Metal-Insulator Transition of Filled Skutterudite Compound SmRu4P12

Filled skutterudite SmRu4P12 exhibits a metal-insulator (M-I) transition near 16 K (=T-M1). We have measured in detail the specific heat C(T), electrical resistivity rho(T) and magnetization M(T) under high magnetic field around M-I transition. The detailed measurements have revealed that this M-I transition is caused by two successive transitions. The single peak in C(T) at zero field changes into a double peak with increasing field. (-d rho/dT) and dM/dT exhibit two anomalies at almost the same positions as the specific heat peaks. The magnetic entropy estimated at zero field reaches R In 4 below T-M1. The crystalline electric field (CEF) ground state is a quartet in cubic symmetry. It is suggested that the M-I transition of SmRu4P12 is related to an orbital ordering originating from the CEF ground state quartet.