Anomalous frequency and temperature-dependent scattering and Hund's coupling in the almost quantum critical heavy-fermion system CeFe2Ge2

We present THz range optical conductivity data of a thin film of the near quantum critical heavy-fermion compound CeFe2Ge2. Our complex conductivity measurements find a deviation from conventional Drude-like transport in a temperature range previously reported to exhibit unconventional behavior. We calculate the frequency-dependent effective mass and scattering rate using an extended Drude model analysis. We find the inelastic scattering rate can be described by a temperature-dependent power law ωn(T), where n(T) approaches ∼1.0±0.2 at 1.5 K. This is compared to the ρ∼T1.5 behavior claimed in dc resistivity data and the ρ∼T2 expected from Fermi-liquid theory. In addition to a low-temperature mass renormalization, we find an anomalous mass renormalization that persists to high temperature. We attribute this to a Hund's coupling in the Fe states in a manner similar to that recently proposed in the ferropnictides. CeFe2Ge2 appears to be a very interesting system where one may study the interplay between the usual 4f lattice Kondo effect and this Hund's enhanced Kondo effect in the 3d states.