The theory of two-phonon critical scattering

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Abstract
The two-phonon critical scattering occurring near a structural phase transition is studied within the framework of a time-dependent Landau-Ginzburg model. It is established that the width of the critical low-frequency scattering appearing in the two-phonon spectrum (accessible to light-scattering experiments) and the width of the critical scattering in the one-phonon spectrum (accessible to neutron-scattering experiments) form a universal ratio, which is calculated to second order in epsilon =4-d. Implications for scattering experiments on structural phase transitions are discussed.