Anomalous heavy-fermion and ordered states in the filled skutterudite PrFe 4 P 12

Specific heat and magnetization measurements have been performed on high-quality single crystals of filled skutterudite ${\mathrm{PrFe}}_{4}{\mathrm{P}}_{12}$ in order to study the high-field heavy-fermion state (HFS) and low-field ordered state (ODS). From a broad hump observed in $C/T$ vs T in HFS for magnetic fields applied along the $〈100〉$ direction, the Kondo temperature of $\ensuremath{\sim}9\mathrm{K}$ and the existence of ferromagnetic Pr-Pr interactions are deduced. The ${}^{141}\mathrm{Pr}$ nuclear Schottky contribution, which works as a highly-sensitive on-site probe for the Pr magnetic moment, sets an upper bound for the ordered moment as $\ensuremath{\sim}0.03{\ensuremath{\mu}}_{B}/\mathrm{Pr}$ ion. This fact strongly indicates that the primary order parameter in the ODS is nonmagnetic, and most probably of quadrupolar origin, combined with other experimental facts. Significantly suppressed heavy-fermion behavior in the ODS suggests a possibility that the quadrupolar degrees of freedom is essential for the heavy quasiparticle band formation in the HFS. Possible crystalline-electric-field level schemes estimated from the anisotropy in the magnetization are consistent with this conjecture.