Optical properties of the charge-density-wave compound ${\text{CeTe}}_2$

The temperature dependence of the optical conductivity of ${\text{CeTe}}_2$ was obtained for the $E\parallel ab$ plane and $E\parallel c$ axis at photon energies ranging from terahertz to visible light. Two gap states are found in the optical conductivity spectra. The magnitudes of the gaps are estimated to be 0.448 and 0.109 eV in the $E\parallel ab$ plane. The former is due to perfect nesting, while the latter is due to imperfect nesting. The magnitude of the gap due to perfect nesting on the $c$ axis is evaluated to be 0.854 eV. The Drude response caused by the ungapped Fermi surface is found only on the $E\parallel ab$ plane since ${\text{CeTe}}_2$ is a quasi-two-dimensional material. The portion of the ungapped Fermi surface across all Fermi surfaces is about 3%.

Figure 1

(Color online) Reflectivity spectra, $R\left(\omega \right)$, of ${\text{CeTe}}_2$ for the $E\parallel ab$ plane on a logarithmic scale for temperatures in the range 7–300 K. Inset: $R\left(\omega \right)$ on a linear scale.

Figure 2

(Color online) Reflectivity spectra, $R\left(\omega \right)$, of ${\text{CeTe}}_2$ for the $E\parallel c$ axis on a logarithmic scale for temperatures in the range 7–300 K.

Figure 3

(Color online) Optical conductivity spectra, ${\sigma }_1\left(\omega \right)$, of ${\text{CeTe}}_2$ for the $E\parallel ab$ plane on a logarithmic scale for temperatures in the range 7–300 K.

Figure 4

(Color online) Optical conductivity spectra, ${\sigma }_1\left(\omega \right)$, of ${\text{CeTe}}_2$ for the $E\parallel c$ axis on a logarithmic scale for temperatures in the range 7–300 K. Inset: ${\sigma }_1\left(\omega \right)$ on a linear scale.

Figure 5

(Color online) Imaginary part, ${\epsilon }_2$, of the dielectric constant for ${\text{CeTe}}_2$ at $T=7\text{K}$ for $E\parallel ab$ plane. A solid line indicates a fitted curve by a Drude model.

Figure 6

(Color online) ${\left(\alpha h\nu \right)}^2$ vs $h\nu$ plot of ${\text{CeTe}}_2$ for the $E\parallel ab$ plane at $T=7$ and 300 K. The solid line represents a linear fit to the data. Inset: ${\left(\alpha h\nu \right)}^2$ vs $h\nu$ plot in lower-energy regions.

Figure 7

(Color online) ${\left(\alpha h\nu \right)}^2$ vs $h\nu$ plot of ${\text{CeTe}}_2$ for the $E\parallel c$ axis at $T=7$ and 300 K. The solid line indicates a linear fit to the data.

Figure 8

(Color online) Fit of the measured optical conductivity, ${\sigma }_1\left(\omega \right)$, of ${\text{CeTe}}_2$ for the $E\parallel ab$ plane using the Drude and Lorentz models showing the experimental data $\left({\sigma }_{\text{exp}}\right)$, the total curve fitted by six Lorentz functions $\left({\sigma }_{\text{LorentzSum}}\right)$, and the Drude-Lorentz components (solid and dotted lines).