Optical spectra of the heavy fermion uniaxial ferromagnet ${\text{UGe}}_2$

We report a detailed study of ${\text{UGe}}_2$ single crystals using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low-frequency interband transition and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp increase in the low-frequency mass and reduction in scattering rate below the upper ferromagnetic transition $T_C=53\text{K}$ indicating the emergence of a heavy fermion state triggered by the ferromagnetic order. The characteristic changes are exhibited most strongly at an energy scale below 12 meV. They recover their unrenormalized value above $T_C$ and for $\omega >40\text{meV}$. In contrast no sign of an anomaly is seen at the lower transition temperature of unknown nature, $T_x\sim 30\text{K}$, observed in transport and thermodynamic experiments.

Figure 1

(Color online) The normal-incidence reflectivity $R\left(\omega \right)$ of ${\text{UGe}}_2$ at three temperatures. The inset shows $R\left(\omega \right)$ at 8.7, 12.3, and 18.6 meV. The reflectivity curves at 100 and 290 K overlap on this scale.

Figure 2

(Color) (a) ${\sigma }_1\left(\omega \right)$ at different temperatures in the energy range of 0–3.7 eV. (b) The enlargement of the low-energy data including the zero-frequency extrapolations. The inset shows ${\sigma }_2\left(\omega \right)$ at different temperatures.

Figure 3

(Color) The effective mass and the scattering rate as a function of photon energy derived from the extended Drude model with $\hslash {\omega }_p=3.5\text{eV}$. The dashed lines below 6 meV show the extrapolation toward zero frequency obtained as described in the text.

Figure 4

(Color online) Temperature dependence of the effective mass and the scattering rate for two frequencies.

Figure 5

(Color online) [(a) and (b)] Schematic band structure within the PAM of the conduction ${ϵ}_c$ and the $5f$ electrons ${ϵ}_f$ at high and low temperature, respectively. Vertical arrow indicates the interband transitions. ${ϵ}_{\uparrow }$ and ${ϵ}_{\downarrow }$ denote the spin-up and spin-down bands. [(c) and (d)] Sketch of the optical conductivity at high and low temperatures.