Neutron Scattering and the Electron-Phonon Coupling in One-Dimensional Conductors

Abstract

I suggest that the Kohn anomaly in tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) occurs in C-H (or C-D) molecular modes rather than in an acoustic phonon. Structure-factor calculations explain why neutron-scattering results from protonated and deuterated samples are very different. A tentative identification of the involved modes is consistent with the TCNQ chains' ordering at $T=54$ K and the TTF chains' ordering at $T=47$ K. A similar analysis of ${\mathrm{K}}_{2}$[${\mathrm{Pt}(\mathrm{CN})}_{4}$]${\mathrm{Br}}_{0.3}$ \ifmmode\cdot\else\textperiodcentered\fi{} 3.2${\mathrm{D}}_{2}$O verifies that the anomaly occurs in the longitudinal acoustic phonon.