Phonon dispersion and Kohn anomaly in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ)

The phonon dispersion relation of the one-dimensional conductor, deuterated tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ), has been studied by inelastic neutron scattering over a temperature range of 60-300 K. Acoustic phonons propagating along the chain direction (the monoclinic $b^{*}$ direction) are studied in detail in search for a giant Kohn anomaly. Below 150 K, a sharp anomaly develops near the transverse branch at $2k_F$ and becomes more pronounced as the Peierls transition at $T=54\mathrm{}$ K is approached. Additional room-temperature measurements on our protonated sample showed excellent agreement with the deuterated crystal.