Observation of a Soft Mode Phonon in MEM(TCNQ) 2 below Spin-Peierls Transition by Far Infrared Spectroscopy

Frequency shift and broadening of a soft mode phonon in MEM(TCNQ) 2 were clearly observed by far infrared spectroscopy below 30 cm -1 , as increasing temperature from 4 K to a spin-Peierls transition temperature. The soft mode phonon frequency at 4 K is 8.5 cm -1 . Taking account of the frequency of 8.5 cm -1 and its mode of motion, it is considered that the dynamical displacement of the soft mode phonon lies almost in a TCNQ plane which contains the static shift associated with the spin-Peierls transition. In addition, two holding phonon bands, whose frequencies with 19.5 and 24.5 cm -1 do not vary with the temperature, were clearly observed only below the spin-Peierls transition temperature.