Lattice dynamics of ${\mathrm{CuGeO}}_3:$ Inelastic neutron scattering and model calculations

The lattice dynamics in ${\mathrm{CuGeO}}_3$ has been analyzed by the combination of inelastic neutron scattering studies and lattice-dynamical model calculations. We report on an almost complete set of dispersion curves along the three orthorhombic directions and along [101]. Some modes exhibit rather low phonon frequencies in relation either with the almost layered character of the crystal structure or with an instability of the ${\mathrm{CuO}}_2$ ribbons and ${\mathrm{GeO}}_4$ chains against twisting. The dispersion of branches associated with the modes directly involved in the spin-Peierls transition allows to explain the particular propagation vector of the structural distortion in the dimerized phase.