Coherent motion of two-site self-trapped excitons and energy transport in wide-band dielectrics

Abstract

The excitonic mechanism of energy transport from a host crystal to impurity is of different character for narrow excitonic bands of organic dielectrics and for a broad band inherent in some inorganic dielectrics. Unlike the former case, the latter presents serious difficulties when explaining the entire set of the relevant experimental data within the traditional notion on exciton dynamics: The predominant mechanism of energy transport cannot be associated, without encountering essential contradictions, either with free or with self-trapped excitons. It is shown that all contradictions are eliminated by taking into account a dual nature of a two-site self-trapped exciton. On one hand, its vibration levels turn to narrow subbands with a very large effective mass; this provides an easy localization of excitons near impurity centers at any energy point of the exciton band. On the other hand, a two-site exciton can move together with its deformation cloud in a coherent way nearly with the sound velocity; this explains a fast energy transport and its decreasing temperature dependence observed experimentally.