickness of the surface region, where the electronic states are different from those in the bulk, is much larger than the IMFP of electrons, the surface electronic states dominate the spectra. In order to examine the bulk electronic states, therefore, the investigator needs to employ photoelectrons with an IMFP that is sufficiently long compared with the thickness of the surface region. There are two means by which to achieve a longer IMFP: decrease the electron kinetic energy below 10 eV, or increase it beyond 1 keV. The former approach, utilizing a 700 MeV compact electron-storage ring, HiSOR, is followed at the Hiroshima Synchrotron Radiation Center (HSRC) at Hiroshima University. High-resolution
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.05.022
ARTICLE IN PRESS K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
170
10 2 Present Bulk sensitive Energy Resolution (meV)
IMFP (A)
103
102
101
100
10 0
10 1
10 2
10 3
Kinetic energy (eV)
10 1
10 0
Future
10 -1
10 -2 0 10
10
1
10
2
10
3
10
4
Kinetic Energy (eV)
Fig. 1. (a) Inelastic mean free path (IMFP) as a function of electron kinetic energy in solids [2]. (b) Typical present energy resolution of photoelectron spectroscopy (circles) and hoped for future improvement (dashed line) as a function of electron kinetic energy. The dotted lines indicate the energy scale of temperatures, kB T ¼ 100 K and 10 K.
photoelectron spectroscopy (PES) in the vacuum ultraviolet (VUV) region is available on the undulator beamlines at HiSOR [3,4]. We are studying the electronic states of strongly correlated electron systems on these beamlines [5–7]. At present, the energy resolution of the PES available at HiSOR is DEo1 meV at hn8 eV, DE15 meV at hn100 eV (Fig. 1(b)). However, the low photon energy permits an examination of the valenceband electronic states near the Fermi level (EF) only. In 2002, in order to extend the photon-energy range and to obtain both bulk-sensitive valenceband and core-level photoelectron spectra, we began a high-resolution HXPES program at the third-generation synchrotron radiation facility, SPring-8, in collaboration with JASRI/SPring-8 and RIKEN/SPring-8 [8–13]. So far, we have used photon energies up to 10 keV (BL29XU) for HXPES studies. We combine high-resolution PES
in the VUV–SX and hard X-ray regions to clarify electronic states that are relevant to unusual and interesting physical properties of strongly correlated electron systems. In the present paper, we will describe an overview of the present status of our HXPES project, with examples from strongly correlated Ce and Yb compounds. The HXPES experiments have been conducted at an X-ray undulator beamline BL29XU in SPring-8. A high-resolution hemispherical electron-energy analyzer (GAMMADATA-SCIENTA SES2002 or R4000) was used for the energy analyses [9,10,12]. The best total energy resolution (photon+electron energy analyzer) of DE75 meV has been realized at hn6 keV [10,12]. It should be noted that this energy resolution is much higher than the lifetime broadening (2G) of the core-level spectra of 3d states of rare-earth compounds, where 2G1 eV [14]. Therefore, we set the energy resolution at
ARTICLE IN PRESS K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
h ν = 5948 eV (As 4p, Sb 5p and Rh 4d) ∆ E = 160 meV
28 K,
hν = 40 eV (Rh 4d) 10 K,
CeRhAs
∆ E = 12 meV
As 4p
Intensity (arb.units)
DE270 meV for our core-level measurements. The samples were mounted on a He-closed cycle refrigerator, and the temperature was controlled from 300 down to 30 K. To avoid growing a significant amount of ice on the surface during low-temperature measurements, we placed our samples in ultrahigh vacuum. We fractured the sample in the vacuum to get clean surfaces. In the most cases, however, the valence band photoelectron spectra obtained from the fractured surface and from the as introduced surface are similar. Fig. 2 shows the valence band HXPES spectra of the Kondo semiconductor CeRhAs and the semimetal CeRhSb, taken at hn5:95 keV [15], and at hn40 eV [16]. Below the Kondo temperature, CeRhAs is a metal with a local moment which is responsible for the Kondo effect. As the temperature decreases, a gradual cross-over from the metallic state to the semiconducting state takes place around a characteristic temperature (T*), which is assumed to be T*370 K for CeRhAs. Based on the photoionization cross-sections [17], the Rh 4d, As 4p and Sb 5p states are strongly represented at hn5:95 keV and the Rh 4d dominates the spectral weight in the spectra taken at hn40 eV. The hn40 eV spectra are quite similar, apart from broader peak width of Rh 4d nonbonding states at EB2 eV in the CeRhAs case. On the other hand, one can see that the spectral features in the energy ranges of EB0–1 eV and EB3–5 eV in the spectra taken at hn5:95 keV, where the Rh4d–As4p and Rh4d–Sb5p hybridized states are located, differ between these two compounds (Fig. 2). The difference in the spectral feature is mainly derived from the difference of As 4p and Sb 5p partial density of state (DOS) in this energy region. The CeRhSb spectrum exhibits a strong peak derived from the Sb 5p DOS just below EF, while the As 4p DOS in CeRhAs does not. The spectral feature in CeRhAs is an important clue concerning the origin of the temperature-dependent metal-to-insulator transition in the Kondo semiconductor. In the HXPES spectra, the DOS of the anion p states located at the top of the valence band is clarified. The valence transition of YbInCu4 at TV42 K has been studied extensively. The transition is an isostructural, first-order phase transition. The
171
As 4p
Rh 4d Sb 5p Sb 5p
CeRhSb
Rh 4d
5
0
Binding Energy (eV) Fig. 2. Photoelectron spectra of CeRhAs and CeRhSb taken at hn ¼ 5:95 keV [15] and 40 eV [16].
change in the valence of Yb has been studied by various spectroscopic methods [18,19]. Fig. 3 shows temperature-dependent Yb 3d XPS spectra [18], where the Yb2+ and Yb3+ signals are well separated. The kinetic energy of the Yb 3d states, ( to probe EK4.4 keV, is large enough (l450 A) the intrinsic bulk spectral intensity ratio of Yb2+/ Yb3+. This ratio rises drastically between 55 and 30 K, resulting in a change in the average valence from Yb+2.90 down to Yb+2.74. This finding is in agreement with a first-order transition at TV. The temperature dependence of the valence transition of Yb measured in this manner was relatively
ARTICLE IN PRESS K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
172
YbInCu 4 3.0
hν = 5951 eV
YbInCu 4
∆E = 270 meV
Thermodynamic data
Yb 3d 3/2
hν = 5951 eV
Yb 3d 5/2
2.8 2.7
HX-PES SX-PES VUV-PES
2.6 2.5 0
50
Yb3+
∆ E = 270 meV
Yb3+ Yb2+
100 150 200 250
Yb2+
Temperature (K)
Cu 3d
55 K 30 K 1620
1600
1580
1560
1540
1520
Binding Energy (eV)
Fig. 3. Core-level photoelectron spectra of YbInCu4 above and below the valence transition (TV42 K) [18]. The inset shows the valence transition estimated by PES [18,19]. VUV–PES data are from [21].
Intensity (arb. units)
Yb valence
Intensity (arb. units)
2.9
55 K
Yb 4f
2+
30 K 55 K
sharp compared with those obtained using lower photon energies (Inset of Fig. 3) [18,19]. The present result is comparable with that obtained by another bulk-sensitive spectroscopic probe, resonant inelastic X-ray scattering [20]. It confirms that the probing depth is indeed large to detect the bulk electronic properties. Since the Yb2+ 3d and Yb3+ 3d components are well separated, the evaluation of the spectral intensity is unambiguous. Fig. 4 provides the valence-band photoelectron spectra. We should note that a very drastic change above and below the transition temperature which was not evident in photoelectron spectra taken at photon energies below 1000 eV [19,21]. This discrepancy indicates that the surface region of this compound is thick and that the IMFP must be longer than at least 20 A˚. The next example is the Kondo semiconductor, YbB12. The T* for YbB12 is considered to be 80 K, judging from the magnetic and transport measurements [22,23]. A photoelectron-spectroscopy study in the VUV region indicates that an energy-gap is formed as the temperature decreases [22]. Fig. 5 presents our Yb 3d5/2 spectra of this compound [23]. On cooling, the Yb2+ intensity slightly increases with respect to the Yb3+
Yb 4f 14
12
10
3+
8
6
4
2
0
Binding Energy (eV) Fig. 4. Valence-band HXPES spectra of YbInCu4 above and below the valence transition (TV42 K) [18]. The valence transition occurs in a narrow temperature range, which is confirmed by a sudden appearance of the Yb2+-derived spectral feature below TV.
intensity, but there is no sudden change as observed in YbInCu4. The valence of the Yb is unambiguously determined as 2.9, a value which is close to that expected for Yb3+, which confirms that the compound is located in the Kondo regime. Fig. 6(a) shows the valence-band HXPES spectra of YbB12 [23]. The spectral intensity from Yb2+ becomes stronger on cooling. However, due to the limited energy resolution, it is difficult to extract further information on the energygap formation in the Yb 4f state. Therefore, we examined the compound using high-resolution UPS at HiSOR, with the results shown in
ARTICLE IN PRESS K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
YbB 12 200 K Yb 3d 5/2
100 K
Intensity (arb. units)
h ν = 5951 eV
22 K
∆ E = 270 meV
Yb
Yb
1540
1535
1530
2+
3+
1525
1520
1515
Binding Energy (eV) Fig. 5. Core-level photoelectron spectra of YbB12. The valence of Yb is estimated to be 2.9 [23].
Fig. 6(b). The energy resolution was set at DE15 meV at hn ¼ 100 eV. It is known that the spectral features derived from the surface region have larger binding energy, by 1 eV, compared with those from the bulk [22], which enables us to compare the Yb2+ 4f7/2 spectra in Fig. 6(b) with those in Fig. 6(a). As the temperature decreases, the plot shows an increase in the spectral intensity, just as in the HXPES results. Due to the higherenergy resolution, we discovered a fine structure peak located at 15 meV, which grew rapidly below 60 K. The appearance of this spectral feature is closely related to the energy-gap formation in the Yb 4f states [23]. In the future, we hope to observe similarly fine spectral features near the Fermi level in the HXPES region. Besides rare-earth compounds described in this paper, we are also using HXPES to study strongly correlated transition-metal oxides. Various HXPES studies on the Mott insulator Y1xCaxTiO3 [24], the pyrochlore molybdenum oxides R2Mo2O7 [25], and the double perovskite Sr2FeMoO6 [26] are in progress.
YbB 12
YbB 12
Intensity (arb. units)
∆ E = 270 meV
(a)
Yb 2+
Yb 2+
4f5/2
4f 7/2
Yb 2+ 4f 7/2 Intensity (arb. units)
h ν = 5951 eV
22 K 100 K 200 K
3
173
2
1 Binding Energy (eV)
h ν = 100 eV ∆ E = 15 meV
9K 60 K 120 K 180 K 250 K 0.2
0 (b)
0.1
0
-0.1
Binding Energy (eV)
Fig. 6. (a) Valence-band HXPES spectra of YbB12 as a function of temperature [23]. The Yb2+ 4f peaks are enhanced on cooling. (b) Valence-band high-resolution UPS spectra of YbB12 as a function of temperature [23]. The additional spectral feature at 15 meV develops below 60 K.
ARTICLE IN PRESS 174
K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
To summarize, we have presented our studies on rare-earth compounds using valence-band and core-level HXPES. Due to the long probing depth available, more than 50 A˚, our HXPES spectra reflect the bulk-derived electronic states. The HXPES technique is a powerful tool for investigating the DOS of the valence band and the core levels free from the surface electronic states. We have also demonstrated that HXPES, combined with VUV–SX PES, is very effective in clarifying the electronic states of strongly correlated electron systems. As a result of the improved energy resolution, the line widths of core-level HXPES spectra are determined by lifetime broadening. However, a further improvement in energy resolution is essential to study the valence-band electronic structure of solids near EF, especially with regard to the transition temperatures of high-TC superconductors or the characteristic temperatures of heavy fermion materials (10100 K). Based on the recent development of the electron energy analyzer with a light source, we hope that, as indicated by the dashed line in Fig. 1(b), the energy resolution of PES will be further improved in the future.
author acknowledges Dr. M. Arita for setting up the sample-preparation chamber and technical supports during experiments. Dr. M. Arita and Dr. A. Ino (Hiroshima Univ.) showed clearly potentialities of the high-resolution valence-band PES in the hard X-ray region for the first time by measuring the Fermi edge of gold in this collaborative program. The author thanks members from HSRC, Prof. H. Sato and Dr. Y. Takeda for preparing figures of YbInCu4 and YbB12 HXPES spectra, and Dr. M. Sawada, Dr. M. Nakatake, and Prof. S. Qiao for the collaborative works and fruitful discussions. We acknowledge Prof. T. Takabatake (Hiroshima Univ.), Prof. F. Iga (Hiroshima Univ.), Prof. K. Hiraoka (Ehime Univ.) and Prof. K. Kojima (Hiroshima University) for collaborative works on Ce and Yb compounds and preparation of single crystals. Experiments at HiSOR were done under approval of HSRC (Proposal no. 03-A-38, 03-A-40). This work was partially supported by the Ministry of Education, Science, Sports and Culture through a Grant-in-Aid for Scientific Research (A) (No. 15206006).
We thank Prof. K. Kobayashi (JASRI/SPring8) for organizing and leading this project. We thank Dr. Y. Takata (RIKEN/SPring-8) for the design and improvement of the HXPES measurement system and organizing collaborative works. We acknowledge Prof. S. Shin (RIKEN/SPring-8) and his group for support and collaboration. We acknowledge Prof. T. Ishikawa (RIKEN/SPring-8) for collaboration and providing us with intense photons suitable for high-resolution HXPES. We thank Dr. K. Tamasaku, Dr. Y. Nishino, and Dr. D. Miwa from Prof. Ishikawa’s group for optimization of the monochromator and supports during experiments. We thank Dr. T. Tokushima (RIKEN/SPring-8), Dr. E. Ikenaga (JASRI/SPring-8) for instrumentation and improvement of the measurement system, and Dr. M. Yabashi (JASRI/SPring-8) for contribution to the optical design of BL29XU. Prof. M. Taniguchi and Prof. H. Namatame are leaders of the HSRC group and they decided to start this joint program. The
References [1] I. Lindau, P. Oianetta, S. Doniach, W.E. Spicer, Nature 250 (1974) 214. [2] M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [3] K. Shimada, M. Arita, Y. Takeda, H. Fujino, K. Kobayashi, T. Narimura, H. Namatame, M. Taniguchi, Surf. Rev. Lett. 9 (2002) 529. [4] M. Arita, K. Shimada, H. Namatame, M. Taniguchi, Surf. Rev. Lett. 9 (2002) 535. [5] Y. Aiura, I. Hase, S.I. Ikeda, H. Bando, Y. Yoshida, M. Higashiguchi, X.Y. Cui, K. Shimada, H. Namatame, M. Taniguchi, Phys. Rev. Lett. 93 (2004) 117005. [6] H. Ishii, H. Kataura, H. Shiozawa, H. Yoshioka, H. Otsubo, Y. Takayama, T. Miyahara, S. Suzuki, Y. Achiba, M. Nakatake, T. Narimura, M. Higashiguchi, K. Shimada, H. Namatame, M. Taniguchi, Nature 426 (2003) 540. [7] K. Shimada, K. Kobayashi, T. Narimura, P. Baltzer, H. Namatame, M. Taniguchi, T. Suemitsu, T. Sasakawa, T. Takabatake, Phys. Rev. B 66 (2002) 155202. [8] K. Kobayashi, M. Yabashi, Y. Takata, D. Miwa, T. Ishikawa, H. Nohira, T. Hattorri, Y. Sugita, O. Nakatsuka, A. Sakai, S. Zaima, Appl. Phys. Lett. 83 (2003) 1005. [9] Y. Takata, K. Tamasaku, T. Tokushima, D. Miwa, S. Shin, T. Ishikawa, M. Yabashi, K. Kobayashi, J.J. Kim, T.
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[10]
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[17] [18]
Yao, T. Yamamoto, M. Arita, H. Namatame, M. Taniguchi, Appl. Phys. Lett. 84 (2004) 4310. Y. Takata, K. Tamasaku, Y. Nishino, D. Miwa, M. Yabashi, E. Ikenaga, K. Horiba, M. Arita, K. Shimada, H. Namatame, H. Nohira, T. Hattori, S. So¨dergren, B. Wannberg, M. Taniguchi, S. Shin, T. Ishikawa, K. Kobayashi, J. Electron Spectrosc. Relat. Phenom. 1063 (2005) 144. K. Kobayashi, Nucl. Instr. and Meth. A (2005). Y. Takata, et al., Nucl. Instr. and Meth. A (2005). T. Ishikawa, et al., Nucl. Instr. and Meth. A (2005). J.C. Fuggle, S.F. Alvarado, Phys. Rev. A 22 (1980) 1615. K. Shimada, et al. unpublished. K. Shimada, M. Higashiguchi, M. Arita, Y. Takeda, H. Namatame, M. Taniguchi, T. Sasakawa, T. Suemitsu, T. Takabatake, J. Electron Spectrosc. Relat. Phenom. 857 (2005) 144. J.J. Yeh, I. Lindau, Atom. Data Nucl. Data Tables 32 (1985) 1. H. Sato, et al., Phys. Rev. Lett. 93 (2004) 246404.
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[19] 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. [20] C. Dallera, M. Grioni, A. Shukla, G. Vanko´, J.L. Sarrao, J.P. Rueff, D.L. Cox, Phys. Rev. Lett. 88 (2002) 196403. [21] F. Reinert, R. Claessen, G. Nicolay, D. Ehm, S. Hu¨fner, W.P. Ellis, G.-H. Gweon, J.W. Allen, B. Kindler, W. Assmus, Phys. Rev. B 58 (1998) 12808. [22] T. Susaki, Y. Takeda, M. Arita, K. Mamiya, A. Fujimori, K. Shimada, H. Namatame, M. Taniguchi, N. Shimizu, F. Iga, T. Takabatake, Phys. Rev. Lett. 82 (1999) 992. [23] Y. Takeda, M. Arita, H. Higashiguchi, K. Shimada, H. Sato, M. Sawada, M. Nakatake, H. Namatame, M. Taniguchi, F. Iga, T. Takabatake, Y. Takata, E. Ikenaga, M. Yabashi, D. Miwa, Y. Nishino, K. Tamasaku, T. Ishikawa, S. Shin, K. Kobayashi, Physica B 351 (2004) 286. [24] M. Arita, et al. unpublished. [25] M. Sawada, et al. unpublished. [26] M. Nakatake, et al. unpublished.
Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175 www.elsevier.com/locate/nima
Hard X-ray valence-band and core-level photoelectron spectroscopy of strongly correlated electron systems Kenya Shimada Hiroshima Synchrotron Radiation Center, Hiroshima University, Kagamiyama 2-313, Higashi-Hiroshima, Hiroshima 739-0046, Japan Available online 6 June 2005
Abstract Valence-band and core-level photoelectron spectroscopy (PES) has been successfully used to study strongly correlated electron systems such as CeRhAs, YbInCu4, and YbB12 at photon energies of hn6 keV with high-energy resolution, DE1602270 meV. Due to the long penetration depth of electrons with high kinetic energy, the photoelectron spectra obtained reflect the bulk electronic properties. The merits of utilizing both hard X-ray PES and VUV–SX PES to investigate the electronic states of strongly correlated electron systems in the bulk are presented. r 2005 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.27.+a; 75.30.Mb; 79.60.i Keywords: Hard X-ray photoelectron spectroscopy; Kondo semiconductor; Valence-transition; Core-level; Valence-band
Since the pioneering work of Lindau et al. involving hard X-ray photoelectron spectroscopy (HXPES) of gold using a photon energy of hn8 keV [1], there have been many improvements in the design of light sources, high-resolution monochromators, and electron-energy analyzers. Electrons with kinetic energies, E K ¼ 202200 eV ( have an inelastic mean free path (IMFP), l5 A (Fig. 1(a)) [2]. An examination of the electronic states using this kinetic energy range reveals that electronic states derived from both the surface and the bulk are reflected in the photoelectron spectra. Tel.: +81 82 424 6293; fax: +81 82 424 6294.
E-mail address: [email protected].
If the thickness of the surface region, where the electronic states are different from those in the bulk, is much larger than the IMFP of electrons, the surface electronic states dominate the spectra. In order to examine the bulk electronic states, therefore, the investigator needs to employ photoelectrons with an IMFP that is sufficiently long compared with the thickness of the surface region. There are two means by which to achieve a longer IMFP: decrease the electron kinetic energy below 10 eV, or increase it beyond 1 keV. The former approach, utilizing a 700 MeV compact electron-storage ring, HiSOR, is followed at the Hiroshima Synchrotron Radiation Center (HSRC) at Hiroshima University. High-resolution
0168-9002/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2005.05.022
ARTICLE IN PRESS K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
170
10 2 Present Bulk sensitive Energy Resolution (meV)
IMFP (A)
103
102
101
100
10 0
10 1
10 2
10 3
Kinetic energy (eV)
10 1
10 0
Future
10 -1
10 -2 0 10
10
1
10
2
10
3
10
4
Kinetic Energy (eV)
Fig. 1. (a) Inelastic mean free path (IMFP) as a function of electron kinetic energy in solids [2]. (b) Typical present energy resolution of photoelectron spectroscopy (circles) and hoped for future improvement (dashed line) as a function of electron kinetic energy. The dotted lines indicate the energy scale of temperatures, kB T ¼ 100 K and 10 K.
photoelectron spectroscopy (PES) in the vacuum ultraviolet (VUV) region is available on the undulator beamlines at HiSOR [3,4]. We are studying the electronic states of strongly correlated electron systems on these beamlines [5–7]. At present, the energy resolution of the PES available at HiSOR is DEo1 meV at hn8 eV, DE15 meV at hn100 eV (Fig. 1(b)). However, the low photon energy permits an examination of the valenceband electronic states near the Fermi level (EF) only. In 2002, in order to extend the photon-energy range and to obtain both bulk-sensitive valenceband and core-level photoelectron spectra, we began a high-resolution HXPES program at the third-generation synchrotron radiation facility, SPring-8, in collaboration with JASRI/SPring-8 and RIKEN/SPring-8 [8–13]. So far, we have used photon energies up to 10 keV (BL29XU) for HXPES studies. We combine high-resolution PES
in the VUV–SX and hard X-ray regions to clarify electronic states that are relevant to unusual and interesting physical properties of strongly correlated electron systems. In the present paper, we will describe an overview of the present status of our HXPES project, with examples from strongly correlated Ce and Yb compounds. The HXPES experiments have been conducted at an X-ray undulator beamline BL29XU in SPring-8. A high-resolution hemispherical electron-energy analyzer (GAMMADATA-SCIENTA SES2002 or R4000) was used for the energy analyses [9,10,12]. The best total energy resolution (photon+electron energy analyzer) of DE75 meV has been realized at hn6 keV [10,12]. It should be noted that this energy resolution is much higher than the lifetime broadening (2G) of the core-level spectra of 3d states of rare-earth compounds, where 2G1 eV [14]. Therefore, we set the energy resolution at
ARTICLE IN PRESS K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
h ν = 5948 eV (As 4p, Sb 5p and Rh 4d) ∆ E = 160 meV
28 K,
hν = 40 eV (Rh 4d) 10 K,
CeRhAs
∆ E = 12 meV
As 4p
Intensity (arb.units)
DE270 meV for our core-level measurements. The samples were mounted on a He-closed cycle refrigerator, and the temperature was controlled from 300 down to 30 K. To avoid growing a significant amount of ice on the surface during low-temperature measurements, we placed our samples in ultrahigh vacuum. We fractured the sample in the vacuum to get clean surfaces. In the most cases, however, the valence band photoelectron spectra obtained from the fractured surface and from the as introduced surface are similar. Fig. 2 shows the valence band HXPES spectra of the Kondo semiconductor CeRhAs and the semimetal CeRhSb, taken at hn5:95 keV [15], and at hn40 eV [16]. Below the Kondo temperature, CeRhAs is a metal with a local moment which is responsible for the Kondo effect. As the temperature decreases, a gradual cross-over from the metallic state to the semiconducting state takes place around a characteristic temperature (T*), which is assumed to be T*370 K for CeRhAs. Based on the photoionization cross-sections [17], the Rh 4d, As 4p and Sb 5p states are strongly represented at hn5:95 keV and the Rh 4d dominates the spectral weight in the spectra taken at hn40 eV. The hn40 eV spectra are quite similar, apart from broader peak width of Rh 4d nonbonding states at EB2 eV in the CeRhAs case. On the other hand, one can see that the spectral features in the energy ranges of EB0–1 eV and EB3–5 eV in the spectra taken at hn5:95 keV, where the Rh4d–As4p and Rh4d–Sb5p hybridized states are located, differ between these two compounds (Fig. 2). The difference in the spectral feature is mainly derived from the difference of As 4p and Sb 5p partial density of state (DOS) in this energy region. The CeRhSb spectrum exhibits a strong peak derived from the Sb 5p DOS just below EF, while the As 4p DOS in CeRhAs does not. The spectral feature in CeRhAs is an important clue concerning the origin of the temperature-dependent metal-to-insulator transition in the Kondo semiconductor. In the HXPES spectra, the DOS of the anion p states located at the top of the valence band is clarified. The valence transition of YbInCu4 at TV42 K has been studied extensively. The transition is an isostructural, first-order phase transition. The
171
As 4p
Rh 4d Sb 5p Sb 5p
CeRhSb
Rh 4d
5
0
Binding Energy (eV) Fig. 2. Photoelectron spectra of CeRhAs and CeRhSb taken at hn ¼ 5:95 keV [15] and 40 eV [16].
change in the valence of Yb has been studied by various spectroscopic methods [18,19]. Fig. 3 shows temperature-dependent Yb 3d XPS spectra [18], where the Yb2+ and Yb3+ signals are well separated. The kinetic energy of the Yb 3d states, ( to probe EK4.4 keV, is large enough (l450 A) the intrinsic bulk spectral intensity ratio of Yb2+/ Yb3+. This ratio rises drastically between 55 and 30 K, resulting in a change in the average valence from Yb+2.90 down to Yb+2.74. This finding is in agreement with a first-order transition at TV. The temperature dependence of the valence transition of Yb measured in this manner was relatively
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YbInCu 4 3.0
hν = 5951 eV
YbInCu 4
∆E = 270 meV
Thermodynamic data
Yb 3d 3/2
hν = 5951 eV
Yb 3d 5/2
2.8 2.7
HX-PES SX-PES VUV-PES
2.6 2.5 0
50
Yb3+
∆ E = 270 meV
Yb3+ Yb2+
100 150 200 250
Yb2+
Temperature (K)
Cu 3d
55 K 30 K 1620
1600
1580
1560
1540
1520
Binding Energy (eV)
Fig. 3. Core-level photoelectron spectra of YbInCu4 above and below the valence transition (TV42 K) [18]. The inset shows the valence transition estimated by PES [18,19]. VUV–PES data are from [21].
Intensity (arb. units)
Yb valence
Intensity (arb. units)
2.9
55 K
Yb 4f
2+
30 K 55 K
sharp compared with those obtained using lower photon energies (Inset of Fig. 3) [18,19]. The present result is comparable with that obtained by another bulk-sensitive spectroscopic probe, resonant inelastic X-ray scattering [20]. It confirms that the probing depth is indeed large to detect the bulk electronic properties. Since the Yb2+ 3d and Yb3+ 3d components are well separated, the evaluation of the spectral intensity is unambiguous. Fig. 4 provides the valence-band photoelectron spectra. We should note that a very drastic change above and below the transition temperature which was not evident in photoelectron spectra taken at photon energies below 1000 eV [19,21]. This discrepancy indicates that the surface region of this compound is thick and that the IMFP must be longer than at least 20 A˚. The next example is the Kondo semiconductor, YbB12. The T* for YbB12 is considered to be 80 K, judging from the magnetic and transport measurements [22,23]. A photoelectron-spectroscopy study in the VUV region indicates that an energy-gap is formed as the temperature decreases [22]. Fig. 5 presents our Yb 3d5/2 spectra of this compound [23]. On cooling, the Yb2+ intensity slightly increases with respect to the Yb3+
Yb 4f 14
12
10
3+
8
6
4
2
0
Binding Energy (eV) Fig. 4. Valence-band HXPES spectra of YbInCu4 above and below the valence transition (TV42 K) [18]. The valence transition occurs in a narrow temperature range, which is confirmed by a sudden appearance of the Yb2+-derived spectral feature below TV.
intensity, but there is no sudden change as observed in YbInCu4. The valence of the Yb is unambiguously determined as 2.9, a value which is close to that expected for Yb3+, which confirms that the compound is located in the Kondo regime. Fig. 6(a) shows the valence-band HXPES spectra of YbB12 [23]. The spectral intensity from Yb2+ becomes stronger on cooling. However, due to the limited energy resolution, it is difficult to extract further information on the energygap formation in the Yb 4f state. Therefore, we examined the compound using high-resolution UPS at HiSOR, with the results shown in
ARTICLE IN PRESS K. Shimada / Nuclear Instruments and Methods in Physics Research A 547 (2005) 169–175
YbB 12 200 K Yb 3d 5/2
100 K
Intensity (arb. units)
h ν = 5951 eV
22 K
∆ E = 270 meV
Yb
Yb
1540
1535
1530
2+
3+
1525
1520
1515
Binding Energy (eV) Fig. 5. Core-level photoelectron spectra of YbB12. The valence of Yb is estimated to be 2.9 [23].
Fig. 6(b). The energy resolution was set at DE15 meV at hn ¼ 100 eV. It is known that the spectral features derived from the surface region have larger binding energy, by 1 eV, compared with those from the bulk [22], which enables us to compare the Yb2+ 4f7/2 spectra in Fig. 6(b) with those in Fig. 6(a). As the temperature decreases, the plot shows an increase in the spectral intensity, just as in the HXPES results. Due to the higherenergy resolution, we discovered a fine structure peak located at 15 meV, which grew rapidly below 60 K. The appearance of this spectral feature is closely related to the energy-gap formation in the Yb 4f states [23]. In the future, we hope to observe similarly fine spectral features near the Fermi level in the HXPES region. Besides rare-earth compounds described in this paper, we are also using HXPES to study strongly correlated transition-metal oxides. Various HXPES studies on the Mott insulator Y1xCaxTiO3 [24], the pyrochlore molybdenum oxides R2Mo2O7 [25], and the double perovskite Sr2FeMoO6 [26] are in progress.
YbB 12
YbB 12
Intensity (arb. units)
∆ E = 270 meV
(a)
Yb 2+
Yb 2+
4f5/2
4f 7/2
Yb 2+ 4f 7/2 Intensity (arb. units)
h ν = 5951 eV
22 K 100 K 200 K
3
173
2
1 Binding Energy (eV)
h ν = 100 eV ∆ E = 15 meV
9K 60 K 120 K 180 K 250 K 0.2
0 (b)
0.1
0
-0.1
Binding Energy (eV)
Fig. 6. (a) Valence-band HXPES spectra of YbB12 as a function of temperature [23]. The Yb2+ 4f peaks are enhanced on cooling. (b) Valence-band high-resolution UPS spectra of YbB12 as a function of temperature [23]. The additional spectral feature at 15 meV develops below 60 K.
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To summarize, we have presented our studies on rare-earth compounds using valence-band and core-level HXPES. Due to the long probing depth available, more than 50 A˚, our HXPES spectra reflect the bulk-derived electronic states. The HXPES technique is a powerful tool for investigating the DOS of the valence band and the core levels free from the surface electronic states. We have also demonstrated that HXPES, combined with VUV–SX PES, is very effective in clarifying the electronic states of strongly correlated electron systems. As a result of the improved energy resolution, the line widths of core-level HXPES spectra are determined by lifetime broadening. However, a further improvement in energy resolution is essential to study the valence-band electronic structure of solids near EF, especially with regard to the transition temperatures of high-TC superconductors or the characteristic temperatures of heavy fermion materials (10100 K). Based on the recent development of the electron energy analyzer with a light source, we hope that, as indicated by the dashed line in Fig. 1(b), the energy resolution of PES will be further improved in the future.
author acknowledges Dr. M. Arita for setting up the sample-preparation chamber and technical supports during experiments. Dr. M. Arita and Dr. A. Ino (Hiroshima Univ.) showed clearly potentialities of the high-resolution valence-band PES in the hard X-ray region for the first time by measuring the Fermi edge of gold in this collaborative program. The author thanks members from HSRC, Prof. H. Sato and Dr. Y. Takeda for preparing figures of YbInCu4 and YbB12 HXPES spectra, and Dr. M. Sawada, Dr. M. Nakatake, and Prof. S. Qiao for the collaborative works and fruitful discussions. We acknowledge Prof. T. Takabatake (Hiroshima Univ.), Prof. F. Iga (Hiroshima Univ.), Prof. K. Hiraoka (Ehime Univ.) and Prof. K. Kojima (Hiroshima University) for collaborative works on Ce and Yb compounds and preparation of single crystals. Experiments at HiSOR were done under approval of HSRC (Proposal no. 03-A-38, 03-A-40). This work was partially supported by the Ministry of Education, Science, Sports and Culture through a Grant-in-Aid for Scientific Research (A) (No. 15206006).
We thank Prof. K. Kobayashi (JASRI/SPring8) for organizing and leading this project. We thank Dr. Y. Takata (RIKEN/SPring-8) for the design and improvement of the HXPES measurement system and organizing collaborative works. We acknowledge Prof. S. Shin (RIKEN/SPring-8) and his group for support and collaboration. We acknowledge Prof. T. Ishikawa (RIKEN/SPring-8) for collaboration and providing us with intense photons suitable for high-resolution HXPES. We thank Dr. K. Tamasaku, Dr. Y. Nishino, and Dr. D. Miwa from Prof. Ishikawa’s group for optimization of the monochromator and supports during experiments. We thank Dr. T. Tokushima (RIKEN/SPring-8), Dr. E. Ikenaga (JASRI/SPring-8) for instrumentation and improvement of the measurement system, and Dr. M. Yabashi (JASRI/SPring-8) for contribution to the optical design of BL29XU. Prof. M. Taniguchi and Prof. H. Namatame are leaders of the HSRC group and they decided to start this joint program. The
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