Coupling of electrons and intramolecular vibrations in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane)

We report experimental evidence for electron-intramolecular vibration coupling in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane). Linear coupling of totally symmetric vibrations of $a_g$ modes to the oscillation in the phase of the charge-density waves gives rise to a set of IR-(infrared-) active modes which grow in intensity with the onset of the Peierls transition. Our experimental estimates of electron-intramolecular coupling constants of the TCNQ molecule are in good agreement with previous values. Moreover, the experimentally determined total electron-intramolecular coupling constant is $\lambda \simeq 0.35$ for TCNQ stacks in TTF-TCNQ. These results indicate that the Peierls distorted state is predominantly stabilized by coupling of the unpaired electrons to the intramolecular vibrations. With the onset of Peierls transition, the two IR-active modes of CN-stretching vibration show a splitting due to a shift in frequency of the mode with the dipole component parallel to the crystallographic $b$ and $c$ axes. The splitting is also seen in the selenium analog TSeF-TCNQ and is consistent with the x-ray diffuse scattering result that the Peierls distortion is due to a rigid molecular translation in the $\mathrm{bc}$ plane. The temperature dependence of both effects show the presence of the sizable amplitude of the fluctuating charge-density waves in the temperature range $53<T<\mathrm{}150\mathrm{}$ K. This result indicates that the collective contribution to dc conductivity is not limited to a small temperature range just above $T_c\simeq 53$ K. We have also commented on some theoretical points of view using the underlying information in the vibrational spectra of TTF-TCNQ. A preliminary report of the experimental results was given by S. Etemad [Bull. Am. Phys. Soc. 23, 381 (1978)].