Lamellar fluctuations melt ferroelectricity

Abstract

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 [1][2][3][4][5]. 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.

Mesoscopic fluctuating domains in strontium titanate

“…Coupling between the electrons and the TO mode plays a key role in a number of theoretical scenarios [21,22,26]. We have shown that a hybridized TO -TA phonon mode is to be taken L140301-4 into account and may even change the understanding of the ground state of quantum paralectrics [59].…”