Low energy electrodynamics of the Kondo-lattice antiferromagnet CeCu${}_2$Ge${}_2$

We present time-domain terahertz spectroscopy data of a thin film of the Kondo-lattice antiferromagnet CeCu${}_2$Ge${}_2$. The low-frequency complex conductivity has been obtained down to temperatures below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is similar to ones found in other heavy-fermion compounds that do not exhibit magnetic order. Using this data in conjunction with dc resistivity measurements, we obtain the frequency dependence of the scattering rate and effective mass through an extended Drude model analysis. The zero-frequency limit of this analysis yields evidence for large mass renormalization even in the magnetic state, the scale of which agrees closely with that obtained from thermodynamic measurements.

Figure 1

Resistivity as a function of temperature for CCG film. Around 110 K there is a broad peak corresponding to crystal-field splitting. At 5.5 K there is a sharp peak attributed to the Kondo-lattice coherence temperature $T^{*}$; a sharp cusp is seen near 4 K, signaling the onset of antiferromagnetic order.

Figure 2

(a)–(f) Real and imaginary parts of the complex conductivity as a function of frequency for six temperatures in the frequency range of 0–1 THz. Solid lines indicate experimental data, while dashed lines show results of a fit described in the text. The values of the dc conductivity are also shown as a solid circle.

Figure 3

(a) The renormalized mass as a function of frequency derived from the extended Drude model for three temperatures. Solid lines indicate experimental data, with dashed lines showing results of a fit described in the text. (b) Renormalized mass at zero frequency as a function of temperature based on the zero-frequency extrapolation of the extended Drude model fits. Note the log scale on temperature axis. (c) Scattering rate as a function of frequency with numerical fits shown as dashed lines for three temperatures. (d) Scattering rate at zero frequency as a function of temperature. The error bars indicate uncertainty, which is primarily due to the ambiguity in determining ${\omega }_p^2$ from FTIR measurements.