- Email: [email protected]
Photoemission study of the temperature-dependent energy-gap formation in the Kondo semiconductor CeRhAs
Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 857–859
Photoemission study of the temperature-dependent energy-gap formation in the Kondo semiconductor CeRhAs Kenya Shimada a, ∗ , Mitsuharu Higashiguchi b , Takamasa Narimura b , Masashi Arita a , Yukiharu Takeda a , Hirofumi Namatame a , Masaki Taniguchi a, b , Tetsuya Sasakawa c , Toshimitsu Suemitsu c , Toshiro Takabatake c a
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-8526, Japan b Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan c Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan Available online 24 February 2005
Abstract The temperature dependence of the Ce 4f state of a Kondo semiconductor CeRhAs has been examined by means of high-resolution temperature-dependent photoemission spectroscopy. The Ce 4f spectral intensity near the Fermi level (EF ) is gradually decreased on cooling indicating temperature-dependent metal-to-insulator transition. The magnitudes of the energy-gap for the f, d, and p states are the same, indicating that these states are strongly hybridized near EF and form the energy-gap. © 2005 Elsevier B.V. All rights reserved. Keywords: CeRhAs; Kondo semiconductor; High-resolution photoemission spectroscopy
Kondo semiconductors have attracted much interest for their temperature-dependent metal-to-insulator transition (MIT) [1,2]. Orthorhombic -TiNiSi type CeNiSn and CeRhSb are the Kondo semimetals with a narrow pseudogap. The anisotropic c–f hybridization has been discussed for these materials based on the periodic Anderson model (PAM) [3,4]. On the other hand, CeRhAs with the same crystal structure has an energy-gap for the whole Brillouin zone (BZ) [1,5]. Recently single crystalline CeRhAs was synthesized and anisotropic physical properties have been elucidated [6]. The energy-gap formation of CeRhAs is associated with structural phase transitions at T1 ∼ 370 K, T2 ∼ 235 K, and T3 ∼ 165 K [6]. It indicates that lattice modulations play an important role for the energy-gap formation [6]. Kumigashira et al. have done high-resolution temperature-dependent photoemission spectroscopy on polycrystalline CeRhSb and CeRhAs using He I␣ resonance line and revealed that the spectral intensity near the Fermi level (EF ) is reduced on cooling [7]. ∗
Corresponding author. Tel.: +81 824 24 6293; fax: +81 824 24 6294. E-mail address: [email protected] (K. Shimada).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.148
Based on the He II␣ − He I␣ difference spectra, the magnitude of the pseudogap in the Ce 4f state (∆f ) is claimed to be smaller than that in the conduction state (∆c ), ∆f < ∆c [8]. We have reported direct observation of the Ce 4f spectra of CeRhSb and CeRhAs single crystals by means of highresolution resonant photoemission spectroscopy [9]. While the Ce 4f spectrum of CeRhSb has a peak structure near EF with a narrow pseudogap (∼13 meV) that of CeRhAs forms a broad energy-gap above the binding energy of ∼90 meV [9]. In the photoemission measurements, the magnitude of the energy-gap (∆PES ∼ 90 meV) is defined by the bindingenergy region for the reduced spectral intensity, which is larger than the energy-gap estimated from transport measurements (∆t ∼ 14–24 meV) [6] determined from low energy excitations at EF . In the present paper, we report a high-resolution temperature-dependent resonant photoemission spectroscopy of single crystalline CeRhAs and directly examine the energygap formation in the Ce 4f state. By changing the incident photon energy, the Rh 4d and As 4p partial density of states (PDOS) have been also clarified.
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K. Shimada et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 857–859
Single crystalline CeRhAs samples were grown by the Bridgman method [6]. Based on the single-impurity Anderson model (SIAM) calculation [10], the Kondo temperature (TK ) of CeRhAs in the metallic phase is estimated to be TK ∼ 3Tχ ∼ 1500 K (∼130 meV) [6,9], where Tχ is the temperature for the maximum magnetic susceptibility (χ) [6]. Photoemission measurements were carried out on a highresolution undulator beamlines (BL-1 [11] and BL-9 [12]) of a compact electron-storage ring (HiSOR) at Hiroshima Synchrotron Radiation Center (HSRC). The experimental energy resolution was set at E = 20, 9, and 5 meV for hν = 122, 40, and 7.8 eV, respectively. Samples were mounted on a cryostat and temperature was controlled between 375 and 10 K. In order to obtain clean surfaces, the samples were fractured in situ in ultrahigh vacuum. Fig. 1(a) shows the Ce 4f spectra of CeRhAs near EF . As clearly shown in the figure, the spectral intensity at EF is gradually reduced on cooling in agreement with the temperature-dependent MIT. To estimate spectral density of states (SDOS), we divided photoemission spectra by the Fermi–Dirac distribution function convoluted with a Gaussian which represents experimental resolution (Fig. 1(b)). The Ce 4f SDOS at 375 K is metallic with no gap structure, while that at 10 K exhibits a clear energy-gap (∆PES ∼ 90 meV) [13]. The spectral intensity at EF is gradually reduced below T1 ∼ 370 K from which χ starts to decrease. The V-shaped energy-gap becomes apparent below ∼200 K which is closer to T2 or T3 , suggesting the importance of lattice modulations. The temperature dependence of the Ce 4f SDOS cannot be described by the SIAM in which a Kondo-resonance peak enhances as temperature decreases [14]. The pseudogap formation of CeNiSn and CeRhSb has been discussed based on the PAM in which almost flat Ce 4f bands hybridize with wide conduction bands: the calculated Ce 4f spectral function has strong intensity near EF with a narrow pseudogap [3,4]. On the other hand, the Ce 4f spectral shape of CeRhAs is significantly different from that given by the PAM [3,4,9]. Next, we examine contributions from the Rh 4d and As 4p states near EF by changing excitation photon energies (Fig. 2(a)). Taking photoionization cross-sections into ac-
Fig. 2. (a) High-resolution photoemission spectra of CeRhAs taken at different incident photon energy and (b) the SDOS ’s obtained from (a).
count, most of the spectral intensity is derived from the As 4p and Rh 4d states taken at hν = 7.8 and 40 eV, respectively [15]. Fig. 2(b) shows the SDOS’s obtained from (a). The magnitudes of the energy-gap for the Ce 4f (∆f ), Rh 4d (∆d ), and As 4p (∆p ) states are the same, ∆f ∼∆d ∼ ∆p ∼ 90 meV, which is different from the previous arguments using He lamp [16]. The present result suggests that the energy-gap is formed by the f, d, and p hybridized dispersive bands near EF [17]. It should be noted that taken at hν ∼ 8 eV, the inelastic meanfree path of photoelectron from EF is expected to be longer ˚ [18]. Photoemission spectra at hν ∼ 8 eV, therethan ∼50 A fore, reflect bulk electronic properties. Band-structure calculations of CeRhAs and CeRhSb have been done extensively by Ishii and Oguchi [19]. The calculation showed an indirect energy-gap (38 meV) in the ground state [19]. Since most of the flat portions of the Ce 4f state are well above EF , the energy-gap is formed by dispersive bands derived from strongly hybridized Ce 4f, Rh 4d and As 4p states [19]. The PDOS’s of the Ce 4f, Rh 4d and As 4p are similar near EF , which explains present photoemission spectral shapes at low-temperature. If As is substituted to Sb, the
Fig. 1. (a) High-resolution temperature-dependent resonant photoemission spectra of CeRhAs and (b) the SDOS’s obtained from (a).
K. Shimada et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 857–859
flat portions of the Ce 4f bands are lowered and located closer to EF . Consequently, the energy-gap is collapsed anisotropically and the Ce 4f PDOS exhibits a sharp peak structure near EF , in agreement with the photoemission results of CeRhSb [9]. The band-structure calculation indicated that the energygap size is sensitive to the lattice parameters [19]. The temperature dependence of the c–f hybridization associated with successive structural phase transitions [6] should be taken into account to describe the energy-gap formation in CeRhAs. In summary, high-resolution temperature-dependent photoemission spectroscopy of a Kondo semiconductor CeRhAs has been performed. A clear MIT in the Ce 4f state has been observed. The magnitudes of the energy-gap in the Ce 4f, Rh 4d, and As 4p states are the same, indicating that the energygap is formed by the bands where the f, d, and p states are strongly hybridized.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements This work was supported by a Grant-in-Aid for COE Research (13CE2002) by the Ministry of Education, Science, and Culture of Japan. We thank the Cryogenic Center, Hiroshima University, for supplying liquid helium. KS would like to thank Dr. F. Ishii and Prof. T. Oguchi for valuable discussion on the band-structure calculation of CeRhAs and CeRhSb. The SR experiments at HiSOR have been done under the approval of HSRC (Proposal no. 01-A-24, 03-A-40).
859
[17]
[18] [19]
T. Takabatake, et al., J. Magn. Magn. Mater. 177–181 (1998) 277. Z. Fisk, et al., Physica B 206–207 (1995) 798. H. Ikeda, K. Miyake, J. Phys. Soc. Jpn. 65 (1996) 1769. J. Moreno, P. Colemen, Phys. Rev. Lett. 84 (2001) 106401. S. Yoshii, et al., Physica B 223–224 (1996) 421. T. Sasakawa, et al., Phys. Rev. B 66 (2002) 041103. H. Kumigashira, et al., Phys. Rev. Lett. 82 (1999) 1943. H. Kumigashira, et al., Phys. Rev. Lett. 87 (2001) 067206. K. Shimada, et al., Phys. Rev. B 66 (2002) 155202. N.E. Bickers, D.L. Cox, J.W. Wilkins, Phys. Rev. Lett. 54 (1985) 230. K. Shimada, et al., Surf. Rev. Lett. 9 (2002) 529. M. Arita, et al., Surf. Rev. Lett. 9 (2002) 535. Strong spectral intensity above EF is derived from the Kondo resonance. See for example F. Reinert, et al., Phys. Rev. Lett. 87 (2001) 106401. J.J. Yeh, Atomic Calculation of Photoionization Cross-Section and Asymmetry Parameters, Gordon and Breach, New York, 1993. The HeI␣ spectrum derived mostly from the Rh 4d state has a pseudogap, ∆ ∼ 90–100 meV, which agrees well with ∆d obtained in the present experiment. In the previous experiment, the energy-gap in the Rh 4d state disappears on heating [7]. In our unpublished data, the As 4p derived spectrum also showed a similar temperature dependence. The energygap in the f, d, and p states, therefore, disappears as temperature increases. The result supports that the energy-gap is formed by the strong hybridization of these states. C.R. Brundle, J. Vac. Sci. Technol. 11 (1974) 212. F. Ishii, Physica B 328 (2003) 154; F. Ishii, T. Oguchi, J. Phys. Soc. Jpn. 73 (2004) 145.
Photoemission study of the temperature-dependent energy-gap formation in the Kondo semiconductor CeRhAs Kenya Shimada a, ∗ , Mitsuharu Higashiguchi b , Takamasa Narimura b , Masashi Arita a , Yukiharu Takeda a , Hirofumi Namatame a , Masaki Taniguchi a, b , Tetsuya Sasakawa c , Toshimitsu Suemitsu c , Toshiro Takabatake c a
Hiroshima Synchrotron Radiation Center, Hiroshima University, Higashi-Hiroshima 739-8526, Japan b Graduate School of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan c Department of Quantum Matter, ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan Available online 24 February 2005
Abstract The temperature dependence of the Ce 4f state of a Kondo semiconductor CeRhAs has been examined by means of high-resolution temperature-dependent photoemission spectroscopy. The Ce 4f spectral intensity near the Fermi level (EF ) is gradually decreased on cooling indicating temperature-dependent metal-to-insulator transition. The magnitudes of the energy-gap for the f, d, and p states are the same, indicating that these states are strongly hybridized near EF and form the energy-gap. © 2005 Elsevier B.V. All rights reserved. Keywords: CeRhAs; Kondo semiconductor; High-resolution photoemission spectroscopy
Kondo semiconductors have attracted much interest for their temperature-dependent metal-to-insulator transition (MIT) [1,2]. Orthorhombic -TiNiSi type CeNiSn and CeRhSb are the Kondo semimetals with a narrow pseudogap. The anisotropic c–f hybridization has been discussed for these materials based on the periodic Anderson model (PAM) [3,4]. On the other hand, CeRhAs with the same crystal structure has an energy-gap for the whole Brillouin zone (BZ) [1,5]. Recently single crystalline CeRhAs was synthesized and anisotropic physical properties have been elucidated [6]. The energy-gap formation of CeRhAs is associated with structural phase transitions at T1 ∼ 370 K, T2 ∼ 235 K, and T3 ∼ 165 K [6]. It indicates that lattice modulations play an important role for the energy-gap formation [6]. Kumigashira et al. have done high-resolution temperature-dependent photoemission spectroscopy on polycrystalline CeRhSb and CeRhAs using He I␣ resonance line and revealed that the spectral intensity near the Fermi level (EF ) is reduced on cooling [7]. ∗
Corresponding author. Tel.: +81 824 24 6293; fax: +81 824 24 6294. E-mail address: [email protected] (K. Shimada).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elspec.2005.01.148
Based on the He II␣ − He I␣ difference spectra, the magnitude of the pseudogap in the Ce 4f state (∆f ) is claimed to be smaller than that in the conduction state (∆c ), ∆f < ∆c [8]. We have reported direct observation of the Ce 4f spectra of CeRhSb and CeRhAs single crystals by means of highresolution resonant photoemission spectroscopy [9]. While the Ce 4f spectrum of CeRhSb has a peak structure near EF with a narrow pseudogap (∼13 meV) that of CeRhAs forms a broad energy-gap above the binding energy of ∼90 meV [9]. In the photoemission measurements, the magnitude of the energy-gap (∆PES ∼ 90 meV) is defined by the bindingenergy region for the reduced spectral intensity, which is larger than the energy-gap estimated from transport measurements (∆t ∼ 14–24 meV) [6] determined from low energy excitations at EF . In the present paper, we report a high-resolution temperature-dependent resonant photoemission spectroscopy of single crystalline CeRhAs and directly examine the energygap formation in the Ce 4f state. By changing the incident photon energy, the Rh 4d and As 4p partial density of states (PDOS) have been also clarified.
858
K. Shimada et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 857–859
Single crystalline CeRhAs samples were grown by the Bridgman method [6]. Based on the single-impurity Anderson model (SIAM) calculation [10], the Kondo temperature (TK ) of CeRhAs in the metallic phase is estimated to be TK ∼ 3Tχ ∼ 1500 K (∼130 meV) [6,9], where Tχ is the temperature for the maximum magnetic susceptibility (χ) [6]. Photoemission measurements were carried out on a highresolution undulator beamlines (BL-1 [11] and BL-9 [12]) of a compact electron-storage ring (HiSOR) at Hiroshima Synchrotron Radiation Center (HSRC). The experimental energy resolution was set at E = 20, 9, and 5 meV for hν = 122, 40, and 7.8 eV, respectively. Samples were mounted on a cryostat and temperature was controlled between 375 and 10 K. In order to obtain clean surfaces, the samples were fractured in situ in ultrahigh vacuum. Fig. 1(a) shows the Ce 4f spectra of CeRhAs near EF . As clearly shown in the figure, the spectral intensity at EF is gradually reduced on cooling in agreement with the temperature-dependent MIT. To estimate spectral density of states (SDOS), we divided photoemission spectra by the Fermi–Dirac distribution function convoluted with a Gaussian which represents experimental resolution (Fig. 1(b)). The Ce 4f SDOS at 375 K is metallic with no gap structure, while that at 10 K exhibits a clear energy-gap (∆PES ∼ 90 meV) [13]. The spectral intensity at EF is gradually reduced below T1 ∼ 370 K from which χ starts to decrease. The V-shaped energy-gap becomes apparent below ∼200 K which is closer to T2 or T3 , suggesting the importance of lattice modulations. The temperature dependence of the Ce 4f SDOS cannot be described by the SIAM in which a Kondo-resonance peak enhances as temperature decreases [14]. The pseudogap formation of CeNiSn and CeRhSb has been discussed based on the PAM in which almost flat Ce 4f bands hybridize with wide conduction bands: the calculated Ce 4f spectral function has strong intensity near EF with a narrow pseudogap [3,4]. On the other hand, the Ce 4f spectral shape of CeRhAs is significantly different from that given by the PAM [3,4,9]. Next, we examine contributions from the Rh 4d and As 4p states near EF by changing excitation photon energies (Fig. 2(a)). Taking photoionization cross-sections into ac-
Fig. 2. (a) High-resolution photoemission spectra of CeRhAs taken at different incident photon energy and (b) the SDOS ’s obtained from (a).
count, most of the spectral intensity is derived from the As 4p and Rh 4d states taken at hν = 7.8 and 40 eV, respectively [15]. Fig. 2(b) shows the SDOS’s obtained from (a). The magnitudes of the energy-gap for the Ce 4f (∆f ), Rh 4d (∆d ), and As 4p (∆p ) states are the same, ∆f ∼∆d ∼ ∆p ∼ 90 meV, which is different from the previous arguments using He lamp [16]. The present result suggests that the energy-gap is formed by the f, d, and p hybridized dispersive bands near EF [17]. It should be noted that taken at hν ∼ 8 eV, the inelastic meanfree path of photoelectron from EF is expected to be longer ˚ [18]. Photoemission spectra at hν ∼ 8 eV, therethan ∼50 A fore, reflect bulk electronic properties. Band-structure calculations of CeRhAs and CeRhSb have been done extensively by Ishii and Oguchi [19]. The calculation showed an indirect energy-gap (38 meV) in the ground state [19]. Since most of the flat portions of the Ce 4f state are well above EF , the energy-gap is formed by dispersive bands derived from strongly hybridized Ce 4f, Rh 4d and As 4p states [19]. The PDOS’s of the Ce 4f, Rh 4d and As 4p are similar near EF , which explains present photoemission spectral shapes at low-temperature. If As is substituted to Sb, the
Fig. 1. (a) High-resolution temperature-dependent resonant photoemission spectra of CeRhAs and (b) the SDOS’s obtained from (a).
K. Shimada et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 857–859
flat portions of the Ce 4f bands are lowered and located closer to EF . Consequently, the energy-gap is collapsed anisotropically and the Ce 4f PDOS exhibits a sharp peak structure near EF , in agreement with the photoemission results of CeRhSb [9]. The band-structure calculation indicated that the energygap size is sensitive to the lattice parameters [19]. The temperature dependence of the c–f hybridization associated with successive structural phase transitions [6] should be taken into account to describe the energy-gap formation in CeRhAs. In summary, high-resolution temperature-dependent photoemission spectroscopy of a Kondo semiconductor CeRhAs has been performed. A clear MIT in the Ce 4f state has been observed. The magnitudes of the energy-gap in the Ce 4f, Rh 4d, and As 4p states are the same, indicating that the energygap is formed by the bands where the f, d, and p states are strongly hybridized.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Acknowledgements This work was supported by a Grant-in-Aid for COE Research (13CE2002) by the Ministry of Education, Science, and Culture of Japan. We thank the Cryogenic Center, Hiroshima University, for supplying liquid helium. KS would like to thank Dr. F. Ishii and Prof. T. Oguchi for valuable discussion on the band-structure calculation of CeRhAs and CeRhSb. The SR experiments at HiSOR have been done under the approval of HSRC (Proposal no. 01-A-24, 03-A-40).
859
[17]
[18] [19]
T. Takabatake, et al., J. Magn. Magn. Mater. 177–181 (1998) 277. Z. Fisk, et al., Physica B 206–207 (1995) 798. H. Ikeda, K. Miyake, J. Phys. Soc. Jpn. 65 (1996) 1769. J. Moreno, P. Colemen, Phys. Rev. Lett. 84 (2001) 106401. S. Yoshii, et al., Physica B 223–224 (1996) 421. T. Sasakawa, et al., Phys. Rev. B 66 (2002) 041103. H. Kumigashira, et al., Phys. Rev. Lett. 82 (1999) 1943. H. Kumigashira, et al., Phys. Rev. Lett. 87 (2001) 067206. K. Shimada, et al., Phys. Rev. B 66 (2002) 155202. N.E. Bickers, D.L. Cox, J.W. Wilkins, Phys. Rev. Lett. 54 (1985) 230. K. Shimada, et al., Surf. Rev. Lett. 9 (2002) 529. M. Arita, et al., Surf. Rev. Lett. 9 (2002) 535. Strong spectral intensity above EF is derived from the Kondo resonance. See for example F. Reinert, et al., Phys. Rev. Lett. 87 (2001) 106401. J.J. Yeh, Atomic Calculation of Photoionization Cross-Section and Asymmetry Parameters, Gordon and Breach, New York, 1993. The HeI␣ spectrum derived mostly from the Rh 4d state has a pseudogap, ∆ ∼ 90–100 meV, which agrees well with ∆d obtained in the present experiment. In the previous experiment, the energy-gap in the Rh 4d state disappears on heating [7]. In our unpublished data, the As 4p derived spectrum also showed a similar temperature dependence. The energygap in the f, d, and p states, therefore, disappears as temperature increases. The result supports that the energy-gap is formed by the strong hybridization of these states. C.R. Brundle, J. Vac. Sci. Technol. 11 (1974) 212. F. Ishii, Physica B 328 (2003) 154; F. Ishii, T. Oguchi, J. Phys. Soc. Jpn. 73 (2004) 145.