Possible Octupole Ordering in Phase II of SmRu₄P₁₂ Suggested from Elastic Constants

Abstract

We have studied the origin of the successive phase transitions in filled skutterudite SmRu₄P₁₂ based on the results of elastic constant measurements. The temperature dependence of the elastic constants shows a sharp drop at the metal–insulator (M–I) transition of T_MI=16.5 K in a zero field. The successive transition at T_N=14 K is indistinct in a low field. The anomaly at T_MI disappears with increasing field. A clear softening toward T_N appears in the elastic constants in a high magnetic field. This result suggests a breakdown of the time reversal symmetry in the temperature range between T_MI and T_N. Such a breakdown is considered to be due to the coupling between the dipole moment and the octupole moment, which is allowed only in T_h symmetry.