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NMR study of β -pyrochlore oxide superconductor KOs2O6
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 698–699 www.elsevier.com/locate/jmmm
NMR study of b-pyrochlore oxide superconductor KOs2 O6 M. Yoshida, K. Arai, R. Kaido, M. Takigawa, S. Yonezawa, Y. Muraoka, Z. Hiroi Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan Available online 30 October 2006
Abstract We report results of NMR measurements on the 39;41 K and 17 O nuclei in KOs2 O6 . We found that the relaxation rate divided by temperature 1=ðT 1 TÞ at the K sites is strongly enhanced compared with the values expected from the Knight shift and the Korringa relation. By comparing the results at the 39 K and 41 K sites, we conclude that the relaxation at the K sites is caused entirely by phonons via the electric quadrupole interaction. Consequently, our results provide information about the dynamics of isolated anharmonic vibration associated with the rattling of K ions. r 2006 Elsevier B.V. All rights reserved. PACS: 76.60.k; 74.70.b; 74.25.Kc Keywords: Superconductivity; Rattring; NMR; b-Pyrochlore oxide; KOs2 O6
Recently, a new class of transition metal oxide superconductors AOs2 O6 (A ¼ K, Rb, and Cs) was discovered by Yonezawa et al. [1]. These compounds have the bpyrochlore structure and the superconducting transition temperatures T c are 9.6, 6.3, and 3.3 K for A ¼ K, Rb, and Cs, respectively. In addition to the superconductivity, possibility of rattling motion, that is the isolated anharmonic vibration of alkali metal ions in a large cage formed by OsO6 octahedra, has been attracting much attention. The temperature dependence of the specific heat indicates important roles of the rattling motion of alkali metal ions for the physical properties of this system at low temperatures [2,3]. Previously, we reported the strongly enhanced NMR relaxation rate divided by temperature 1=ðT 1 TÞ at the 39 K sites compared with the values expected from the Knight shift and the Korringa relation [4]. In the present study, we have investigated 1=ðT 1 TÞ at the 41 K and 17 O sites in 17 O-enriched powder of KOs2 O6 . Fig. 1 shows the temperature dependences of 1=ðT 1 TÞ at the 39 K sites. The open circles represent the data of 5 T previously reported for the normal powder sample without isotope enrichment [4]. The open triangles and solid squares represent the 1=ðT 1 TÞ for the 17 O-enriched powder Corresponding author. Tel./fax: +81 4 7136 3228.
E-mail address: [email protected] (M. Yoshida). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.186
at 5 and 8.5 T, respectively. Their common features are the broad maximum near 15 K and rapid decrease below T c . However, the 17 O-enriched powder shows somewhat larger values of 1=ðT 1 TÞ than the normal powder near the maximum. This indicates that the relaxation rate depends on the procedure for the sample preparation or possibly the content of 17 O. On the other hand, the relaxation rate is insensitive to the magnetic field. In general, such behavior of 1=ðT 1 TÞ at the 39 K sites is suggestive of development of antiferromagnetic spin fluctuations as the temperature decreases [4]. However, the relaxation rate at the 17 O sites shows no signature for strong spin fluctuations. Fig. 2 shows the temperature dependences of 1=ðT 1 TÞ at the 39;41 K and 17 O sites at 8.5 T in the 17 O-enriched powder. The solid circles, solid squares, and open squares represent the 1=ðT 1 TÞ at the 41 K, 39 K, and 17 O sites, respectively. The strong enhancement and broad maximum at around 15 K were not observed at the 17 O sites. The relaxation at the 17 O sites can be understood in terms of the Korringa mechanism. This indicates either the antiferromagnetic fluctuations do not couple to the nuclei or the anomaly at the K sites has nonmagnetic origin, for example, relaxation via phonons. In order to investigate the contribution of phonons to the relaxation at the K sites, 1=T 1 at the 41 K sites have also been measured. The temperature dependence of 1=ðT 1 TÞ
ARTICLE IN PRESS M. Yoshida et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 698–699
Fig. 1. Temperature dependences of 1=ðT 1 TÞ at the
39
K sites.
Fig. 2. Temperature dependences of 1=ðT 1 TÞ at the 39;41 K and 17 O sites at 8.5 T. The inset shows the temperate dependence of ð39 T 1 =41 T 1 Þ. The solid and dashed lines show ð41 Q=39 QÞ2 of 1.48 and ð41 g=39 gÞ2 of 0.30, respectively [5].
for 41 K is shown in Fig. 2 along with the results of 39 K and 17 O. In general, 1=T 1 is the sum of the contributions from the magnetic hyperfine interaction and the electric quadrupole interaction, 1=T 1 ¼ ð1=T 1 ÞM þ ð1=T 1 ÞQ . ð1=T 1 ÞM and ð1=T 1 ÞQ are proportional to the squares of the magnetogyric ratio g2 and the nuclear quadrupole moment Q2 , respectively. The comparison between 1=T 1 for 41 K and 39 K provides decisive information about which of the interactions dominates the relaxation, because the ratios of these nuclear moments are remarkably different, ð41 g=39 gÞ2 ¼ 0:30 and ð41 Q=39 QÞ2 ¼ 1:48 [5]. The inset of Fig. 2 shows the temperate dependence of ð39 T 1 =41 T1 Þ.Over the whole temperature range, the ratio shows the constant value 1.48, which agrees with ð41 Q=39 QÞ2 . This means that ð1=T 1 ÞM is negligibly smaller than ð1=T 1 ÞQ for the K sites. The present results show that the anomalous behavior of 1=ðT 1 TÞ at the K sites originates from the relaxation via
699
Fig. 3. Temperature dependences of 1=ðT 1 TÞ at the 39 K sites in the lower-temperature region. The arrows indicate T c in the magnetic field.
phonons. For metallic compounds, it is extremely rare that the relaxation is caused entirely by phonons. Therefore, the large phonon contribution observed in KOs2 O6 implies that there is a relaxation process peculiar to the rattling motion of alkali ions. A phenomenological analysis for the relaxation via rattling phonons is now in progress. Another important feature of 1=ðT 1 TÞ at the K sites is the rapid decrease below T c . Fig. 3 shows the temperature dependence of 1=ðT 1 TÞ at the 39 K sites in the lower-temperature region. We can see that 1=ðT 1 TÞ decrease rapidly immediately below T c without a coherence peak. In general, behavior of the relaxation rate below T c is related to the anisotropy of the superconducting gap. In this case, however, this decrease is not due to the superconducting gap, because ð1=T 1 ÞM is negligibly small. This behavior of 1=ðT 1 TÞ below T c demonstrates that the rattling motion is significantly affected by the superconductivity. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 16076204 ‘‘Invention of Anomalous Quantum Materials’’) from the Ministry of Culture, Sports, Science and Technology of Japan.
References [1] S. Yonezawa, Y. Muraoka, Y. Matsushita, Z. Hiroi, J. Phys. Condens. Matter 16 (2004) L9. [2] Z. Hiroi, S. Yonezawa, J. Yamaura, T. Muramatsu, Y. Muraoka, J. Phys. Soc. Japan 74 (2005) 1682. [3] Z. Hiroi, S. Yonezawa, J. Yamaura, T. Muramatsu, Y. Muraoka, J. Phys. Soc. Japan 74 (2005) 1255. [4] K. Arai, J. Kikuchi, K. Kodama, M. Takigawa, S. Yonezawa, Y. Muraoka, Z. Hiroi, Physica B 359–361 (2005) 488. [5] P.A. Bonczyk, V.W. Hughes, Phys. Rev. 161 (1967) 15.
Journal of Magnetism and Magnetic Materials 310 (2007) 698–699 www.elsevier.com/locate/jmmm
NMR study of b-pyrochlore oxide superconductor KOs2 O6 M. Yoshida, K. Arai, R. Kaido, M. Takigawa, S. Yonezawa, Y. Muraoka, Z. Hiroi Institute for Solid State Physics, University of Tokyo, Kashiwa, Chiba 277-8581, Japan Available online 30 October 2006
Abstract We report results of NMR measurements on the 39;41 K and 17 O nuclei in KOs2 O6 . We found that the relaxation rate divided by temperature 1=ðT 1 TÞ at the K sites is strongly enhanced compared with the values expected from the Knight shift and the Korringa relation. By comparing the results at the 39 K and 41 K sites, we conclude that the relaxation at the K sites is caused entirely by phonons via the electric quadrupole interaction. Consequently, our results provide information about the dynamics of isolated anharmonic vibration associated with the rattling of K ions. r 2006 Elsevier B.V. All rights reserved. PACS: 76.60.k; 74.70.b; 74.25.Kc Keywords: Superconductivity; Rattring; NMR; b-Pyrochlore oxide; KOs2 O6
Recently, a new class of transition metal oxide superconductors AOs2 O6 (A ¼ K, Rb, and Cs) was discovered by Yonezawa et al. [1]. These compounds have the bpyrochlore structure and the superconducting transition temperatures T c are 9.6, 6.3, and 3.3 K for A ¼ K, Rb, and Cs, respectively. In addition to the superconductivity, possibility of rattling motion, that is the isolated anharmonic vibration of alkali metal ions in a large cage formed by OsO6 octahedra, has been attracting much attention. The temperature dependence of the specific heat indicates important roles of the rattling motion of alkali metal ions for the physical properties of this system at low temperatures [2,3]. Previously, we reported the strongly enhanced NMR relaxation rate divided by temperature 1=ðT 1 TÞ at the 39 K sites compared with the values expected from the Knight shift and the Korringa relation [4]. In the present study, we have investigated 1=ðT 1 TÞ at the 41 K and 17 O sites in 17 O-enriched powder of KOs2 O6 . Fig. 1 shows the temperature dependences of 1=ðT 1 TÞ at the 39 K sites. The open circles represent the data of 5 T previously reported for the normal powder sample without isotope enrichment [4]. The open triangles and solid squares represent the 1=ðT 1 TÞ for the 17 O-enriched powder Corresponding author. Tel./fax: +81 4 7136 3228.
E-mail address: [email protected] (M. Yoshida). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.186
at 5 and 8.5 T, respectively. Their common features are the broad maximum near 15 K and rapid decrease below T c . However, the 17 O-enriched powder shows somewhat larger values of 1=ðT 1 TÞ than the normal powder near the maximum. This indicates that the relaxation rate depends on the procedure for the sample preparation or possibly the content of 17 O. On the other hand, the relaxation rate is insensitive to the magnetic field. In general, such behavior of 1=ðT 1 TÞ at the 39 K sites is suggestive of development of antiferromagnetic spin fluctuations as the temperature decreases [4]. However, the relaxation rate at the 17 O sites shows no signature for strong spin fluctuations. Fig. 2 shows the temperature dependences of 1=ðT 1 TÞ at the 39;41 K and 17 O sites at 8.5 T in the 17 O-enriched powder. The solid circles, solid squares, and open squares represent the 1=ðT 1 TÞ at the 41 K, 39 K, and 17 O sites, respectively. The strong enhancement and broad maximum at around 15 K were not observed at the 17 O sites. The relaxation at the 17 O sites can be understood in terms of the Korringa mechanism. This indicates either the antiferromagnetic fluctuations do not couple to the nuclei or the anomaly at the K sites has nonmagnetic origin, for example, relaxation via phonons. In order to investigate the contribution of phonons to the relaxation at the K sites, 1=T 1 at the 41 K sites have also been measured. The temperature dependence of 1=ðT 1 TÞ
ARTICLE IN PRESS M. Yoshida et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 698–699
Fig. 1. Temperature dependences of 1=ðT 1 TÞ at the
39
K sites.
Fig. 2. Temperature dependences of 1=ðT 1 TÞ at the 39;41 K and 17 O sites at 8.5 T. The inset shows the temperate dependence of ð39 T 1 =41 T 1 Þ. The solid and dashed lines show ð41 Q=39 QÞ2 of 1.48 and ð41 g=39 gÞ2 of 0.30, respectively [5].
for 41 K is shown in Fig. 2 along with the results of 39 K and 17 O. In general, 1=T 1 is the sum of the contributions from the magnetic hyperfine interaction and the electric quadrupole interaction, 1=T 1 ¼ ð1=T 1 ÞM þ ð1=T 1 ÞQ . ð1=T 1 ÞM and ð1=T 1 ÞQ are proportional to the squares of the magnetogyric ratio g2 and the nuclear quadrupole moment Q2 , respectively. The comparison between 1=T 1 for 41 K and 39 K provides decisive information about which of the interactions dominates the relaxation, because the ratios of these nuclear moments are remarkably different, ð41 g=39 gÞ2 ¼ 0:30 and ð41 Q=39 QÞ2 ¼ 1:48 [5]. The inset of Fig. 2 shows the temperate dependence of ð39 T 1 =41 T1 Þ.Over the whole temperature range, the ratio shows the constant value 1.48, which agrees with ð41 Q=39 QÞ2 . This means that ð1=T 1 ÞM is negligibly smaller than ð1=T 1 ÞQ for the K sites. The present results show that the anomalous behavior of 1=ðT 1 TÞ at the K sites originates from the relaxation via
699
Fig. 3. Temperature dependences of 1=ðT 1 TÞ at the 39 K sites in the lower-temperature region. The arrows indicate T c in the magnetic field.
phonons. For metallic compounds, it is extremely rare that the relaxation is caused entirely by phonons. Therefore, the large phonon contribution observed in KOs2 O6 implies that there is a relaxation process peculiar to the rattling motion of alkali ions. A phenomenological analysis for the relaxation via rattling phonons is now in progress. Another important feature of 1=ðT 1 TÞ at the K sites is the rapid decrease below T c . Fig. 3 shows the temperature dependence of 1=ðT 1 TÞ at the 39 K sites in the lower-temperature region. We can see that 1=ðT 1 TÞ decrease rapidly immediately below T c without a coherence peak. In general, behavior of the relaxation rate below T c is related to the anisotropy of the superconducting gap. In this case, however, this decrease is not due to the superconducting gap, because ð1=T 1 ÞM is negligibly small. This behavior of 1=ðT 1 TÞ below T c demonstrates that the rattling motion is significantly affected by the superconductivity. This work was supported by a Grant-in-Aid for Scientific Research on Priority Areas (No. 16076204 ‘‘Invention of Anomalous Quantum Materials’’) from the Ministry of Culture, Sports, Science and Technology of Japan.
References [1] S. Yonezawa, Y. Muraoka, Y. Matsushita, Z. Hiroi, J. Phys. Condens. Matter 16 (2004) L9. [2] Z. Hiroi, S. Yonezawa, J. Yamaura, T. Muramatsu, Y. Muraoka, J. Phys. Soc. Japan 74 (2005) 1682. [3] Z. Hiroi, S. Yonezawa, J. Yamaura, T. Muramatsu, Y. Muraoka, J. Phys. Soc. Japan 74 (2005) 1255. [4] K. Arai, J. Kikuchi, K. Kodama, M. Takigawa, S. Yonezawa, Y. Muraoka, Z. Hiroi, Physica B 359–361 (2005) 488. [5] P.A. Bonczyk, V.W. Hughes, Phys. Rev. 161 (1967) 15.