Inelastic Neutron Scattering Study of the 2k_F Instability in K₂Pt(CN)₄Br_0.30 x D₂0 (KCP)

Besides the modifications of the longitudinal acoustic dispersion in the c‐direction, ill‐elastic neutron scattering measurements of the 2k_F instability in KCP between room temperature and 60 K reveal an additional excitation at a frequency of 0.6 THz for the wave vector 2k_F. This excitation obviously corresponds to the additional structure earlier observed at low temperatures by infrared measurements but the extension in wave vector 2k_F – 0.2c* < q_c < 2k_F + 0.1c*, the very small dispersion, and the fact that, though much weaker than at low temperature, it is still observable at room temperature, casts serious doubts on its interpretation in terms of the pinned charge density wave (pinned Fröhlich mode). The existence of a low frequency longitudinal optical phonon branch is more likely, despite the unexplained weak intensity of the corresponding phonons.