Transverse acoustic modes in TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane)

The transverse acoustic modes [$\mathrm{TA}\left(a^{*}\right)$] of tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) propagating along the chain direction ($b^{*}$) and polarized along ${\overrightarrow{\mathrm{a}}}^{*}$ are studied between room temperature and 60 K. No anomalous behavior is observed at $q=2\mathrm{}{k}_F$ in the temperature region studied. These results, taken in conjunction with other neutron and x-ray experiments, show that the Kohn anomaly in TTF-TCNQ occurs in the $\mathrm{TA}\left(c^{*}\right)$ branch which is the transverse branch polarized principally along $c^{*}$.