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Pressure-dependent electrical resistivity of the filled skutterudite compound CeRu4Sb12
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) e81–e82
Pressure-dependent electrical resistivity of the filled skutterudite compound CeRu4Sb12 Nobuyuki Kuritaa,*, Masato Hedoa, Yoshiya Uwatokoa, Miki Kobayashib, # c Hitoshi Sugawarab, Hideyuki Satob, Nobuo Mori a
Institute for Solid State Physics, University of Tokyo, Kashiwanoha 1-5-1, Kashiwa, Chiba 277-8581, Japan Department of Physics, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192-0397, Japan c Department of Physics, Faculty of Sciences, Saitama University, Shimo-Okubo 225, Saitama City, 338-8570, Japan b
Abstract The electrical resistivity of the filled skutterudite compound CeRu4 Sb12 was measured in the temperature range 2–300 K and under hydrostatic pressures up to 8 GPa: Above 6 GPa; we observed metal-semiconductor transition in CeRu4 Sb12 at low temperature and a energy gap estimated from activation law was enhanced with increasing pressure at a rate of 12:2 K=GPa: Semiconductive behavior of CeRu4 Sb12 under high pressure may have the same origin as other Ce-based skutterudite compounds which show semiconducting behavior. r 2003 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.27.þa Keywords: Filled skutterudite; High pressure; Electrical resistivity; CeRu4 Sb12
Ternary intermetallic compounds RT4 X12 (R ¼rare earth; T ¼ Fe; Ru, Os; X ¼ P; As, Sb) with the skutterudite structure have various physical properties [1]. Among them, most of the Ce-based compounds CeT4 X12 show semiconductive behavior which might be attributed to the large c–f hybridization strength originated from the unique structure. Actually, the magnitude of energy gap increases with decreasing lattice constant [2]. CeRu4 Sb12 is, exceptionally, metallic not explained by the band calculation which predicts it semiconducting [3]. Moreover, CeRu4 Sb12 shows anomalous such as heavy fermion [4] and non-Fermi liquid (NFL) [5] behaviors at ambient pressure. Recently, Kobayashi et al. reported NFL behavior which was suppressed with an extension of Fermi liquid regime under 1:3 GPa [4]. These features must be related to the delicate electronic structure in the vicinity of the Fermi level. This is supported by Shubnikov–de Haas and the de Haas–van Alphen experiments in CeRu4 Sb12 which *Corresponding author. Tel.: +81-4-7136-3332; fax: +81-47136-3333. E-mail address: [email protected] (N. Kurita).
clarified small Fermi surface at low temperature with highly enhanced effective mass [6]. It is expected that c–f hybridization plays important roles in the electrical mechanism of CeRu4 Sb12 : Therefore, we have studied the transport properties under applied high pressure which can have a profound effect on the c–f hybridization. High-quality single crystals of CeRu4 Sb12 were grown by Sb-self flux method. The quality of this sample is basically the same as one used in Refs. [4,6]. The electrical resistivity was measured from 2 to 300 K under applied hydrostatic pressures up to 8 GPa by the standard DC four-probe method. The hydrostatic pressure was generated by using a cubic anvil device with a 250 ton press [7]. We used a 1:1 mixture of Fluorinert FC-70 and 77 as a pressure transmitting medium. Fig. 1 shows a temperature dependence of the electrical resistivity of CeRu4 Sb12 under several constant pressure (a) 1:5–5 GPa; (b) 5–8 GPa; respectively. At 1:5 GPa; the resistivity exhibits a maximum ðrmax Þ around 80 K ðTmax Þ above which it shows log T dependence due to Kondo scattering, based on the same mechanism at ambient pressure [8]. Below 80 K it decreases rapidly. The resistivity has small pressure
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.173
ARTICLE IN PRESS e82
N. Kurita et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e81–e82
Fig. 2. ln r versus 1=T for CeRu4 Sb12 at 6–8 GPa: The inset shows the pressure dependence of the energy gap.
Fig. 1. Temperature dependence of electrical resistivity in CeRu4 Sb12 under several constant pressures. (a) 1:5–5 GPa; (b) 5–8 GPa: The inset in (b) shows the resistivity at low temperature under 6 GPa:
dependence, up to 5 GPa; except that Tmax tends to be gradually remarkable with increasing pressure. As shown in Fig. 1(b) inset, the resistivity under 6 GPa has a small maximum around 8 K: With further increasing pressures, the resistivity is enhanced as a whole and drastically at the low-temperature regime. Semiconducting behavior was stabilized under high pressure in CeRu4 Sb12 : While, the resistivity at 8 GPa shows a tendency to saturate with decreasing temperature. In order to obtain the magnitude of the energy gap at low temperature, we assume CeRu4 Sb12 being simple activated semiconductor and plot ln r as a function of 1=T in Fig. 2. A linear relation is observed in a narrow and limited temperature range which is shown by solid lines in Fig. 2. Fits to an activated conduction law r ¼ expðD=TÞ yield small band gap D ¼ 0:3; 9:1 and 24:7 K under 6; 7 and 8 GPa; respectively. A pressure coefficient of the energy gap dD=dP is about 12:2 K=GPa; as shown in Fig. 2 inset. The magnitude of D under high pressure is close to that of CeOs4 Sb12 ; DB10 K [9]. This is consistent with the relationship between D and a lattice constant in CeT4 X12 as mentioned at the begining. The problem is that Tmax is slightly shifted to lower temperature side as increasing pressures. This fact is
contrary to the conventional Ce-based Kondo compounds like CeAl3 [10] and CeRhSb [7], though no clear explanation has been made. And we cannot explain why the resistivity under high pressure tends to saturate at low temperature. Taking the high quality of this sample into account, it may be caused not only by impurity bands but by other complicated electrical structure. Investigation of the physical properties under higher pressure can lead us to solving them. In summary, we have investigated pressure effect on the electrical resistivity of the filled skutterudite compound CeRu4 Sb12 and found that it is a small band-gap semiconductor above 6 GPa: D is enhanced with increasing pressures at a rate of 12:2 K=GPa: The electrical mechanism of CeRu4 Sb12 being small, the band-gap semiconductor under high pressure may have the same origin as other Ce-based skutterudite compounds which show a semiconducting behavior. More researches, such as the Hall effect under high pressure, are necessary to understand the electrical mechanism of CeRu4 Sb12 more precisely and are in progress.
References [1] H. Sato, et al., Phys. Rev. B (2000) 15125 and references therein. [2] I. Shirotani, et al., J. Solid State Chem. 142 (1999) 146. [3] H. Harima, unpublished. [4] M. Kobayashi, et al., Physica B 329–333 (2003) 605, and references therein. [5] N. Takeda, et al., J. Phys.: Condens. Matter 13 (2001) 5971. [6] K. Abe, et al., Physica B 312–313 (2002) 256. [7] Y. Uwatoko, et al., Physica B 239 (1997) 95 and references therein. [8] K. Abe, et al., J. Phys.: Condens. Matter 14 (2002) 11757. [9] E.D. Bauer, et al., J. Phys.: Condens. Matter 13 (2001) 4495. [10] G. Oomi, et al., J. Phys. Soc. Jpn. 65 (1996) 2732.
Journal of Magnetism and Magnetic Materials 272–276 (2004) e81–e82
Pressure-dependent electrical resistivity of the filled skutterudite compound CeRu4Sb12 Nobuyuki Kuritaa,*, Masato Hedoa, Yoshiya Uwatokoa, Miki Kobayashib, # c Hitoshi Sugawarab, Hideyuki Satob, Nobuo Mori a
Institute for Solid State Physics, University of Tokyo, Kashiwanoha 1-5-1, Kashiwa, Chiba 277-8581, Japan Department of Physics, Tokyo Metropolitan University, Minami-Osawa 1-1, Hachioji, Tokyo 192-0397, Japan c Department of Physics, Faculty of Sciences, Saitama University, Shimo-Okubo 225, Saitama City, 338-8570, Japan b
Abstract The electrical resistivity of the filled skutterudite compound CeRu4 Sb12 was measured in the temperature range 2–300 K and under hydrostatic pressures up to 8 GPa: Above 6 GPa; we observed metal-semiconductor transition in CeRu4 Sb12 at low temperature and a energy gap estimated from activation law was enhanced with increasing pressure at a rate of 12:2 K=GPa: Semiconductive behavior of CeRu4 Sb12 under high pressure may have the same origin as other Ce-based skutterudite compounds which show semiconducting behavior. r 2003 Elsevier B.V. All rights reserved. PACS: 71.20.Eh; 71.27.þa Keywords: Filled skutterudite; High pressure; Electrical resistivity; CeRu4 Sb12
Ternary intermetallic compounds RT4 X12 (R ¼rare earth; T ¼ Fe; Ru, Os; X ¼ P; As, Sb) with the skutterudite structure have various physical properties [1]. Among them, most of the Ce-based compounds CeT4 X12 show semiconductive behavior which might be attributed to the large c–f hybridization strength originated from the unique structure. Actually, the magnitude of energy gap increases with decreasing lattice constant [2]. CeRu4 Sb12 is, exceptionally, metallic not explained by the band calculation which predicts it semiconducting [3]. Moreover, CeRu4 Sb12 shows anomalous such as heavy fermion [4] and non-Fermi liquid (NFL) [5] behaviors at ambient pressure. Recently, Kobayashi et al. reported NFL behavior which was suppressed with an extension of Fermi liquid regime under 1:3 GPa [4]. These features must be related to the delicate electronic structure in the vicinity of the Fermi level. This is supported by Shubnikov–de Haas and the de Haas–van Alphen experiments in CeRu4 Sb12 which *Corresponding author. Tel.: +81-4-7136-3332; fax: +81-47136-3333. E-mail address: [email protected] (N. Kurita).
clarified small Fermi surface at low temperature with highly enhanced effective mass [6]. It is expected that c–f hybridization plays important roles in the electrical mechanism of CeRu4 Sb12 : Therefore, we have studied the transport properties under applied high pressure which can have a profound effect on the c–f hybridization. High-quality single crystals of CeRu4 Sb12 were grown by Sb-self flux method. The quality of this sample is basically the same as one used in Refs. [4,6]. The electrical resistivity was measured from 2 to 300 K under applied hydrostatic pressures up to 8 GPa by the standard DC four-probe method. The hydrostatic pressure was generated by using a cubic anvil device with a 250 ton press [7]. We used a 1:1 mixture of Fluorinert FC-70 and 77 as a pressure transmitting medium. Fig. 1 shows a temperature dependence of the electrical resistivity of CeRu4 Sb12 under several constant pressure (a) 1:5–5 GPa; (b) 5–8 GPa; respectively. At 1:5 GPa; the resistivity exhibits a maximum ðrmax Þ around 80 K ðTmax Þ above which it shows log T dependence due to Kondo scattering, based on the same mechanism at ambient pressure [8]. Below 80 K it decreases rapidly. The resistivity has small pressure
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.173
ARTICLE IN PRESS e82
N. Kurita et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) e81–e82
Fig. 2. ln r versus 1=T for CeRu4 Sb12 at 6–8 GPa: The inset shows the pressure dependence of the energy gap.
Fig. 1. Temperature dependence of electrical resistivity in CeRu4 Sb12 under several constant pressures. (a) 1:5–5 GPa; (b) 5–8 GPa: The inset in (b) shows the resistivity at low temperature under 6 GPa:
dependence, up to 5 GPa; except that Tmax tends to be gradually remarkable with increasing pressure. As shown in Fig. 1(b) inset, the resistivity under 6 GPa has a small maximum around 8 K: With further increasing pressures, the resistivity is enhanced as a whole and drastically at the low-temperature regime. Semiconducting behavior was stabilized under high pressure in CeRu4 Sb12 : While, the resistivity at 8 GPa shows a tendency to saturate with decreasing temperature. In order to obtain the magnitude of the energy gap at low temperature, we assume CeRu4 Sb12 being simple activated semiconductor and plot ln r as a function of 1=T in Fig. 2. A linear relation is observed in a narrow and limited temperature range which is shown by solid lines in Fig. 2. Fits to an activated conduction law r ¼ expðD=TÞ yield small band gap D ¼ 0:3; 9:1 and 24:7 K under 6; 7 and 8 GPa; respectively. A pressure coefficient of the energy gap dD=dP is about 12:2 K=GPa; as shown in Fig. 2 inset. The magnitude of D under high pressure is close to that of CeOs4 Sb12 ; DB10 K [9]. This is consistent with the relationship between D and a lattice constant in CeT4 X12 as mentioned at the begining. The problem is that Tmax is slightly shifted to lower temperature side as increasing pressures. This fact is
contrary to the conventional Ce-based Kondo compounds like CeAl3 [10] and CeRhSb [7], though no clear explanation has been made. And we cannot explain why the resistivity under high pressure tends to saturate at low temperature. Taking the high quality of this sample into account, it may be caused not only by impurity bands but by other complicated electrical structure. Investigation of the physical properties under higher pressure can lead us to solving them. In summary, we have investigated pressure effect on the electrical resistivity of the filled skutterudite compound CeRu4 Sb12 and found that it is a small band-gap semiconductor above 6 GPa: D is enhanced with increasing pressures at a rate of 12:2 K=GPa: The electrical mechanism of CeRu4 Sb12 being small, the band-gap semiconductor under high pressure may have the same origin as other Ce-based skutterudite compounds which show a semiconducting behavior. More researches, such as the Hall effect under high pressure, are necessary to understand the electrical mechanism of CeRu4 Sb12 more precisely and are in progress.
References [1] H. Sato, et al., Phys. Rev. B (2000) 15125 and references therein. [2] I. Shirotani, et al., J. Solid State Chem. 142 (1999) 146. [3] H. Harima, unpublished. [4] M. Kobayashi, et al., Physica B 329–333 (2003) 605, and references therein. [5] N. Takeda, et al., J. Phys.: Condens. Matter 13 (2001) 5971. [6] K. Abe, et al., Physica B 312–313 (2002) 256. [7] Y. Uwatoko, et al., Physica B 239 (1997) 95 and references therein. [8] K. Abe, et al., J. Phys.: Condens. Matter 14 (2002) 11757. [9] E.D. Bauer, et al., J. Phys.: Condens. Matter 13 (2001) 4495. [10] G. Oomi, et al., J. Phys. Soc. Jpn. 65 (1996) 2732.