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Infrared study on electronic structure of SrT4Sb12 (T=Fe , Ru)
ARTICLE IN PRESS
Physica B 383 (2006) 137–139 www.elsevier.com/locate/physb
Infrared study on electronic structure of SrT4 Sb12 (T ¼ Fe, Ru) S. Kimuraa,b,, H.J. Imb, Y. Sakuraia, T. Mizunoa,c, K. Takegaharad, H. Harimae, K. Hayashif, E. Matsuokaf, T. Takabatakef a UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan Department of Structural Molecular Science, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan c Department of Physics, Okayama University of Science, Okayama 700-0005, Japan d Department of Materials Science and Technology, Hirosaki University, Hirosaki, Aomori 036-8561, Japan e Department of Physics, Kobe University, Nada, Kobe 657-8501, Japan f ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
b
Abstract The temperature dependent optical conductivity spectrum of strontium filled skutterudites SrT4 Sb12 ðT ¼ Fe; RuÞ has been measured to investigate the origin of the heavy-fermion-like physical properties and the enhancement of the thermopower at around 50 K. The optical conductivity spectra of T ¼ Fe and Ru at the temperature of 7 K have peak structures at _o ¼ 24 and 190 meV, respectively. From the band structure calculation, the peaks correspond to the density of states (DOS) mainly originating from the Fe 3d and Ru 4d characters locating in the unoccupied states, i.e., SrFe4 Sb12 has a very narrower band near the Fermi level ðE F Þ than SrRu4 Sb12 . The shape of DOS near E F is concluded to be the origin of the unconventional physical properties. r 2006 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 78.20.e Keywords: Filled skutterudite; Optical property; Electronic structure
Filled skutterudite compounds AT4 X12 (A ¼ rareearth, alkali, alkaline-earth, T ¼ transition metals, X ¼ pnictgen) have attracted recent attention because of their various physical properties. In the case of alkali and alkaline-earth filled iron–antimony skutterudites, Aþ Fe4 Sb12 and A2þ Fe4 Sb12 , weak itinerant ferromagnetism originating from the Fe 3d appears. Especially, in the case of alkalineearth ions, the electrical resistivity with a shoulder structure at and with the quadratic dependence below 70 K, the thermopower with a local maximum at around 50 K, the electronic specific heat coefficient g of 100 mJ=mol K2 and the maximum magnetic susceptibility at 50 K commonly imply the low energy spin fluctuations described by the selfconsistent renormalization (SCR) theory [1].
Corresponding author. UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan. Tel.: +81 564 55 7202; fax: +81 564 54 7079. E-mail address: [email protected] (S. Kimura).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.03.080
In A2þ Fe4 Sb12 , the ratio between the enhanced coefficient A of the quadratic electrical resistivity ðr ¼ AT 2 Þ and g is close to the Kadowaki–Woods value ½1:0 105 mO cm K2 =ðmJ=mol K2 Þ2 . This indicates that the heavy-fermion-like electronic structure appears near the Fermi level (E F ). Actually, optical conductivity spectra ½sðoÞ of A2þ ¼ Ca and Ba commonly have a set of a peak at _o ¼ 0 eV due to the renormalized carriers, a dip at 10 meV and a hump at 20 meV (peak-dip-hump (PDH) structure) which is usually observed in heavy fermion systems [2]. On the other hand, alkaline-earth ruthenium filled skutterudites, A2þ Ru4 Sb12 , have the same physical properties as normal metals. This means that the anomalous physical properties of A2þ Fe4 Sb12 do not originate from the A2þ guest ions but from the Fe–Sb cage. In this paper, to investigate the electronic structure of the Fe–Sb cage, the temperature dependence of near-normal reflectivity spectra ½RðoÞ of SrFe4 Sb12 and SrRu4 Sb12 were measured and sðoÞ via the Kramers–Kronig analysis were compared with the band structure calculation.
ARTICLE IN PRESS 138
S. Kimura et al. / Physica B 383 (2006) 137–139
Fig. 1. Temperature dependent optical conductivity spectrum ½sðoÞ of SrFe4 Sb12 (solid lines) with the corresponding direct current conductivity (sDC , marks). The fitted Drude functions at 7 and 300 K are also shown by dashed lines.
The samples of high density were synthesized by the method reported before [1]. The optical measurement was performed in the photon energy range of 2 meV–30 eV. Below 1.5 eV, two interferometers (Martin–Puplett and Michelson type) were used at temperatures of 7 and 300 K [3]. Above 1.5 eV, a synchrotron radiation beam line 7B at UVSOR-II, the Institute for Molecular Science was used only at 300 K [4]. The band structure calculation was performed by full-potential LAPW method [5]. Temperature dependent sðoÞ of SrFe4 Sb12 is shown in Fig. 1. At 300 K, sðoÞ monotonically decreases with increasing photon energy below 350 meV except for fine peaks at 20–30 meV originating from optical phonons. At 7 K, sðoÞ changes to the structure with a peak at 0 eV, a dip at around 10 meV and a hump at around 20 meV (PDH structure) with considering the high direct current conductivity ðsDC Þ. That is the same as other A2þ Fe4 Sb12 for A ¼ Ca and Ba [2]. Such temperature dependence does not appear in SrRu4 Sb12 (not shown). To determine the origin of the PDH structure of SrFe4 Sb12 , sðoÞ of SrFe4 Sb12 and SrRu4 Sb12 were compared with the band structure calculation as shown in Fig. 2. The sðoÞ of SrFe4 Sb12 ðSrRu4 Sb12 Þ at the temperature of 7 K has hump and shoulder structures at sðoÞ ¼ 24 meV and 1.0 eV (190 meV and 1.2 eV), respectively. From the band calculation, the peaks correspond to the unoccupied density of states (DOS) mainly originating from the Fe(Ru) 3d (4d) character, i.e., the shape of DOS of the top of the valence band below 0.5 eV and of the bottom of the conduction band above 0.8 eV is in good
Fig. 2. Optical conductivity [sðoÞ] spectra (solid lines) of SrFe4 Sb12 and SrRu4 Sb12 compared with the density of states (dotted lines) above the Fermi level ðE F Þ. Inset is the expansion of the low energy part of the figure of SrFe4 Sb12 .
agreement with sðoÞ. This means that the PDH structure in SrFe4 Sb12 originates from the ‘‘V’’-shape narrow dip structure of DOS on E F . The detail is plotted in the inset of Fig. 2. The hump energy of sðoÞ agrees with the shoulder structure below the main peak of DOS. This means that the hump in sðoÞ originates from the optical transition at the singularity of the joint DOS in the band structure and/or from its exciton. The ‘‘V’’-shape narrow dip structure at E F is considered to be the origin of the enhancement of the thermopower of this material. However, the heavy-fermion-like character including the mass enhancement cannot be explained by the shape of DOS. To clarify the origin of the heavy-fermion-like character, the detailed temperature dependence of sðoÞ should be studied in relation to the other physical properties. To summarize, optical conductivity ½sðoÞ spectra of SrFe4 Sb12 and SrRu4 Sb12 were measured in the wide energy range of 2 meV–30 eV. In SrFe4 Sb12 , a characteristic PDH structure below 50 meV similar to heavy fermion materials appears at low temperature. In comparison with the band structure calculation, sðoÞ reflects the unoccupied electronic structure and the origin is the ‘‘V’’-shape narrow dip structure of DOS at E F .
ARTICLE IN PRESS S. Kimura et al. / Physica B 383 (2006) 137–139
This work was a joint study program of the Institute for Molecular Science (2005). References [1] E. Matsuoka, K. Hayashi, A. Ikeda, K. Tanaka, T. Takabatake, M. Matsumura, J. Phys. Soc. Japan 74 (2005) 1382.
139
[2] J. Sichelschmidt, V. Voevodin, H.J. Im, S. Kimura, H. Rosner, A. Leithe-Jasper, W. Schnelle, U. Burkhardt, J.A. Mydosh, Yu. Grin, F. Steglich, Phys. Rev. Lett. 96 (2006) 037406. [3] S. Kimura, in preparation. [4] K. Fukui, H. Miura, H. Nakagawa, I. Shimoyama, K. Nakagawa, H. Okamura, T. Nanba, M. Hasumoto, T. Kinoshita, Nucl. Instr. and Meth. A 467–468 (2001) 601. [5] K. Takegahara, H. Harima, J. Phys. Soc. Japan 71 (Suppl.) (2002) 240.
Physica B 383 (2006) 137–139 www.elsevier.com/locate/physb
Infrared study on electronic structure of SrT4 Sb12 (T ¼ Fe, Ru) S. Kimuraa,b,, H.J. Imb, Y. Sakuraia, T. Mizunoa,c, K. Takegaharad, H. Harimae, K. Hayashif, E. Matsuokaf, T. Takabatakef a UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan Department of Structural Molecular Science, The Graduate University for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan c Department of Physics, Okayama University of Science, Okayama 700-0005, Japan d Department of Materials Science and Technology, Hirosaki University, Hirosaki, Aomori 036-8561, Japan e Department of Physics, Kobe University, Nada, Kobe 657-8501, Japan f ADSM, Hiroshima University, Higashi-Hiroshima 739-8530, Japan
b
Abstract The temperature dependent optical conductivity spectrum of strontium filled skutterudites SrT4 Sb12 ðT ¼ Fe; RuÞ has been measured to investigate the origin of the heavy-fermion-like physical properties and the enhancement of the thermopower at around 50 K. The optical conductivity spectra of T ¼ Fe and Ru at the temperature of 7 K have peak structures at _o ¼ 24 and 190 meV, respectively. From the band structure calculation, the peaks correspond to the density of states (DOS) mainly originating from the Fe 3d and Ru 4d characters locating in the unoccupied states, i.e., SrFe4 Sb12 has a very narrower band near the Fermi level ðE F Þ than SrRu4 Sb12 . The shape of DOS near E F is concluded to be the origin of the unconventional physical properties. r 2006 Elsevier B.V. All rights reserved. PACS: 71.27.+a; 78.20.e Keywords: Filled skutterudite; Optical property; Electronic structure
Filled skutterudite compounds AT4 X12 (A ¼ rareearth, alkali, alkaline-earth, T ¼ transition metals, X ¼ pnictgen) have attracted recent attention because of their various physical properties. In the case of alkali and alkaline-earth filled iron–antimony skutterudites, Aþ Fe4 Sb12 and A2þ Fe4 Sb12 , weak itinerant ferromagnetism originating from the Fe 3d appears. Especially, in the case of alkalineearth ions, the electrical resistivity with a shoulder structure at and with the quadratic dependence below 70 K, the thermopower with a local maximum at around 50 K, the electronic specific heat coefficient g of 100 mJ=mol K2 and the maximum magnetic susceptibility at 50 K commonly imply the low energy spin fluctuations described by the selfconsistent renormalization (SCR) theory [1].
Corresponding author. UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan. Tel.: +81 564 55 7202; fax: +81 564 54 7079. E-mail address: [email protected] (S. Kimura).
0921-4526/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2006.03.080
In A2þ Fe4 Sb12 , the ratio between the enhanced coefficient A of the quadratic electrical resistivity ðr ¼ AT 2 Þ and g is close to the Kadowaki–Woods value ½1:0 105 mO cm K2 =ðmJ=mol K2 Þ2 . This indicates that the heavy-fermion-like electronic structure appears near the Fermi level (E F ). Actually, optical conductivity spectra ½sðoÞ of A2þ ¼ Ca and Ba commonly have a set of a peak at _o ¼ 0 eV due to the renormalized carriers, a dip at 10 meV and a hump at 20 meV (peak-dip-hump (PDH) structure) which is usually observed in heavy fermion systems [2]. On the other hand, alkaline-earth ruthenium filled skutterudites, A2þ Ru4 Sb12 , have the same physical properties as normal metals. This means that the anomalous physical properties of A2þ Fe4 Sb12 do not originate from the A2þ guest ions but from the Fe–Sb cage. In this paper, to investigate the electronic structure of the Fe–Sb cage, the temperature dependence of near-normal reflectivity spectra ½RðoÞ of SrFe4 Sb12 and SrRu4 Sb12 were measured and sðoÞ via the Kramers–Kronig analysis were compared with the band structure calculation.
ARTICLE IN PRESS 138
S. Kimura et al. / Physica B 383 (2006) 137–139
Fig. 1. Temperature dependent optical conductivity spectrum ½sðoÞ of SrFe4 Sb12 (solid lines) with the corresponding direct current conductivity (sDC , marks). The fitted Drude functions at 7 and 300 K are also shown by dashed lines.
The samples of high density were synthesized by the method reported before [1]. The optical measurement was performed in the photon energy range of 2 meV–30 eV. Below 1.5 eV, two interferometers (Martin–Puplett and Michelson type) were used at temperatures of 7 and 300 K [3]. Above 1.5 eV, a synchrotron radiation beam line 7B at UVSOR-II, the Institute for Molecular Science was used only at 300 K [4]. The band structure calculation was performed by full-potential LAPW method [5]. Temperature dependent sðoÞ of SrFe4 Sb12 is shown in Fig. 1. At 300 K, sðoÞ monotonically decreases with increasing photon energy below 350 meV except for fine peaks at 20–30 meV originating from optical phonons. At 7 K, sðoÞ changes to the structure with a peak at 0 eV, a dip at around 10 meV and a hump at around 20 meV (PDH structure) with considering the high direct current conductivity ðsDC Þ. That is the same as other A2þ Fe4 Sb12 for A ¼ Ca and Ba [2]. Such temperature dependence does not appear in SrRu4 Sb12 (not shown). To determine the origin of the PDH structure of SrFe4 Sb12 , sðoÞ of SrFe4 Sb12 and SrRu4 Sb12 were compared with the band structure calculation as shown in Fig. 2. The sðoÞ of SrFe4 Sb12 ðSrRu4 Sb12 Þ at the temperature of 7 K has hump and shoulder structures at sðoÞ ¼ 24 meV and 1.0 eV (190 meV and 1.2 eV), respectively. From the band calculation, the peaks correspond to the unoccupied density of states (DOS) mainly originating from the Fe(Ru) 3d (4d) character, i.e., the shape of DOS of the top of the valence band below 0.5 eV and of the bottom of the conduction band above 0.8 eV is in good
Fig. 2. Optical conductivity [sðoÞ] spectra (solid lines) of SrFe4 Sb12 and SrRu4 Sb12 compared with the density of states (dotted lines) above the Fermi level ðE F Þ. Inset is the expansion of the low energy part of the figure of SrFe4 Sb12 .
agreement with sðoÞ. This means that the PDH structure in SrFe4 Sb12 originates from the ‘‘V’’-shape narrow dip structure of DOS on E F . The detail is plotted in the inset of Fig. 2. The hump energy of sðoÞ agrees with the shoulder structure below the main peak of DOS. This means that the hump in sðoÞ originates from the optical transition at the singularity of the joint DOS in the band structure and/or from its exciton. The ‘‘V’’-shape narrow dip structure at E F is considered to be the origin of the enhancement of the thermopower of this material. However, the heavy-fermion-like character including the mass enhancement cannot be explained by the shape of DOS. To clarify the origin of the heavy-fermion-like character, the detailed temperature dependence of sðoÞ should be studied in relation to the other physical properties. To summarize, optical conductivity ½sðoÞ spectra of SrFe4 Sb12 and SrRu4 Sb12 were measured in the wide energy range of 2 meV–30 eV. In SrFe4 Sb12 , a characteristic PDH structure below 50 meV similar to heavy fermion materials appears at low temperature. In comparison with the band structure calculation, sðoÞ reflects the unoccupied electronic structure and the origin is the ‘‘V’’-shape narrow dip structure of DOS at E F .
ARTICLE IN PRESS S. Kimura et al. / Physica B 383 (2006) 137–139
This work was a joint study program of the Institute for Molecular Science (2005). References [1] E. Matsuoka, K. Hayashi, A. Ikeda, K. Tanaka, T. Takabatake, M. Matsumura, J. Phys. Soc. Japan 74 (2005) 1382.
139
[2] J. Sichelschmidt, V. Voevodin, H.J. Im, S. Kimura, H. Rosner, A. Leithe-Jasper, W. Schnelle, U. Burkhardt, J.A. Mydosh, Yu. Grin, F. Steglich, Phys. Rev. Lett. 96 (2006) 037406. [3] S. Kimura, in preparation. [4] K. Fukui, H. Miura, H. Nakagawa, I. Shimoyama, K. Nakagawa, H. Okamura, T. Nanba, M. Hasumoto, T. Kinoshita, Nucl. Instr. and Meth. A 467–468 (2001) 601. [5] K. Takegahara, H. Harima, J. Phys. Soc. Japan 71 (Suppl.) (2002) 240.