The Dynamics of Jahn—Teller Crystals

This chapter deals with the spectrum of elementary excitations of Jahn—Teller elastics; in general they represent a set of dynamically coupled electron-phonon (or, as they are sometimes called, vibron-phonon) modes active in structural phase transition. The notion of elementary excitations in a crystal, one of the most fundamental precepts in solid-state physics, plays an extremely important role in the problem of phase transitions. This is attributable to the relationship of this notion to the soft-mode concept [1–3], according to which the frequency of one of the elementary excitations of the crystal tends to zero as the system approaches the phase transition temperature, thus accounting for the instability against the creation of a new phase.