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

We have studied the origin of the successive phase transitions in filled skutterudite SmRu 4 P 12 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.