Optical properties of the Ce and La ditelluride charge density wave compounds

The La and Ce ditellurides $\mathrm{La}{\mathrm{Te}}_2$ and $\mathrm{Ce}{\mathrm{Te}}_2$ are deep in the charge-density-wave (CDW) ground state even at $300\mathrm{K}$. We have collected their electrodynamic response over a broad spectral range from the far infrared up to the ultraviolet. We establish the energy scale of the single particle excitation across the CDW gap. Moreover, we find that the CDW collective state gaps a large portion of the Fermi surface. Based on the observation of a power-law behavior in the optical conductivity, similar to that found for the related rare-earth tritellurides but over a reduced frequency range, we suggest that interactions and umklapp processes may play a role in the onset of the CDW broken-symmetry ground state.

Figure 1

(Color online) $R\left(\omega \right)$ of $\mathrm{La}{\mathrm{Te}}_2$ and $\mathrm{Ce}{\mathrm{Te}}_2$ at $300\mathrm{K}$. The inset is a blow-up of $R\left(\omega \right)$ emphasizing the broad absorption feature in the midinfrared spectral range. The arrows mark the shallow minimum in $R\left(\omega \right)$ at the onset of the broad absorption, peaked between 5000 and $6000{\mathrm{cm}}^{-1}$ and overlapped to the plasma edge feature.

Figure 2

(Color online) Room temperature ${\sigma }_1\left(\omega \right)$ of $\mathrm{La}{\mathrm{Te}}_2$ and $\mathrm{Ce}{\mathrm{Te}}_2$. Arrows mark the position of the midinfrared peak for each compound (see text). Inset: Drude-Lorentz fit for $\mathrm{Ce}{\mathrm{Te}}_2$, showing the experimental data, the total fitted curve, and the Drude-Lorentz components.

Figure 3

(Color online) ${\sigma }_1\left(\omega \right)$ of $R{\mathrm{Te}}_2$ ($R=\mathrm{La}$ and Ce) plotted on a bilogarithmic scale. The $y$ axis is scaled by the maximum of the midinfrared peak in ${\sigma }_1$, while the energy axis is scaled by the frequency where the maximum in ${\sigma }_1\left(\omega \right)$ occurs. The solid lines are power-law fits to the data (the exponents are given in the figure).