Statics and dynamics of fluctuation quenching in elastic media under an ordering force: Application to light scattering in KH2PO4

Phase transitions in elastic media, taking place under the application of the force conjugated to the order parameter (the ordering force), are discussed theoretically. The force imposes to the solid the symmetry of the new phase, and this produces a linear coupling between order parameter and scalar variables such as density and energy. If there exists a diverging susceptibility $\chi (T,\overrightarrow{\mathrm{q}})$ in the absence of the force, it will generally become quenched by the induced coupling to density in the elastic solid. Specifically, one finds that $\chi$ acquires a discontinuity at the wave vector $q=0\mathrm{}$, with only $\chi \mathrm{}(T,0\mathrm{})\mathrm{}$ that can diverge. This fluctuation quenching is discussed in general, and detailed calculations are given for the ferroelastic transition in K${\mathrm{H}}_2$P${\mathrm{O}}_4$. As light scattering is a choice investigation method in that case, particular results are given for the scattering intensity, as well as for the size and motion of the scattering cone under the applied force. The force also modifies the dynamics in an essential way, as order-parameter fluctuations become linearly coupled to entropy fluctuations. This coupling leads to a thermal central peak whose strength and width are derived.