Ordering and crossover in a pseudo-sextet localized electron model - Possible scenario for SmRu₄P₁₂ skutterudite

A crystalline electric field (CEF) model of localized 4f⁵ electrons is proposed for understanding intriguing ordering phenomena in SmRu₄P₁₂. We take the CEF quartet and doublet (pseudo-sextet) of the Hund's rule ground state with J = 5/2, and include intersite interactions between multipoles by the mean-field theory. The model leads to a multipole order with representation Γ_5u, and shows the following features: (i) increasing transition temperature T_MI with increasing magnetic field; (ii) an anomaly in specific heat and other thermodynamic quantities at lower temperature T^*, (iii) sharpening of the anomaly at T^* as the magnetic field increases. These behaviors reproduce salient features observed experimentally in SmRu₄P₁₂. Possible mechanisms for spontaneous magnetic moment are discussed.