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Low-energy optical conductivity of Yb4As3
Physica B 312–313 (2002) 356–358
Low-energy optical conductivity of Yb4As3 Shin-ichi Kimuraa,b,*, Mitsuru Okunoa, Hideki Iwataa, Tatsuhiko Nishia, Hidekazu Aokic, Akira Ochiaid a
Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan b PRESTO, Japan Science and Technology Corporation, Japan c Max-Planck-Institut fur . Chemische Physik fester Stoffe, Dresden 01187, Germany d Center for Low Temperature Science, Tohoku University, Sendai 980-8578, Japan
Abstract To investigate the anomalous transport property and the electronic structure near the Fermi level of Yb4 As3 ; we have measured the temperature dependence of reflectivity spectra of Yb4 ðAs1x Px Þ3 (x ¼ 0; 0.05, 0.15) in the far-infrared region. In Yb4 As3 ; a Drude-curve with very low carrier density and long relaxation time appears at temperatures below 70 K. Coincidently, a peak appears around 15 meV. The peak is considered to originate from the hybridization between Yb3þ 4f-hole and 5d states on the Yb3þ chain. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Yb4 As3 ; Electronic structure; Hybridization gap
Yb4 As3 is one of strongly correlated 4f electron systems with low carrier density [1]. The optical spectrum in the infrared region strongly varies as the temperature decreases [2,3]. According to the optical conductivity (sðoÞ) spectra, a peak was observed at 0.4 eV. The origin is the change of the band structure near the Fermi level due to the charge ordering effect [4]. According to the band calculation [5], the Yb3þ 5d state on the Fermi level is predicted to exist on the Fermi level. However, no signal of the Yb3þ 5d state has been observed in any measurements. Then, to investigate the contribution of the Yb3þ 5d state to the transport property, we measured the temperature dependence of the sðoÞ spectrum of Yb4 ðAs1x Px Þ3 (x ¼ 0; 0.05, 0.15) in the far-infrared region at several temperatures between 7 and 70 K. The reflectivity measurements of Yb4 ðAs1x Px Þ3 were done in the energy range of 5 meV–1.5 eV at tempera*Corresponding author. Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan. Tel.: +81-78-803-1464; fax: +81-78-8035649. E-mail address: [email protected] (S. Kimura).
tures of 7–70 K by using a conventional infrared Fourier spectrometer. The measured samples were polished up to mirror surfaces by diamond rapping films in a helium atmosphere and were mounted to a closed-cycle helium cryostat in situ for avoiding oxidization. The sðoÞ spectra were obtained by the Kramers–Kronig analysis of the reflectivity spectra. Fig. 1 indicates the sðoÞ spectra of Yb4 As3 at 7 and 70 K, Yb4 ðAs0:95 P0:05 Þ3 at 7 K and Yb4 ðAs0:85 P0:15 Þ3 at 7 K by solid lines. The sðoÞ spectra mainly consist of three parts. First is a Drude part which appears in Yb4 As3 below 10 meV, second is the TO-phonon part of sharp peaks in all materials between 10 and 35 meV, and the last is the flat part due to the interband transition above 40 meV. In addition, a shoulder structure was recognized in Yb4 As3 around 30 meV at 7 K. The shoulder becomes small at 70 K. Since the Drude curve becomes gentle as the temperature increases, the decreasing of the shoulder simultaneously occurs with the gentle of the Drude curve. This indicates that the shoulder structure is considered to relate to the transport property. To evaluate the temperature dependence of the shoulder structure, the TO-phonon peaks and the Drude part are subtracted from the sðoÞ spectrum as follows.
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 0 7 5 - 4
S. Kimura et al. / Physica B 312–313 (2002) 356–358
Fig. 1. Optical conductivity (sðoÞ) spectra of Yb4 As3 at 7 and 70 K, Yb4 ðAs0:95 P0:05 Þ3 at 7 K and Yb4 ðAs0:85 P0:15 Þ3 at 7 K (solid lines). Dotted line is the summation of the TO-phonon between Yb and As and the invariable interband transition parts which was evaluated by using the sðoÞ spectra of Yb4 (As0.85P0.15)3 and Yb4 ðAs0:95 P0:05 Þ3 (see the text for detail). Dashed line is the fitted Drude function with the effective carrier mass of 0.7m0 : The subtraction of dotted line and dashed line from the sðoÞ of spectrum Yb4 As3 at 7 K is shown by dots.
First, we evaluate the sðoÞ structures due to the TOphonon of Yb and As ions and due to the interband transition which is common in all materials. The spectrum can be obtained from the function of ½sðo; Yb4 ðAs0:95 P0:05 Þ3 Þ3 sðo; Yb4 ðAs0:85 P0:15 Þ3 Þ=2; which is the operation to eliminated the TO-phonons of the Yb–P modes at 32 meV, because both of Yb4 ðAs0:95 P0:05 Þ3 and Yb4 ðAs0:85 P0:15 Þ3 have no carrier absorption at 7 K. The obtained spectrum is shown by dotted line in Fig. 1. By the analysis, the peaks of the sðoÞ spectrum due to the TO-phonons of the Yb – P mode is disappeared. Second, we subtract the Drudecurve from the sðoÞ spectrum of Yb4 As3 : The fitting was done using the sðoÞ spectrum below 10 meV and the sð0Þ value. The fitting curve is shown by the dashed line in Fig. 1. The carrier mass was evaluated to be 0.7 times larger than the rest mass of an electron (m0 ). The value is similar to the effective mass of the cyclotron resonance (0:72m0 ) [6] and that of the Shubnikov–de Haas data (0:4m0 ) [7]. In any case, the carrier mass is not heavier than the rest mass of an electron. This result indicates that the Yb 4f state does not contribute to the carriers. Finally, we subtract these curves from the sðoÞ spectrum
357
Fig. 2. Temperature dependence of the 15 meV-peak (dots in Fig. 1) of Yb4 As3 (solid lines). The dotted line which is the 15 meV-peak at 7 K is shown for comparison.
of Yb4 As3 : The obtained sðoÞ spectrum is shown by dots in Fig. 1. The dots indicate the subtracted spectrum has the peak at 15 meV and the energy gap at 5 meV. In the same way, the subtracted sðoÞ spectrum of Yb4 As3 as a function of temperature is shown in Fig. 2. To increase the temperature, the energy gap of the peak disappears at the temperature between 25 and 35 K and the peak shape becomes gentle above 35 K. The transition temperature is estimated to be about 30 K. The transition temperature is similar to the exchange energy (J=26 K) of the spin excitation observed in the neutron scattering [8]. This means that the peak relates to the formation of the Yb3þ chain. Therefore the peak originates from the electronic structure on the Yb3þ chain. One possibility is the Yb3þ 4f-hole–5d hybridization state, i.e., the sðoÞ peak originates from the transition between the bonding and the antibonding states. In this case, the Yb5d state may not appear on the Fermi level. To confirm the expectation, the optical measurement should be done by using a single domain sample of Yb4 As3 in the future. In conclusion, the temperature dependence of the optical conductivity spectrum of Yb4 As3 was measured in the far-infrared region. One peak around 15 meV was observed and it becomes gentle above 35 K. Since the
358
S. Kimura et al. / Physica B 312–313 (2002) 356–358
transition temperature is similar to the exchange energy of the spin excitation on the Yb3þ chain, the peak is considered to originate from the Yb3þ 4f-hole–5d hybridization state on the Yb3þ chain.
References [1] A. Ochiai, T. Suzuki, T. Kasuya, J. Phys. Soc. Japan 59 (1990) 4219. [2] S. Kimura, M. Ikezawa, A. Ochiai, T. Suzuki, J. Phys. Soc. Japan 65 (1996) 3591.
[3] S. Kimura, A. Ochiai, T. Suzuki, Physica B 230–232 (1997) 705. [4] S. Kimura, T. Nishi, M. Okuno, H. Iwata, H. Aoki, A. Ochiai, submitted for publication. [5] H. Harima, J. Phys. Soc. Japan 67 (1998) 37. [6] H. Matsui, A. Ochiai, H. Harima, H. Aoki, T. Suzuki, T. Yamada, N. Toyoda, J. Phys. Soc. Japan 66 (1997) 3729. [7] H. Aoki, Doctor Thesis, Tohoku University, 2000. [8] M. Kohgi, K. Iwasa, J.-M. Mignot, A. Ochiai, T. Suzuki, Phys. Rev. B 56 (1997) R11388.
Low-energy optical conductivity of Yb4As3 Shin-ichi Kimuraa,b,*, Mitsuru Okunoa, Hideki Iwataa, Tatsuhiko Nishia, Hidekazu Aokic, Akira Ochiaid a
Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan b PRESTO, Japan Science and Technology Corporation, Japan c Max-Planck-Institut fur . Chemische Physik fester Stoffe, Dresden 01187, Germany d Center for Low Temperature Science, Tohoku University, Sendai 980-8578, Japan
Abstract To investigate the anomalous transport property and the electronic structure near the Fermi level of Yb4 As3 ; we have measured the temperature dependence of reflectivity spectra of Yb4 ðAs1x Px Þ3 (x ¼ 0; 0.05, 0.15) in the far-infrared region. In Yb4 As3 ; a Drude-curve with very low carrier density and long relaxation time appears at temperatures below 70 K. Coincidently, a peak appears around 15 meV. The peak is considered to originate from the hybridization between Yb3þ 4f-hole and 5d states on the Yb3þ chain. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Yb4 As3 ; Electronic structure; Hybridization gap
Yb4 As3 is one of strongly correlated 4f electron systems with low carrier density [1]. The optical spectrum in the infrared region strongly varies as the temperature decreases [2,3]. According to the optical conductivity (sðoÞ) spectra, a peak was observed at 0.4 eV. The origin is the change of the band structure near the Fermi level due to the charge ordering effect [4]. According to the band calculation [5], the Yb3þ 5d state on the Fermi level is predicted to exist on the Fermi level. However, no signal of the Yb3þ 5d state has been observed in any measurements. Then, to investigate the contribution of the Yb3þ 5d state to the transport property, we measured the temperature dependence of the sðoÞ spectrum of Yb4 ðAs1x Px Þ3 (x ¼ 0; 0.05, 0.15) in the far-infrared region at several temperatures between 7 and 70 K. The reflectivity measurements of Yb4 ðAs1x Px Þ3 were done in the energy range of 5 meV–1.5 eV at tempera*Corresponding author. Graduate School of Science and Technology, Kobe University, 1-1 Rokkodai, Nada-ku, Kobe 657-8501, Japan. Tel.: +81-78-803-1464; fax: +81-78-8035649. E-mail address: [email protected] (S. Kimura).
tures of 7–70 K by using a conventional infrared Fourier spectrometer. The measured samples were polished up to mirror surfaces by diamond rapping films in a helium atmosphere and were mounted to a closed-cycle helium cryostat in situ for avoiding oxidization. The sðoÞ spectra were obtained by the Kramers–Kronig analysis of the reflectivity spectra. Fig. 1 indicates the sðoÞ spectra of Yb4 As3 at 7 and 70 K, Yb4 ðAs0:95 P0:05 Þ3 at 7 K and Yb4 ðAs0:85 P0:15 Þ3 at 7 K by solid lines. The sðoÞ spectra mainly consist of three parts. First is a Drude part which appears in Yb4 As3 below 10 meV, second is the TO-phonon part of sharp peaks in all materials between 10 and 35 meV, and the last is the flat part due to the interband transition above 40 meV. In addition, a shoulder structure was recognized in Yb4 As3 around 30 meV at 7 K. The shoulder becomes small at 70 K. Since the Drude curve becomes gentle as the temperature increases, the decreasing of the shoulder simultaneously occurs with the gentle of the Drude curve. This indicates that the shoulder structure is considered to relate to the transport property. To evaluate the temperature dependence of the shoulder structure, the TO-phonon peaks and the Drude part are subtracted from the sðoÞ spectrum as follows.
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 0 7 5 - 4
S. Kimura et al. / Physica B 312–313 (2002) 356–358
Fig. 1. Optical conductivity (sðoÞ) spectra of Yb4 As3 at 7 and 70 K, Yb4 ðAs0:95 P0:05 Þ3 at 7 K and Yb4 ðAs0:85 P0:15 Þ3 at 7 K (solid lines). Dotted line is the summation of the TO-phonon between Yb and As and the invariable interband transition parts which was evaluated by using the sðoÞ spectra of Yb4 (As0.85P0.15)3 and Yb4 ðAs0:95 P0:05 Þ3 (see the text for detail). Dashed line is the fitted Drude function with the effective carrier mass of 0.7m0 : The subtraction of dotted line and dashed line from the sðoÞ of spectrum Yb4 As3 at 7 K is shown by dots.
First, we evaluate the sðoÞ structures due to the TOphonon of Yb and As ions and due to the interband transition which is common in all materials. The spectrum can be obtained from the function of ½sðo; Yb4 ðAs0:95 P0:05 Þ3 Þ3 sðo; Yb4 ðAs0:85 P0:15 Þ3 Þ=2; which is the operation to eliminated the TO-phonons of the Yb–P modes at 32 meV, because both of Yb4 ðAs0:95 P0:05 Þ3 and Yb4 ðAs0:85 P0:15 Þ3 have no carrier absorption at 7 K. The obtained spectrum is shown by dotted line in Fig. 1. By the analysis, the peaks of the sðoÞ spectrum due to the TO-phonons of the Yb – P mode is disappeared. Second, we subtract the Drudecurve from the sðoÞ spectrum of Yb4 As3 : The fitting was done using the sðoÞ spectrum below 10 meV and the sð0Þ value. The fitting curve is shown by the dashed line in Fig. 1. The carrier mass was evaluated to be 0.7 times larger than the rest mass of an electron (m0 ). The value is similar to the effective mass of the cyclotron resonance (0:72m0 ) [6] and that of the Shubnikov–de Haas data (0:4m0 ) [7]. In any case, the carrier mass is not heavier than the rest mass of an electron. This result indicates that the Yb 4f state does not contribute to the carriers. Finally, we subtract these curves from the sðoÞ spectrum
357
Fig. 2. Temperature dependence of the 15 meV-peak (dots in Fig. 1) of Yb4 As3 (solid lines). The dotted line which is the 15 meV-peak at 7 K is shown for comparison.
of Yb4 As3 : The obtained sðoÞ spectrum is shown by dots in Fig. 1. The dots indicate the subtracted spectrum has the peak at 15 meV and the energy gap at 5 meV. In the same way, the subtracted sðoÞ spectrum of Yb4 As3 as a function of temperature is shown in Fig. 2. To increase the temperature, the energy gap of the peak disappears at the temperature between 25 and 35 K and the peak shape becomes gentle above 35 K. The transition temperature is estimated to be about 30 K. The transition temperature is similar to the exchange energy (J=26 K) of the spin excitation observed in the neutron scattering [8]. This means that the peak relates to the formation of the Yb3þ chain. Therefore the peak originates from the electronic structure on the Yb3þ chain. One possibility is the Yb3þ 4f-hole–5d hybridization state, i.e., the sðoÞ peak originates from the transition between the bonding and the antibonding states. In this case, the Yb5d state may not appear on the Fermi level. To confirm the expectation, the optical measurement should be done by using a single domain sample of Yb4 As3 in the future. In conclusion, the temperature dependence of the optical conductivity spectrum of Yb4 As3 was measured in the far-infrared region. One peak around 15 meV was observed and it becomes gentle above 35 K. Since the
358
S. Kimura et al. / Physica B 312–313 (2002) 356–358
transition temperature is similar to the exchange energy of the spin excitation on the Yb3þ chain, the peak is considered to originate from the Yb3þ 4f-hole–5d hybridization state on the Yb3þ chain.
References [1] A. Ochiai, T. Suzuki, T. Kasuya, J. Phys. Soc. Japan 59 (1990) 4219. [2] S. Kimura, M. Ikezawa, A. Ochiai, T. Suzuki, J. Phys. Soc. Japan 65 (1996) 3591.
[3] S. Kimura, A. Ochiai, T. Suzuki, Physica B 230–232 (1997) 705. [4] S. Kimura, T. Nishi, M. Okuno, H. Iwata, H. Aoki, A. Ochiai, submitted for publication. [5] H. Harima, J. Phys. Soc. Japan 67 (1998) 37. [6] H. Matsui, A. Ochiai, H. Harima, H. Aoki, T. Suzuki, T. Yamada, N. Toyoda, J. Phys. Soc. Japan 66 (1997) 3729. [7] H. Aoki, Doctor Thesis, Tohoku University, 2000. [8] M. Kohgi, K. Iwasa, J.-M. Mignot, A. Ochiai, T. Suzuki, Phys. Rev. B 56 (1997) R11388.