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$$mu$$SR法によるSmRu$$_4$$P$$_{12}$$の多極子状態の研究

Possible multipole state of SmRu$$_4$$P$$_{12}$$ probed by $$mu$$SR

伊藤 孝  ; 髭本 亘 ; 大石 一城*; Heffner, R. H.; 西田 信彦*; 佐藤 一彦*; 菅原 仁*; 青木 勇二*; 菊地 大輔*; 佐藤 英行*

Ito, Takashi; Higemoto, Wataru; Oishi, Kazuki*; Heffner, R. H.; Nishida, Nobuhiko*; Sato, Kazuhiko*; Sugawara, Hitoshi*; Aoki, Yuji*; Kikuchi, Daisuke*; Sato, Hideyuki*

充填スクッテルダイト化合物SmRu$$_4$$P$$_{12}$$$$T_{rm MI}=$$16.5Kにおいて金属絶縁体転移を起こす。この転移に伴い、時間反転対称性の破れ、異常な弾性特性、複雑な磁場温度相図などが観測されており、磁気八極子秩序の可能性が議論されている。われわれはこの物質における多極子の状態を調べるために$$mu$$SR測定を行い、秩序相において強い縦緩和を観測した。縦緩和率は$$T^{prime}sim 3$$K付近において極大値に達した後、低温に向かって減少を示す。高次多極子の低エネルギー励起の可能性と、それがローカルプローブに及ぼす影響について議論する。

The filled skutterudite compound SmRu$$_4$$P$$_{12}$$ exhibits a metal-insulator (MI) transition at $$T_{rm MI}$$ $$sim$$ 16.5 K. An entropy release $$sim Rln4$$ below $$T_{rm MI}$$ suggests that the crystalline-electric-field ground state is a $$Gamma_{67}$$ quartet with various multipole degrees of freedom. A possibility of magnetic octupole ordering has been intensively discussed to explain the time-reversal-symmetry breaking, anomalous elastic properties, and a complex field-temperature phase diagram. We will report $$mu$$SR measurements on single crystalline samples of SmRu$$_4$$P$$_{12}$$ to investigate the nature of the multipoles in the ordered state. A remarkable longitudinal relaxation was observed in the ordered state, the magnitude of which was reduced below $$T^{prime}$$ $$sim$$ 3 K. We will discuss possible low-energy excitations of magnetic multipoles in the ordered state and their influence on the local magnetic probe on the basis of symmetry of the multipoles.

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