Magneto-elastic excitations in spin-Peierls systems

Within the random phase approximation to the spin-Peierls transition two parameter regimes of phonon softening and hardening are present. Magneto-elastic excitations are discussed in detail for phonons coupled to the exactly solvable model of XY spin chains for both regimes, leading to a modified interpretation of the 30 cm${}^{-1}$ mode in CuGeO${}_3$. Frustrated Heisenberg chains coupled to phonons satisfactory describe the pre-transitional quasi-elastic scattering in CuGeO${}_3$. A real space interpretation of the quasi-elastic scattering is given justifying effective Ising model approaches.