  In the unconventional f-electron-associated charge order phase of filled
skutterudite PrRu4P12, the low-temperature behaviors of the triplet
crystalline-electric-field ground state of Pr ions have been studied by
specific heat and magnetization measurements using high quality single
crystals. Specific heat shows an anomalous Schottky-type peak structure at 0.30
K in zero field in spite of the absence of any symmetry breaking. Magnetization
curve at 0.06 K shows a remarkable rounding below 1 T. It has been revealed
that these anomalies provide compelling evidence for the formation of a lattice
of Pr 4f-electron-nuclear hyperfine-coupled multiplets, the first known
thermodynamical observation of its kind.
