Scalar order: Possible candidate for order parameters in skutterudites

Phenomenological Landau analysis shows that the properties of ordered phases in some skutterudites are consistently accounted for by a scalar-order parameter which preserves the cubic symmetry, even in the ordered phase. A universal value is found for the anisotropy ratio of the transition temperature in a magnetic field, homogeneous magnetization, and induced staggered magnetization. The difference in magnetic behavior between PrFe ₄P₁₂ and PrRu₄P₁₂ near their phase transitions is explained within a single framework. For the low-field phase of PrFe₄P₁₂, the scalar order with Γ_1g symmetry can explain (i) the absence of field-induced dipoles perpendicular to the magnetic field, (ii) isotropic magnetic susceptibility in the ordered phase, (iii) the field-angle dependence of the transition temperature, and (iv) the splitting pattern of the ³¹P nuclear magnetic resonance (NMR) spectra. It is proposed how the order parameter in SmRu₄P ₁₂ is identified by NMR analysis of a single crystal.