Specific-Heat Evidence for Octupolar Ordering in SmRu4P12

We measured specific heat in the anomalous ordered phase ( T < 16.5 K) of filled skutterudite SmRu 4 P 12 . From the hyperfine-enhanced Sm nuclear contribution, the ordered Sm dipole moment has been accurately determined to be 0.29 ±0.01 µ B , which is strongly reduced for usual dipolar ordering. We demonstrate that this finding along with other physical quantities provide strong evidence that the primary order parameter is Γ 4 u -type octupole. The ordered state provides the first example of an octupolar ordering accompanied by nonzero parasitic dipole moments.