The critical dynamics of SrTiO3: a study of neutron- and Raman-determined soft-mode lineshapes

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Abstract
The authors report new Raman and neutron scattering measurements made on the cubic perovskite SrTiO3. By comparing the soft-mode lineshapes in the low-temperature phase they have been able to observe the breakdown of the one-phonon approximation conventionally used in the analysis of Raman scattering due to the scattering from critical two-phonon and interference scattering processes. They have determined the limiting behaviour of the soft-mode frequencies and widths on the approaching the transition temperature, Tc. Below Tc the soft mode has components of Eg and A2g symmetry. On approaching Tc from below, the Eg mode frequency saturates at a value of 0.16 THz. On approaching Tc from above, the widths of the phonons are about 0.20 THz. They have found that there is appreciable scattering at low energies, which cannot be accounted for by a simple classically damped phonon model. This extra scattering may be interpreted as a broad, critical central peak with a width comparable to that of the soft phonons.