- Email: [email protected]
Transport properties of PrxOs4Sb12 single crystals with high Pr-site filling fraction grown under high pressure
ARTICLE IN PRESS Physica B 404 (2009) 2999–3001
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Transport properties of Prx Os4 Sb12 single crystals with high Pr-site filling fraction grown under high pressure Kenya Tanaka a,, Takahiro Namiki a, Takashi Saito a, Sho Tatsuoka a, Atsushi Imamura a, Keitaro Kuwahara b, Yuji Aoki a, Hideyuki Sato a a b
Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Institute of Applied Beam Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan
a r t i c l e in fo
PACS: 71.27.þa 71.20.Eh 74.70.Tx Keywords: Skutterudite PrOs4 Sb12 High-pressure synthesis Heavy-fermion superconductor Crystalline electric field
abstract We have succeeded in growing Prx Os4 Sb12 single crystals under 4 GPa with high Pr-site filling fraction x. The electrical resistance measurements clearly show that the superconducting (SC) transition is sharper and the onset temperatures is lower in the single crystal samples grown under high pressure compared to that of the sample grown under ambient pressure. These results suggest that the double SC transition ascribed to sample inhomogeneity is suppressed in the sample grown under high pressure. The change of 4f-electron crystalline electric field energy splitting between the G1 ground state and the Gð2Þ 4 first excited state in the sample made under high pressure is proposed as one of the possible origins of the suppression of the double SC transition. & 2009 Elsevier B.V. All rights reserved.
1. Introduction The filled skutterudite compounds, especially Pr-based ones, have attracted considerable attention because of their various novel features, i.e., the heavy-fermion superconductivity and the field induced quadrupolar ordered state in PrOs4 Sb12 , the scalar type ordered phase and the field induced heavy-fermion state in PrFe4 P12 , and the metal–non-metal phase transition in PrRu4 P12 [1–5]. In these systems, the strong hybridization between conduction electrons and 4f-electrons and the multipolar degrees of freedom of 4f-electrons under small crystalline electric field (CEF) are thought to play key roles in realization of their novel features. There found noticeable sample dependence in PrT4 Sb12 (T ¼ Fe, Os). In Prx Fe4 Sb12 , we found a drastic change in the magnetic ground state from ferromagnetic to non-magnetic by increasing Pr-site filling fraction x from 0:8 to 1 [6–8]. Recently, anomalous sample dependence of a double superconducting (SC) transition ascribed to sample inhomogeneity was reported in Prx Os4 Sb12 [9]. Pr-site vacancy is thought to be one of the possible origins of the problems [8,9]. In order to clarify the issues, it is necessary to prepare single crystals with high x.
Corresponding author. Tel.: +81 426 77 2487; fax: +81 426 77 2483.
E-mail address: [email protected] (K. Tanaka). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.07.023
In this paper, we report a method of growing single crystal of Prx Os4 Sb12 under high pressure and results of electrical resistance measurement of the single crystals. The single crystals grown under high pressure have sharper SC transition and their onset temperatures decrease, suggesting a suppression of the double SC transition. As one of the possible origins of these results, a change of CEF energy splitting (DCEF ) between the G1 ground state and the Gð2Þ 4 first excited state in the sample made under high pressure is proposed.
2. Single crystal growth High-pressure synthesis is a powerful technique to reduce Pr-site vacancy [8]. The aim of this work is to grow single crystals of PrOs4 Sb12 by Sb self-flux method under high pressure. The samples prepared under 4 GPa are categorized by two synthesis condition. In condition one, the starting materials of powdered PrSb2 , Os, and Sb were placed in BN crucible in the atomic ratio Pr:Os:Sb ¼ 1:4:20. These were heated up to 800 3 C and kept for 30 min, and then cooled down to 750 3 C with cooling rate 0:3 3 C=h. We have succeeded in growing single crystal of Prx Os4 Sb12 of size 300 mm cubic. These samples were denoted as SP-1. In condition two, the starting materials of the chips of Pr, powdered Os and Sb were placed in BN crucible in the atomic ratio 1:4:30. These were heated up to 1300 3 C and kept for 2 h, and then were quickly cooled down to 850 3 C and cooled to 800 3 C with cooling rate
ARTICLE IN PRESS 3000
K. Tanaka et al. / Physica B 404 (2009) 2999–3001
1:7 3 C=h. The single crystals of size 50021000 mm imperfect cuboids were obtained. These samples were denoted as SP-2. The photograph of the single crystals of Prx Os4 Sb12 of SP-2 is shown in Fig. 1. For comparison, we prepared samples of Prx Os4 Sb12 by the conventional Sb self-flux method under ambient pressure [10]. These samples grown under ambient pressure were denoted as SP-3.
3. Estimation of Pr-site filling fraction In order to estimate x of SP-1, -2, and -3, we performed scanning electron microscopy with energy dispersive analysis. From the results of scanning macroscopic area of 100 mm2 at about 20 spots per sample, the average of x of SP-1, -2, and -3 were estimated to be 0.93(2), 0.95(1), and 0.93(1), respectively. The x of the SP-2 is slightly higher than that of other samples. The synthesis conditions and physical properties of SP-1, -2, and -3 are listed in Table 1.
4. Experimental results To investigate the effect of x on SC transition temperature Tc in Prx Os4 Sb12 , we measured the electrical resistance by ordinary DCor AC- four probe method. Fig. 2 shows the temperature dependence of the electrical resistance rðTÞ normalized with the value of r(290 K). Overall temperature dependence is not much different among the three samples. The residual resistance ratio r ð290 KÞ=rð2 KÞ of SP-1, -2, and -3 are 20, 6, and 17, respectively. The inset of Fig. 2 shows low temperature expansion around Tc of rðTÞ=rð2 KÞ vs. T plot. The SP-3 shows a double SC transition behavior at Tc1 1:85 K and Tc2 1:73 K, where Tc is defined in the onset of SC transition. The resemble behavior in rðTÞ is reported in Ref. [11]. In contrast to the broad SC transition in the SP-3, the transitions are sharper in SP-1 and -2. The SP-2 shows a weaker double SC transition behavior at Tc1 1:78 K and Tc2 1:73 K. The SP-3 shows the most sharpest SC transition at Tc 1:72 K with a transition width of 50 mK. The sharper SC transition along with their lower onset temperatures in
1 PrxOs4Sb12 SP-1
0.8
SP-2 SP-3 ρ/ρ290K
0.6
0.4 ρ/ρ2K
0.8
0.2
0.4 0 1.6
0 0
1.7
100
1.8 200
1.9 300
T (K)
Fig. 1. Photograph of the grown single crystals of Prx Os4 Sb12 (SP-2).
Fig. 2. Temperature dependence of the electrical resistance rðTÞ normalized with rð290 KÞ of Prx Os4 Sb12 . Inset shows the low temperature expansion of rðTÞ=rð2 KÞ vs. T plot.
Table 1 Sample synthesis conditions and its physical properties of Prx Os4 Sb12 . T (K)
RRR
1.78 1.73
7.8
20
0.95(1)
– 1.72
10.1
6
0.93(1)
1.85 1.73
7.2
17
Sample name Synthesis pressure
Starting Pr shape Pr:Os:Sb
Heat-treatment condition
Obtained sample size Size in r measurement
x value
Tc1 (K) Tc2 (K)
SP-1 4 GPa
Powdered PrSb2 1:4:20
800 3 C for 30 min 800-750 3 C, 0:3 3 C=h
300 mm cubic
0.93(2)
SP-2 4 GPa
Chips 1:4:30
1300 3 C for 2 h 850-800 3 C, 1:7 3 C=h
1000 mm cubic 620 160 180 mm3
SP-3 Ambient
Ingot 1:4:20
950 3 C for 10 h 950-650 3 C, 1 3 C=h
1000 mmcubic 280 250 180 mm3
230 210 180 mm3
Starting Pr shape and Pr:Os:Sb: The shape of starting Pr element and the atomic ratio of Pr, Os, and Sb. Heat-treatment condition: The heat-treatment condition in single crystal growth such as a highest temperatures, a keeping time, and a speed of slow cooling. Obtained sample size and Size in r measurement: The maximum size of obtained sample and the sample dimension used in the r measurement. x value: The filling fraction of Pr-site estimated by scanning electron microscopy with energy dispersive analysis. Tc1 and Tc2 : The on-set SC transition temperature in the electrical resistance measurements. T : The temperature as two straight lines’ intersection in the dr=dT vs. T plot shown in the inset of Fig. 3. RRR: Residual resistance ratio r(290 K)/r(2 K).
ARTICLE IN PRESS K. Tanaka et al. / Physica B 404 (2009) 2999–3001
physical properties such as magnetic susceptibility, specific heat, and inelastic neutron scattering are necessary.
PrxOs4Sb12 SP-1
0.08
6. Summary
SP-2 SP-3
In summary, we have succeeded in growing the single crystals of Prx Os4 Sb12 under 4 GPa. The electrical resistance measurement of three Prx Os4 Sb12 prepared by different synthesis condition suggests that the double superconducting transition ascribed to sample inhomogeneity is suppressed in the sample made under high pressure. The change of DCEF in the sample made under high pressure may play an important role in the SC transition of Prx Os4 Sb12 .
ρ*/ρ*290K
T*
0.04 dρ*/dT
SP-3
Acknowlegments
T*~7.2 K 0
0 0
3001
10
10
20 20
T (K) Fig. 3. The r =r ð290 KÞ vs. T plot of Prx Os4 Sb12 , where r is subtraction of residual resistance from rðTÞ. Inset shows the dr=dT vs. T plot of SP-3.
SP-1 and -2 suggest that the double SC transition in Prx Os4 Sb12 is suppressed in the samples grown under high pressure.
5. Discussion There remains the most important question: Why is the double SC transition suppressed in the sample made under high pressure? One of the possible origins is a change in 4f-electron CEF state as in the case of Prx Fe4 Sb12 [8]. To clarify the 4f-electron contribution to rðTÞ, r ðTÞ=r ð290 KÞ is shown in Fig. 3, where r ðTÞ rðTÞ r0 , r0 is determined by T-square extrapolation of r below 3 K. The roll-off behavior has been ascribed to the increase in the conduction electron scattering accompanied by the 4f-electron CEF excitation from the G1 ground state to the Gð2Þ 4 first excited state [12]. The arrows show the temperature T as two straight lines’ intersection in the dr=dT vs. T plot shown in the inset of Fig. 3. The T of SP-2 is higher than that of SP-3. The change of roll-off behavior resembles the change of roll-off behavior in PrðOs1y Ruy Þ4 Sb12 with y [13,14], suggesting that the DCEF increases in SP-2. In the superconducting mechanism of Pr-based filled skutterudite, DCEF is considered to play an essential role in determining the value of Tc [15,16]. In order to discuss more accurate CEF state of Prx Os4 Sb12 , higher quality sample than the present samples and/or further measurements of other
The authors thank the Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo for facilities. This work was supported by a Grant-in-Aid for Scientific Research Priority Area ‘‘Skutterudite’’ (no. 15072206), ‘‘Ubiquitous’’ (no. 20045015), and no. 20029018 of the Ministry of Education, Culture, Sports, Science and Technology, Japan. Kenya Tanaka is supported by a Grant-in-Aid for Japan Society for the Promotion of Science Fellows. References [1] Y. Aoki, H. Sugawara, H. Harima, H. Sato, J. Phys. Soc. Japan 74 (2005) 209. [2] M.B. Maple, J. Phys. Soc. Japan 74 (2005) 222. [3] M.B. Maple, Z. Henkie, W.M. Yuhasz, P.-C. Ho, T. Yanagisawa, T.A. Sayles, N.P. Butch, J.R. Jeffries, A. Pietraszko, J. Magn. Magn. Mater. 310 (2007) 182. [4] H. Sato, D. Kikuchi, K. Tanaka, H. Aoki, K. Kuwahara, Y. Aoki, M. Kohgi, H. Sugawara, K. Iwasa, J. Magn. Magn. Mater. 310 (2007) 188. [5] J. Phys. Soc. Japan 77 (2008) (Suppl. A), Proceedings of International Conference on Skutterudite, 2007. [6] E. Bauer, St. Berger, Ch. Paul, M.D. Mea, G. Hilscher, H. Michor, M. Reissner, W. Steiner, A. Grytsiv, P. Rogl, E.W. Scheidt, Phys. Rev. B 66 (2002) 214421. [7] N.P. Butch, W.M. Yuhasz, P.-C. Ho, J.R. Jeffries, N.A. Frederick, T.A. Sayles, X.G. Zheng, M.B. Maple, J.B. Betts, A.H. Lacerda, F.M. Woodward, J.W. Lynn, P. Rogl, G. Giester, Phys. Rev. B 71 (2005) 2144117. [8] K. Tanaka, Y. Kawahito, Y. Yonezawa, D. Kikuchi, H. Aoki, K. Kuwahara, M. Ichihara, H. Sugawara, Y. Aoki, H. Sato, J. Phys. Soc. Japan 76 (2007) 103704. [9] M.-A. Me asson, D. Braithwaite, G. Lapertot, J.P. Brison, J. Flouquet, P. Bordet, H. Sugawara, P.C. Canfield, Phys. Rev. B 77 (2008) 134517. [10] H. Sugawara, M. Kobayashi, S. Osaki, S.R. Saha, T. Namiki, Y. Aoki, H. Sato, Phys. Rev. B 72 (2005) 014519. [11] G. Seyfarth, J.P. Brison, M.-A. Me asson, D. Braithwaite, G. Lapertot, J. Flouquet, Phys. Rev. Lett. 97 (2006) 236403. [12] N.A. Frederick, M.B. Maple, J. Phys. Condens. Matter 15 (2003) 4789. [13] H. Akita, G. Yoshino, A. Ochiai, Physica B 378–380 (2006) 197. [14] A. Ochiai, H. Akita, G. Yoshino, S. Nakamura, T. Nojima, J. Magn. Magn. Mater. 310 (2007) 258. [15] T. Namiki, Y. Aoki, H. Sato, C. Sekine, I. Shirotani, T.D. Matsuda, Y. Haga, T. Yagi, J. Phys. Soc. Japan 76 (2007) 093704. [16] J. Chang, I. Eremin, P. Thalmeier, P. Fulde, Phys. Rev. B 76 (2007) 220510.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Transport properties of Prx Os4 Sb12 single crystals with high Pr-site filling fraction grown under high pressure Kenya Tanaka a,, Takahiro Namiki a, Takashi Saito a, Sho Tatsuoka a, Atsushi Imamura a, Keitaro Kuwahara b, Yuji Aoki a, Hideyuki Sato a a b
Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan Institute of Applied Beam Science, Ibaraki University, Mito, Ibaraki 310-8512, Japan
a r t i c l e in fo
PACS: 71.27.þa 71.20.Eh 74.70.Tx Keywords: Skutterudite PrOs4 Sb12 High-pressure synthesis Heavy-fermion superconductor Crystalline electric field
abstract We have succeeded in growing Prx Os4 Sb12 single crystals under 4 GPa with high Pr-site filling fraction x. The electrical resistance measurements clearly show that the superconducting (SC) transition is sharper and the onset temperatures is lower in the single crystal samples grown under high pressure compared to that of the sample grown under ambient pressure. These results suggest that the double SC transition ascribed to sample inhomogeneity is suppressed in the sample grown under high pressure. The change of 4f-electron crystalline electric field energy splitting between the G1 ground state and the Gð2Þ 4 first excited state in the sample made under high pressure is proposed as one of the possible origins of the suppression of the double SC transition. & 2009 Elsevier B.V. All rights reserved.
1. Introduction The filled skutterudite compounds, especially Pr-based ones, have attracted considerable attention because of their various novel features, i.e., the heavy-fermion superconductivity and the field induced quadrupolar ordered state in PrOs4 Sb12 , the scalar type ordered phase and the field induced heavy-fermion state in PrFe4 P12 , and the metal–non-metal phase transition in PrRu4 P12 [1–5]. In these systems, the strong hybridization between conduction electrons and 4f-electrons and the multipolar degrees of freedom of 4f-electrons under small crystalline electric field (CEF) are thought to play key roles in realization of their novel features. There found noticeable sample dependence in PrT4 Sb12 (T ¼ Fe, Os). In Prx Fe4 Sb12 , we found a drastic change in the magnetic ground state from ferromagnetic to non-magnetic by increasing Pr-site filling fraction x from 0:8 to 1 [6–8]. Recently, anomalous sample dependence of a double superconducting (SC) transition ascribed to sample inhomogeneity was reported in Prx Os4 Sb12 [9]. Pr-site vacancy is thought to be one of the possible origins of the problems [8,9]. In order to clarify the issues, it is necessary to prepare single crystals with high x.
Corresponding author. Tel.: +81 426 77 2487; fax: +81 426 77 2483.
E-mail address: [email protected] (K. Tanaka). 0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.07.023
In this paper, we report a method of growing single crystal of Prx Os4 Sb12 under high pressure and results of electrical resistance measurement of the single crystals. The single crystals grown under high pressure have sharper SC transition and their onset temperatures decrease, suggesting a suppression of the double SC transition. As one of the possible origins of these results, a change of CEF energy splitting (DCEF ) between the G1 ground state and the Gð2Þ 4 first excited state in the sample made under high pressure is proposed.
2. Single crystal growth High-pressure synthesis is a powerful technique to reduce Pr-site vacancy [8]. The aim of this work is to grow single crystals of PrOs4 Sb12 by Sb self-flux method under high pressure. The samples prepared under 4 GPa are categorized by two synthesis condition. In condition one, the starting materials of powdered PrSb2 , Os, and Sb were placed in BN crucible in the atomic ratio Pr:Os:Sb ¼ 1:4:20. These were heated up to 800 3 C and kept for 30 min, and then cooled down to 750 3 C with cooling rate 0:3 3 C=h. We have succeeded in growing single crystal of Prx Os4 Sb12 of size 300 mm cubic. These samples were denoted as SP-1. In condition two, the starting materials of the chips of Pr, powdered Os and Sb were placed in BN crucible in the atomic ratio 1:4:30. These were heated up to 1300 3 C and kept for 2 h, and then were quickly cooled down to 850 3 C and cooled to 800 3 C with cooling rate
ARTICLE IN PRESS 3000
K. Tanaka et al. / Physica B 404 (2009) 2999–3001
1:7 3 C=h. The single crystals of size 50021000 mm imperfect cuboids were obtained. These samples were denoted as SP-2. The photograph of the single crystals of Prx Os4 Sb12 of SP-2 is shown in Fig. 1. For comparison, we prepared samples of Prx Os4 Sb12 by the conventional Sb self-flux method under ambient pressure [10]. These samples grown under ambient pressure were denoted as SP-3.
3. Estimation of Pr-site filling fraction In order to estimate x of SP-1, -2, and -3, we performed scanning electron microscopy with energy dispersive analysis. From the results of scanning macroscopic area of 100 mm2 at about 20 spots per sample, the average of x of SP-1, -2, and -3 were estimated to be 0.93(2), 0.95(1), and 0.93(1), respectively. The x of the SP-2 is slightly higher than that of other samples. The synthesis conditions and physical properties of SP-1, -2, and -3 are listed in Table 1.
4. Experimental results To investigate the effect of x on SC transition temperature Tc in Prx Os4 Sb12 , we measured the electrical resistance by ordinary DCor AC- four probe method. Fig. 2 shows the temperature dependence of the electrical resistance rðTÞ normalized with the value of r(290 K). Overall temperature dependence is not much different among the three samples. The residual resistance ratio r ð290 KÞ=rð2 KÞ of SP-1, -2, and -3 are 20, 6, and 17, respectively. The inset of Fig. 2 shows low temperature expansion around Tc of rðTÞ=rð2 KÞ vs. T plot. The SP-3 shows a double SC transition behavior at Tc1 1:85 K and Tc2 1:73 K, where Tc is defined in the onset of SC transition. The resemble behavior in rðTÞ is reported in Ref. [11]. In contrast to the broad SC transition in the SP-3, the transitions are sharper in SP-1 and -2. The SP-2 shows a weaker double SC transition behavior at Tc1 1:78 K and Tc2 1:73 K. The SP-3 shows the most sharpest SC transition at Tc 1:72 K with a transition width of 50 mK. The sharper SC transition along with their lower onset temperatures in
1 PrxOs4Sb12 SP-1
0.8
SP-2 SP-3 ρ/ρ290K
0.6
0.4 ρ/ρ2K
0.8
0.2
0.4 0 1.6
0 0
1.7
100
1.8 200
1.9 300
T (K)
Fig. 1. Photograph of the grown single crystals of Prx Os4 Sb12 (SP-2).
Fig. 2. Temperature dependence of the electrical resistance rðTÞ normalized with rð290 KÞ of Prx Os4 Sb12 . Inset shows the low temperature expansion of rðTÞ=rð2 KÞ vs. T plot.
Table 1 Sample synthesis conditions and its physical properties of Prx Os4 Sb12 . T (K)
RRR
1.78 1.73
7.8
20
0.95(1)
– 1.72
10.1
6
0.93(1)
1.85 1.73
7.2
17
Sample name Synthesis pressure
Starting Pr shape Pr:Os:Sb
Heat-treatment condition
Obtained sample size Size in r measurement
x value
Tc1 (K) Tc2 (K)
SP-1 4 GPa
Powdered PrSb2 1:4:20
800 3 C for 30 min 800-750 3 C, 0:3 3 C=h
300 mm cubic
0.93(2)
SP-2 4 GPa
Chips 1:4:30
1300 3 C for 2 h 850-800 3 C, 1:7 3 C=h
1000 mm cubic 620 160 180 mm3
SP-3 Ambient
Ingot 1:4:20
950 3 C for 10 h 950-650 3 C, 1 3 C=h
1000 mmcubic 280 250 180 mm3
230 210 180 mm3
Starting Pr shape and Pr:Os:Sb: The shape of starting Pr element and the atomic ratio of Pr, Os, and Sb. Heat-treatment condition: The heat-treatment condition in single crystal growth such as a highest temperatures, a keeping time, and a speed of slow cooling. Obtained sample size and Size in r measurement: The maximum size of obtained sample and the sample dimension used in the r measurement. x value: The filling fraction of Pr-site estimated by scanning electron microscopy with energy dispersive analysis. Tc1 and Tc2 : The on-set SC transition temperature in the electrical resistance measurements. T : The temperature as two straight lines’ intersection in the dr=dT vs. T plot shown in the inset of Fig. 3. RRR: Residual resistance ratio r(290 K)/r(2 K).
ARTICLE IN PRESS K. Tanaka et al. / Physica B 404 (2009) 2999–3001
physical properties such as magnetic susceptibility, specific heat, and inelastic neutron scattering are necessary.
PrxOs4Sb12 SP-1
0.08
6. Summary
SP-2 SP-3
In summary, we have succeeded in growing the single crystals of Prx Os4 Sb12 under 4 GPa. The electrical resistance measurement of three Prx Os4 Sb12 prepared by different synthesis condition suggests that the double superconducting transition ascribed to sample inhomogeneity is suppressed in the sample made under high pressure. The change of DCEF in the sample made under high pressure may play an important role in the SC transition of Prx Os4 Sb12 .
ρ*/ρ*290K
T*
0.04 dρ*/dT
SP-3
Acknowlegments
T*~7.2 K 0
0 0
3001
10
10
20 20
T (K) Fig. 3. The r =r ð290 KÞ vs. T plot of Prx Os4 Sb12 , where r is subtraction of residual resistance from rðTÞ. Inset shows the dr=dT vs. T plot of SP-3.
SP-1 and -2 suggest that the double SC transition in Prx Os4 Sb12 is suppressed in the samples grown under high pressure.
5. Discussion There remains the most important question: Why is the double SC transition suppressed in the sample made under high pressure? One of the possible origins is a change in 4f-electron CEF state as in the case of Prx Fe4 Sb12 [8]. To clarify the 4f-electron contribution to rðTÞ, r ðTÞ=r ð290 KÞ is shown in Fig. 3, where r ðTÞ rðTÞ r0 , r0 is determined by T-square extrapolation of r below 3 K. The roll-off behavior has been ascribed to the increase in the conduction electron scattering accompanied by the 4f-electron CEF excitation from the G1 ground state to the Gð2Þ 4 first excited state [12]. The arrows show the temperature T as two straight lines’ intersection in the dr=dT vs. T plot shown in the inset of Fig. 3. The T of SP-2 is higher than that of SP-3. The change of roll-off behavior resembles the change of roll-off behavior in PrðOs1y Ruy Þ4 Sb12 with y [13,14], suggesting that the DCEF increases in SP-2. In the superconducting mechanism of Pr-based filled skutterudite, DCEF is considered to play an essential role in determining the value of Tc [15,16]. In order to discuss more accurate CEF state of Prx Os4 Sb12 , higher quality sample than the present samples and/or further measurements of other
The authors thank the Materials Design and Characterization Laboratory, Institute for Solid State Physics, University of Tokyo for facilities. This work was supported by a Grant-in-Aid for Scientific Research Priority Area ‘‘Skutterudite’’ (no. 15072206), ‘‘Ubiquitous’’ (no. 20045015), and no. 20029018 of the Ministry of Education, Culture, Sports, Science and Technology, Japan. Kenya Tanaka is supported by a Grant-in-Aid for Japan Society for the Promotion of Science Fellows. References [1] Y. Aoki, H. Sugawara, H. Harima, H. Sato, J. Phys. Soc. Japan 74 (2005) 209. [2] M.B. Maple, J. Phys. Soc. Japan 74 (2005) 222. [3] M.B. Maple, Z. Henkie, W.M. Yuhasz, P.-C. Ho, T. Yanagisawa, T.A. Sayles, N.P. Butch, J.R. Jeffries, A. Pietraszko, J. Magn. Magn. Mater. 310 (2007) 182. [4] H. Sato, D. Kikuchi, K. Tanaka, H. Aoki, K. Kuwahara, Y. Aoki, M. Kohgi, H. Sugawara, K. Iwasa, J. Magn. Magn. Mater. 310 (2007) 188. [5] J. Phys. Soc. Japan 77 (2008) (Suppl. A), Proceedings of International Conference on Skutterudite, 2007. [6] E. Bauer, St. Berger, Ch. Paul, M.D. Mea, G. Hilscher, H. Michor, M. Reissner, W. Steiner, A. Grytsiv, P. Rogl, E.W. Scheidt, Phys. Rev. B 66 (2002) 214421. [7] N.P. Butch, W.M. Yuhasz, P.-C. Ho, J.R. Jeffries, N.A. Frederick, T.A. Sayles, X.G. Zheng, M.B. Maple, J.B. Betts, A.H. Lacerda, F.M. Woodward, J.W. Lynn, P. Rogl, G. Giester, Phys. Rev. B 71 (2005) 2144117. [8] K. Tanaka, Y. Kawahito, Y. Yonezawa, D. Kikuchi, H. Aoki, K. Kuwahara, M. Ichihara, H. Sugawara, Y. Aoki, H. Sato, J. Phys. Soc. Japan 76 (2007) 103704. [9] M.-A. Me asson, D. Braithwaite, G. Lapertot, J.P. Brison, J. Flouquet, P. Bordet, H. Sugawara, P.C. Canfield, Phys. Rev. B 77 (2008) 134517. [10] H. Sugawara, M. Kobayashi, S. Osaki, S.R. Saha, T. Namiki, Y. Aoki, H. Sato, Phys. Rev. B 72 (2005) 014519. [11] G. Seyfarth, J.P. Brison, M.-A. Me asson, D. Braithwaite, G. Lapertot, J. Flouquet, Phys. Rev. Lett. 97 (2006) 236403. [12] N.A. Frederick, M.B. Maple, J. Phys. Condens. Matter 15 (2003) 4789. [13] H. Akita, G. Yoshino, A. Ochiai, Physica B 378–380 (2006) 197. [14] A. Ochiai, H. Akita, G. Yoshino, S. Nakamura, T. Nojima, J. Magn. Magn. Mater. 310 (2007) 258. [15] T. Namiki, Y. Aoki, H. Sato, C. Sekine, I. Shirotani, T.D. Matsuda, Y. Haga, T. Yagi, J. Phys. Soc. Japan 76 (2007) 093704. [16] J. Chang, I. Eremin, P. Thalmeier, P. Fulde, Phys. Rev. B 76 (2007) 220510.