FLUCTUATION EFFECTS ON THE RAMAN-SCATTERING FROM THE CHARGE-DENSITY-WAVE SYSTEM TTF-TCNQ

The resonant Raman spectrum of TTF-TCNQ has been measured from room te mperature to IO K, using a Fourier Raman spectrometer. Only features d ue to the TCNQ molecule are observed. New Raman lines appear at temper atures below 150 K as the fluctuating charge-density wave (CDW) occurs . The intensity of the new Raman lines increases with decreasing tempe rature. In addition to the Raman allowed modes, we observe numerous Li nes originating from the usually infrared-active-only modes, which bec ome Raman active via the Frohlich interaction in the fluctuating and s tatic CDW phases. The appearance of the strong out-of-plane intramolec ular B-3u vibrational modes of TCNQ confirms an earlier x-ray study th at found that the CDW on the TCNQ chain involved such an out-of-plane distortion of the TCNQ molecule. The condensation of the longitudinal acoustic phonon is also observed in the Raman spectrum.