Abstract
The damping of collective modes in TbVO4 near the structural phase transition is calculated with the use of a perturbation method applied to a pseudospin-phonon model. A pseudofermion representation is utilised for the pseudospin operator in order to take into account an intrinsic effect of fluctuation field caused by surrounding pseudospins on a given pseudospin. It is shown that the spectrum of singlet pair excitations splits into a pair of spin waves by the fluctuation field and at the same time it gives the damping of the collective modes of the order of the experimental values. The damping of both the acoustic phonon and the central doublet excitation are also derived by using relationships which hold exactly between pseudospin and phonon Green functions.
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