Lattice instabilities in coupled pseudospin–phonon systems

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Abstract
General features of coupled pseudospin-phonon systems are discussed. The pseudospin variable is inroduced in a purely formal procedure so that the results ae applicable to various physical systems undgoing structural phase transitions. The dynamical proprties of the coupled systems ae discussed in the terms of susceptibility tensors with emphasis on he appearance of soft excitations due to the interaction between pseudospins and phonons. Experimentally, the properties of th coupled systms are mainly investigated with various scattering techniques. Some of of the observed results of typical coupled systems are discussed on the basis of the general theory.