FAR-INFRARED REFLECTION SPECTRA OF 1T-TAS2 AND 1T-TASE2 IN COMMENSURATE AND INCOMMENSURATE CHARGE-DENSITY-WAVE STATES

Ten strong infrared-active modes have been observed in both 1T-TaS2 and 1T-TaSe2 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 modes by the CDW-induced lattice distortion. In 1T-TaSe2, 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. © 1979 The American Physical Society.