Phonon-assisted electronic excitation transfer near distortive phase transitions

An anomalous temperature dependence of the phonon-assisted electronic excitation transfer rate is to be expected near a distortive phase transition, when the energy mismatch between donor and acceptor energy levels is close to that of a soft optical mode, or an order-parameter-sensitive hard mode. In the case of electron coupling to soft acoustical modes, it is shown that the Debye model used in current theories breaks down near T_c.