We reconsider the Cross and Fischer approach to spin-Peierls transitions. We show that a soft phonon occurs only if Omega_0<2.2 T_SP. For CuGeO_3 this condition is not fulfilled and the calculated temperature dependence of the Peierls-active phonon modes is in excellent agreement with experiment. A central peak of a width ~0.2 meV is predicted at T_SP. Good agreement is found between theory and experiment for the pretransitional Peierls-fluctuations. Finally, we consider the problem of quantum criticality in CuGeO_3.
@article{arxiv.cond-mat/9804092,
title = {Dynamics of the Peierls-active phonon modes in CuGeO_3},
author = {Claudius Gros and Ralph Werner},
journal= {arXiv preprint arXiv:cond-mat/9804092},
year = {2009}
}