- Email: [email protected]
Direct observation of pseudogap of SmB6 using ultrahigh-resolution photoemission spectroscopy
Physica B 312–313 (2002) 329–330
Direct observation of pseudogap of SmB6 using ultrahigh-resolution photoemission spectroscopy S. Souma*, H. Kumigashira, T. Ito, T. Takahashi, S. Kunii Department of Physics, Tohoku University, Sendai 980-8578, Japan
Abstract We have performed a temperature-dependent ultrahigh-resolution photoemission spectroscopy on SmB6 to study the ‘‘Kondo-insulator’’ nature. We found a sharp Sm 4f-derived peak about 18 meV away from EF and a pseudogap at EF at low temperature. The pseudogap is gradually filled-in by the transfer of spectral weight from the sharp peak at high temperatures. This indicates that the pseudogap is a Kondo-insulator gap originating in the hybridization between the Sm 4f states and the conduction electrons. r 2002 Elsevier Science B.V. All rights reserved. Keywords: SmB6 ; Photoemission spectroscopy; Kondo insulator
SmB6 is categorized into a ‘‘classical’’ Kondo insulator and has attracted much attention because of the anomalous physical properties such as the activation behavior of the electrical resistivity sensitive to the pressure and/or the external magnetic field [1–3]. These anomalous properties have been regarded to originate in a temperature-dependent small (pseudo)gap at the Fermi level ðEF Þ [2]. Although an insufficient energy resolution in photoemission spectroscopy (PES) has hindered the precise investigation [4], a recent highresolution photoemission spectroscopy (PES) has revealed a small gap at EF [5] at low temperature. Since one of the key characters to identify the Kondoinsulator gap is the temperature evolution in contrast to the essentially temperature-independent conventional energy gap, it is important to investigate how the observed small gap evolves as a function of temperature. In this paper, we report a temperature-dependent ultrahigh-resolution ðDEB8 meVÞ PES study on SmB6 : SmB6 single crystals were grown by the floating-zone method. Photoemission measurements were carried out using a SCIENTA SES-200 analyzer with a GAMMA-
*Corresponding author. Tel.: +81-22-217-6417; fax: +8122-217-6419. E-mail address: [email protected] (S. Souma).
DATA discharge lamp and a toroidal grating monochromator. A fresh and clean surface for PES measurement was obtained by in situ scraping by a diamond file under ultrahigh vacuum of 5 1011 Torr. Fig. 1 shows PES spectra near EF of SmB6 measured at 13.5 K using He Ia (21.218 eV) and He IIa (40.814 eV) photons. The inset shows the He II spectrum in an expanded energy scale compared with that of gold measured under the same condition. We find that both He I and II spectra show two prominent peaks at about 18 and 160 meV, respectively, consistent with the previous report [5]. The remarkable enhancement of the two peaks in the He II spectrum suggests their Sm-4f origin [6], and the peaks at 18 and 160 meV are ascribed to the final state multiplet of Sm2þ ð6 H5=2 and 6 H7=2 ; respectively). The most important finding in Fig. 1 is that the leading-edge midpoint of PES spectrum (6 H5=2 peak) is not on EF but a little (B5 meV) away from EF in contrast with mixed-valent Sm compounds where a ‘‘Kondo-resonance peak’’ is found to be situated at EF : Thus, the present PES results clearly indicate that SmB6 has a (pseudo) gap of about 18 meV at 13.5 K. It is remarked that this gap energy (18 meV) is consistent with the reported optical gap energy (19 meV) [7] since the Hall coefficient indicates that SmB6 has a n-type conduction [2,7]. On the other hand, a fairly good agreement of the gap value (18 meV) with the magnetic excitation energy (14 meV) observed by neutron scatter-
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 1 1 5 - 2
S. Souma et al. / Physica B 312–313 (2002) 329–330
330
SmB 6
T = 13.5 K
SmB 6
Density of States (arb. units)
1.5
He II Intensity (arb. units)
6
H 7/2
He I
6
H 5/2
He II
He II
1.0 1.6
0.5 1.2 1.0
Au
SmB6
0.8
0.0
300 30
300
20
10
200
13.5 K 30 K 50 K 70 K 100 K 150 K 200 K 250 K 300 K
1.4
40
20
200
EF
100
EF
Binding Energy (meV)
EF
100
EF
Binding Energy (meV) Fig. 1. Ultrahigh-resolution photoemission spectra near EF of SmB6 measured with He Ia and He IIa photons at 13.5 K. Inset shows the He II spectrum in an expanded energy scale compared with that of gold.
ing [8] suggests that the pseudogap is strongly correlated with the formation of a Kondo singlet [2]. Since the most crucial point to distinguish between a Kondo-insulator gap and a conventional energy gap is the behavior as a function of temperature, we have measured the temperature dependence of PES spectrum. Fig. 2 shows the temperature dependence of the density of states (DOS) near EF obtained by dividing the He II spectra with the Fermi-Dirac (FD) function at each temperature convoluted with the instrumental resolution. This procedure removes a possible complication from the FD function which has a sharp drop at EF : As found in Fig. 2, there is a clear pseudogap at EF and a sharp peak at 18 meV in the DOS at 13.5 K, and the gap is gradually filled-in by a transfer of spectral weight from the sharp peak at elevating temperature. The pseudogap almost disappears at 250 K. This temperature evolution of pseudogap is strongly reminiscent of the spectral function calculated based on the Anderson lattice model (ALM) [9], indicating that the pseudogap in SmB6 is formed through the c–f hybridization and is certainly a ‘‘Kondo-insulator gap’’. It should be noted that an exiton–phonon approach has been adopted in the discussion about the low-temperature electrical properties of SmB6 associated with intragap states within the
Fig. 2. Temperature dependence of the density of states near EF of SmB6 derived from the PES spectra. Inset shows the expansion near EF :
Kondo-insulator gap [10]. We find in Fig. 2 that the sharp peak at 18 meV abruptly loses its intensity with increasing temperature and looks to nearly disappear at 100–150 K. This temperature coincides well with the characteristic temperature at which the thermal expansion shows an anomaly [2]. This suggests that the valence fluctuation plays an important role for the pseudogap formation in SmB6 : H.K. and T.I. thank the Japan Society for the Promotion of Science for financial support. This work was supported by grant from the Ministry of Education, Science and Culture of Japan.
References [1] J.C. Nickerson, et al., Phys. Rev. B 3 (1971) 2030. [2] P. Wachter, in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 19, North-Holland, Amsterdam, 1994, p. 177. [3] J.C. Cooley, et al., Phys. Rev. Lett. 74 (1995) 1629. [4] J.N. Chazalaviel, et al., Phys. Rev. B 14 (1976) 4586. [5] J.D. Denlinger, et al., Physica B 281&282 (2000) 716. [6] J.J. Yeh, I. Lindau, Atomic Data and Nuclear Data Tables 32 (1985) 1. [7] B. Gorshunov, et al., Phys. Rev. B 59 (1999) 1808. [8] J.-M. Mignot, et al., Physica B 199&200 (1994) 430. [9] M.J. Rozenberg, G. Kotliar, H. Kajueter, Phys. Rev. B 54 (1996) 8452. [10] N.E. Sluchanko, et al., Phys. Rev. B 61 (2000) 9906.
Direct observation of pseudogap of SmB6 using ultrahigh-resolution photoemission spectroscopy S. Souma*, H. Kumigashira, T. Ito, T. Takahashi, S. Kunii Department of Physics, Tohoku University, Sendai 980-8578, Japan
Abstract We have performed a temperature-dependent ultrahigh-resolution photoemission spectroscopy on SmB6 to study the ‘‘Kondo-insulator’’ nature. We found a sharp Sm 4f-derived peak about 18 meV away from EF and a pseudogap at EF at low temperature. The pseudogap is gradually filled-in by the transfer of spectral weight from the sharp peak at high temperatures. This indicates that the pseudogap is a Kondo-insulator gap originating in the hybridization between the Sm 4f states and the conduction electrons. r 2002 Elsevier Science B.V. All rights reserved. Keywords: SmB6 ; Photoemission spectroscopy; Kondo insulator
SmB6 is categorized into a ‘‘classical’’ Kondo insulator and has attracted much attention because of the anomalous physical properties such as the activation behavior of the electrical resistivity sensitive to the pressure and/or the external magnetic field [1–3]. These anomalous properties have been regarded to originate in a temperature-dependent small (pseudo)gap at the Fermi level ðEF Þ [2]. Although an insufficient energy resolution in photoemission spectroscopy (PES) has hindered the precise investigation [4], a recent highresolution photoemission spectroscopy (PES) has revealed a small gap at EF [5] at low temperature. Since one of the key characters to identify the Kondoinsulator gap is the temperature evolution in contrast to the essentially temperature-independent conventional energy gap, it is important to investigate how the observed small gap evolves as a function of temperature. In this paper, we report a temperature-dependent ultrahigh-resolution ðDEB8 meVÞ PES study on SmB6 : SmB6 single crystals were grown by the floating-zone method. Photoemission measurements were carried out using a SCIENTA SES-200 analyzer with a GAMMA-
*Corresponding author. Tel.: +81-22-217-6417; fax: +8122-217-6419. E-mail address: [email protected] (S. Souma).
DATA discharge lamp and a toroidal grating monochromator. A fresh and clean surface for PES measurement was obtained by in situ scraping by a diamond file under ultrahigh vacuum of 5 1011 Torr. Fig. 1 shows PES spectra near EF of SmB6 measured at 13.5 K using He Ia (21.218 eV) and He IIa (40.814 eV) photons. The inset shows the He II spectrum in an expanded energy scale compared with that of gold measured under the same condition. We find that both He I and II spectra show two prominent peaks at about 18 and 160 meV, respectively, consistent with the previous report [5]. The remarkable enhancement of the two peaks in the He II spectrum suggests their Sm-4f origin [6], and the peaks at 18 and 160 meV are ascribed to the final state multiplet of Sm2þ ð6 H5=2 and 6 H7=2 ; respectively). The most important finding in Fig. 1 is that the leading-edge midpoint of PES spectrum (6 H5=2 peak) is not on EF but a little (B5 meV) away from EF in contrast with mixed-valent Sm compounds where a ‘‘Kondo-resonance peak’’ is found to be situated at EF : Thus, the present PES results clearly indicate that SmB6 has a (pseudo) gap of about 18 meV at 13.5 K. It is remarked that this gap energy (18 meV) is consistent with the reported optical gap energy (19 meV) [7] since the Hall coefficient indicates that SmB6 has a n-type conduction [2,7]. On the other hand, a fairly good agreement of the gap value (18 meV) with the magnetic excitation energy (14 meV) observed by neutron scatter-
0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 0 1 ) 0 1 1 1 5 - 2
S. Souma et al. / Physica B 312–313 (2002) 329–330
330
SmB 6
T = 13.5 K
SmB 6
Density of States (arb. units)
1.5
He II Intensity (arb. units)
6
H 7/2
He I
6
H 5/2
He II
He II
1.0 1.6
0.5 1.2 1.0
Au
SmB6
0.8
0.0
300 30
300
20
10
200
13.5 K 30 K 50 K 70 K 100 K 150 K 200 K 250 K 300 K
1.4
40
20
200
EF
100
EF
Binding Energy (meV)
EF
100
EF
Binding Energy (meV) Fig. 1. Ultrahigh-resolution photoemission spectra near EF of SmB6 measured with He Ia and He IIa photons at 13.5 K. Inset shows the He II spectrum in an expanded energy scale compared with that of gold.
ing [8] suggests that the pseudogap is strongly correlated with the formation of a Kondo singlet [2]. Since the most crucial point to distinguish between a Kondo-insulator gap and a conventional energy gap is the behavior as a function of temperature, we have measured the temperature dependence of PES spectrum. Fig. 2 shows the temperature dependence of the density of states (DOS) near EF obtained by dividing the He II spectra with the Fermi-Dirac (FD) function at each temperature convoluted with the instrumental resolution. This procedure removes a possible complication from the FD function which has a sharp drop at EF : As found in Fig. 2, there is a clear pseudogap at EF and a sharp peak at 18 meV in the DOS at 13.5 K, and the gap is gradually filled-in by a transfer of spectral weight from the sharp peak at elevating temperature. The pseudogap almost disappears at 250 K. This temperature evolution of pseudogap is strongly reminiscent of the spectral function calculated based on the Anderson lattice model (ALM) [9], indicating that the pseudogap in SmB6 is formed through the c–f hybridization and is certainly a ‘‘Kondo-insulator gap’’. It should be noted that an exiton–phonon approach has been adopted in the discussion about the low-temperature electrical properties of SmB6 associated with intragap states within the
Fig. 2. Temperature dependence of the density of states near EF of SmB6 derived from the PES spectra. Inset shows the expansion near EF :
Kondo-insulator gap [10]. We find in Fig. 2 that the sharp peak at 18 meV abruptly loses its intensity with increasing temperature and looks to nearly disappear at 100–150 K. This temperature coincides well with the characteristic temperature at which the thermal expansion shows an anomaly [2]. This suggests that the valence fluctuation plays an important role for the pseudogap formation in SmB6 : H.K. and T.I. thank the Japan Society for the Promotion of Science for financial support. This work was supported by grant from the Ministry of Education, Science and Culture of Japan.
References [1] J.C. Nickerson, et al., Phys. Rev. B 3 (1971) 2030. [2] P. Wachter, in: Handbook on the Physics and Chemistry of Rare Earths, Vol. 19, North-Holland, Amsterdam, 1994, p. 177. [3] J.C. Cooley, et al., Phys. Rev. Lett. 74 (1995) 1629. [4] J.N. Chazalaviel, et al., Phys. Rev. B 14 (1976) 4586. [5] J.D. Denlinger, et al., Physica B 281&282 (2000) 716. [6] J.J. Yeh, I. Lindau, Atomic Data and Nuclear Data Tables 32 (1985) 1. [7] B. Gorshunov, et al., Phys. Rev. B 59 (1999) 1808. [8] J.-M. Mignot, et al., Physica B 199&200 (1994) 430. [9] M.J. Rozenberg, G. Kotliar, H. Kajueter, Phys. Rev. B 54 (1996) 8452. [10] N.E. Sluchanko, et al., Phys. Rev. B 61 (2000) 9906.