Inelastic neutron scattering from PrFe4 P12 at low temperatures and under high magnetic fields

PrFe4 P12 exhibits a unique transition at 6.5 K with an order parameter whose nature is still controversial. In order to elucidate the origin of the transition, we have carried out inelastic neutron scattering experiments in a range of temperatures and magnetic fields. Our data reveal a different type of low-lying excitations centered at 1.5 and 3.5 meV in the ordered phase with an unusual Q dependence. With increasing field or temperature, the well-defined excitations develop into a quasielastic response, while the ordered phase becomes a paramagnetic state with a very large characteristic energy scale of Γ 0.8 meV, consistent with a large Sommerfeld coefficient γ=1.4 J/mol K2.