Unconventional emergence of elastic softening induced by magnetic fields in the unusual heavy-fermion compound PrFe4P12

Ultrasonic measurement on the filled skutterudite compound PrFe4P12 exhibits a mysterious temperature dependence of the elastic constant (C11 - C12)/2. Pronounced elastic softening at low temperatures is revived by applying a magnetic field. This fact strongly suggests the 4f-multiplet ground state of the Pr ion split by the crystalline electric field (CEF) to be a Γ3 non-Kramers doublet. The expectation value of a quadrupole moment with Γ3 symmetry in the CEF ground state, which leads to elastic softening at low temperature, was evaluated by theoretical fitting to the present results. This may imply that suppression of the electric quadrupole Kondo effect occurs in PrFe4P12 and the quadrupole moment becomes steady due to the application of a magnetic field. (letter to the editor)