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

We present THz range optical conductivity data of a thin film of the near quantum critical heavy-fermion compound ${\mathrm{CeFe}}_2{\mathrm{Ge}}_2$. 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 ${\omega }^{n\left(T\right)}$, where $n\left(T\right)$ approaches $\sim 1.0\pm 0.2$ 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 ferropnictides. ${\mathrm{CeFe}}_2{\mathrm{Ge}}_2$ 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.

Figure 1

dc resistivity as a function of temperature for the ${\mathrm{CeFe}}_2{\mathrm{Ge}}_2$ film. Inset: Resistivity minus residual resistivity as a function of temperature. A fit to the data in the temperature range 2–15 K is shown as a dashed line.

Figure 2

(a)–(f) Real and imaginary parts of the complex conductivity as functions of frequency for select temperatures. Solid lines indicate experimental data, while dashed lines show results of a two-component Drude fit described in the text. The values of the dc conductivity are shown as solid circles.

Figure 3

(a) Scattering rates as functions of frequency. Experimental data are shown as solid lines; extended Drude model fits, described in the text, to the data are shown as dashed lines. dc data points are shown as solid circles. (b) Scattering rates minus dc values vs frequency with power-law fits, according to the equation $\frac{1}{\tau (\omega ,T)}=\frac{1}{\tau (0,T)}+A{\omega }^n$ for temperatures below 12 K. (c) The power-law exponents of the fits to the scattering rates as a function of temperature. Error bars were estimated by additionally fitting the data from 0.2 THz to both 1.25 and 1.75 THz and taking the average of the values [25].

Figure 4

(a) Renormalized masses as functions of frequency. (b) The $\omega \to 0$ limit of the frequency-dependent renormalized masses as functions of temperature extracted from extended Drude model fits. Error bars reflect the uncertainty in the extended Drude model fits, while the gray shadow reflects the uncertainty from the range of possible values of ${\omega }_p$. The red points are from a data set that was taken at an earlier time than the data represented in blue.