Structural phase transitions, soft modes and quasi-elastic scattering

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Abstract
Experimental measurements of light and neutron scattering in materials which undergo displacive structural phase transitions have shown that there is frequently quasi-elastic response in addition to the scattering predicted by the soft mode theory. These measurements are reviewed and the phenomenological theory described. The anharmonic lattice dynamical treatment of these effects is used to show that within the framework of a Landau theory quasi-elastic scattering is expected both above and below Tc at some phase transitions, while at others only below Tc. If critical fluctuations cause a breakdown of the Landau theory close to Tc, then additional quasi-elastic scattering may be expected. The importance of these critical fluctuations may be assessed with the techniques of renormalization group theory.