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Photoemission lineshape of CeNiSn near the Fermi level
Physica B 259—261 (1999) 1116—1117
Photoemission lineshape of CeNiSn near the Fermi level J.-S. Kang *, Y. Inada, Y. O nuki, C.G. Olson, S.K. Kwon, B.I. Min Department of Physics, The Catholic University of Korea, Puchon 422-743, South Korea Graduate School of Science, Osaka University, Toyonaka 560, Japan Ames Laboratory, Iowa State University, Ames, IA 50011, USA Department of Physics, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract The high-resolution photoemission spectroscopy study of single crystalline CeNiSn indicates a finite density of states at E , implying a semi-metallic ground state. This finding is supported by the lineshape analysis using the DOS models. $ The Ce 4f spectrum exhibits three peak structures, in which the peak near-E is found to reflect a substantially large Ce $ 4f—Sn sp hybridization. 1999 Elsevier Science B.V. All rights reserved. Keywords: Photoemission; CeNiSn; Pseudo-gap
CeNiSn has attracted much attention because of its very small pseudo-gap of a few K [1], and a strong anisotropy in magnetic and electronic properties. Early transport, thermodynamic, and magnetic measurements on CeNiSn indicated the energy-gap formation in the nonmagnetic ground state below ¹ +6 K (¹ : the gap formation temperature) [2—5]. In contrast, recent experiments on the temperature(T)-dependence of the electrical resistivity in a purified single crystal of CeNiSn indicated a metallic-like ground state [6]. CeNiSn thus seems to exhibit an apparent contradiction between the metallic conductivity and the gap-wise behavior of the thermodynamic properties and the spin response. To resolve the contradiction on the nature of the ground state of CeNiSn, it is important to investigate its electronic structure. Band structure calculations [7,8] predict CeNiSn to be either a semi-metal or a small-gap insulator with a V-shaped DOS near E . A few photo$ emission spectroscopy (PES) studies on polycrystalline CeNiSn have been reported [9,10]. However, neither the predicted anisotropic electronic structure nor the Vshaped DOS near E has been confirmed yet. In view of $ * Corresponding author: Fax: #82-32-340-3111; e-mail: [email protected].
the recent reports of the metallic-like behavior, it is important to carry out PES experiments on high-quality single crystal samples. In this paper we report a high resolution PES study of high-quality single crystalline CeNiSn and the analysis of the lineshape near E using $ two different DOS models. We have also determined the Ce 4f partial spectral weight (PSW) distribution, based on the resonant photoemission study near the Ce 4dP4f absorption edge. Fig. 1a compares the high-resolution PES spectrum of CeNiSn (dots) with that of Pt metal (solid line) in the vicinity of E . These data were taken with the same $ experimental conditions; at ¹ +20 K, and with .#1 FWHM&45 meV. In this comparison, two spectra are scaled to each other at 400 meV below E . Fig. 1a shows $ that the onset and the slope of the spectrum of CeNiSn near E are very similar to those of Pt metal, with no sign $ of a gap at E . This finding can be seen more explicitly in $ Fig. 1b and c, where the spectrum of CeNiSn is analyzed using two different DOS models (dashed lines). The first model [“pseudo-gap model” in Fig. 1b] assumes a Vshaped DOS with a residual DOS at E of width 10 meV $ below E . The second model [Fig. 1c] assumes a linear $ DOS with a finite DOS at E . Both theory curves are $ multiplied with a 20 K Fermi function and convoluted with a Gaussian function of FWHM"45 meV (solid
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 8 2 1 - 7
J.-S. Kang et al. / Physica B 259—261 (1999) 1116—1117
1117
Fig. 2. Comparison of the extracted Ce 4f spectrum (dots) with the Ce 4dP4f on-resonance (solid line) and off-resonance (dashed line) spectra.
Fig. 1. (a) High-resolution photoemission spectrum of CeNiSn (dots) in the vicinity of E , and that of Pt metal (solid line). (b) $ hl"14 eV spectrum (dots), compared to the V-shaped DOS with a residual DOS (“pseudo-gap model”). (c) Similarly for the linear DOS model.
lines) to simulate the experimental temperature and the instrumental resolution, respectively. Note that the first model yields a deviated onset near E , in contrast to the $ conventional metallic Fermi edge in the measured spectrum. Other models with a real gap of order of 10 meV but without a residual DOS yield even larger discrepancies with experiment (not shown here). It is clear that a simple linear DOS yields a better fit to the experimental spectrum than a pseudo-gap model, suggesting a finite metallic DOS at E in CeNiSn. This is consistent with the $ recent report of a metallic ground state [6]. Even though our PES spectrum is measured above ¹ , it should at least reflect the ground state DOS. Therefore, the present analysis seems to refute the existence of a pseudo-gap of width of order of&10 meV at E . Our analysis, however, $ does not eliminate the possibility of the pseudo-gap of width (5 meV at E or the gap that exists over only $ part of the Fermi surface. Fig. 2 presents the extracted Ce 4f PSW (dots), in comparison with the Ce 4dP4f on-resonance (solid line) and off-resonance (dashed line) spectra, with a resolution of &250 meV. Three prominent features are observed in the Ce 4f PSW at about 0.3 eV (“A”), 0.9 eV (“B”), and 2.7 eV (“C”) below E . The peaks “A” and “C” are remi$ niscent of the well-known double-peak structures observed in Ce metal and Ce compounds. The peak “C” corresponds to the 4fP4f transition. The 4f spectral
intensity close to E (“A”) corresponds to the 4fcK\ $ final state, reflecting the hybridization between Ce 4f and conduction band electrons, such as Ce 5d, Sn sp, and Ni 3d electrons. The large separation between Ce sites in CeNiSn (d"3.82 As ) suggests that the direct interaction between near-neighbor Ce 4f electrons is negligible. Further, the localized character of the Ni 3d wave function and the shorter nearest-neighbor separation between Ce and Sn atoms (3.27 As ) than that between Ce and Ce atoms (3.82 As ) suggest that the Ce 4f—Sn sp hybridization might be the important factor to determine the intensity of the near-E peak “A”. Finally, the peak “B” reflects $ both the Ce 4f—Ni 3d hybridization and the Ce 5d-Ni 3d hybridization [11]. This work has been supported by the Korea Research Foundation (1997-001-D00139), and the Korea Science and Engineering Foundation (96-0702-01-01-3).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. Takabatake et al., Phys. Rev. B 45 (1992) 5740. T.E. Mason et al., Phys. Rev. Lett. 69 (1992) 490. S. Nishigori et al., J. Phys. Soc. Japan 65 (1996) 2614. M. Kyogaku et al., J. Phys. Soc. Japan 59 (1990) 1728. K. Izawa et al., J. Phys. Soc. Japan 65 (1996) 3119. Y. Inada et al., J. Phys. Soc. Japan 65 (1996) 1158. A. Yanase, H. Harima, Prog. Theor. Phys. Suppl. 108 (1992) 19. T.J. Hammond et al., Phys. Rev. B 51 (1995) 2994. S. Nohara et al., Phys. Rev. B 47 (1993) 1754. A. Slebarski et al., Phys. Rev. B 56 (1997) 7245. J.-S. Kang et al., Phys. Rev. B 58 (1998) 4426.
Photoemission lineshape of CeNiSn near the Fermi level J.-S. Kang *, Y. Inada, Y. O nuki, C.G. Olson, S.K. Kwon, B.I. Min Department of Physics, The Catholic University of Korea, Puchon 422-743, South Korea Graduate School of Science, Osaka University, Toyonaka 560, Japan Ames Laboratory, Iowa State University, Ames, IA 50011, USA Department of Physics, Pohang University of Science and Technology, Pohang 790-784, South Korea
Abstract The high-resolution photoemission spectroscopy study of single crystalline CeNiSn indicates a finite density of states at E , implying a semi-metallic ground state. This finding is supported by the lineshape analysis using the DOS models. $ The Ce 4f spectrum exhibits three peak structures, in which the peak near-E is found to reflect a substantially large Ce $ 4f—Sn sp hybridization. 1999 Elsevier Science B.V. All rights reserved. Keywords: Photoemission; CeNiSn; Pseudo-gap
CeNiSn has attracted much attention because of its very small pseudo-gap of a few K [1], and a strong anisotropy in magnetic and electronic properties. Early transport, thermodynamic, and magnetic measurements on CeNiSn indicated the energy-gap formation in the nonmagnetic ground state below ¹ +6 K (¹ : the gap formation temperature) [2—5]. In contrast, recent experiments on the temperature(T)-dependence of the electrical resistivity in a purified single crystal of CeNiSn indicated a metallic-like ground state [6]. CeNiSn thus seems to exhibit an apparent contradiction between the metallic conductivity and the gap-wise behavior of the thermodynamic properties and the spin response. To resolve the contradiction on the nature of the ground state of CeNiSn, it is important to investigate its electronic structure. Band structure calculations [7,8] predict CeNiSn to be either a semi-metal or a small-gap insulator with a V-shaped DOS near E . A few photo$ emission spectroscopy (PES) studies on polycrystalline CeNiSn have been reported [9,10]. However, neither the predicted anisotropic electronic structure nor the Vshaped DOS near E has been confirmed yet. In view of $ * Corresponding author: Fax: #82-32-340-3111; e-mail: [email protected].
the recent reports of the metallic-like behavior, it is important to carry out PES experiments on high-quality single crystal samples. In this paper we report a high resolution PES study of high-quality single crystalline CeNiSn and the analysis of the lineshape near E using $ two different DOS models. We have also determined the Ce 4f partial spectral weight (PSW) distribution, based on the resonant photoemission study near the Ce 4dP4f absorption edge. Fig. 1a compares the high-resolution PES spectrum of CeNiSn (dots) with that of Pt metal (solid line) in the vicinity of E . These data were taken with the same $ experimental conditions; at ¹ +20 K, and with .#1 FWHM&45 meV. In this comparison, two spectra are scaled to each other at 400 meV below E . Fig. 1a shows $ that the onset and the slope of the spectrum of CeNiSn near E are very similar to those of Pt metal, with no sign $ of a gap at E . This finding can be seen more explicitly in $ Fig. 1b and c, where the spectrum of CeNiSn is analyzed using two different DOS models (dashed lines). The first model [“pseudo-gap model” in Fig. 1b] assumes a Vshaped DOS with a residual DOS at E of width 10 meV $ below E . The second model [Fig. 1c] assumes a linear $ DOS with a finite DOS at E . Both theory curves are $ multiplied with a 20 K Fermi function and convoluted with a Gaussian function of FWHM"45 meV (solid
0921-4526/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 8 ) 0 0 8 2 1 - 7
J.-S. Kang et al. / Physica B 259—261 (1999) 1116—1117
1117
Fig. 2. Comparison of the extracted Ce 4f spectrum (dots) with the Ce 4dP4f on-resonance (solid line) and off-resonance (dashed line) spectra.
Fig. 1. (a) High-resolution photoemission spectrum of CeNiSn (dots) in the vicinity of E , and that of Pt metal (solid line). (b) $ hl"14 eV spectrum (dots), compared to the V-shaped DOS with a residual DOS (“pseudo-gap model”). (c) Similarly for the linear DOS model.
lines) to simulate the experimental temperature and the instrumental resolution, respectively. Note that the first model yields a deviated onset near E , in contrast to the $ conventional metallic Fermi edge in the measured spectrum. Other models with a real gap of order of 10 meV but without a residual DOS yield even larger discrepancies with experiment (not shown here). It is clear that a simple linear DOS yields a better fit to the experimental spectrum than a pseudo-gap model, suggesting a finite metallic DOS at E in CeNiSn. This is consistent with the $ recent report of a metallic ground state [6]. Even though our PES spectrum is measured above ¹ , it should at least reflect the ground state DOS. Therefore, the present analysis seems to refute the existence of a pseudo-gap of width of order of&10 meV at E . Our analysis, however, $ does not eliminate the possibility of the pseudo-gap of width (5 meV at E or the gap that exists over only $ part of the Fermi surface. Fig. 2 presents the extracted Ce 4f PSW (dots), in comparison with the Ce 4dP4f on-resonance (solid line) and off-resonance (dashed line) spectra, with a resolution of &250 meV. Three prominent features are observed in the Ce 4f PSW at about 0.3 eV (“A”), 0.9 eV (“B”), and 2.7 eV (“C”) below E . The peaks “A” and “C” are remi$ niscent of the well-known double-peak structures observed in Ce metal and Ce compounds. The peak “C” corresponds to the 4fP4f transition. The 4f spectral
intensity close to E (“A”) corresponds to the 4fcK\ $ final state, reflecting the hybridization between Ce 4f and conduction band electrons, such as Ce 5d, Sn sp, and Ni 3d electrons. The large separation between Ce sites in CeNiSn (d"3.82 As ) suggests that the direct interaction between near-neighbor Ce 4f electrons is negligible. Further, the localized character of the Ni 3d wave function and the shorter nearest-neighbor separation between Ce and Sn atoms (3.27 As ) than that between Ce and Ce atoms (3.82 As ) suggest that the Ce 4f—Sn sp hybridization might be the important factor to determine the intensity of the near-E peak “A”. Finally, the peak “B” reflects $ both the Ce 4f—Ni 3d hybridization and the Ce 5d-Ni 3d hybridization [11]. This work has been supported by the Korea Research Foundation (1997-001-D00139), and the Korea Science and Engineering Foundation (96-0702-01-01-3).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]
T. Takabatake et al., Phys. Rev. B 45 (1992) 5740. T.E. Mason et al., Phys. Rev. Lett. 69 (1992) 490. S. Nishigori et al., J. Phys. Soc. Japan 65 (1996) 2614. M. Kyogaku et al., J. Phys. Soc. Japan 59 (1990) 1728. K. Izawa et al., J. Phys. Soc. Japan 65 (1996) 3119. Y. Inada et al., J. Phys. Soc. Japan 65 (1996) 1158. A. Yanase, H. Harima, Prog. Theor. Phys. Suppl. 108 (1992) 19. T.J. Hammond et al., Phys. Rev. B 51 (1995) 2994. S. Nohara et al., Phys. Rev. B 47 (1993) 1754. A. Slebarski et al., Phys. Rev. B 56 (1997) 7245. J.-S. Kang et al., Phys. Rev. B 58 (1998) 4426.