F-electron-nuclear hyperfine-coupled multiplets in the unconventional charge order phase of filled skutterudite PrRu₄P₁₂

The filled skutterudite PrRu₄P₁₂ is known to undergo an unconventional charge order phase transition at 63 K, below which two sublattices with distinct /-electron crystalline-electric-field ground states are formed. In this paper, we study experimentally and theoretically the properties of the charge order phase at very low temperature, particularly focusing on the nature of the degenerate triplet ground state on one of the sublattices. First, we present experimental results of specific heat and magnetization measured with high quality single crystals. In spite of the absence of any symmetry breaking, the specific heat shows a peak structure at T_p - -0.30 K in zero field; it shifts to higher temperatures as the magnetic field is applied. In addition, the magnetization curve has a remarkable rounding below 1 T. Then, we study the origin of these experimental findings by considering the hyperfine interaction between 4/ electron and nuclear spin. We demonstrate that the puzzling behaviors at low temperatures can be well accounted for by the formation of 4f-electron-nuclear hyperfine-coupled multiplets, the first thermodynamical observation of its kind.