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

TL;DR

This study investigates the optical conductivity of CeFe$_2$Ge$_2$, revealing deviations from conventional transport, anomalous mass renormalization, and the influence of Hund's coupling, highlighting complex interactions near quantum criticality.

Contribution

It provides the first THz range optical conductivity analysis of CeFe$_2$Ge$_2$, uncovering unconventional scattering behavior and the role of Hund's coupling in a heavy fermion system.

Findings

Deviation from Drude-like transport at certain temperatures

Inelastic scattering rate follows a temperature-dependent power law

Persistent high-temperature mass renormalization attributed to Hund's coupling

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.