Lamellar fluctuations melt ferroelectricity

We consider a Ginzburg-Landau theory for a ferroelectric phase transition whose primary order parameter (electrical polarization) is coupled to gradients of elastic strain (flexoelectricity). At the harmonic level, such couplings lead to a hybridization of acoustic and optic phonon modes and can lead to phases with modulated lattice structures that precede the symmetry broken state due to level repulsion. Here, we show that at the mean field plus Gaussian approximation, the fluctuations of polarization of the parent phase tend to diverge at the onset of the transition and therefore the long-range-ordered modulated phase is avoided. We discuss the implications for the nearly ferroelectric SrTiO$_3$ and KTaO$_3$, and propose that these systems are melted versions of an underlying modulated state which is dominated by non-zero momentum thermal fluctuations except at the very lowest temperatures.Comment: 15 pages, 3 figures, 1 table, supplementary information include