Far-infrared reflection spectra of $1T$-Ta${\mathrm{S}}_2$ and $1T$-Ta${\mathrm{Se}}_2$ in commensurate and incommensurate charge-density-wave states

Ten strong infrared-active modes have been observed in both $1T$-Ta${\mathrm{S}}_2$ and $1T$-Ta${\mathrm{Se}}_2$ in their commensurate charge-density-wave (CDW) states. The frequency ratios of corresponding modes in the two crystals indicate that the four low-frequency and six high-frequency modes arise from the acoustic- and optic-phonon branches, respectively, of the undistorted crystal folded into $k=0\mathrm{}$ modes by the CDW-induced lattice distortion. In $1T$-Ta${\mathrm{Se}}_2$, a strong absorption is observed, corresponding to each mode, as the temperature is lowered to 4.2 K. This appears to be related to a strong scattering of the change carriers responsible for most of the reflectivity at each phonon frequency.