Anomalous frequency and temperature dependent scattering and Hund's coupling in the almost quantum critical heavy fermion system CeFe$_2$Ge$_2$

Abstract

We present THz range optical conductivity data of a thin film of the near quantum critical heavy fermion compound CeFe$_2$Ge$_2$. 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 $\omega^{n(T)}$ where $n(T)$ approaches $\sim1.0 \pm 0.2$ at 1.5 K. This is compared to the $\rho \sim T^{1.5}$ behavior claimed in dc resistivity data and the $\rho \sim T^{2}$ 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 ferro-pnictides. CeFe$_2$Ge$_2$ 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.