- Email: [email protected]
High-resolution photoemission study of the hybridization gap in the Kondo semiconductor CeRhAs
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e57–e58 www.elsevier.com/locate/jmmm
High-resolution photoemission study of the hybridization gap in the Kondo semiconductor CeRhAs K. Shimadaa,, M. Higashiguchib, M. Aritaa, H. Namatamea, M. Taniguchia,b, S.-i. Fujimoric, Y. Saitohc, A. Fujimoric,d, Y. Takatae, S. Shine, K. Kobayashif, E. Ikenagaf, M. Yabashif, K. Tamasakug, Y. Nishinog, D. Miwag, T. Ishikawag, T. Sasakawah, T. Takabatakeh a
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-0046, Japan b Graduate School of Science, Hiroshima University, Higashi-Horoshima, Hiroshima 739-8526, Japan c Synchrotron Radiation Research Unit, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan d Department of Complexity Science and Engineering, University of Tokyo, Kashiwa, Chiba 277-8561, Japan e Soft X-Ray Spectroscopy Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Hyogo 679-5148, Japan f JASRI/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Hyogo 679-5148, Japan g Coherent X-Ray Optics Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Hyogo 679-5148, Japan h Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan Available online 2 November 2006
Abstract We have examined the electronic states of the Kondo semiconductor CeRhAs and the semimetal CeRhSb by means of high-resolution photoemission spectroscopy using tunable photon energies from hn ¼ 40 up to 5948 eV. On the basis of the photon-energy dependence of the photoionization cross-section, we have elucidated the p–d–f hybridized states in these compounds. r 2006 Elsevier B.V. All rights reserved. PACS: 71.27.þa; 75.20.Hr; 75.30.Mb; 79.60.i Keywords: CeRhAs; Resonance photoemission; Kondo semiconductor
The Kondo semiconductor CeRhAs and semimetal CeRhSb of the orthorhombic e-TiNiSi type have attracted much interest for their temperature-dependent energy-gap or pseudogap formations [1–3]. The Kondo temperatures for CeRhAs and CeRhSb are estimated to be T K 1500 KðkB T K 130 meVÞ [3] and 400 KðkB T K 34 meVÞ [2], respectively. The unit-cell volume increases on going from CeRhAs to CeRhSb, which leads to a decreased c–f hybridization strength [4]. Although high-resolution photoemission spectroscopy (PES) of these compounds has been conducted extensively [5–7], the partial density-of-states (DOS) for the whole valence bands have not been revealed so far. In order to understand the origin of the energy-gap formation, it is Corresponding author. Tel.: +81 82 424 6293; fax: +81 82 424 6294.
E-mail address: [email protected] (K. Shimada). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.201
important to clarify the p–d–f hybridized states at the top of the valence band. Since the photoionization crosssections of the As 4p and Sb 5p states with respect to those of Rh 4d or Ce 4f states increase above hn ¼ 1000 eV [8], PES with hard X -rays is useful to identify the p-derived spectral features. In the present paper, we report high-resolution PES of CeRhAs and CeRhSb using excitation photon energies ranging from hn ¼ 40 up to 5948 eV. We discuss the p–d–f hybridization in these compounds. Single crystals of CeRhAs and CeRhSb were grown by the Bridgman method [1–3]. The VUV PES (hn ¼ 40, 115 eV; energy resolution, DE ¼ 12–18 meV), soft X-ray PES (hn ¼ 870–881 eV; DE ¼ 100 meV), and hard X-ray PES (hn ¼ 5948 eV; DE ¼ 150 meV) measurements were performed at the beamlines BL-1 of HiSOR [9], BL23SU of SPring-8 [10], and BL29XU of SPring-8 [11],
ARTICLE IN PRESS K. Shimada et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e57–e58
CeRhAs
CeRhSb
hν=881 eV, 15 K (Ce 4f)
hν=881 eV, 15 K (Ce 4f)
hν=40 eV, 10 K (Rh 4d) hν=870 eV, 15 K hν=115 eV, 10 K
Intensity (arb. units)
Intensity (arb.units)
e58
hν=5948 eV, 28 K (Rh 4d + Sb 5p)
hν=5948 eV, 28 K (Rh 4d + As 4p) hν=40 eV
(a)
5 0 Binding Energy (eV)
hν=40 eV, 10 K (Rh 4d) hν=870 eV, 15 K hν=115 eV, 10 K
hν=40 eV
(b)
5 0 Binding Energy (eV)
Fig. 1. High-resolutions PES spectra of (a) CeRhAs and (b) CeRhSb taken at hn ¼ 40; 115; 870; 881, and 5948 eV.
respectively. The sample temperature was kept at 10–28 K. Clean sample surfaces were obtained by fracturing samples in the ultrahigh vacuum. Fig. 1(a) and (b) show PES spectra of CeRhAs and CeRhSb. The hn ¼ 881 eV spectra represent on-resonance spectra in the Ce 3d–4f resonance regime. The contribution from the surface electronic state is estimated to be less than 30% of the whole spectral weight at this photoelectron kinetic energy. Most of the spectral features near the Fermi level ðE F Þ reflect the Ce 4f states in the bulk. One can see a sharp peak structure with a width of o0:5 eV near E F , which is derived from the Ce 4f1 final states. The Ce 4f0 spectral features at a binding energy of E B 2 eV are weak, indicating a strong c–f hybridization in these compounds. We should note that the Ce 4f1 spectral feature of CeRhAs has a broader width than that of CeRhSb. Based on the photoionization cross-sections [8], the Rh 4d-derived states are strongly reflected in the hn ¼ 40 eV spectra, and almost vanished in the hn ¼ 115 eV spectra. The hn ¼ 870 eV spectra are off-resonance spectra in the Ce 3d–4f resonance regime, and the Ce 4f derived states are significantly suppressed. On going from hn ¼ 40 to 870 eV, and then back to 115 eV spectra, the Rh 4d derived peak at 2 eV is reduced with respect to the spectral intensities at E B ¼ 3–7 eV and E B ¼ 0–1.5 eV. On the basis of these observations, the Rh 4d derived peak at 2 eV has a nonbonding nature. The non-bonding Rh 4d states of CeRhAs have broader widths than those of CeRhSb, which suggests increased d–d overlap in the former. The As 4p and Sb 5p states at E B ¼ 3–7 eV and E B ¼ 0–1.5 eV are clearly resolved in the hn5948 eV spectra, a photon energy at which these states have almost comparable cross-sections to those of the Rh 4d states [8]. We regard the p–d–f hybridized states at E B ¼ 3–7 eV as
‘‘bonding’’ states, and those at E B ¼ 0–1.5 eV as ‘‘antibonding’’ states. Note that the Ce 4f states make a negligible contribution to the bonding states. The hn ¼ 5948 eV spectra are clearly different between CeRhAs and CeRhSb near E F : a broad peak exists at 1 eV in CeRhAs and two distinct peaks exist at E B 0:5 eV and 1 eV in CeRhSb. The magnitude of the energy gap in the p, d, and f states in CeRhAs has been reported to be 90 meV, which decreases on heating [7]. On the other hand, high-resolution lowenergy PES of CeRhSb has indicated that the Rh 4d and Sb 5p states have a pseudogap of 30 meV [5] or 70 meV [12], both of which are larger than the magnitude of the pseudogap in the Ce 4f states 13 meV [7]. Taking into account the present results, the energy gap of CeRhAs is formed via strong p–d–f hybridization. On the other hand, the Ce 4f states is predominant in the pseudogap of CeRhSb. According to the results of a band-structure calculation (LSDA) [13], CeRhAs has a gap, and most of the flat portions of the Ce 4f-derived bands are located in the unoccupied states. On the other hand, the Ce 4f states in CeRhSb are closer to E F , and the flat portion of the Ce 4f bands appears near E F . The pseudogap is, therefore, mainly derived from the Ce 4f states [13], in agreement with our observations. In conclusion, we have conducted high-resolution PES measurements on CeRhAs and CeRhSb using a wide range of photon energies. We have clarified the p–d–f hybridized states near E F in these compounds. This work was partly supported by a Grant-in-Aid for Scientific Research (No. 17654060) and for COE Research (13CE2002) by MEXT of Japan. The experiments at SPring-8 have been done as a collaborative program between JAERI/ SPring-8, JASRI/SPring-8, RIKEN/SPring-8 and HSRC. The SR experiments at HiSOR have been done under the approval of HSRC (Proposal Nos. 03-A-38. 03-A-40). References [1] T. Takabatake, et al., J. Magn. Magn. Mater. 177–181 (1998) 277. [2] T. Takabatake, et al., Physica B 206–207 (1995) 804; T. Takabatake, et al., Physica B 223–224 (1996) 413. [3] T. Sasakawa, et al., Phys. Rev. B 66 (2002) 041103(R). [4] P. Salamakha, et al., J. Alloy Comp. 313 (2000) L5. [5] H. Kumigashira, et al., Phys. Rev. Lett. 82 (1999) 1943; H. Kumigashira, et al., Phys. Rev. Lett. 87 (2001) 067205. [6] A. Sekiyama, et al., Physica B 359–361 (2005) 115. [7] K. Shimada, et al., Phys. Rev. B 66 (2002) 155202; K. Shimada, et al., J. Electron Spectrosc. Relat. Phenomen. 144–147 (2005) 857. [8] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [9] K. Shimada, et al., Nucl. Instr. and Meth. A 467–468 (2001) 517; K. Shimada, et al., Surf. Rev. Lett. 9 (2002) 529. [10] Y. Saitoh, et al., Nucl. Instr. and Meth. A 474 (2001) 253. [11] Y. Takata, et al., Nucl. Instr. and Meth. A 547 (2005) 50. [12] We have measures PES spectra at hn ¼ 8 and 40 eV and evaluated the magnitudes of the pseudogaps in Sb 5p and Rh 4d. [13] F. Ishii, T. Oguchi, J. Phys. Soc. Japan 73 (2004) 145.
Journal of Magnetism and Magnetic Materials 310 (2007) e57–e58 www.elsevier.com/locate/jmmm
High-resolution photoemission study of the hybridization gap in the Kondo semiconductor CeRhAs K. Shimadaa,, M. Higashiguchib, M. Aritaa, H. Namatamea, M. Taniguchia,b, S.-i. Fujimoric, Y. Saitohc, A. Fujimoric,d, Y. Takatae, S. Shine, K. Kobayashif, E. Ikenagaf, M. Yabashif, K. Tamasakug, Y. Nishinog, D. Miwag, T. Ishikawag, T. Sasakawah, T. Takabatakeh a
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-0046, Japan b Graduate School of Science, Hiroshima University, Higashi-Horoshima, Hiroshima 739-8526, Japan c Synchrotron Radiation Research Unit, Japan Atomic Energy Agency, Sayo, Hyogo 679-5148, Japan d Department of Complexity Science and Engineering, University of Tokyo, Kashiwa, Chiba 277-8561, Japan e Soft X-Ray Spectroscopy Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Hyogo 679-5148, Japan f JASRI/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Hyogo 679-5148, Japan g Coherent X-Ray Optics Laboratory, RIKEN/SPring-8, 1-1-1 Kouto, Mikazuki-cho, Hyogo 679-5148, Japan h Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8530, Japan Available online 2 November 2006
Abstract We have examined the electronic states of the Kondo semiconductor CeRhAs and the semimetal CeRhSb by means of high-resolution photoemission spectroscopy using tunable photon energies from hn ¼ 40 up to 5948 eV. On the basis of the photon-energy dependence of the photoionization cross-section, we have elucidated the p–d–f hybridized states in these compounds. r 2006 Elsevier B.V. All rights reserved. PACS: 71.27.þa; 75.20.Hr; 75.30.Mb; 79.60.i Keywords: CeRhAs; Resonance photoemission; Kondo semiconductor
The Kondo semiconductor CeRhAs and semimetal CeRhSb of the orthorhombic e-TiNiSi type have attracted much interest for their temperature-dependent energy-gap or pseudogap formations [1–3]. The Kondo temperatures for CeRhAs and CeRhSb are estimated to be T K 1500 KðkB T K 130 meVÞ [3] and 400 KðkB T K 34 meVÞ [2], respectively. The unit-cell volume increases on going from CeRhAs to CeRhSb, which leads to a decreased c–f hybridization strength [4]. Although high-resolution photoemission spectroscopy (PES) of these compounds has been conducted extensively [5–7], the partial density-of-states (DOS) for the whole valence bands have not been revealed so far. In order to understand the origin of the energy-gap formation, it is Corresponding author. Tel.: +81 82 424 6293; fax: +81 82 424 6294.
E-mail address: [email protected] (K. Shimada). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.201
important to clarify the p–d–f hybridized states at the top of the valence band. Since the photoionization crosssections of the As 4p and Sb 5p states with respect to those of Rh 4d or Ce 4f states increase above hn ¼ 1000 eV [8], PES with hard X -rays is useful to identify the p-derived spectral features. In the present paper, we report high-resolution PES of CeRhAs and CeRhSb using excitation photon energies ranging from hn ¼ 40 up to 5948 eV. We discuss the p–d–f hybridization in these compounds. Single crystals of CeRhAs and CeRhSb were grown by the Bridgman method [1–3]. The VUV PES (hn ¼ 40, 115 eV; energy resolution, DE ¼ 12–18 meV), soft X-ray PES (hn ¼ 870–881 eV; DE ¼ 100 meV), and hard X-ray PES (hn ¼ 5948 eV; DE ¼ 150 meV) measurements were performed at the beamlines BL-1 of HiSOR [9], BL23SU of SPring-8 [10], and BL29XU of SPring-8 [11],
ARTICLE IN PRESS K. Shimada et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e57–e58
CeRhAs
CeRhSb
hν=881 eV, 15 K (Ce 4f)
hν=881 eV, 15 K (Ce 4f)
hν=40 eV, 10 K (Rh 4d) hν=870 eV, 15 K hν=115 eV, 10 K
Intensity (arb. units)
Intensity (arb.units)
e58
hν=5948 eV, 28 K (Rh 4d + Sb 5p)
hν=5948 eV, 28 K (Rh 4d + As 4p) hν=40 eV
(a)
5 0 Binding Energy (eV)
hν=40 eV, 10 K (Rh 4d) hν=870 eV, 15 K hν=115 eV, 10 K
hν=40 eV
(b)
5 0 Binding Energy (eV)
Fig. 1. High-resolutions PES spectra of (a) CeRhAs and (b) CeRhSb taken at hn ¼ 40; 115; 870; 881, and 5948 eV.
respectively. The sample temperature was kept at 10–28 K. Clean sample surfaces were obtained by fracturing samples in the ultrahigh vacuum. Fig. 1(a) and (b) show PES spectra of CeRhAs and CeRhSb. The hn ¼ 881 eV spectra represent on-resonance spectra in the Ce 3d–4f resonance regime. The contribution from the surface electronic state is estimated to be less than 30% of the whole spectral weight at this photoelectron kinetic energy. Most of the spectral features near the Fermi level ðE F Þ reflect the Ce 4f states in the bulk. One can see a sharp peak structure with a width of o0:5 eV near E F , which is derived from the Ce 4f1 final states. The Ce 4f0 spectral features at a binding energy of E B 2 eV are weak, indicating a strong c–f hybridization in these compounds. We should note that the Ce 4f1 spectral feature of CeRhAs has a broader width than that of CeRhSb. Based on the photoionization cross-sections [8], the Rh 4d-derived states are strongly reflected in the hn ¼ 40 eV spectra, and almost vanished in the hn ¼ 115 eV spectra. The hn ¼ 870 eV spectra are off-resonance spectra in the Ce 3d–4f resonance regime, and the Ce 4f derived states are significantly suppressed. On going from hn ¼ 40 to 870 eV, and then back to 115 eV spectra, the Rh 4d derived peak at 2 eV is reduced with respect to the spectral intensities at E B ¼ 3–7 eV and E B ¼ 0–1.5 eV. On the basis of these observations, the Rh 4d derived peak at 2 eV has a nonbonding nature. The non-bonding Rh 4d states of CeRhAs have broader widths than those of CeRhSb, which suggests increased d–d overlap in the former. The As 4p and Sb 5p states at E B ¼ 3–7 eV and E B ¼ 0–1.5 eV are clearly resolved in the hn5948 eV spectra, a photon energy at which these states have almost comparable cross-sections to those of the Rh 4d states [8]. We regard the p–d–f hybridized states at E B ¼ 3–7 eV as
‘‘bonding’’ states, and those at E B ¼ 0–1.5 eV as ‘‘antibonding’’ states. Note that the Ce 4f states make a negligible contribution to the bonding states. The hn ¼ 5948 eV spectra are clearly different between CeRhAs and CeRhSb near E F : a broad peak exists at 1 eV in CeRhAs and two distinct peaks exist at E B 0:5 eV and 1 eV in CeRhSb. The magnitude of the energy gap in the p, d, and f states in CeRhAs has been reported to be 90 meV, which decreases on heating [7]. On the other hand, high-resolution lowenergy PES of CeRhSb has indicated that the Rh 4d and Sb 5p states have a pseudogap of 30 meV [5] or 70 meV [12], both of which are larger than the magnitude of the pseudogap in the Ce 4f states 13 meV [7]. Taking into account the present results, the energy gap of CeRhAs is formed via strong p–d–f hybridization. On the other hand, the Ce 4f states is predominant in the pseudogap of CeRhSb. According to the results of a band-structure calculation (LSDA) [13], CeRhAs has a gap, and most of the flat portions of the Ce 4f-derived bands are located in the unoccupied states. On the other hand, the Ce 4f states in CeRhSb are closer to E F , and the flat portion of the Ce 4f bands appears near E F . The pseudogap is, therefore, mainly derived from the Ce 4f states [13], in agreement with our observations. In conclusion, we have conducted high-resolution PES measurements on CeRhAs and CeRhSb using a wide range of photon energies. We have clarified the p–d–f hybridized states near E F in these compounds. This work was partly supported by a Grant-in-Aid for Scientific Research (No. 17654060) and for COE Research (13CE2002) by MEXT of Japan. The experiments at SPring-8 have been done as a collaborative program between JAERI/ SPring-8, JASRI/SPring-8, RIKEN/SPring-8 and HSRC. The SR experiments at HiSOR have been done under the approval of HSRC (Proposal Nos. 03-A-38. 03-A-40). References [1] T. Takabatake, et al., J. Magn. Magn. Mater. 177–181 (1998) 277. [2] T. Takabatake, et al., Physica B 206–207 (1995) 804; T. Takabatake, et al., Physica B 223–224 (1996) 413. [3] T. Sasakawa, et al., Phys. Rev. B 66 (2002) 041103(R). [4] P. Salamakha, et al., J. Alloy Comp. 313 (2000) L5. [5] H. Kumigashira, et al., Phys. Rev. Lett. 82 (1999) 1943; H. Kumigashira, et al., Phys. Rev. Lett. 87 (2001) 067205. [6] A. Sekiyama, et al., Physica B 359–361 (2005) 115. [7] K. Shimada, et al., Phys. Rev. B 66 (2002) 155202; K. Shimada, et al., J. Electron Spectrosc. Relat. Phenomen. 144–147 (2005) 857. [8] J.J. Yeh, I. Lindau, At. Data Nucl. Data Tables 32 (1985) 1. [9] K. Shimada, et al., Nucl. Instr. and Meth. A 467–468 (2001) 517; K. Shimada, et al., Surf. Rev. Lett. 9 (2002) 529. [10] Y. Saitoh, et al., Nucl. Instr. and Meth. A 474 (2001) 253. [11] Y. Takata, et al., Nucl. Instr. and Meth. A 547 (2005) 50. [12] We have measures PES spectra at hn ¼ 8 and 40 eV and evaluated the magnitudes of the pseudogaps in Sb 5p and Rh 4d. [13] F. Ishii, T. Oguchi, J. Phys. Soc. Japan 73 (2004) 145.