Lattice-Dynamics Of K₂pt(CN)₄Br_0.3.3.2D₂o (KCP) Studied By Inelastic Neutron-Scattering

The lattice-dynamicalproperties of the "quasi-one-dimensional conductor" K₂Pt(CN)₄Br0.3.3.2D₂O(KCP₎ has been studied by neutron inelastic scattering at temperatures between20 and 240 K with special attention to the relation to the partialthree-dimensional ordering at T≈100K. We have investigated the general lattice dynamics by measuring theacoustic-phonon dispersion relations. Secondly, extensive measurements havebeen carried out to clarify the nature of the excitations in the vicinity of the 2k_F anomaly, which are restricted to extremely narrow wave-vector regions. In these regions we present normalized intensity contours, which may be directlycompared with theoretical calculations. At all temperatures below 160 K we find a maximum in the scattering at the 2k_F anomaly with an energy of 2.5 meV, as has recently been found by Comès etal. At lower temperatures the inelastic scattering becomes well separated in energy from the elastic scattering, signifying a phonon gap. We find itplausible to ascribe the apparent disappearance of this gap at higher temperatures to phonon lifetime effects, leading to the conclusion that thephonon frequency does not, at any temperature, condense to ω=0. At all temperatures, we find the scattering in the 2k_F anomaly to be connected with the regular phonons. Furthermore, the inelastic scattering intensities at the 2k_F anomaly is found to vary slowly with the wave-vector component perpendicular to the Pt chains and does not seen to reflect the buildup of the transverse,static correlations at lower temperatures. Our results extend previous neutroninelastic studies in several ways, and are found to be in fair agreement withthe recent infrared (ir) and Raman scattering data.