Temperature dependence of free carriers and optical phonons in ${\mathrm{LiInSe}}_2$ in the terahertz frequency regime

We investigate free carriers dynamics and anharmonic decay of optical phonons in ${\mathrm{LiInSe}}_2$ by using low temperature THz time-domain spectroscopy. The quantitatively determined refractive index and absorption coefficient are significantly influenced by temperature. By fitting the evaluated dielectric functions with a Drude-Lorentz model, the temperature dependencies of the plasmon frequency, scattering rates of free carriers, activation energies of carriers, and frequencies and linewidths of optical phonons are investigated. The contributions of three- and four-phonon processes to the temperature dependent optical phonon frequency and linewidth variations are quantitatively determined.

Figure 1

Schematic illustration of unit cell structure of LISe crystal (adopted from Ref. [8]).

Figure 2

Refractive index (a) and absorption coefficient (b) of LISe along the (001) axis from 20 to 300 K.

Figure 3

Experimental data (symbols) and DL fitting (solid curve) of the dielectric function of LISe along the (001) axis at 20 K and 200 K.

Figure 4

Temperature dependence of the plasmon frequency (a) and scattering rate (b) of free carriers in LISe along the (001) axis. Error bars are determined from the DL fit. The red solid curve in (a) is the fit using Eq. (3) with $\mathrm{\Delta }E=17.7$ meV.

Figure 5

Temperature dependence of the center frequency (a) and linewidth (b) of the dominant phonon resonance at around 1.34 THz in LISe along the (001) axis. Error bars are determined from the DL fit. The red solid curve is the theoretical fit using both three- and four-phonon processes. The blue dashed curve is the theoretical fit using only three-phonon processes.

Figure 6

Refractive index (a) and absorption coefficient (b) of LISe along the (100) axis from 20 to 300 K.

Figure 7

Experimental data (symbols) and DL fitting (solid curve) of the dielectric function of LISe along the (100) axis at 20 K and 200 K.

Figure 8

Temperature dependence of the plasmon frequency (a) and scattering rate (b) of free carriers in LISe along the (100) axis. Error bars are determined from the DL fit. The red solid curve in (a) is the theoretical fit using Eq. (3) with $\mathrm{\Delta }E=14.6$ meV.

Figure 9

Temperature dependence of the center frequency (a) and linewidth (b) of the dominant phonon resonance at around 2.10 THz in LISe along the (100) axis. Error bars determined from the DL fit are almost completely covered by the size of the black symbol. The red solid curve is the theoretical fit using both three- and four-phonon processes. The blue dashed curve is the theoretical fit using only three-phonon processes.