Dynamics of cooperative Jahn-Teller T-systems

We consider a cubic crystal with a triply degenerate electronic ground-state in each unit cell (T-state). A linear Jahn-Teller interaction with acoustic phonons is assumed (T-e coupling). Below a critical temperature Tsub(c) a tetragonal cooperative ordering is established. If a satic crystal field splits the electronic states, an anticrossing of the cooperative excitation branches and the phonons in the small k-regime is found. The resulting 'mixed modes' are calculated (RPA and isotropic Debye model). The dispersion curves are given for specific k-directions and the temperature dependence is discussed. (orig.)